Defective G protein activation of the cAMP pathway in rat kidney during genetic hypertension

Attenuated diuresis and natriuresis in response to glucagon-like peptide-1 in hypertensive rats are associated with lower expression of the glucagon-like peptide-1 receptor in the renal vasculature

Accumulating evidence from clinical and experimental studies indicates that the incretin glucagon-like peptide-1 (GLP-1) elicits blood-pressure lowering effects via its diuretic, natriuretic and vasodilatory properties. The present study investigated whether acute infusion of GLP-1 induces diuresis and natriuresis in spontaneously hypertensive rats (SHRs). Additionally, we examined whether GLP-1 influences the vascular reactivity of the renal arteries of normotensive and hypertensive rats and elucidated the underlying mechanisms. We found that the increase in urinary output and urinary sodium excretion in response to systemic infusion of GLP-1 for 30min in SHRs was much less pronounced than in normotensive rats. The diuretic and natriuretic actions of GLP-1 in normotensive rats were accompanied by increases in GFR and RBF and a reduction in RVR through activation of the cAMP signaling pathway. However, no changes in renal hemodynamics were observed in SHRs. Similarly, GLP-1 induced an endothelium-independent relaxation effect in the renal arteries of normotensive rats, whereas the renal vasculature of SHRs was unresponsive to this vasodilator. The absence of a GLP-1-induced renal artery vasodilator effect in SHRs was associated with lower expression of the GLP-1 receptor, blunted GLP-1-induced increases in cAMP production and higher activity and expression of the GLP-1 inactivating enzyme dipeptidyl peptidase IV relative to the renal arteries of normotensive rats. Collectively, these results demonstrate that the renal acute responses to GLP-1 are attenuated in SHRs. Thus, chronic treatment with incretin-based agents may rely upon the upregulation of GLP-1/GLP-1 receptor signaling in the kidneys of hypertensive patients and experimental models.

Renal autoregulation in health and disease

Intrarenal autoregulatory mechanisms maintain renal blood flow (RBF) and glomerular filtration rate (GFR) independent of renal perfusion pressure (RPP) over a defined range (80-180 mmHg). Such autoregulation is mediated largely by the myogenic and the macula densa-tubuloglomerular feedback (MD-TGF) responses that regulate preglomerular vasomotor tone primarily of the afferent arteriole. Differences in response times allow separation of these mechanisms in the time and frequency domains. Mechanotransduction initiating the myogenic response requires a sensing mechanism activated by stretch of vascular smooth muscle cells (VSMCs) and coupled to intracellular signaling pathways eliciting plasma membrane depolarization and a rise in cytosolic free calcium concentration ([Ca(2+)]i). Proposed mechanosensors include epithelial sodium channels (ENaC), integrins, and/or transient receptor potential (TRP) channels. Increased [Ca(2+)]i occurs predominantly by Ca(2+) influx through L-type voltage-operated Ca(2+) channels (VOCC). Increased [Ca(2+)]i activates inositol trisphosphate receptors (IP3R) and ryanodine receptors (RyR) to mobilize Ca(2+) from sarcoplasmic reticular stores. Myogenic vasoconstriction is sustained by increased Ca(2+) sensitivity, mediated by protein kinase C and Rho/Rho-kinase that favors a positive balance between myosin light-chain kinase and phosphatase. Increased RPP activates MD-TGF by transducing a signal of epithelial MD salt reabsorption to adjust afferent arteriolar vasoconstriction. A combination of vascular and tubular mechanisms, novel to the kidney, provides for high autoregulatory efficiency that maintains RBF and GFR, stabilizes sodium excretion, and buffers transmission of RPP to sensitive glomerular capillaries, thereby protecting against hypertensive barotrauma. A unique aspect of the myogenic response in the renal vasculature is modulation of its strength and speed by the MD-TGF and by a connecting tubule glomerular feedback (CT-GF) mechanism. Reactive oxygen species and nitric oxide are modulators of myogenic and MD-TGF mechanisms. Attenuated renal autoregulation contributes to renal damage in many, but not all, models of renal, diabetic, and hypertensive diseases. This review provides a summary of our current knowledge regarding underlying mechanisms enabling renal autoregulation in health and disease and methods used for its study.

Effect of dipeptidyl peptidase 4 inhibition on arterial blood pressure is context dependent

Because the effects of dipeptidyl peptidase 4 (DPP4) inhibitors on blood pressure are controversial, we examined the long-term effects of sitagliptin (80 mg/kg per day) on blood pressure (radiotelemetry) in spontaneously hypertensive rats (SHR), Wistar-Kyoto rats, and Zucker Diabetic-Sprague Dawley rats (metabolic syndrome model). In SHR, chronic (3 weeks) sitagliptin significantly increased systolic, mean, and diastolic blood pressures by 10.3, 9.2, and 7.9 mm Hg, respectively, a response abolished by coadministration of BIBP3226 (2 mg/kg per day; selective Y1-receptor antagonist). Sitagliptin also significantly increased blood pressure in SHR treated with hydralazine (vasodilator; 25 mg/kg per day) or enalapril (angiotensin-converting enzyme inhibitor; 10 mg/kg per day). In Wistar-Kyoto rats, chronic sitagliptin slightly decreased systolic, mean, and diastolic blood pressures (-1.8, -1.1, and -0.4 mm Hg, respectively). In Zucker Diabetic-Sprague Dawley rats, chronic sitagliptin decreased systolic, mean, and diastolic blood pressures by -7.7, -5.8, and -4.3 mm Hg, respectively, and did not alter the antihypertensive effects of chronic enalapril. Because DPP4 inhibitors impair the metabolism of neuropeptide Y1-36 (NPY1-36; Y1-receptor agonist) and glucagon-like peptide (GLP)-1(7-36)NH2 (GLP-1 receptor agonist), we examined renovascular responses to NPY1-36 and GLP-1(7-36)NH2 in isolated perfused SHR and Zucker Diabetic-Sprague Dawley kidneys pretreated with norepinephrine (to induce basal tone). In Zucker Diabetic-Sprague Dawley kidneys, NPY1-36 and GLP-1(7-36)NH2 exerted little, if any, effect on renovascular tone. In contrast, in SHR kidneys, both NPY1-36 and GLP-1(7-36)NH2 elicited potent and efficacious vasoconstriction.

IN CONCLUSION

(1) The effects of DPP4 inhibitors on blood pressure are context dependent; (2) The context-dependent effects of DPP4 inhibitors are due in part to differential renovascular responses to DPP4’s most important substrates (NPY1–36 and GLP-1(7–36)NH2) [corrected]; (3) Y1 receptor antagonists may prevent the prohypertensive and possibly augment the antihypertensive effects of DPP4 inhibitors.

Dopamine and renal function and blood pressure regulation

Dopamine is an important regulator of systemic blood pressure via multiple mechanisms. It affects fluid and electrolyte balance by its actions on renal hemodynamics and epithelial ion and water transport and by regulation of hormones and humoral agents. The kidney synthesizes dopamine from circulating or filtered L-DOPA independently from innervation. The major determinants of the renal tubular synthesis/release of dopamine are probably sodium intake and intracellular sodium. Dopamine exerts its actions via two families of cell surface receptors, D1-like receptors comprising D1R and D5R, and D2-like receptors comprising D2R, D3R, and D4R, and by interactions with other G protein-coupled receptors. D1-like receptors are linked to vasodilation, while the effect of D2-like receptors on the vasculature is variable and probably dependent upon the state of nerve activity. Dopamine secreted into the tubular lumen acts mainly via D1-like receptors in an autocrine/paracrine manner to regulate ion transport in the proximal and distal nephron. These effects are mediated mainly by tubular mechanisms and augmented by hemodynamic mechanisms. The natriuretic effect of D1-like receptors is caused by inhibition of ion transport in the apical and basolateral membranes. D2-like receptors participate in the inhibition of ion transport during conditions of euvolemia and moderate volume expansion. Dopamine also controls ion transport and blood pressure by regulating the production of reactive oxygen species and the inflammatory response. Essential hypertension is associated with abnormalities in dopamine production, receptor number, and/or posttranslational modification.

Regulation of 3',5'-cAMP in preglomerular smooth muscle and endothelial cells from genetically hypertensive rats

Our previous studies show that inhibition of phosphodiesterase 4 (PDE4) augments agonist-induced renovascular 3',5'-cAMP secretion more in isolated, perfused kidneys from spontaneously hypertensive rats (SHR) versus Wistar-Kyoto normotensive rats (WKY); however, whether this is because of PDE4 inhibition in renovascular smooth muscle cells or endothelial cells is unknown. Therefore, we examined the effects of 3-isobutyl-1-methylxanthine (broad-spectrum PDE inhibitor) and RO 20-1724 (selective PDE4 inhibitor) on isoproterenol-induced 3',5'-cAMP levels in cultured WKY and SHR preglomerular vascular smooth muscle and endothelial cells. 3-Isobutyl-1-methylxanthine and RO 20-1724 augmented isoproterenol-induced 3',5'-cAMP levels similarly in WKY versus SHR endothelial cells. In contrast, 3-isobutyl-1-methylxanthine and RO 20-1724 augmented isoproterenol-induced 3',5'-cAMP levels significantly more in SHR, compared to WKY, smooth muscle cells (P<0.0001). In both cell types from both rat strains, mRNA levels for the PDE4B subtype exceeded levels for the PDE4A, PDE4C, and PDE4D subtypes, and small interfering RNA knockdown of PDE4B mRNA in SHR smooth muscle cells increased isoproterenol-induced 3',5'-cAMP. mRNA levels for the PDE4B2 variant exceeded levels for the PDE4B1, PDE4B3, PDE4B4, and PDE4B5 variants. In vivo, infusions of RO 20-1724 increased the urinary excretion of 3',5'-cAMP more in SHR than WKY (P=0.0211). We conclude that (1) the greater effect of PDE4 inhibition on renovascular 3',5'-cAMP is mediated by inhibition of PDE4 in renovascular smooth muscle cells, not endothelial cells; (2) the major PDE4 subtype in both renovascular smooth muscle and endothelial cells is PDE4B with variant PDE4B2 likely being dominant; and (3) inhibition of PDE4 in vivo increases renal 3',5'-cAMP levels more in genetically hypertensive rats.

