Enhanced renal angiotensin II subtype 1 receptor responses in the spontaneously hypertensive rat

Venous endothelial function in cardiovascular disease

The essential role of the endothelium in vascular homeostasis is associated with the release of endothelium-dependent relaxing and contractile factors (EDRF and EDCF, respectively). Different from arteries, where these factors are widely studied, the vasoactive factors derived from the venous endothelium have been given less attention. There is evidence for a role of the nitric oxide (NO), endothelium-dependent hyperpolarization (EDH) mechanism, and cyclooxygenase (COX)-derived metabolites as EDRFs; while the EDCFs need to be better evaluated since no consensus has been reached about their identity in venous vessels. The imbalance between the synthesis, bioavailability, and/or action of EDRFs and/or EDCFs results in a pathological process known as endothelial dysfunction, which leads to reduced vasodilation and/or increased vasoconstriction. In the venous system, endothelial dysfunction is relevant since reduced venodilation may increase venous tone and decrease venous compliance, thus enhancing mean circulatory filling pressure, which maintains or modify cardiac workload contributing to the etiology of cardiovascular diseases. Interestingly, some alterations in venous function appear at the early stages (or even before) the establishment of these diseases. However, if the venous endothelium dysfunction is involved in these alterations is not yet fully understood and requires further studies. In this sense, the present study aims to review the current knowledge on venous endothelial function and dysfunction, and the general state of the venous tone in two important cardiovascular diseases of high incidence and morbimortality worldwide: hypertension and heart failure.

α 2-Adrenoceptors: Challenges and Opportunities-Enlightenment from the Kidney

It was indeed a Don Quixote-like pursuit of the mechanism of essential hypertension when we serendipitously discovered α₂-adrenoceptors (α₂-ARs) in skin-lightening experiments in the frog. Now α₂-ARs lurk on the horizon involving hypertension causality, renal denervation for hypertension, injury from falling in the elderly and prazosin's mechanism of action in anxiety states such as posttraumatic stress disorder (PTSD). Our goal here is to focus on this horizon and bring into clear view the role of α₂-AR-mediated mechanisms in these seemingly unrelated conditions. Our narrative begins with an explanation of how experiments in isolated perfused kidneys led to the discovery of a sodium-retaining process, a fundamental mechanism of hypertension, mediated by α₂-ARs. In this model system and in the setting of furosemide-induced sodium excretion, α₂-AR activation inhibited adenylate cyclase, suppressed cAMP formation, and caused sodium retention. Further investigations led to the realization that renal α₂-AR expression in hypertensive animals is elevated, thus supporting a key role for kidney α₂-ARs in the pathophysiology of essential hypertension. Subsequent studies clarified the molecular pathways by which α₂-ARs activate prohypertensive biochemical systems. While investigating the role of α₁-adrenoceptors (α₁-ARs) versus α₂-ARs in renal sympathetic neurotransmission, we noted an astonishing result: in the kidney α₁-ARs suppress the postjunctional expression of α₂-ARs. Here, we describe how this finding relates to a broader understanding of the role of α₂-ARs in diverse disease states. Because of the capacity for qualitative and quantitative monitoring of α₂-AR-induced regulatory mechanisms in the kidney, we looked to the kidney and found enlightenment.

RACK1 regulates angiotensin II-induced contractions of SHR preglomerular vascular smooth muscle cells

The preglomerular microcirculation of spontaneously hypertensive rats (SHR) is hypersensitive to angiotensin (ANG) II, and studies have shown that this is likely due to enhanced coincident signaling between G protein subunits αq (Gαq; released by ANG II) and βγ (Gβγ; released by Gi-coupled receptors) to active phospholipase C (PLC). Here we investigated the molecular basis for the enhanced coincident signaling between Gβγ and Gαq in SHR preglomerular vascular smooth muscle cells (PGVSMCs). Because receptor for activated C kinase 1 (RACK1; a scaffolding protein) organizes interactions between Gβγ, Gαq, and PLC, we included RACK1 in this investigation. Cell fractionation studies demonstrated increased levels of membrane (but not cytosolic) Gβ, Gαq, PLCβ3, and RACK1 in SHR PGVSMCs compared with Wistar-Kyoto rat PGVSMCs. In SHR PGVSMCs, coimmunoprecipitation demonstrated RACK1 binding to Gβ and PLCβ3, but only at cell membranes. Pertussis toxin (which blocks Gβγ) and U73122 (which blocks PLC) reduced membrane RACK1; however, RACK1 knockdown (shRNA) did not affect membrane levels of Gβ, Gαq, or PLCβ3 In a novel gel contraction assay, RACK1 knockdown in SHR PGVSMCs attenuated contractions to ANG II and abrogated the ability of neuropeptide Y (which signals via Gβγ) to enhance ANG II-induced contractions. We conclude that in SHR PGVSMCs the enlarged pool of Gβγ and PLCβ3 recruits RACK1 to membranes and RACK1 then organizes signaling. Consequently, knockdown of RACK1 prevents coincident signaling between ANG II and the Gi pathway. This is the first study to implicate RACK1 in vascular smooth muscle cell contraction and suggests that RACK1 inhibitors could be effective cardiovascular drugs.

p66Shc regulates renal vascular tone in hypertension-induced nephropathy

Renal preglomerular arterioles regulate vascular tone to ensure a large pressure gradient over short distances, a function that is extremely important for maintaining renal microcirculation. Regulation of renal microvascular tone is impaired in salt-sensitive (SS) hypertension-induced nephropathy, but the molecular mechanisms contributing to this impairment remain elusive. Here, we assessed the contribution of the SH2 adaptor protein p66Shc (encoded by Shc1) in regulating renal vascular tone and the development of renal vascular dysfunction associated with hypertension-induced nephropathy. We generated a panel of mutant rat strains in which specific modifications of Shc1 were introduced into the Dahl SS rats. In SS rats, overexpression of p66Shc was linked to increased renal damage. Conversely, deletion of p66Shc from these rats restored the myogenic responsiveness of renal preglomerular arterioles ex vivo and promoted cellular contraction in primary vascular smooth muscle cells (SMCs) that were isolated from renal vessels. In primary SMCs, p66Shc restricted the activation of transient receptor potential cation channels to attenuate cytosolic Ca2+ influx, implicating a mechanism by which overexpression of p66Shc impairs renal vascular reactivity. These results establish the adaptor protein p66Shc as a regulator of renal vascular tone and a driver of impaired renal vascular function in hypertension-induced nephropathy.