Dopamine-mediated inhibition of renal Na+/K+-ATPase in HK-2 cells is reduced by ouabain

1. Abnormal renal sodium handling is considered a major contributing factor in hypertension associated with chronic ouabain treatment. However, the molecular mechanisms involved in abnormal renal sodium handling have not been elucidated. Therefore, we investigated whether chronic ouabain treatment perturbs dopamine D(1) receptor function. 2. The expression and phosphorylation levels of the D(1) receptor in cells of the human proximal tubule cell line (HK-2) were determined using western blot analysis and reverse transcription polymerase chain reaction. The activity of the renal sodium/potassium pump (Na(+)/K(+)-ATPase) was measured using a colourimetric assay, and cyclic adenosine monophosphate accumulation was determined by performing a radioimmunoassay. 3. We showed that chronic ouabain treatment decreased the protein and mRNA expression levels of the D(1) receptor and increased the basal phosphorylation of the D(1) receptor in HK-2 cells. We also showed that in the presence of ouabain, HK-2 cells did not reveal the cyclic adenosine monophosphate accumulation and Na(+)/K(+)-ATPase inhibition induced by the D(1) receptor agonist fenoldopam. 4. We hypothesize that the ouabain-induced decrease in renal D(1) receptor function is responsible for the increase in renal sodium reabsorption, which eventually leads to ouabain-induced hypertension.

Regulation of renovascular adenosine 3',5'-cyclic monophosphate in spontaneously hypertensive rats

This study tested the hypothesis that regulation of 3',5'-cAMP levels in the kidney vasculature is abnormal in spontaneously hypertensive rats. In isolated, perfused kidneys from adult rats (16 weeks of age), isoproterenol similarly increased renal venous 3',5'-cAMP secretion from kidneys of hypertensive versus normotensive Wistar-Kyoto rats. However, a broad-spectrum phosphodiesterase inhibitor (isobutyl-1-methylxanthine) augmented isoproterenol (3 mumol/L)-induced increases in renal venous 3',5'-cAMP secretion more so in kidneys from adult hypertensive versus age-matched normotensive rats (31-fold and 5-fold, respectively; P<0.0001). In contrast to isoproterenol, broad-spectrum phosphodiesterase inhibition augmented forskolin-induced increases in renal venous 3',5'-cAMP secretion similarly in kidneys from adult hypertensive versus age-matched normotensive rats. In kidneys from adults of both strains, the effects of isobutyl-1-methylxanthine on isoproterenol-induced 3',5'-cAMP responses were mimicked by the inhibition of phosphodiesterase 4 (RO 20-1724) but not by the inhibition of phosphodiesterase 1 (3,8-methoxymethyl-3-isobutyl-1-methylxanthine) or phosphodiesterase 3 (milrinone). In kidneys from young (5 weeks of age), adult, and old (39 weeks of age) rats, RO 20-1724 augmented isoproterenol-induced renal 3',5'-cAMP secretion more so in kidneys from hypertensive rats. In adult hypertensive rats, arterial blood pressure and renal vascular resistance were elevated compared with age-matched normotensive rats, and intravenous infusions of RO 20-1724 reduced blood pressure and renal vascular resistance in hypertensive rats but had little effect on these variables in normotensive rats. We conclude that, in the renal vasculature of spontaneously hypertensive rats (young, adult, and old), there is increased activity of a compartment of phosphodiesterase 4. Selective inhibition of renal vascular phosphodiesterase 4 may represent a new strategy for improving renal hemodynamics in genetic hypertension.

The dopaminergic system in hypertension

Dopamine plays an important role in the pathogenesis of hypertension by regulating epithelial sodium transport, vascular smooth muscle contractility and production of reactive oxygen species and by interacting with the renin–angiotensin and sympathetic nervous systems. Dopamine receptors are classified into D1-like (D1 and D5) and D2-like (D2, D3 and D4) subtypes based on their structure and pharmacology. Each of the dopamine receptor subtypes participates in the regulation of blood pressure by mechanisms specific for the subtype. Some receptors regulate blood pressure by influencing the central and/or peripheral nervous system; others influence epithelial transport and regulate the secretion and receptors of several humoral agents. This review summarizes the physiology of the different dopamine receptors in the regulation of blood pressure, and the relationship between dopamine receptor subtypes and hypertension.

Identification of human dopamine receptors agonists from Chinese herbs

AIM

To find human dopamine receptors, especially D1-like receptor specific agonists from Chinese herbs as potential antihypertension drug leads.

METHODS

Two D1-like receptor cell lines carrying a beta-lactamase reporter gene, and a D2 receptor cell line coexpressing a promiscuous G protein G15 were constructed using HEK293 cells. A natural compound library made from fractionated samples of herbal extracts was used for high-throughput screening (HTS) against one of the cell lines, HEK/D5R/CRE-blax. The interested hits were evaluated for their activities against various dopamine receptors.

RESULTS

Fourteen hits were identified from primary screening, of which 2 of the better hit samples, HD0522 and HD0059, were selected for further material and activity analysis, and to obtain 2 compounds that appeared as 2 single peaks in HPLC, HD0522H01 and HD0059H01. HD0059H01 could activate D1, D2, and D5 receptors, with EC(50 ) values of 2.28 microg/mL, 0.85 microg/mL, and 1.41 microg/mL, respectively. HD0522H01 could only activate D1R and D5R with EC(50 ) values of 2.95 microg/mL and 8.38 microg/mL.

CONCLUSION

We established cellbased assays for 3 different human dopamine receptors and identified specific agonists HD0522H01 and HD0059H01 through HTS. The specific agonist to D1-like receptors, HD0522H01, may become a new natural product-based drug lead for antihypertension treatment.

Mechanisms of disease: the role of GRK4 in the etiology of essential hypertension and salt sensitivity

Hypertension and salt sensitivity of blood pressure are two conditions the etiologies of which are still elusive because of the complex influences of genes, environment, and behavior. Recent understanding of the molecular mechanisms that govern sodium homeostasis is shedding new light on how genes, their protein products, and interacting metabolic pathways contribute to disease. Sodium transport is increased in the proximal tubule and thick ascending limb of Henle of the kidney in human essential hypertension. This Review focuses on the counter-regulation between the dopaminergic and renin-angiotensin systems in the renal proximal tubule, which is the site of about 70% of total renal sodium reabsorption. The inhibitory effect of dopamine is most evident under conditions of moderate sodium excess, whereas the stimulatory effect of angiotensin II is most evident under conditions of sodium deficit. Dopamine and angiotensin II exert their actions via G protein-coupled receptors, which are in turn regulated by G protein-coupled receptor kinases (GRKs). Polymorphisms that lead to aberrant action of GRKs cause a number of conditions, including hypertension and salt sensitivity. Polymorphisms in one particular member of this family-GRK4-have been shown to cause hyperphosphorylation, desensitization and internalization of a member of the dopamine receptor family, the dopamine 1 receptor, while increasing the expression of a key receptor of the renin-angiotensin system, the angiotensin II type 1 receptor. Novel diagnostic and therapeutic approaches for identifying at-risk subjects, followed by selective treatment of hypertension and salt sensitivity, might center on restoring normal receptor function through blocking the effects of GRK4 polymorphisms.

Increased renal vascular reactivity to ANG II after unilateral nephrectomy in the rat involves 20-HETE

This study examined the role of intrarenal ANG II in the renal vascular reactivity changes occurring in the remaining kidney undergoing adaptation following contralateral nephrectomy. Renal blood flow responses to intrarenal injections of ANG II (0.25 to 5 ng) were measured in anesthetized euvolemic male Wistar rats 1, 4, 12, and 24 wk after uninephrectomy (UNX) or sham procedure (SHAM). At week 4, renal vasoconstriction induced by 2 ng ANG II was greater in UNX (69 +/- 5%) than in SHAM rats (50 +/- 3%; P < 0.01). This response was inhibited, by 50 and 66%, and by 20 and 25%, in SHAM and UNX rats, after combined injections of ANG II and losartan, or PD-123319 (P < 0.05), respectively. Characteristics of ANG II receptor binding in isolated preglomerular resistance vessels were similar in the two groups. After prostanoid inhibition with indomethacin, renal vasoconstriction was enhanced by 42 +/- 8% (P < 0.05), only in SHAM rats, whereas after 20-HETE inhibition with HET0016, it was reduced by 53 +/- 16% (P < 0.05), only in UNX rats. These differences vanished after concomitant prostanoid and 20-HETE inhibition in the two groups. After UNX, renal cortical protein expression of cytochrome P-450 2c23 isoform (CYP2c23) and cyclooxygenase-1 (COX-1) was unaltered, but it was decreased for CYP4a and increased for COX-2. In conclusion, renal vascular reactivity to ANG II was significantly increased in the postuninephrectomy adapted kidney, independently of protein expression, but presumably involving interactions between 20-HETE and COX in the renal microvasculature and changes in the paracrine activity of ANG II and 20-HETE.

Functional genomics of the dopaminergic system in hypertension

Abnormalities in dopamine production and receptor function have been described in human essential hypertension and rodent models of genetic hypertension. Under normal conditions, D(1)-like receptors (D(1) and D(5)) inhibit sodium transport in the kidney and intestine. However, in the Dahl salt-sensitive and spontaneously hypertensive rats (SHRs) and in humans with essential hypertension, the D(1)-like receptor-mediated inhibition of epithelial sodium transport is impaired because of an uncoupling of the D(1)-like receptor from its G protein/effector complex. The uncoupling is receptor specific, organ selective, nephron-segment specific, precedes the onset of hypertension, and cosegregates with the hypertensive phenotype. The defective transduction of the renal dopaminergic signal is caused by activating variants of G protein-coupled receptor kinase type 4 (GRK4: R65L, A142V, A486V). The GRK4 locus is linked to and GRK4 gene variants are associated with human essential hypertension, especially in salt-sensitive hypertensive subjects. Indeed, the presence of three or more GRK4 variants impairs the natriuretic response to dopaminergic stimulation in humans. In genetically hypertensive rats, renal inhibition of GRK4 expression ameliorates the hypertension. In mice, overexpression of GRK4 variants causes hypertension either with or without salt sensitivity according to the variant. GRK4 gene variants, by preventing the natriuretic function of the dopaminergic system and by allowing the antinatriuretic factors (e.g., angiotensin II type 1 receptor) to predominate, may be responsible for salt sensitivity. Subclasses of hypertension may occur because of additional perturbations caused by variants of other genes, the quantitative interaction of which may vary depending upon the genetic background.

Interaction of Angiotensin II Type 1 and D 5 Dopamine Receptors in Renal Proximal Tubule Cells

Angiotensin II type 1 (AT 1 ) receptor and D 1 and D 3 dopamine receptors directly interact in renal proximal tubule (RPT) cells from normotensive Wistar-Kyoto rats (WKY). There is indirect evidence for a D 5 and AT 1 receptor interaction in WKY and spontaneously hypertensive rats (SHR). Therefore, we sought direct evidence of an interaction between AT 1 and D 5 receptors in RPT cells. D 5 and AT 1 receptors colocalized in WKY cells. Angiotensin II decreased D 5 receptors in WKY cells in a time- and concentration-dependent manner (EC 50 =2.7×10 −9 M; t 1/2 =4.9 hours), effects that were blocked by an AT 1 receptor antagonist (losartan). In SHR, angiotensin II (10 −8 M/24 hours) also decreased D 5 receptors (0.96±0.08 versus 0.72±0.08; n=12) and to the same degree as in WKY cells (1.44±0.07 versus 0.92±0.08). However, basal D 5 receptors were decreased in SHR RPT cells (SHR 0.96±0.08; WKY 1.44±0.07; n=12 per strain; P <0.05) and renal brush border membranes of SHR compared with WKY (SHR 0.54±0.16 versus WKY 1.46±0.10; n=5 per strain; P <0.05). Angiotensin II decreased AT 1 receptor expression in WKY (1.00±0.04 versus 0.72±0.08; n=8; P <0.05) but increased it in SHR (0.96±0.04 versus 1.32±0.08; n=8; P <0.05). AT 1 and D 5 receptors also interacted in vivo; renal D 5 receptor protein was higher in mice lacking the AT 1A receptor (AT 1A −/−; 1.61±0.31; n=6) than in wild-type littermates used as controls (AT 1A +/+; 0.81±0.08; n=6; P <0.05), and renal cortical AT 1 receptor protein was higher in D 5 receptor null mice than in wild-type littermates (1.18±0.08 versus 0.84±0.07; n=4; P <0.05). We conclude that D 5 and AT 1 receptors interact with each other. Altered interactions between AT 1 and dopamine receptors may play a role in the pathogenesis of hypertension.

The renal vascular response to ANG II injection is reduced in the nonclipped kidney of two-kidney, one-clip hypertension

The ANG II receptor 1 (AT(1)R) level in the nonclipped kidney of two-kidney, one-clip hypertension (2K1C) has shown to be unchanged despite a high circulating angiotensin (ANG) II level. To examine the vasoreactive response to ANG II in this kidney, injections of ANG II into renal artery were performed 6 wk after clipping of the kidney and compared with normotensive controls. The renal blood flow (RBF) response to 2.5 ng ANG II was measured by a Transonic transit-time flowmeter, before and after indomethacin and candesartan treatment, and analyzed by a computer program. The RBF response to 5 ng arginine-vasopressin (AVP) was examined for comparison with ANG II. The mRNA for AT(1A) and AT(1B) as well as Western blotting for AT(1)R in renal resistance vessels were determined, and plasma renin activity (PRA) was measured. Systolic blood pressure was 183 +/- 4 mmHg in 2K1C rats compared with 113 +/- 1 mmHg in controls (P < 0.001). PRA was significantly increased in 2K1C animals (P < 0.05). Injection of ANG II reduced RBF with 10 +/- 2% in the nonclipped kidney and 24 +/- 3% in controls (P < 0.001). After indomethacin, the RBF response increased from 10 +/- 2 to 20 +/- 3% (P < 0.02) in 2K1C rats and from 24 +/- 3 to 34 +/- 6% in controls (P < 0.01). The doses of candesartan needed to completely inhibit RBF response to ANG II were 30 microg/kg in the nonclipped kidney and 100 microg/kg in controls (P < 0.001). Western blotting and mRNA for AT(1A) and AT(1B) in the nonclipped kidney were similar to the controls. The results indicate that despite no difference in total AT(1)R levels, functional AT(1)R is downregulated in the nonclipped kidney of 2K1C rats.

D5 dopamine receptor knockout mice and hypertension

Abnormalities in dopamine production and receptor function have been described in human essential hypertension and rodent models of genetic hypertension. All of the five dopamine receptor genes (D1, D2, D3, D4, and D5) expressed in mammals and some of their regulators are in loci linked to hypertension in humans and in rodents. Under normal conditions, D1-like receptors (D1 and D5) inhibit sodium transport in the kidney and the intestine. However, in the Dahl salt-sensitive and spontaneously hypertensive rats, and humans with essential hypertension, the D1-like receptor-mediated inhibition of sodium transport is impaired because of an uncoupling of the D1-like receptor from its G protein/effector complex. The uncoupling is genetic, and receptor-, organ-, and nephron segment-specific. In human essential hypertension, the uncoupling of the D1 receptor from its G protein/effector complex is caused by an agonist-independent serine phosphorylation/desensitization by constitutively active variants of the G protein-coupled receptor kinase type 4. The D5 receptor is also important in blood pressure regulation. Disruption of the D5 or the D1 receptor gene in mice increases blood pressure. However, unlike the D1 receptor, the hypertension in D5 receptor null mice is caused by increased activity of the sympathetic nervous system, apparently due to activation of oxytocin, V1 vasopressin, and non-N-methyl D-aspartate receptors in the central nervous system. The cause of the activation of these receptors remains to be determined.

Salt-induced ANG II suppression impairs the response of cerebral artery smooth muscle cells to prostacyclin

Recent studies have demonstrated that cerebral arteries from rats fed a high-salt (HS) diet exhibit impaired vasodilation and altered electrophysiological response to reduction in Po2. The present study examined whether an increase in salt intake alters the response of vascular smooth muscle cells (VSMC) to prostacyclin, a crucial mediator of hypoxic dilation in cerebral arteries. VSMC were isolated from cerebral arteries of male Sprague-Dawley rats maintained on an HS (4% NaCl) or a low-salt diet (0.4% NaCl) for 3 days. The stable prostacyclin analog iloprost (10 ng/ml) inhibited serotonin (0.1–10 μM)-induced contractions and the increase in intracellular Ca2+concentration ([Ca2+]i) in VSMC isolated from arteries of animals fed the low-salt diet. In contrast, iloprost had no effect on serotonin-induced contractions and increases in [Ca2+]iin VSMC isolated from arteries of rats fed the HS diet. Preventing the fall in ANG in rats fed the HS diet by infusion of a low dose of ANG II (5 ng·kg−1·min−1iv) restored the inhibitory effect of iloprost on serotonin-induced contractions and increases in [Ca2+]iin VSMC from animals fed the HS diet. These effects were reversed by AT1receptor blockade with losartan. These results indicate that ANG II suppression secondary to elevated dietary salt intake impairs vascular relaxation and Ca2+regulation by prostacyclin.

Enhanced Ca2+ response to AVP in preglomerular vessels from rats with genetic hypertension during different hydration states

Exaggerated arginine vasopressin (AVP)-induced calcium signaling and renal vasoconstriction, characteristic in young spontaneously hypertensive rats (SHR) during euvolemia, are related to greater amounts of V1a receptor mRNA and V1a protein in preglomerular resistance arterioles. The present study determined whether V1a receptor density and calcium signal transduction in the renal vasculature of young SHR is regulated appropriately during physiological changes in hydration state. [3H]AVP ligand binding documented two- to threefold greater density of V1a receptors in euvolemic SHR vs. Wistar-Kyoto (WKY) rats. Parallel changes in V1a receptor density were observed in both strains during chronic water loading (plus approximately 50 fmol/mg) and during dehydration (minus approximately 50 fmol/mg). Affinity was unchanged. Real-time RT-PCR demonstrated that V1a mRNA in preglomerular arterioles was three times greater in euvolemic SHR. Dehydration decreased expression approximately 50% in renal vessels independent of rat strain; water loading increased V1a mRNA. Thus V1a receptor regulation correlated with changes in mRNA in a normal manner in response to chronic changes in AVP concentration, albeit set at a higher level in SHR. In dehydrated animals, AVP increased the cytosolic Ca2+ concentration ([Ca2+]i) by 60 +/- 5 and 112 +/- 13 nM cytosolic Ca2+ in WKY and SHR, respectively (P < 0.01), whereas in hydrated animals the [Ca2+]i increase was 168 +/- 10 and 220 +/- 18 nM, respectively (P < 0.05). In all hydration states, calcium signaling was greater in SHR compared with WKY (P < 0.05). Calcium signaling paralleled changes in the receptor density and mRNA. Mechanisms other than hydration state per se are likely to be responsible for the two- to threefold difference in the V1a receptor density between WKY and SHR in the renal vasculature at the critical age of 6 wk.

The Abnormal Renal Vasodilator Response to D1-Like Receptor Stimulation in Conscious SHR Can Be Normalized by AT1 Blockade

BACKGROUND

We previously showed that the renal vasodilator response to a D1-like receptor agonist is blunted in conscious SHR compared with WKY rats. The mechanism of this impaired dopaminergic responsiveness in SHR is unclear. An altered balance between the renin-angiotensin-aldosterone system (RAAS) and the dopaminergic system may be involved. To determine the interaction between the RAAS and the dopaminergic system in the blunted D1-like responsiveness in SHR, we studied the renal vasodilator response to the D1-like receptor agonist fenoldopam before and after 7 days of pretreatment with the AT1-receptor antagonist (AT1-A) L158,809 in conscious SHR and WKY rats.

METHODS

Effective renal plasma flow (ERPF) was measured by the clearance of I-hippuran. Mean arterial pressure (MAP) was measured via an intraarterial catheter.

RESULTS

Without pretreatment, MAP was reduced to comparable degrees by fenoldopam in WKY (-7 +/- 4%, ns) and SHR (-6 +/- 1%, P < 0.05). However, ERPF was significantly more increased (P < 0.006) by fenoldopam in WKY (+26 +/- 2%, P < 0.0001) than in SHR (+2 +/- 2%, ns). AT1-A treatment reduced MAP and increased ERPF and glomerular filtration rate significantly in both strains. Pretreatment with AT1-A significantly potentiated the fenoldopam-induced rise in ERPF in SHR, but not in WKY, without affecting the blood pressure responses in either strain. As a result, during pretreatment with an AT1-A, the rise in ERPF by fenoldopam was similar in both strains (SHR +25 +/- 2%, P < 0.0001; WKY +33 +/- 2%, P < 0.0001).

CONCLUSIONS

These results suggest that the RAAS accounts for the blunted renal vasodilator response to a D1-like receptor agonist in SHR. A dysbalance between the dopaminergic system and the RAAS may be involved in the abnormal renal hemodynamic regulation in SHR.