Vascular mechanisms involved in angiotensin II-induced venoconstriction in hypertensive rats

To investigate the venoconstrictor effect of angiotensin II (Ang II) in spontaneously hypertensive rats (SHR), we used preparations of mesenteric venular beds and the circular muscle of the portal veins. Vessels were tested with Ang II in the presence or absence of losartan, PD 123319, HOE 140, L-NAME, indomethacin, or celecoxib. In the mesenteric venular bed of SHR, the effect of Ang II (0.1 nmol) was nearly abolished by losartan and enhanced by HOE 140, indomethacin, and celecoxib, while PD123319 and L-NAME had no effect. In portal vein preparations, cumulative-concentration response curves (CCRC) to Ang II (0.1-100 nmol/L) exhibited a lower maximal response (E(max)) in SHR compared to Wistar rats. AT(1) receptor expression was similar in the two strains, while AT(2) receptor levels were lower in SHR portal veins when compared to Wistar. In SHR portal veins, losartan shifted the CCRC to Ang II to the right, while indomethacin and HOE 140 increased the E(max) to Ang II. PD 123319, celecoxib, and L-NAME had no effect. Taken together, our results suggest that Ang II-induced venoconstriction in SHR is mediated by activation of AT(1) receptors and this effect may be counterbalanced by kinin B(2) receptor and COX metabolites. Furthermore, our data indicate that there are different cellular and molecular mechanisms involved in the regulation of venous tonus of normotensive and hypertensive rats. These differences probably reflect distinct factors that influence arterial and venous bed in hypertension.

Renal denervation modulates angiotensin receptor expression in the renal cortex of rabbits with chronic heart failure

Excessive sympathetic drive is a hallmark of chronic heart failure (HF). Disease progression can be correlated with plasma norepinephrine concentration. Renal function is also correlated with disease progression and prognosis. Because both the renal nerves and renin-angiotensin II system are activated in chronic HF we hypothesized that excessive renal sympathetic nerve activity decreases renal blood flow in HF and is associated with changes in angiotensin II type 1 receptor (AT1R) and angiotensin II type 2 receptor (AT2R) expression. The present study was carried out in conscious, chronically instrumented rabbits with pacing-induced HF. We found that rabbits with HF showed a decrease in mean renal blood flow (19.8±1.6 in HF vs. 32.0±2.5 ml/min from prepace levels; P<0.05) and an increase in renal vascular resistance (3.26±0.29 in HF vs. 2.21±0.13 mmHg·ml(-1)·min in prepace normal rabbits; P<0.05) while the blood flow and resistance was not changed in HF rabbits with the surgical renal denervation. Renal AT1R expression was increased by ∼67% and AT2R expression was decreased by ∼87% in rabbits with HF; however, kidneys from denervated rabbits with HF showed a near normalization in the expression of these receptors. These results suggest renal sympathetic nerve activity elicits a detrimental effect on renal blood flow and may be associated with alterations in the expression of angiotensin II receptors.

Angiotensin II stimulates thick ascending limb NO production via AT(2) receptors and Akt1-dependent nitric-oxide synthase 3 (NOS3) activation

Angiotensin II (Ang II) acutely stimulates thick ascending limb (TAL) NO via an unknown mechanism. In endothelial cells, activation of Ang II type 2 receptor (AT(2)) stimulates NO. Akt1 activates NOS3 by direct phosphorylation. We hypothesized that Ang II stimulates TAL NO production via AT(2)-mediated Akt1 activation, which phosphorylates NOS3 at serine 1177. We measured NO production by fluorescence microscopy. In isolated TALs, Ang II (100 nm) increased NO production by 1.1 +/- 0.2 fluorescence units/min (p < 0.01). Ang II increased cGMP accumulation by 4.9 +/- 1.3 fmol/microg (p < 0.01). Upon adding the AT(2) antagonist PD123319 (1 microm), Ang II failed to stimulate NO (0.1 +/- 0.1 fluorescence units/min; p < 0.001 versus Ang II); adding the AT(1) antagonist losartan (1 microm) resulted in Ang II stimulating NO by 0.9 +/- 0.1 fluorescence units/min. Akt inhibitor (5 microm) blocked Ang II-stimulated NO (-0.1 +/- 0.2 fluorescence units/min versus inhibitor alone). Phospho-Akt1 increased by 72% after 5 min (p < 0.006), returning to basal after 10 min. Phospho-Akt2 did not change after 5 min but increased by 115 and 163% after 10 and 15 min (p < 0.02). Phospho-Akt3 did not change. An AT(2) agonist increased pAkt1 by 78% (p < 0.02), PI3K inhibition blocked this effect. In TALs transduced with dominant negative Akt1, Ang II failed to stimulate NO (0.1 +/- 0.2 fluorescence units/min versus 1.2 +/- 0.2 for controls; p < 0.001). Ang II increased phospho-NOS3 at serine 1177 by 130% (p < 0.01) and 150% after 5 and 10 min (p < 0.02). Ang II increased phosphoNOS3 at serine 633 by 50% after 5 min (p < 0.01). Akt inhibition prevented NOS3 phosphorylation. We concluded that Ang II enhances TAL NO production via activation of AT(2) and Akt1-dependent phosphorylation of NOS3 at serines 1177 and 633.