Modulation of cyclic AMP production by signal transduction pathways in preglomerular microvessels and microvascular smooth muscle cells

Cyclic AMP affects microvascular smooth muscle contraction and growth. Therefore, it is important to elucidate mechanisms regulating cyclic AMP production in microvascular smooth muscle. In this study, we determined whether several signal transduction pathways regulate receptor-induced cyclic AMP in isolated preglomerular microvessels and microvascular smooth muscle cells. Preglomerular microvessels were incubated with isoproterenol (beta-adrenoceptor agonist) and with and without U73122 (phospholipase C inhibitor), GF109203X (protein kinase C inhibitor), 1-butanol (phospholipase D inhibitor), CGP77675 (c-src inhibitor), HA1077 (Rho kinase inhibitor), Y27632 (Rho kinase inhibitor), LY294002 (phosphatidylinositol-3-kinase inhibitor), dipenyleneiodonium (NADPH oxidase inhibitor), or Tempol (superoxide dismutase mimetic). Cultured preglomerular microvascular smooth muscle cells were incubated with isoproterenol or forskolin (direct activator of adenylyl cyclase) and with or without U73122, C(2)-ceramide (phospholipase D inhibitor), or PP1 [src family inhibitor, 1-(1,1-dimethylethyl)-1-(4-methylphenyl)-1H-pyrazolo[3,4-d]pyrimidin-4-amine]. All studies were conducted with 3-isobutyl-1-methylxanthine (broad-spectrum phosphodiesterase inhibitor) to eliminate changes in cyclic AMP degradation. In microvessels isoproterenol-induced cyclic AMP was not affected by Y27632, HA1007, LY294002, dipenylene-iodonium, or Tempol; was increased by U73122 and GF109203X; and was decreased by 1-butanol and CGP77675. In cells, U73122 increased and C(2)-ceramide and PP1 decreased isoproterenol-induced cyclic AMP. Forskolin-induced cyclic AMP was not altered. These results indicate that receptor-mediated activation of adenylyl cyclase is 1) not modulated by Rho kinase, phosphatidylinositol-3-kinase, NADPH oxidase, or superoxide; 2) is attenuated by phospholipase C and protein kinase C; and 3) is augmented by phospholipase D and src. Phospholipase C, phospholipase D, and src modulate receptor-induced cyclic AMP by affecting beta-adrenoreceptor/G protein/adenylyl cyclase coupling rather than by directly affecting adenylyl cyclase activity.

Age-dependent regulation of vasopressin V1areceptors in preglomerular vessels from the spontaneously hypertensive rat

Experiments were performed to get insight into the role of AVP receptor V1aregulation with age, i.e., during development and maintenance of high blood pressure. Previous studies showed an increased gene expression and renal vascular response to AVP in young spontaneously hypertensive rats (SHR). The age regulation of the V1areceptor was examined in preglomerular vessels from 5-, 10-, 20-, and 70-wk-old SHR using normotensive Wistar-Kyoto rats (WKY) as controls. Real-time PCR and ligand binding were used for analysis of receptor expression, and the change in cytosolic calcium concentration during stimulation of isolated preglomerular vessels with AVP was studied. Studies showed an increase of the V1areceptor protein and mRNA from 5-and 10-wk-old SHR compared with vessels from 20- and 70-wk-old SHR. In 5-wk-old SHR receptor density was 84 ± 13 fmol/mg protein, and 38 ± 11 fmol/mg protein in 70-wk-old SHR ( P < 0.05). mRNA in the 5- and 70-wk-old SHR was 15,854 ± 629 and 3,181 ± 224 V1amRNA/108 18S ribosomal RNA, respectively ( P < 0.001). Values from WKY at all ages were similar to 20- and 70-wk-old SHR. During stimulation with AVP, the change in cytosolic calcium in vessels from 5-wk-old SHR increased 234 ± 59 nM, whereas the increase was 89 ± 9 nM in 70-wk-old SHR ( P = 0.03). These results indicate that the V1areceptor is increased at protein and mRNA level during development of hypertension in SHR but is normalized when hypertension is established.

Dopamine D 1 Receptor Augmentation of D 3 Receptor Action in Rat Aortic or Mesenteric Vascular Smooth Muscles

Dopamine is an important modulator of blood pressure, in part, by regulating vascular resistance. To test the hypothesis that D 1 and D 3 receptors interact in vascular smooth muscle cells, we studied A10 cells, a rat aortic smooth muscle cell line, and rat mesenteric arteries that express both dopamine receptor subtypes. Fenoldopam, a D 1 -like receptor agonist, increased both D 1 and D 3 receptor protein in a time-dependent and a concentration-dependent manner in A10 cells. The effect of fenoldopam was specific because a D 1 -like receptor antagonist, SCH23390 (10 −7 M/24 h), completely blocked the stimulatory effect of fenoldopam (10 −7 M/24 h) (D 3 receptor: control=21±1 density units [DU]); SCH23390=23±2 DU; fenoldopam=33±2 DU; fenoldopam+SCH23390=23±2 DU; n=10). D 1 and D 3 receptors physically interacted with each other because fenoldopam (10 −7 M/24 h) increased D 1 /D 3 receptor coimmunoprecipitation (35±5 versus 65±5 DU; n=8). A D 3 receptor agonist, PD128907, relaxed mesenteric arterial rings independent of the endothelium, effects that were blocked by a D 3 receptor antagonist, U99194A. Costimulation of D 1 and D 3 receptors led to additive vasorelaxation. We conclude that the D 1 receptor regulates the D 3 receptor by physical interaction and receptor expression. D 1 receptor stimulation augments D 3 receptor vasorelaxant effects. An interaction of D 1 and D 3 receptors may be involved in the regulation of blood pressure.

Aberrant D1 and D3 dopamine receptor transregulation in hypertension

Dopamine plays a role in the regulation of blood pressure by inhibition of sodium transport in renal proximal tubules (RPTs) and relaxation of vascular smooth muscles. Because dopamine receptors can regulate and interact with each other, we studied the interaction of D(1) and D(3) receptors in immortalized RPT cells and mesenteric arteries from Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHRs), and in human coronary artery smooth muscle cells (CASMCs). In WKY rats, the D(1)-like agonist, fenoldopam, increased D(3) receptor protein in a time-dependent and concentration-dependent manner (EC(50)=4.5x10(-9) M, t(1/2)=15.8 hours). In SHRs, fenoldopam (10(-5) M) actually decreased the expression of D(3) receptors. D(1) and D(3) receptor co-immunoprecipitation was increased by fenoldopam (10(-7) M/24 h) in WKY rats but not in SHRs. The effects of fenoldopam in CASMCs were similar as those in WKY RPT cells (ie, fenoldopam increased D(1) and D(3) receptor proteins). Both D(3) (PD128907, Emax=80%+/-6%, pED(50)=5+/-0.1) and D(1)-like receptor (fenoldopam, Emax=81%+/-8%, pED(50)=5+/-0.2, n=12) agonists relaxed mesenteric arterial rings. Co-stimulation of D(1) and D(3) receptors led to additive vasorelaxation in WKY rats, but not in SHRs. D(1) and D(3) receptors interact differently in WKY and SHRs. Altered interactions between D(1) and D(3) receptors may play a role in the pathogenesis of genetic hypertension, including human hypertension, because these receptors also interact in human vascular smooth muscle cells.

Renal extraction of angiotensin II

Angiotensin II regulates many aspects of renal function and thereby influences long-term blood pressure. The effects of angiotensin II on the kidney have been exhaustively studied; however, the converse (i.e., effects of the kidney on angiotensin II) has received little attention. Accordingly, the focus of this study was to determine whether renal degradation of angiotensin II is regulated by chronic levels of angiotensin II or long-term levels of blood pressure. Twenty hypertensive rats and 22 normotensive rats were treated for 1 week with either vehicle, angiotensin II (50 ng/kg/min, subcutaneously) or captopril (100 mg/kg/day, orally). Right kidney vascular resistance was measured during infusions of angiotensin II into the left renal artery or vena cava at the level of left renal vein. Dose-response data were curve-fitted, and the extraction of angiotensin II by the left kidney was calculated by comparing the doses of angiotensin II required to elicit equal increases in right renal vascular resistance during intravenous versus left intrarenal artery infusions. Renal extraction of angiotensin II was high (mean, 81%) and demonstrated little animal-to-animal variation (coefficient of variation, 23%; standard deviation, 19%). Renal extraction of angiotensin II was independent of hypertension (P = 0.257) or previous chronic exposure to angiotensin II or captopril (P = 0.270), and there was no interaction between hypertension and chronic exposure to angiotensin II or captopril (P = 0.950). We conclude that renal degradation of angiotensin II is constitutively high, is unaffected by chronic levels of arterial blood pressure, and is independent of long-term changes in levels of angiotensin II.

Perturbation of D1 dopamine and AT1 receptor interaction in spontaneously hypertensive rats

The dopaminergic and renin-angiotensin systems interact to regulate blood pressure. Because this interaction may be perturbed in genetic hypertension, we studied D1 dopamine and AT1 angiotensin receptors in immortalized renal proximal tubule (RPT) and A10 aortic vascular smooth muscle cells. In normotensive Wistar-Kyoto (WKY) rats, the D1-like agonist fenoldopam increased D1 receptors but decreased AT1 receptors. These effects were blocked by the D1-like antagonist SCH 23390 (10(-7) mol/L per 24 hours). In spontaneously hypertensive rat (SHR) RPT cells, fenoldopam also decreased AT1 receptors but no longer stimulated D1 receptor expression. Basal levels of AT1/D1 receptor coimmunoprecipitation were greater in WKY RPT cells (29+/-2 density units, DU) than in SHR RPT cells (21+/-2 DU, n=7 per group, P<0.05). The coimmunoprecipitation of D1 and AT1 receptors was increased by fenoldopam (10(-7) mol/L per 24 hours) in WKY RPT cells but decreased in SHR RPT cells. The effects of fenoldopam in RPT cells from WKY rats were similar in aortic vascular smooth muscle cells from normotensive BD IX rats, that is, fenoldopam decreased AT1 receptors and increased D1 receptors. Our studies show differential regulation of the expression of D1 and AT1 receptors in RPT cells from WKY and SHR. This regulation and D1/AT1 receptor interaction are different in RPT cells of WKY and SHR. An altered interaction of D1 and AT1 receptors may play a role in the impaired sodium excretion and enhanced vasoconstriction in hypertension.

Increased renal vascular sensitivity to angiotensin II in hypertension is due to decreased response to prostaglandins

OBJECTIVES

An enhanced sensitivity to angiotensin II in the renal circulation has been demonstrated in the pre-hypertensive phase both in the spontaneously hypertensive rat and in man. To further characterize this abnormality and the role of prostanoids, renal haemodynamics in normotensive young men with a positive (PFH) or a negative (NFH) family history of hypertension were studied.

METHODS

Renal vascular reactivity was assessed during infusion of angiotensin II with and without inhibition of prostaglandin synthesis. Normotensive men with PFH (n = 13) and with NFH (n = 10) with a mean age of 38 years were given on two different occasions: (i). angiotensin II infusion i.v. (0.1, 0.5 and 1.0 ng/kg per min) and (ii). angiotensin II infusion after inhibition of prostaglandin synthesis with indomethacin (150 mg daily three consecutive days). Glomerular filtration rate (GFR) and renal plasma flow were measured with renal clearances of chromium edetic acid and para-aminohippuric acid.

RESULTS

Before angiotensin II challenge, the groups did not differ with respect to blood pressure, body mass index, plasma renin activity, GFR, renal blood flow (RBF) or urinary sodium excretion. There was no significant difference in systolic or diastolic blood pressure response to angiotensin II between the two groups. In PFH, the lowest angiotensin II dose caused a significant decrease in RBF and increase in renal vascular resistance (RVR) from baseline (P < 0.01 for both). In NFH, only the highest angiotensin II dose produced a significant decrease in RBF and increase in RVR (P < 0.01 for both). During inhibition of prostaglandin synthesis, all three angiotensin II doses caused a significant decrease in RBF (P < 0.02) and increase in RVR (P < 0.02) also in NFH. The renal haemodynamic difference between PFH and NFH was thus eliminated.

CONCLUSIONS

These findings indicate that young human subjects with a positive family history of hypertension have a defective vasodilator prostaglandin system, which is responsible for increased renal vascular sensitivity to angiotensin II. Enhanced renal vasoconstriction may be an early event leading to the generation of primary hypertension.

High-salt diet impairs vascular relaxation mechanisms in rat middle cerebral arteries

Male Sprague-Dawley rats were maintained on a low-salt (LS) diet (0.4% NaCl) or a high-salt (HS) diet (4% NaCl) for 3 days or 4 wk. PO(2) reduction to 40-45 mmHg, the stable prostacyclin analog iloprost (10 pg/ml), and stimulatory G protein activation with cholera toxin (1 ng/ml) caused vascular smooth muscle (VSM) hyperpolarization, increased cAMP production, and dilation in cerebral arteries from rats on a LS diet. Arteries from rats on a HS diet exhibited VSM depolarization and constriction in response to hypoxia and iloprost, failed to dilate or hyperpolarize in response to cholera toxin, and cAMP production did not increase in response to hypoxia, iloprost, or cholera toxin. Low-dose angiotensin II infusion (5 ng x kg(-1) x min(-1) i.v.) restored normal responses to reduced PO(2) and iloprost in arteries from animals on a HS diet. These observations suggest that angiotensin II suppression with a HS diet leads to impaired relaxation of cerebral arteries in response to vasodilator stimuli acting at the cell membrane.

Angiotensin II regulation of AT1 and D3 dopamine receptors in renal proximal tubule cells of SHR

Dopamine and angiotensin II negatively interact to regulate sodium excretion and blood pressure. D3 dopamine receptors downregulate angiotensin type 1 (AT1) receptors in renal proximal tubule cells from normotensive Wistar-Kyoto rats. We determined whether AT1 receptors regulate D3 receptors and whether the regulation is different in cultured renal proximal tubule cells from normotensive and spontaneously hypertensive rats. Angiotensin II (10(-8)M/24 hours) decreased D3 receptors in both normotensive (control, 36+/-3; angiotensin II, 24+/-3 U) and hypertensive (control, 30+/-3; angiotensin II, 11+/-3 U; n=9 per group) rats; effects that were blocked by the AT1 receptor antagonist, losartan (10(-8)M/24 hours). However, the reduction in D3 expression was greater in hypertensive (60+/-10%) than in normotensive rats (32+/-9%). In normotensive rats, angiotensin II (10(-8)M/24hr) also decreased AT1 receptors. In contrast, in cells from hypertensive rats, angiotensin II increased AT1 receptors. AT1 and D3 receptors co-immunoprecipitated in renal proximal tubule cells from both strains. Angiotensin II decreased D3/AT1 receptor co-immunoprecipitation similarly in both rat strains, but basal D3/AT1 co-immunoprecipitation was 6 times higher in normotensive than in hypertensive rats. Therefore, AT1 and D3 receptor interaction is qualitatively and quantitatively different between normotensive and hypertensive rats; angiotensin II decreases AT1 expression in normotensive but increases it in hypertensive rats. In addition, angiotensin II decreases D3 expression to a greater extent in hypertensive than in normotensive rats. Aberrant interactions between D3 and AT1 receptors may play a role in the pathogenesis of hypertension.

Dopamine and the kidney: a role in hypertension?

PURPOSE OF REVIEW

Defective transduction of the dopamine receptor signal in the kidney has been shown to be important in the pathogenesis of hypertension This review will discuss the genetic mechanism for the defective renal dopaminergic function and the interaction with other gene variant products in the pathogenesis of salt sensitivity and essential hypertension.

RECENT FINDINGS

Single nucleotide polymorphisms of G protein-coupled receptor kinase type 4 (GRK4) phosphorylate, desensitize, and diminish the inhibitory action of D receptors on sodium transport in the kidney. Inhibition of GRK4 expression normalizes renal proximal tubule D receptor function in humans and rodents and ameliorates the hypertension in genetically hypertensive rats. Expression of the GRK4 variant, GRK4gammaA142V, produces hypertension and impairs the natriuretic effect of D receptor stimulation in mice. In humans, GRK4 single nucleotide polymorphisms are associated with essential hypertension, particularly salt sensitive hypertension. The prediction of the hypertensive phenotype is most accurate when elements of the renin-angiotensin system and GRK4 are included in the analysis.

SUMMARY

GRK4 single nucleotide polymorphisms, by preventing the natriuretic function of the dopaminergic system and by allowing the antinatriuretic function of angiotensin II type 1 receptors to predominate, may be responsible for salt sensitivity. Hypertension develops with additional perturbations caused by the variants of other genes (e.g., alpha-adducin, angiotensin converting enzyme, angiotensinogen, angiotensin II type 1 receptor, aldosterone synthase, 11beta-hydroxysteroid dehydrogenase type 2), the quantitative interaction of which may vary depending upon the genetic background.

D2 -like receptor stimulation decreases effective renal plasma flow and glomerular filtration rate in spontaneously hypertensive rats

In spontaneously hypertensive rats (SHRs) the dopaminergic D1-like renal vasodilator response is impaired. The renal vascular response to D2-like receptor stimulation in vivo is incompletely known. Therefore, renal hemodynamics were studied in conscious SHRs during continuous infusion of D2-like agonist N,N-Di-n-propyldopamine (DPDA) (10 microg/kg/min) with Wistar-Kyoto (WKY) rats as controls. As sodium status may affect dopaminergic responses, rats were studied during both low- and high-sodium diets. D2-like stimulation reduced mean arterial pressure and effective renal plasma flow and glomerular filtration rate (GFR) similarly in SHR and WKY rats. Renal vascular resistance increased significantly in both strains. The response to DPDA is modified by sodium status, with a more pronounced fall in blood pressure (in WKYs and SHRs) and GFR (in WKYs) during high-sodium conditions. The responses were blocked by co-infusion with D2 antagonist domperidone. Thus, D2-like renal vascular responses are normal in SHRs irrespective of sodium intake. The combination of a preserved D2-like renal vasoconstrictive and an impaired D1-like renal vasodilatory response may contribute to maintenance of hypertension in SHRs.

Dopamine receptor-coupling defect in hypertension

Dopamine synthesized in non-neural tissues, eg, renal proximal tubule, functions in an autocrine or paracrine manner. The effects of dopamine are transduced by two classes of receptors (D1- and D2-like) that belong to the superfamily of G protein-coupled receptors. In genetic hypertension, the D1 receptor, a member of the D1-like receptor family, is uncoupled from its G protein complex, resulting in a decreased ability to regulate renal sodium transport. The impaired D1 receptor/G protein coupling in renal proximal tubules in genetic hypertension is secondary to abnormal phosphorylation and desensitization of the D1 receptor caused by activating single nucleotide polymorphisms of a G protein-coupled receptor kinase, GRK type 4.

Store-operated Ca2+ entry is exaggerated in fresh preglomerular vascular smooth muscle cells of SHR

BACKGROUND

Regulation of preglomerular vasomotor tone vessels ultimately control glomerular filtration rate, sodium reabsorption and systemic blood pressure. To gain insight into the complex renal hemodynamic factors that may result in hypertension, we studied calcium signaling pathways.

METHODS

Fresh, single, preglomerular vascular smooth muscle cells (VSMC) were isolated from 5- to 6-week-old SHR and WKY utilizing a magnetized microsphere/sieving technique. Cytosolic Ca2+ ([Ca2+]i) was measured with fura-2 ratiometric fluorescence. To examine store-operated calcium entry (SOC), VSMC were activated in calcium-free buffer containing nifedipine. To deplete the sarcoplasmic reticulum (SR) of Ca2+, vasopressin-1 receptor agonist [V1R; inositol trisphosphate (IP3)-mediated mobilization], ryanodine (non-IP3 induced mobilization), and cyclopiazonic acid (CPA; Ca2+-ATPase inhibition) were utilized. Addition of external calcium followed by quenching of the fura/Ca2+ signal with Mn2+ permitted assessment of divalent cation entry via SOC.

RESULTS

V1R caused greater mobilization in SHR than WKY (P < 0.01) as well as greater calcium entry (P < 0.001). Ryanodine and CPA both caused SR calcium depletion that was not statistically different between strains, but absolute calcium entry through SOC was more than double in SHR following either maneuver (P < 0.001). 2-Amino-ethoxybiphenyl borane (2-APB), an inhibitor not only of IP3 receptors, but also of SOC, blocked calcium entry in the ryanodine and CPA experiments independent of IP3. As well, Gd3+, a selective inhibitor of SOC, inhibited the Ca2+ response. We also studied L-channel calcium entry stimulated by V1R. The total calcium response was greater in SHR as was the absolute inhibition by nifedipine. As a percent of the total response, participation of L-type channels sensitive to nifedipine was about 45% in both strains of rat.

CONCLUSION

Utilizing three separate mechanisms to deplete the SR of Ca2+ in order to activate SOC, we show for the first time, that SOC is exaggerated in preglomerular VSMC of young SHR.