Involvement of protein kinase C-CPI-17 in androgen modulation of angiotensin II-renal vasoconstriction

AIMS

Previous studies suggested that androgens augmented renal vascular responses to angiotensin II (Ang II). The protein kinase C (PKC)-CPI-17 pathway is involved in vascular constriction. We tested the hypothesis that this pathway may contribute to androgenic amplification of Ang II-renal vasoconstriction in the New Zealand genetically hypertensive (NZGH) rat.

METHODS AND RESULTS

NZGH underwent sham operation, castration, or castration with testosterone replacement at 5 weeks of age. When the rats were 16-17 weeks of age, mean arterial pressure (MAP) and renal vascular resistance (RVR) responses to intravenous Ang II infusion (20, 40, and 80 ng/kg/min) were recorded before and after treatment with a PKC inhibitor, chelerythrine. mRNA expression of PKC isoforms and CPI-17 protein expression were analysed in renal cortex. MAP and RVR responses to Ang II were enhanced in androgen-replete NZGH. The Ang II-induced increase in RVR was significantly lower in castrated NZGH (ranged from 100 +/- 8% to 161 +/- 9% of baseline) than in sham-operated NZGH (ranged between 123 +/- 3% and 237 +/- 19% of baseline). Testosterone treatment restored RVR responses to Ang II in castrated rats. Chelerythrine treatment markedly reduced the MAP and RVR responses to Ang II in each group and attenuated the differential MAP and RVR responses to Ang II amongst the three groups. PKCdelta and PKCepsilon mRNA levels were significantly reduced by castration and increased by testosterone treatment. In contrast, no significant differences in protein expression were detected for these PKC isoforms. Castration decreased while testosterone treatment increased CPI-17 and phospho-CPI-17 expression.

CONCLUSION

Collectively, these results suggest that androgens modulate renal vascular responses to Ang II in part via an effect on the PKC-CPI-17 signalling pathway.

H2O2 stimulation of the Cl-/HCO3- exchanger by angiotensin II and angiotensin II type 1 receptor distribution in membrane microdomains

The present study tested the hypothesis that angiotensin II (Ang II)-induced oxidative stress and Ang II-stimulated Cl(-)/HCO(3)(-) exchanger are increased and related to the differential membrane Ang II type 1 (AT(1)) receptor and reduced nicotinamide-adenine dinucleotide phosphate oxidase expression in immortalized renal proximal tubular epithelial (PTE) cells from the spontaneously hypertensive rat (SHR) relative to its normotensive control (Wistar Kyoto rat [WKY]). The exposure of cells to Ang II increased Cl(-)/HCO(3)(-) exchanger activity with EC(50)s of 0.10 and 12.2 nmol/L in SHR and WKY PTE cells, respectively. SHR PTE cells were found to overexpress nicotinamide-adenine dinucleotide phosphate oxidase 2 and 4 and were endowed with an enhanced ability to generate H(2)O(2). The reduced nicotinamide-adenine dinucleotide phosphate oxidase inhibitor apocynin reduced the production of H(2)O(2) in SHR PTE cells and abolished their hypersensitivity to Ang II. The expression of the glycosylated form of the AT(1) receptor in both lipid and nonlipid rafts were higher in SHR cells than in WKY PTE cells. Pretreatment with apocynin reduced the abundance of AT(1) receptors in both microdomains, mainly the glycosylated form of the AT(1) receptor in lipid rafts, in SHR cells but not in WKY PTE cells. In conclusion, differences between WKY and SHR PTE cells in their sensitivity to Ang II correlate with the higher H(2)O(2) generation that provokes an enhanced expression of glycosylated and nonglycosylated AT(1) receptor forms in lipid rafts.

Enhanced AT1receptor-mediated vasocontractile response to ANG II in endothelium-denuded aorta of obese Zucker rats

In the present study, we tested the hypothesis that ANG II causes a greater vasoconstriction in obese Zucker rats, a model of type 2 diabetes, with mild hypertension. Measurement of isometric tension in isolated aortic rings with intact endothelium revealed a modest but not significantly greater ANG II-induced contraction in obese than lean rats. Removal of endothelium or inhibition of nitric oxide (NO) synthase by NG-nitro-l-arginine methyl ester (l-NAME) enhanced 1) ANG II-induced contraction in both lean and obese rats, being significantly greater in obese rats (Emaxg/g tissue, denuded: lean 572 ± 40 vs. obese 664 ± 16; l-NAME: lean 535 ± 14 vs. obese 818 ± 23) and 2) ANG II sensitivity in obese compared with lean rats, as revealed by the pD2values. Endothelin-1 and KCl elicited similar contractions in the aortic rings of lean and obese rats. ACh, a NO-dependent relaxing hormone, produced greater relaxation in the aortic rings of obese than lean rats, whereas sodium nitroprusside, an NO donor, elicited similar relaxations in both rat strains. The expression of the ANG type 1 (AT1) receptor protein and mRNA in the endothelium-intact aorta was significantly greater in obese than lean rats, whereas the endothelium-denuded rings expressed modest but not significantly greater levels of AT1receptors in obese than lean rats. The endothelial NO synthase protein and mRNA expression levels were higher in the aorta of obese than lean animals. We conclude that, although ANG II produces greater vasoconstriction in obese rat aortic rings, enhanced endothelial AT1receptor-mediated NO production appears to counteract the increased ANG II-induced vasoconstriction, suggesting that arterial AT1receptor may not be a contributing factor to hypertension in this model of obesity.