G protein-coupled receptor kinase 4 gene variants in human essential hypertension

Essential hypertension has a heritability as high as 30-50%, but its genetic cause(s) has not been determined despite intensive investigation. The renal dopaminergic system exerts a pivotal role in maintaining fluid and electrolyte balance and participates in the pathogenesis of genetic hypertension. In genetic hypertension, the ability of dopamine and D(1)-like agonists to increase urinary sodium excretion is impaired. A defective coupling between the D(1) dopamine receptor and the G protein/effector enzyme complex in the proximal tubule of the kidney is the cause of the impaired renal dopaminergic action in genetic rodent and human essential hypertension. We now report that, in human essential hypertension, single nucleotide polymorphisms of a G protein-coupled receptor kinase, GRK4gamma, increase G protein-coupled receptor kinase (GRK) activity and cause the serine phosphorylation and uncoupling of the D(1) receptor from its G protein/effector enzyme complex in the renal proximal tubule and in transfected Chinese hamster ovary cells. Moreover, expressing GRK4gammaA142V but not the wild-type gene in transgenic mice produces hypertension and impairs the diuretic and natriuretic but not the hypotensive effects of D(1)-like agonist stimulation. These findings provide a mechanism for the D(1) receptor coupling defect in the kidney and may explain the inability of the kidney to properly excrete sodium in genetic hypertension.

G protein-coupled receptor kinase 4 gene variants in human essential hypertension

Essential hypertension has a heritability as high as 30-50%, but its genetic cause(s) has not been determined despite intensive investigation. The renal dopaminergic system exerts a pivotal role in maintaining fluid and electrolyte balance and participates in the pathogenesis of genetic hypertension. In genetic hypertension, the ability of dopamine and D(1)-like agonists to increase urinary sodium excretion is impaired. A defective coupling between the D(1) dopamine receptor and the G protein/effector enzyme complex in the proximal tubule of the kidney is the cause of the impaired renal dopaminergic action in genetic rodent and human essential hypertension. We now report that, in human essential hypertension, single nucleotide polymorphisms of a G protein-coupled receptor kinase, GRK4gamma, increase G protein-coupled receptor kinase (GRK) activity and cause the serine phosphorylation and uncoupling of the D(1) receptor from its G protein/effector enzyme complex in the renal proximal tubule and in transfected Chinese hamster ovary cells. Moreover, expressing GRK4gammaA142V but not the wild-type gene in transgenic mice produces hypertension and impairs the diuretic and natriuretic but not the hypotensive effects of D(1)-like agonist stimulation. These findings provide a mechanism for the D(1) receptor coupling defect in the kidney and may explain the inability of the kidney to properly excrete sodium in genetic hypertension.

GTP-binding proteins in cell survival and demise: the emerging picture in the pancreatic beta-cell

It is widely believed that guanine nucleotide-binding regulatory proteins (G-proteins) play central roles as "molecular switches" in a variety of cellular processes ranging from signal transduction to protein and vesicle trafficking. To achieve these regulatory functions, G-proteins form complexes with a wide range of effector molecules whose activities are altered upon interaction with the G-protein. These effector molecules can be either soluble or membrane bound, and it is likely that some are localized to secretory granules where they direct the movement, docking, and fusion of granules during exocytosis. The effector molecules regulated by G-proteins are diverse and include phospholipases, protein kinases, protein phosphatases, ion channels, adenylate cyclases, cytoskeletal elements, as well as secretory vesicle and plasma membrane-associated fusion-proteins. The majority of studies performed in the pancreatic beta-cell have focused on the role of G-proteins in the regulation of insulin secretion, whereas very little attention has been focused on their potential involvement in other cellular processes. Such studies have identified and implicated both heterotrimeric (comprising alpha, beta, and gamma subunits) and monomeric (low molecular mass) G-proteins in the regulation of insulin secretion, but intriguing recent evidence has also begun to emerge which favors the view that they may be involved in the maintenance of beta-cell viability. In the present commentary, we will review this evidence and discuss the current understanding of the role of G-proteins in the life and death of the beta-cell.

Impaired renal D(1)-like and D(2)-like dopamine receptor interaction in the spontaneously hypertensive rat

D(1)-like (D(1), D(5)) and D(2)-like (D(2), D(3), D(4)) dopamine receptors interact in the kidney to produce a natriuresis and a diuresis. Disruption of D(1) or D(3) receptors in mice results in hypertension that is caused, in part, by a decreased ability to excrete an acute saline load. We studied D(1)-like and D(2)-like receptor interaction in anesthetized spontaneously hypertensive rats (SHR) by the intrarenal infusion of Z-1046 (a novel dopamine receptor agonist with rank order potency of D(3)> or =D(4)>D(2)>D(5)>D(1)). Z-1046 increased glomerular filtration rate (GFR), urine flow, and sodium excretion in normotensive Wistar-Kyoto rats but not in SHRs. The lack of responsiveness to Z-1046 in SHRs was not an epiphenomenon, because intrarenal cholecystokinin infusion increased GFR, urine flow, and sodium excretion to a similar extent in the two rat strains. We conclude that renal D(1)-like and D(2)-like receptor interaction is impaired in SHRs. The impaired D(1)-like and D(2)-like receptor interaction in SHRs is not caused by alterations in the coding sequence of the D(3) receptor, the D(2)-like receptor expressed in rat renal tubules that has been shown to be involved in sodium transport. Because the diuretic and natriuretic effects of D(1)-like receptors are, in part, caused by an interaction with D(2)-like receptors, it is possible that the decreased Z-1046 action in SHRs is secondary to the renal D(1)-like receptor dysfunction in this rat strain.

Enhanced interaction between renovascular alpha(2)-adrenoceptors and angiotensin II receptors in genetic hypertension

In spontaneously hypertensive rats (SHR), hypertension is mediated in part by an enhanced renovascular response to angiotensin (Ang) II. Pertussis toxin normalizes renovascular responses to Ang II and lowers blood pressure in SHR, suggesting a role for altered G(i) signaling in the enhanced renovascular response to Ang II in SHR. To further investigate this hypothesis, we measured reductions in renal blood flow and increases in renovascular resistance in response to intrarenal infusions of Ang II in the presence and absence of coactivation of alpha(2)-adrenoceptors (ie, receptors selectively coupled to G(i)) with UK 14,304 in adrenalectomized, renal-denervated, captopril-pretreated SHR and normotensive Wistar-Kyoto rats. In SHR, but not Wistar-Kyoto rats, UK 14,304 markedly enhanced renovascular responses to Ang II and vasopressin. However, UK 14,304 did not enhance renovascular responses to methoxamine (alpha(1)-adrenoceptor agonist) in either strain. In uninephrectomized, normotensive Sprague-Dawley animals and in Sprague-Dawley rats with nongenetic hypertension induced by uninephrectomy, chronic administration of deoxycorticosterone acetate, and 1% saline as drinking water, UK 14,304 had little or no effect on renovascular responses to Ang II. In SHR, intrarenal infusions of U73122, a phospholipase C/D inhibitor, blocked the enhancement of renovascular responses to Ang II by UK 14,304. We conclude that activation of alpha(2)-adrenoceptors selectively enhances renovascular responses to Ang II and vasopressin in vivo in animals with genetic hypertensive but not in normotensive animals or animals with acquired hypertension. These results suggest that in SHR, there is a genetically mediated enhanced cross talk between the G(i) signal transduction pathway and signal transduction pathways activated by Ang II and vasopressin, but not methoxamine, and involving phospholipase C and/or D.

Altered growth response to prostaglandin E2 and its receptor signaling in mesangial cells from stroke-prone spontaneously hypertensive rats

OBJECTIVE

Prostaglandin (PG) E2, a major arachidonic acid metabolite in the kidney, acts on four receptor subtypes (EP1, EP2, EP3 and EP4). One of major causes of end-stage renal failure is hypertensive renal disease, in which enhanced renal PGE2 production has been shown. In this study, to explore the pathophysiological significance of EP subtypes in the kidney, we examined the role of EP subtypes on proliferation of mesangial cells (MCs) from stroke-prone spontaneously hypertensive rats (SHRSPs), which show faster growth than those from normotensive Wistar-Kyoto rats (WKYs).

DESIGN AND METHODS

Using MCs from SHRSPs and WKYs, we investigated DNA synthesis and its upstream event, the phosphorylation of extracellular signal-regulated kinase (ERK), together with the gene expression of EP subtypes.

RESULTS

Sulprostone, an EP1 agonist, dose-dependently increased DNA synthesis and the phosphorylation of ERK in MCs from both strains. The EP4 agonist, 11-deoxy-PGE1, inhibited sulprostone-induced phosphorylation of ERK in WKY-MCs. In contrast, 11-deoxy-PGE1 failed to inhibit the ERK activity in SHRSP-MCs. Interestingly, cAMP production mediated by EP4 was markedly attenuated in SHRSP-MCs as compared with that in WKY-MCs, despite the overproduction of endogenous PGE2 in SHRSP-MCs. Similar gene expressions of EP1 and EP4 and only faint expression of EP3 were detected in MCs from both strains.

CONCLUSIONS

These results indicate that the PGE2/EP4 system counteracts the PGE2/EP1 system at the level of the intracellular signaling pathway. The altered EP4 signaling may play a critical role in the exaggerated mesangial growth in SHRSPs.

Iloprost inhibits inositol-1,4,5-trisphosphate-mediated calcium mobilization stimulated by angiotensin II in cultured preglomerular vascular smooth muscle cells

In a previous study of cultured preglomerular vascular smooth muscle cells, it was demonstrated that, although the stable prostacyclin analog iloprost alone had no effect on the intracellular calcium concentration ([Ca2+](i)), it did significantly attenuate the increase in [Ca2+](i) stimulated by angiotensin II (AngII). In this study, the mechanisms by which iloprost interacts with calcium signaling pathways stimulated by AngII were examined. [Ca2+](i) was assessed using the calcium-sensitive fluorescent dye fura-2. Initial studies identified two major components of the [Ca2+](i) response to AngII in this homogeneous preparation of vascular smooth muscle cells from renal resistance vessels. Mobilization of internal stores was evident as an immediate TMB-8-sensitive peak increase in [Ca2+](i) (52 +/- 6 to 297 +/- 26 nM, P: < 0.001) in a calcium-free medium. After [Ca2+](i) had returned to baseline levels during continued AngII stimulation, a nifedipine-sensitive entry pathway was revealed by the sustained stimulatory effect of added external calcium, which increased [Ca2+](i) to 112 +/- 13 nM (P: < 0.001). Coadministration of iloprost with AngII attenuated both the immediate peak (154 +/- 14 nM) and sustained plateau (61 +/- 9 nM) phases. Increases in endogenous levels of cAMP induced by the phosphodiesterase inhibitor milrinone mirrored the actions of iloprost, suggesting that the prostacyclin analog exerted its actions via cAMP activation. Blockade of cAMP-dependent protein kinase with KT 5720 reversed the effects of both iloprost and milrinone. When iloprost or milrinone was introduced after the initial mobilization peak had dissipated, the plateau phase of calcium entry was unchanged (92 +/- 9 nM). The concept that iloprost does not directly modulate calcium entry was further supported by data showing that the activation of L-type calcium channels by BAY-K 8644 was unchanged during iloprost treatment. On the basis of the observation that iloprost did not alter thapsigargin stimulation of Ca(2+)-ATPase activity, it is concluded that the actions of cAMP are distinct from increasing calcium uptake into the sarcoplasmic reticulum. This study provides new information on the ability of iloprost to primarily attenuate inositol-1,4,5-triphosphate-mediated calcium mobilization via cAMP, with secondary inhibition of L-type calcium entry channels. These data clarify the mechanism by which prostaglandins buffer AngII constriction in resistance arterioles.

Dopamine(1) receptor, G(salpha), and Na(+)-H(+) exchanger interactions in the kidney in hypertension

The ability of dopamine(1) (D(1)) receptors to inhibit luminal Na(+)-H(+) exchanger (NHE) activity in renal proximal tubules and induce a natriuresis is impaired in spontaneously hypertensive rats (SHR). However, it is not clear whether the defect is at the level of the D(1) receptor, G(salpha), or effector proteins. The coupling of the D(1) receptor to G(salpha) and NHE3 was studied in renal brush border membranes (BBM), devoid of cytoplasmic second messengers. D(1) receptor, G(salpha), and NHE3 expressions were similar in SHR and their normotensive controls, Wistar-Kyoto rats (WKY). Guanosine-5'-O:-(3-thiotriphosphate) (GTPgammaS) decreased NHE activity and increased NHE3 linked with G(salpha) similarly in WKY and SHR, indicating normal G(salpha) and NHE3 regulation in SHR. However, D(1) agonists increased NHE3 linked with G(salpha) in WKY but not in SHR, and the inhibitory effects of D(1) agonists on NHE activity were less in SHR than in WKY. Moreover, GTPgammaS enhanced the inhibitory effect of D(1) agonist on NHE activity in WKY but not in SHR, suggesting an uncoupling of the D(1) receptor from G(salpha)/NHE3 in SHR. Similar results were obtained with the use of immortalized renal proximal tubule cells from WKY and SHR. We conclude that the defective D(1) receptor function in renal proximal tubules in SHR is proximal to G(salpha)/effectors and presumably at the receptor level. The mechanism(s) responsible for the uncoupling of the D(1) receptor from G proteins remains to be determined. Because the primary structure of the D(1) receptor is not different between normotensive and hypertensive rats, differences in D(1) receptor posttranslational modification are possible.

Upregulation of V(1) receptors in renal resistance vessels of rats developing genetic hypertension

Previous studies have demonstrated that arginine vasopressin (AVP) produces exaggerated renal vasoconstriction in young spontaneously hypertensive rats (SHR) relative to normotensive rats. The exaggerated renal vascular reactivity does not appear to be due to a primary defect in postreceptor calcium signal transduction. Although the magnitudes of vascular responses differ, the relative proportions of calcium entry and mobilization pathways evoked by AVP in renal resistance vessels are similar in these rat strains. The purpose of the present study was to evaluate possible differences in V(1) mRNA and receptor density and affinity in preglomerular resistance vessels (<50 microm) obtained from young Wistar-Kyoto (WKY) and SHR. Quantitative RT-PCR analysis revealed twofold greater expression of the V(1a) receptor gene in preglomerular arterioles of 7-wk-old SHR compared with WKY. In vitro radiolabeled ligand binding studies were performed under equilibrium conditions on preglomerular resistance arterioles freshly isolated from kidneys of 7-wk-old rats. The results indicate that AVP receptor density (B(max)) is two to three times greater in SHR than in WKY (248 +/- 24 vs. 91 +/- 11 fmol/mg protein, P < 0.001). The affinity does not differ between strains (K(d) = 0.5 nM). Displacement studies yielded similar results for SHR and WKY. Unlabeled AVP completely displaced [(3)H]AVP binding, with an IC(50) of 2.5 x 10(-10) M. Expression of AVP receptor types in afferent arterioles was evaluated using the V(1) receptor agonist, [Phe(2), Ile(3),Org(8)]vasopressin, the V(1) receptor antagonist, [d(CH(2))(5), Tyr(Me)(2), Tyr(NH(2))(9)]Arg(8)-vasopressin, and the V(2) receptor agonist, desamino-[D-Arg(8)]vasopressin. Both the V(1) agonist and antagonist displaced up to 90% of the AVP binding with IC(50) values of 4 x 10(-8) and 8 x 10(-7) M, respectively. The V(2) receptor agonist was a weak inhibitor, displacing less than 15% of AVP binding at a high concentration of 10(-4) M. These results demonstrate that virtually all AVP receptors in the preglomerular arterioles are of the V(1) type. Collectively, our results provide evidence that the enhanced renal reactivity to AVP is mediated by a higher density of V(1) receptors associated with increased gene expression in renal resistance vessels of SHR developing genetic hypertension.

Renal dopamine and sodium homeostasis

During the past decade, it has become evident that dopamine plays an important role in the regulation of fluid and electrolyte balance and blood pressure. Dopamine exerts its actions through two families of dopamine receptors, designated D1-like and D2-like, which are identical in the brain and in peripheral tissues. The two D1-like receptors--D1 and D5 receptors--expressed in mammals are linked to stimulation of adenylyl cyclase. The three D2-like receptors--D2, D3, and D4,--are linked to inhibition of adenylyl cyclase. Dopamine affects fluid and electrolyte balance by regulation of renal excretion of electrolytes and water through actions on renal hemodynamics and tubular epithelial transport and by modulation of the secretion and/or action of vasopressin, renin, aldosterone, catecholamines, and endothelin B receptors (ETB) receptors. It also affects fluid and sodium intake by way of "appetite" centers in the brain and alterations of gastrointestinal tract transport. The production of dopamine in neural and non-neural tissues and the presence of receptors in these tissues suggest that dopamine can act in an autocrine or paracrine fashion. This renal autocrine-paracrine function, which becomes most evident during extracellular fluid volume expansion, is lost in essential hypertension and in some animal models of genetic hypertension. This deficit may be caused by abnormalities in renal dopamine production and polymorphisms or abnormal post-translational modification and regulation of dopamine receptor subtypes.

D1 dopamine receptor signalling defect in spontaneous hypertension

Dopamine modulates cardiovascular function by actions in the central and peripheral nervous system, by altering the secretion/release of prolactin, pro-opiomelanocortin, vasopressin, aldosterone, and renin, and by directly affecting renal function. Dopamine produced by the renal proximal tubule exerts an autocrine/paracrine action via two classes of dopamine receptors, D1-like (D1 and D5) and D2-like (D2, D3, and D4), that are differentially expressed along the nephron. The autocrine/paracrine function of dopamine, manifested by tubular rather than by haemodynamic mechanisms, becomes most evident during extracellular fluid volume expansion. This renal autocrine/paracrine function is lost in essential hypertension and in some animal models of genetic hypertension. The molecular basis for the dopaminergic dysfunction in hypertension may involve an abnormal post-translational modification of dopamine receptors.

Prostaglandins buffer ANG II-mediated increases in cytosolic calcium in preglomerular VSMC

In order to exert an appropriate biological effect, the action of the vasoconstrictive hormone angiotensin II (ANG II) is modulated by vasoactive factors such as prostaglandins PGE2 and PGI2. The present study investigates whether prostaglandins alter ANG II-mediated increases in cytosolic calcium concentration ([Ca2+]i) in vascular smooth muscle cells (VSMC) isolated from rat renal preglomerular arterioles. [Ca2+]i was assessed using the calcium-sensitive dye fura 2 and a microscope-based photometer system. ANG II (10(-7) M) caused a biphasic, time-dependent [Ca2+]i response: an initial peak increase from 52 +/- 7 to 264 +/- 25 nM, followed by a sustained plateau of 95 +/- 9 nM in cultured VSMC. Coadministration of PGE2 or PGI2 or synthetic mimetics caused dose-dependent decreases in the peak [Ca2+]i response to ANG II, with attenuation of 40-50%. This degree of inhibition was even more pronounced in individual freshly isolated preglomerular VSMC. Increasing cAMP levels in cultured VSMC, by using either a cell-permeable analog or inhibiting phosphodiesterase activity, mirrored the antagonistic effects of prostaglandins on ANG II-stimulated increases in [Ca2+]i. Radioimmunoassays demonstrate that ANG II (10(-7) M) stimulates production of PGI2 and PGE2; the stable prostacyclin metabolite 6-keto-PGF(1alpha) was released in 10-fold greater concentrations than PGE(2.) Indomethacin blockade of prostaglandin production potentiated both the peak (264 to 337 +/- 26 nM) and sustained [Ca2+]i responses (95 to 181 +/- 22 nM) to ANG II. When prostaglandin analogs were added during indomethacin treatment, the ANG II response was restored to the typical pattern. In conclusion, we demonstrate that modulation of intracellular calcium levels is one mechanism by which prostaglandins can buffer ANG II-mediated constriction in renal preglomerular VSMC. PGI2 is more potent than PGE2 in this regard.

Impaired prostaglandin E(2)/prostaglandin I(2) receptor-G(s) protein interactions in isolated renal resistance arterioles of spontaneously hypertensive rats

The protective effect of vasodilator agents linked to the cAMP pathway is less effective for buffering the vasoconstrictor effect of angiotensin II in young animals with genetic hypertension. To determine the underlying cellular mechanism, experiments were performed on freshly isolated preglomerular resistance arterioles obtained from kidneys of 7-week-old spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKY). Specific high-affinity saturable binding of (3)H-prostaglandin (PG) E(2) revealed 1 receptor class in renal microvessels; PGE(2) receptor density was similar in SHR and WKY (106 versus 115 fmol/mg; P>0.8), as was receptor affinity (3.6 versus 3.5 nmol/L; P>0.7). Basal cAMP activity was similar in renal arterioles from SHR and WKY. A major finding was that PGE(2), PGI(2), and isoproterenol produced weaker stimulation of cAMP formation in arteriolar cells of SHR (P<0.02). In contrast, GTPgammas and forskolin stimulated cAMP generation to a similar degree in both rat strains, which suggests normal adenylate cyclase activity in hypertension-prone SHR. Immunoblots revealed the presence of 3 classes of G proteins (G(s), G(i), and G(q)) in preglomerular arterioles. The relative amounts of discernible G-protein alpha-subunits in renal resistance vessels did not differ between SHR and WKY. These results extend previous in vivo studies of abnormal renal vascular reactivity in SHR and more directly localize defective coupling of the prostaglandin and beta-adrenergic receptors to a stimulatory G protein and cAMP production in freshly isolated preglomerular arteriolar cells of young SHR. This dysfunction may be due to an abnormal interaction between prostaglandin receptors and G(s) protein that leads to inefficient coupling of initiating steps in the cAMP-protein kinase A cascade during the development of hypertension.