Role of renal sympathetic nerves in regulating renovascular responses to angiotensin II in spontaneously hypertensive rats

The purpose of this study was to test the hypothesis that renal sympathetic nerves modulate angiotensin II-induced renal vasoconstriction in kidneys from genetically hypertensive rats via Y1 receptors activating the Gi pathway. In isolated, perfused kidneys from spontaneously hypertensive rats, the naturally occurring renal sympathetic cotransmitter neuropeptide Y at 6 nM enhanced angiotensin II (0.3 nM)-induced changes in perfusion pressure by 47 +/- 7 mm Hg, and this effect was inhibited by BIBP3226 [N2-(diphenylacetyl)-N-[(4-hydroxyphenyl)-methyl]-D-arginine amide)], a selective Y1 receptor antagonist (1 microM). We next examined whether periarterial nerve stimulation (5 Hz) enhances renal vascular responses to a physiological level of angiotensin II (100 pM). Kidneys were pretreated with prazosin (a selective alpha1-adrenoceptor antagonist) to block nerve stimulation-induced changes in perfusion pressure. In kidneys from spontaneously hypertensive rats, but not normotensive rats, periarterial nerve stimulation significantly augmented angiotensin II-induced changes in perfusion pressure (177 +/- 26% of response in absence of stimulation). BIBP3226, but not rauwolscine (a selective alpha2-adrenoceptor antagonist), abolished periarterial nerve stimulation-induced enhancement of angiotensin II-mediated renal vasoconstriction. Pretreatment of hypertensive animals with pertussis toxin 3 days prior to kidney perfusion significantly (p < 0.000001) decreased mean blood pressure (203 +/- 2 versus 145 +/- 6 mm Hg in nonpretreated versus pertussis toxin-pretreated spontaneously hypertensive rats) and abolished periarterial nerve stimulation-induced enhancement of angiotensin II-mediated renal vasoconstriction. We conclude that, in spontaneously hypertensive rats but not normotensive rats, sympathetic nerve stimulation enhances renal vascular responses to physiological levels of angiotensin II via a mechanism mainly involving Y1 receptors coupled to Gi proteins.

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.

Enhanced angiotensin II-induced activation of Na+, K+-ATPase in the proximal tubules of obese Zucker rats

Renal angiotensin II (AII) is suggested to play a role in the enhanced sodium reabsorption that causes a shift in pressure natriuresis in obesity related hypertension; however, the mechanism is not known. Therefore, to assess the influence of AII on tubular sodium transport, we determined the effect of AII on the Na+, K+-ATPase activity (NKA), an active transporter regulated by the AT1 receptor activity, in the isolated proximal tubules of lean and obese Zucker rats. Also, we determined the levels of the tubular AT1 receptor and associated signal transducing G proteins, as the initial signaling components that mediate the effects of AII on Na+, K+-ATPase activity. In the isolated proximal tubules, AII produced greater stimulation of the NKA activity in obese compared with lean rats. Determination of the AT1 receptors by Scatchard analysis of the [125I] Sar-Ang II binding and Western blot analysis in the basolateral (BLM) and brush border membrane (BBM) revealed a modest but significant increase (23%) in the AT1 receptor number mainly in the BLM of obese compared with lean rats. The AII affinity for AT1 receptors, as determined by IC50 values of AII to displace [125I] Sar-Ang II binding in BLM and BBM were similar in lean and obese rats. Western blot analysis revealed significant increases in Gialpha1, Gialpha2, Gialpha3, and Gq/11alpha in BLM and Gialpha1, Gialpha3, and Gq/11alpha in BBM of obese as compared with lean rats. The increase in the levels of the AT1 receptor and G proteins, mainly in the BLM, may be contributing to the enhanced AII-induced activation of NKA in the proximal tubules of obese rats. This phenomenon, in part, may be responsible for the increased sodium reabsorption and the development of hypertension in obese Zucker rats.

Pancreatic Polypeptide-Fold Peptide Receptors and Angiotensin II–Induced Renal Vasoconstriction

The Gi pathway augments renal vasoconstriction induced by angiotensin II in spontaneously hypertensive but not normotensive Wistar-Kyoto rats. Because the Gi-coupled pancreatic polypeptide (PP)-fold peptide receptors Y1 and Y2 are expressed in kidneys and are activated by endogenous PP-fold peptides, we tested the hypothesis that these receptors regulate angiotensin II-induced renal vasoconstriction in kidneys from hypertensive but not normotensive rats. A selective Y1-receptor agonist [(Leu31,Pro34)-neuropeptide Y; 6 to 10 nmol/L] greatly potentiated angiotensin II-induced changes in perfusion pressure in isolated, perfused kidneys from hypertensive but not normotensive rats. A selective Y2-receptor agonist (peptide YY(3-36); 6 nM) only slightly potentiated angiotensin II-induced renal vasoconstriction and only in kidneys from hypertensive rats. Neither the Y1-receptor nor the Y2-receptor agonist increased basal perfusion pressure. BIBP3226 (1 micromol/L, highly selective Y1-receptor antagonist) and BIIE0246 (1 micromol/L, highly selective Y2-receptor antagonist) completely abolished potentiation by (Leu31,Pro34)-neuropeptide Y and peptide YY(3-36), respectively. Y1-receptor and Y2-receptor mRNA and protein levels were expressed in renal microvessels and whole kidneys, but the abundance was similar in kidneys from hypertensive and normotensive rats. Both Y1-receptor-induced and Y2-receptor-induced potentiation of angiotensin II-mediated renal vasoconstriction was completely abolished by pretreatment with pertussis toxin (30 microg/kg IV, blocks Gi proteins). These data indicate that, in kidneys from genetically hypertensive but not normotensive rats, Y1-receptor activation markedly enhances angiotensin II-mediated renal vasoconstriction by a mechanism involving Gi. Although Y2 receptors can also potentiate angiotensin II-mediated renal vasoconstriction via Gi, the effect is modest compared with Y1 receptors. These findings may have important implications for the etiology of genetic hypertension.