Ontogenetic aspects of hypertension development: analysis in the rat

In this review, we attempt to outline the age-dependent interactions of principal systems controlling the structure and function of the cardiovascular system in immature rats developing hypertension. We focus our attention on the cardiovascular effects of various pharmacological, nutritional, and behavioral interventions applied at different stages of ontogeny. Several distinct critical periods (developmental windows), in which particular stimuli affect the further development of the cardiovascular phenotype, are specified in the rat. It is evident that short-term transient treatment of genetically hypertensive rats with certain antihypertensive drugs in prepuberty and puberty (at the age of 4-10 wk) has long-term beneficial effects on further development of their cardiovascular apparatus. This juvenile critical period coincides with the period of high susceptibility to the hypertensive effects of increased salt intake. If the hypertensive process develops after this critical period (due to early antihypertensive treatment or late administration of certain hypertensive stimuli, e.g., high salt intake), blood pressure elevation, cardiovascular hypertrophy, connective tissue accumulation, and end-organ damage are considerably attenuated compared with rats developing hypertension during the juvenile critical period. As far as the role of various electrolytes in blood pressure modulation is concerned, prohypertensive effects of dietary Na+ and antihypertensive effects of dietary Ca2+ are enhanced in immature animals, whereas vascular protective and antihypertensive effects of dietary K+ are almost independent of age. At a given level of dietary electrolyte intake, the balance between dietary carbohydrate and fat intake can modify blood pressure even in rats with established hypertension, but dietary protein intake affects the blood pressure development in immature animals only. Dietary protein restriction during gestation, as well as altered mother-offspring interactions in the suckling period, might have important long-term hypertensive consequences. The critical periods (developmental windows) should be respected in the future pharmacological or gene therapy of human hypertension.

Idiopathic edema

Idiopathic edema is a syndrome of real or perceived excessive weight gain. This article reviews what is known about the possible causes, evaluation, and treatment. Although the cause is unknown but often thought to be due to secondary hyperaldosteronism, primary abnormalities of the hypothalamus, thyroid, dopaminergic release or renal dopaminergic metabolism, vascular basement membrane, or capillary sphincter control could perhaps contribute in some patients. The diagnosis requires careful attention to possible abnormalities of the liver, heart, kidneys, gastrointestinal tract, thyroid, and pancreas. The history must include an evaluation for risks of bulimia and purging; diuretic and laxative screening should be performed. Specific records of daily weights, urinary outputs, and menstral cycle dates are useful. Treatment may include dietary counseling to provide weight control and a constant carbohydrate intake, treatments for depression, compression stockings, spironolactone, amiloride, angiotensin II inhibitors, or sympathomimetic agents, depending on the severity and timing of the patient's symptoms. Unfortunately, idiopathic edema may be a multifactorial disorder that has not been completely delineated. Further research into possible causative mechanisms is required before a more useful algorithm for evaluation and treatment is available.

Impaired renal vascular response to a D1-like receptor agonist but not to an ACE inhibitor in conscious spontaneously hypertensive rats

The natriuretic response to a dopamine 1-like receptor agonist is blunted in spontaneously hypertensive rats (SHRs). Whether the renal vasodilator response to D1-like receptor stimulation in SHRs is defective also is unclear. To determine whether the renal hemodynamic response to a D1-like receptor is impaired in SHR, we examined the effect of a continuous infusion of the D1-like receptor agonist fenoldopam (2 microg/kg/min) on systemic and renal hemodynamics in conscious SHRs and Wistar-Kyoto (WKY) rats. As an active control, we used an equivalent antihypertensive dosage of captopril (10 mg/kg). Fenoldopam significantly increased effective renal plasma flow (ERPF) in WKY rats (+22 +/- 5%; p < 0.01), whereas this response was absent in SHRs (+7 +/- 3%; NS). Mean arterial pressure (MAP) was significantly reduced in SHRs (-11 +/- 2%; p < 0.001), demonstrating a systemic vasodilator response to fenoldopam in SHRs. The reduction in renal vascular resistance (RVR) was more pronounced in WKY rats (-24 +/- 2%) than in SHRs (-13 +/- 4%; p < 0.05). Captopril significantly increased ERPF in SHRs (+16 +/- 3%; p < 0.001), demonstrating a preserved renal vasodilatory capacity in SHRs. The blunting of the renal vasodilatory response to fenoldopam in SHRs is present during a high as well as a low sodium intake. In conscious SHRs, the renal vasodilatory response to a D1-like receptor agonist is impaired, whereas the blood pressure response is more pronounced. The preserved renal vasodilatory response to captopril indicates that the defective vasodilatory response in SHRs is functional rather than due to altered structural properties of the renal vascular bed.

Association of the G(s)alpha gene with essential hypertension and response to beta-blockade

We examined whether the GNAS1 locus, encoding the G(s) protein alpha-subunit (G(s)alpha), is implicated in the genetic causes of essential hypertension. A common silent polymorphism (ATT-->ATC, Ile(131)) was identified in exon 5 of the G(s)alpha gene by single-strand conformation polymorphism analysis and DNA sequencing. This polymorphism consists of the presence (+) or absence (-) of a restriction site for FokI. Only 1 other rare allele was found in the coding region; the high GC content of the 5' noncoding sequence prevented mutation scanning of the promoter region of the gene. There was a significant difference in frequency of the FokI alleles between 268 white hypertensives (FokI+:FokI-, 51%:49%) and a matched group of 231 control subjects (FokI+:FokI-, 58%:42%) (P=0.02). Multiple regression analysis showed that the FokI genotype was independently related to the level of untreated systolic blood pressure in 294 well-characterized white hypertensives (P=0.01) but not in normotensives. The influence of the FokI allele on blood pressure (BP) response to beta-blockade was examined in 114 of the patients randomly assigned to this class of drug. Significant differences in frequency of the FokI allele were observed in the good responders (FokI+:FokI-, 62.5%:37.5%, n=36) versus the poor responders (FokI+:FokI-, 41.7%:58.3%, n=30) after beta-blocker therapy (P=0.02). In a multiple regression analysis, the G(s)alpha genotype was the only independent predictor of BP response. These results suggest that the GNAS1 locus might carry a functional variant that influences BP variation and response to beta-blockade in essential hypertension.

Pertussis toxin-sensitive G-proteins and regulation of blood pressure in the spontaneously hypertensive rat

1. Increased Gi-protein-mediated receptor-effector coupling in the vasculature of the spontaneously hypertensive rat (SHR) has been proposed as a contributing factor in the maintenance of elevated blood pressure. If increased Gi-protein-mediated activity plays an important role in hypertension in SHR, then inhibition of Gi-proteins by pertussis toxin would be expected to decrease blood pressure in this genetic hypertensive model. To address this hypothesis, studies were undertaken comparing the cardiovascular effects of pertussis toxin in SHR and normotensive Wistar-Kyoto (WKY) rats. 2. Spontaneously hypertensive and WKY rats were instrumented with radiotelemetry devices and blood pressure measurements were recorded in conscious rats. Following a single injection of pertussis toxin (10 micrograms/kg, i.v.), mean arterial blood pressure fell from 161 +/- 3 to 146 +/- 1 mmHg in the SHR and the effect was sustained for more than 2 weeks. In contrast, 10 micrograms/kg, i.v., pertussis toxin produced no significant effect on blood pressure in WKY rats (103 +/- 4 vs 101 +/- 5 mmHg). 3. In a separate study, SHR and WKY rats were administered 30 micrograms/kg, i.v., pertussis toxin or 150 microL/kg, i.v., saline and, 3-5 days later, rats were anaesthetized and instrumented to permit measurement of blood pressure and renal function. At this higher dose, pertussis toxin reduced blood pressure in both strains of rat, although the effect was markedly greater in SHR (approximately 40 mmHg decrease) compared with WKY rats (approximately 15 mmHg decrease). In SHR, pertussis toxin increased renal blood flow (from 5.7 +/- 0.3 to 7.5 +/- 0.8 mL/min per g kidney) and decreased renal vascular resistance (from 31 +/- 2 to 19 +/- 2 mmHg/mL per min per g kidney). In WKY rats, pertussis toxin had no significant effect on renal parameters. 4. Results from these studies indicate that a pertussis toxin-sensitive Gi-protein-mediated pathway contributes to the maintenance of hypertension and elevated renal vascular tone in the SHR.

Dopamine-1 receptor coupling defect in renal proximal tubule cells in hypertension

The ability of the dopamine-1 (D1)-like receptor to stimulate adenylyl cyclase (AC) and phospholipase C (PLC), inhibit sodium transport in the renal proximal tubule (RPT), and produce natriuresis is attenuated in several rat models of hypertension. Since the inhibitory effect of D1-like receptors on RPT sodium transport is also reduced in some patients with essential hypertension, we measured D1-like receptor coupling to AC and PLC in cultures of human RPT cells from normotensive (NT) and hypertensive (HT) subjects. Basal cAMP concentrations were the same in NT (n=6) and HT (n=4). However, the D1-like receptor agonist fenoldopam increased cAMP production to a greater extent in NT (maximum response=67+/-1%) than in HT (maximum response=17+/-5%), with a potency ratio of 105. Dopamine also increased cAMP production to a greater extent in NT (32+/-3%) than in HT (14+/-3%). The fenoldopam-mediated increase in cAMP production was blocked by SCH23390 (a D1-like receptor antagonist) and by antisense D1 oligonucleotides in both HT and NT, indicating action at the D1 receptor. The stimulatory effects of forskolin and parathyroid hormone-related protein of cAMP accumulation were not statistically different in NT and HT, indicating receptor specificity and an intact G-protein/AC pathway. The fenoldopam-stimulated PLC activity was not impaired in HT, and the primary sequence and expression of the D1 receptor were the same in NT and HT. However, D1 receptor serine phosphorylation in the basal state was greater in HT than in NT and was not responsive to fenoldopam stimulation in HT. These studies demonstrate the expression of D1 receptors in human RPT cells in culture. The uncoupling of the D1 receptor in both rats (previously described) and humans (described here) suggests that this mechanism may be involved in the pathogenesis of hypertension; the uncoupling may be due to ligand-independent phosphorylation of the D1 receptor in hypertension.