Mechanism of the vascular angiotensin II/alpha2-adrenoceptor interaction

alpha(2)-Adrenoceptors potentiate vascular responses to angiotensin II. The goal of this study was to test the hypothesis that the phospholipase C (PLC)/protein kinase C (PKC)/c-src/phosphatidylinositol 3-kinase (PI3K) pathway contributes to the vascular angiotensin II/alpha(2)-adrenoceptor interaction. In rats in vivo, intrarenal infusions of angiotensin II (10 ng/kg/min) increased renal vascular resistance by 5.8 +/- 0.5 units, and this response was enhanced (p < 0.05) to 9.1 +/- 1.2 units by UK-14,304 [5-bromo-N-(4,5-dihydro-1H-imidazol-2-yl)-6-quinoxalinamine; 3 microg/kg/min; alpha(2)-adrenoceptor agonist]. Intrarenal infusions of U-73122 [1-[6-[[(17beta)-3-methoxyestra-1,3,5(10)-trien-17-yl]amino]-hexyl]-1H-pyrrole-2,5-dione; 3 microg/min; PLC inhibitor], GF109203X [bisindolylmaleimide I; 10 microg/min; PKC inhibitor], CGP77675 [1-(2-{4-[4-amino-5-(3-methoxyphenyl)pyrrolo[2,3-d]pyrimidin-7-yl]phenyl}ethyl)piperidin-4-ol; 5 microg/min; c-src inhibitor], and wortmannin (1 microg/min; PI3K inhibitor) abolished the angiotensin II/alpha(2)-adrenoceptor interaction. In isolated perfused rat kidneys, angiotensin II (0.3, 1, and 3 nM) increased perfusion pressure (by 15 +/- 8, 39 +/- 4, and 93 +/- 9 mm Hg, respectively), and UK-14,304 (1 microM) potentiated these responses (to 36 +/- 4, 67 +/- 7, and 135 +/- 17 mm Hg, respectively). This angiotensin II/alpha(2)-adrenoceptor interaction was abolished by U-73122 (10 microM), GF109203X (3 microM), CGP77675 (5 microM), and wortmannin (0.2 microM). Preglomerular microvascular smooth muscle cells expressed phospholipase (PLC)-beta(2), PLC-beta(3), c-src, phospho(tyrosine 416)-c-src, and PI3K. In these cells, angiotensin II (0.1 microM) and UK-14,304 (1 microM) per se did not increase phospho-c-src; however, the combination of angiotensin II plus UK-14,304 doubled phospho-c-src, and this interaction was abolished by U-73122 (10 microM) and GF109203X (3 microM). In conclusion, the PLC/PKC/c-src/PI3K pathway may contribute importantly to the interaction between alpha(2)-adrenoceptors and angiotensin II on renal vascular resistance.

Effects of des-aspartate-angiotensin I on the actions of angiotensin III in the renal and mesenteric vasculature of normo- and hypertensive rats

An earlier study showed that des-aspartate-angiotensin I (DAA-I) attenuated the pressor action of angiotensin III in aortic rings of the spontaneously hypertensive rat (SHR) but not the normotensive Wistar Kyoto (WKY) rat. The present study investigated similar properties of DAA-I in isolated perfused kidneys and mesenteric beds of WKY and SHR. In the renal vasculature, angiotensin III induced a dose-dependent pressor response, which was more marked in the SHR than WKY in terms of significant greater magnitude of response and lower threshold. DAA-I attenuated the pressor action of angiotensin III in both the WKY and SHR. The attenuation in SHR was much more marked, occurring at doses as low as 10(-15) M DAA-I, while effective attenuation was only seen with 10(-9) M in WKY. The effects of DAA-I was not inhibited by PD123319 and indomethacin, indicating that its action was not mediated by angiotensin AT2 receptors and prostaglandins. However, the direct pressor action of angiotensin III in the SHR but not the WKY was attenuated by indomethacin suggesting that this notable difference could be due to known decreased response of renal vasculature to vasodilator prostaglandins in the SHR. Pressor responses to angiotensin III in the mesenteric vascular bed was also dose dependent, but smaller in magnitude compared to the renal response. The responses in the SHR, though generally smaller, were not significantly different from those of the WKY. This trend is in line with the similar observations with angiotensin III and II by other investigators. In terms of the effect of DAA-I, indomethacin and PD123319 on angiotensin III action, similar patterns to those of the renal vasculature were observed. This reaffirms that in the perfused kidney and mesenteric bed, where the majority of the vessels are contractile, femtomolar concentrations of DAA-I attenuates the pressor action of angiotensin III. The attenuation is not indomethacin sensitive and does not involve the angiotensin AT2 receptor. The findings suggest that DAA-I possesses protective vascular actions and is involved in the pathophysiology of hypertension.

Impaired substance P release from renal sensory nerves in SHR involves a pertussis toxin-sensitive mechanism

Stretching the renal pelvic wall activates renal mechanosensory nerves by a PGE2-mediated release of substance P via activation of the cAMP-PKA pathway. Renal pelvic ANG II modulates the responsiveness of renal sensory nerves by suppressing the PGE2-mediated activation of adenylyl cyclase via a pertussis toxin (PTX)-sensitive mechanism. In SHR, activation of renal mechanosensory nerves is impaired. This is due to suppressed release of substance P in response to increased pelvic pressure. The present study was performed to investigate whether the PGE2-mediated release of substance P was suppressed in SHR vs. WKY and, if so, whether the impaired PGE2-mediated release of substance P was due to ANG II activating a PTX-sensitive mechanism. In an isolated renal pelvic wall preparation, PGE2, 0.14 μM, increased substance P release from 9 ± 3 to 22 ± 3 pg/min ( P < 0.01) in Wistar-Kyoto rats (WKY), but had no effect in spontaneously hypertensive rats (SHR). A tenfold higher concentration of PGE2, 1.4 μM, was required to increase substance P release in SHR, from 7 ± 1 to 22 ± 3 pg/min ( P < 0.01). In SHR, treating renal pelvises with losartan enhanced the release of substance P produced by subthreshold concentration of PGE2, 0.3 μM, from 16 ± 2 to 26 ± 3 pg/min ( P < 0.01). Likewise, treating renal pelvises with PTX enhanced the PGE2-mediated release of substance P from 10 ± 1 to 33 ± 3 pg/min ( P < 0.01) in SHR. In WKY, neither losartan nor PTX had an effect on the release of substance P produced by subthreshold concentrations of PGE2, 0.03 μM. In conclusion, the impaired responsiveness of renal sensory nerves in SHR involves endogenous ANG II suppressing the PGE2-mediated release of substance P via a PTX-sensitive mechanism.

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.

Effects of angiotensin II receptor antagonists on insulin resistance syndrome and leptin in sucrose-fed spontaneously hypertensive rats

In order to investigate the usefulness of angiotensin II type 1 receptor (AT1) antagonists (ARA) in the treatment of hypertension with insulin resistance syndrome, we studied the effects of a high dose sucrose diet and ARA on insulin sensitivity, plasma lipids, and leptin in spontaneous hypertensive rats (SHR) and Wistar-Kyoto rats (WKY). SHR and WKY were divided into three groups and treated for 12 weeks: those fed a standard chow, those given a sucrose-rich chow or those given a sucrose-rich chow and ARA. While in SHR the weight of both subcutaneous and mesenteric adipose tissue was greater in the sucrose-rich chow fed animals than in the standard chow fed animals, ARA treatment significantly decreased the weights of both subcutaneous and mesenteric adipose tissue. ARA treatment decreased free fatty acid and triglyceride in SHR, and increased high density lipoprotein cholesterol in SHR and WKY. Homeostasis model assessment-insulin resistance (HOMA-IR) index, plasma levels of leptin, and leptin mRNA in mesenteric adipose tissue were significantly greater in the sucrose-rich chow fed animals than in the standard chow fed animals, and significantly lower in the ARA-treated sucrose-rich chow fed animals than in the sucrose-rich chow fed animals in both SHR and WKY. ARA improved insulin resistance, and reduced plasma leptin and leptin mRNA in adipose tissue. These results suggest that the improvement of insulin resistance by ARA may be attributed, at least in part, to the reduction of adipose tissue weight. It is concluded that ARA is useful in the treatment of patients with hypertension and concomitant insulin resistance syndrome.

alpha 2-Adrenoceptors potentiate angiotensin II- and vasopressin-induced renal vasoconstriction in spontaneously hypertensive rats

Hypertension in spontaneously hypertensive rats (SHRs) is due in part to enhanced effects of vasoactive peptides on the renal vasculature. We hypothesize that the G(i) signal transduction pathway enhances renovascular responses to vasoactive peptides in SHRs more so than in normotensive Wistar-Kyoto (WKY) rats. To test this hypothesis, we examined in isolated perfused kidneys from SHRs and WKY rats the renovascular responses (assessed as changes in renal perfusion pressure in mm Hg) to angiotensin II (10 nM) and vasopressin (3 nM) in the presence and absence of UK-14,304 [5-bromo-N-(4,5-dihydro-1H-imidazol-2-yl)-6-quinoxalinamine; an agonist that selectively activates the G(i) pathway by stimulating alpha(2)-adrenoceptors]. In SHR, but not WKY, kidneys, UK-14,304 (10 nM) enhanced (P < 0.05) renovascular responses to angiotensin II (control WKY, 43 +/- 6; UK-14,304-treated WKY, 52 +/- 19; control SHR, 66 +/- 17; UK-14,304-treated SHR, 125 +/- 16) and vasopressin (control WKY, 42 +/- 17; UK-14,304-treated WKY, 36 +/- 11; control SHR, 16 +/- 8; UK-14,304-treated SHR, 83 +/- 17). Pretreatment of SHRs with pertussis toxin (30 microg/kg, intravenously, 3-4 days before study) to inactivate G(i) blocked the effects of UK-14,304. Western blot analysis of receptor expression in whole kidney and preglomerular microvessels revealed similar levels of expression of AT(1), V(1a), and alpha(2A) receptors in SHRs compared with WKY rats. We conclude that activation of alpha(2)-adrenoceptors selectively enhances renovascular responses to angiotensin II and vasopressin in SHRs via an enhanced cross talk between the G(i) signal transduction pathway and signal transduction pathways activated by angiotensin II and vasopressin.

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.

Abnormal renal medullary response to angiotensin II in SHR is corrected by long-term enalapril treatment

This study tested the hypotheses that renal medullary blood flow (MBF) in spontaneously hypertensive rats (SHR) has enhanced responsiveness to angiotensin (ANG) II and that long-term treatment with enalapril can correct this. MBF, measured by laser Doppler flowmetry in anesthetized rats, was not altered significantly by ANG II in Wistar-Kyoto (WKY) rats, but was reduced dose dependently (25% at 50 ng. kg(-1). min(-1)) in SHR. Infusion of N(G)-nitro-L-arginine methyl ester (L-NAME) into the renal medulla unmasked ANG II sensitivity in WKY rats while L-arginine given into the renal medulla abolished the responses to ANG II in SHR. In 18- to 19-wk-old SHR treated with enalapril (25 mg. kg(-1). day(-1) when 4 to 14 wk old), ANG II did not alter MBF significantly, but sensitivity to ANG II was unmasked after L-NAME was infused into the renal medulla. Endothelium-dependent vasodilation (assessed with aortic rings) was significantly greater in treated SHR when compared with that in control SHR. These results indicate that MBF in SHR is sensitive to low-dose ANG II and suggest that this effect may be due to an impaired counterregulatory effect of nitric oxide. Long-term treatment with enalapril improves endothelium-dependent vascular relaxation and decreases the sensitivity of MBF to ANG II. These effects may be causally related to the persistent antihypertensive action of enalapril in SHR.

Angiotensin II signaling to phospholipase D in renal microvascular smooth muscle cells in SHR

Angiotensin II (Ang II)-induced phospholipase D (PLD) activity is greater in aortic smooth muscle from spontaneously hypertensive rats (SHR) versus normotensive Wistar-Kyoto rats (WKY). Whether and how this signaling pathway is altered in preglomerular microvascular smooth muscle cells (PGSMCs), a cell type that may participate in genetic hypertension, is unknown. The goals of the present study were to determine in SHR and WKY PGSMCs the following: (1) whether Ang II induces PLD activity; (2) whether the effect of Ang II on PLD activity is greater in SHR; (3) which PLD isoform is stimulated by Ang II; (4) what signaling pathway mediates Ang II-induced PLD stimulation; and (5) whether the signaling pathways mediating Ang II-induced PLD activity are different in SHR and WKY. The EC(50) for Ang II-induced PLD stimulation in SHR was 10-fold lower than the EC(50) in WKY, and both were inhibited by L-158,805, an AT(1) antagonist. Inhibitors of phosphoinositol-3-kinase and protein kinase C did not block Ang II-induced PLD activity in SHR and WKY PGSMCs. Catalytically-inactive constructs of PLD2 and RhoA, but not PLD1, ADP ribosylation factor 1 (ARF1), ARF6, or ADP ribosylation factor nucleotide exchange factor (ARNO) blocked Ang II-induced PLD activity in SHR and WKY PGSMCs. Brefeldin A completely blocked Ang II-induced PLD activity in SHR but only slightly reduced Ang II-induced PLD activity in WKY PGSMCs. Therefore, we conclude that in PGSMCs, the effect of Ang II on PLD activity is (1) greater in SHR; (2) mediated by AT(1) receptors signaling to PLD2; (3) transduced primarily by Rho proteins; and (4) inhibited in SHR by brefeldin A.

Angiotensin-(1-7) regulates the levels of angiotensin II receptor subtype AT1 mRNA differentially in a strain-specific fashion

Ang-(1-7) is an effector peptide of the renin-angiotensin system with several distinct actions that are likely mediated by a specific receptor. Regulatory effects of angiotensin (Ang) peptides, Ang-(1-7) and Ang II, on Ang receptor subtype 1 (AT1) mRNA expression were investigated in vascular smooth muscle cells (VSMC) from four University of Akron (Akr) rat strains (WKY, SHR and two backcross consomic lines SHR/y and SHR/a), and in SHR and WKY cells from Charles River Laboratories (Crl). In WKY/Akr and SHR/Akr, Ang-(1-7) treatment increased the levels of AT1 mRNA. This effect was inhibited by the specific Ang-(1-7) antagonist, A-779, in WKY/Akr but not SHR/Akr. Ang II had no effect in Akr cells, but it down-regulated AT1 mRNA in WKY/Crl and SHR/Crl VSMC. Ang-(1-7) did not affect AT1 mRNA levels in Crl lines. In conclusion, Ang-(1-7) regulates the AT1 receptor either directly or indirectly in a strain-specific fashion. The Ang-(1-7) antagonist, A-779, blocks the actions of Ang-(1-7) only in VSMC from WKY/Akr rats, suggesting either that the binding sites for Ang-(1-7) have different properties in SHR/Akr and WKY/Akr cell lines, or that some of the effects of Ang-(1-7) are not receptor mediated. Further, we found differences between Akr cells and Crl cells that are consistent with their genetic heterogeneity.

Role of the angiotensin AT(1) receptor in rat aortic and cardiac PAI-1 gene expression

Although the renin-angiotensin system has been implicated in increasing plasminogen activator inhibitor-1 (PAI-1) expression, the role of the angiotensin type 1 (AT(1)) receptor is controversial. This report examines the effects of angiotensin peptides, angiotensin-converting enzyme inhibition, and AT(1) antagonism on rat aortic and cardiac PAI-1 gene expression. In vitro, angiotensin (Ang) I, Ang II, and angiotensin Arg(2)-Phe(8) (Ang III) were potent agonists of PAI-1 mRNA expression in rat aortic smooth muscle cells (RASMCs), and stimulation of PAI-1 by these peptides was blocked by the AT(1) antagonist candesartan. Angiotensin Val(3)-Phe(8) (Ang IV) and angiotensin Asp(1)-Pro(7) (Ang [1-7]) did not affect PAI-1 expression in RASMCs. In neonatal rat cardiomyocytes, Ang II increased PAI-1 mRNA expression by 4-fold (P<0.01), and this response was completely blocked by AT(1) receptor antagonism. Continuous intrajugular infusion of Ang II into Sprague-Dawley rats for 3 hours increased aortic and cardiac PAI-1 mRNA expression by 17- and 9 fold, respectively, and these Ang II responses were completely blocked by coinfusion with candesartan. Aortic and cardiac PAI-1 expressions were compared in spontaneously hypertensive rats and Wistar-Kyoto rats. PAI-1 expression in the aorta and heart from spontaneously hypertensive rats was 5.8-fold and 2-fold higher, respectively, than in control Wistar-Kyoto rats (P<0.05). Candesartan treatment for 1 week reduced aortic and cardiac PAI-1 expression in spontaneously hypertensive rats by 94% and 72%, respectively (P<0.05), but did not affect vascular PAI-1 levels in Wistar-Kyoto rats. These results demonstrate a role for the AT(1) receptor in mediating the effects of Ang II on aortic and cardiac PAI-1 gene expression.

Afferent arteriolar reactivity to angiotensin II is enhanced during the early phase of angiotensin II hypertension

Increased renal microvascular reactivity may contribute to the blunted pressure natriuretic response and increase in blood pressure during the development of angiotensin II hypertension. The current studies were performed to determine renal microvascular reactivity during the early phases of angiotensin II-infused hypertension. Male-Sprague Dawley rats received angiotensin II (60 ng/min) or vehicle via an osmotic minipump. Normotensive and angiotensin II hypertensive rats were studied 1 and 2 weeks after implantation of the minipump. Systolic blood pressure averaged 117 +/- 4 mm Hg (n = 31) before pump implantation. Angiotensin II infusion increased systolic blood pressure to 149 +/- 3 and 187 +/- 5 mm Hg on infusion days 6 and 12, respectively. Renal microvascular responses to angiotensin II and norepinephrine at renal perfusion pressures of 100 and 150 mm Hg were observed using the in vitro juxtamedullary nephron preparation. Afferent arteriolar diameters of 1-week normotensive animals averaged 22 +/- 1 microm and after 2 weeks of vehicle infusion averaged 21 +/- 1 microm at a perfusion pressure of 100 mm Hg. In animals infused with angiotensin II for 1 or 2 weeks, diameters of the afferent arterioles perfused at a pressure of 100 mm Hg were 20% and 9% smaller, respectively. Additionally, 1- and 2-week hypertensive animals had an enhanced responsiveness of the renal microvasculature to angiotensin II. At a perfusion pressure of 100 mm Hg, angiotensin II (10 nmol/L) decreased afferent arteriolar diameter by 26 +/- 5% and 22 +/- 3% in the 1- and 2-week angiotensin II hypertensive rats, respectively. In 1- and 2-week normotensive animals, angiotensin II (10 nmol/L) decreased afferent arteriolar diameter by 18 +/- 2% and 15 +/- 2%, respectively, at a perfusion pressure of 100 mm Hg. In contrast, the afferent arteriolar response to norepinephrine was not altered in angiotensin II hypertensive rats. These data demonstrate an elevated renal microvascular resistance and enhanced vascular reactivity that is selective for angiotensin II in the early phases of hypertension development after infusion of angiotensin II. Thus, an alteration in renal microvascular function contributes to the blunted pressure natriuretic response and progressive development of hypertension.

Effect of cardiac receptor stimulation on renal vascular resistance in the pregnant rat

Stimulation of cardiac receptors (CR) evokes blunted reflex reductions in mean arterial pressure (MAP) in pregnant compared with virgin rats. Because CR-mediated sympathoinhibition has preferential effects on the kidney, we tested whether, during pregnancy, renal vascular resistance (RVR) changes less in response to CR stimulation and investigated possible mechanisms. MAP, right atrial pressure, renal sympathetic nerve activity (RSNA), renal blood flow (RBF), and RVR were measured in anesthetized animals in response to CR stimulation by graded atrial injections of saline. Baseline MAP and RVR and reflex changes in these variables during CR stimulation were reduced in late-pregnant vs. virgin rats (P<0.05). Reflex changes in RSNA were attenuated in pregnant rats, but changes in RBF as a function of RSNA were similar in both groups. ANG II AT(1)-receptor blockade increased basal RBF more in virgin rats (P<0.05), but between-group differences in reflex changes in MAP, RSNA, and RVR were maintained after AT(1) blockade. Thus during CR simulation, reflex changes in RVR were reduced in pregnant versus virgin rats. This difference does not appear to involve differential effects of ANG II.

Differing effects of enalapril and losartan on renal medullary blood flow and renal interstitial hydrostatic pressure in spontaneously hypertensive rats

OBJECTIVE

To determine the effect of short-term angiotensin converting enzyme inhibition (enalapril) or angiotensin II AT1 receptor blockade (losartan) on medullary hemodynamics in the spontaneously hypertensive rat (SHR).

DESIGN

Laser-Doppler flowmetry allowed for the characterization of medullary blood flow (MBF) over a wide range of renal arterial pressure (RAP), and was used for comparison among treatment groups. Renal interstitial hydrostatic pressure (RIHP) was also determined over a wide range of RAP.

METHOD

Enalapril or losartan was given to male 12-13-week-old SHR for 3 days (25 mg/kg per day in drinking water). Rats were anesthetized with Inactin, renal function was measured at resting levels of RAP and then RAP was varied over a range of 50-150 mmHg in 25 mmHg steps. MBF and RIHP were determined at each pressure.

RESULTS

Resting mean arterial pressure (MAP) (mmHg +/- SE) for enalapril- and for losartan-treated SHR [114 +/- 3 (n = 18) and 124 +/- 3 (n = 20), respectively] were both significantly lower than for untreated SHR [159 +/- 5 (n = 20)]. Renal function at resting levels of MAP was not significantly different among groups. Enalapril and losartan both increased MBF by 30% at levels of RAP of 125 mmHg and over. Enalapril did not alter the relation between RAP and RIHP, but losartan shifted the RAP versus RIHP curve by approximately 40 mmHg to lower levels of RAP. Acute administration of the B2 kinin receptor antagonist HOE 140 [20 microg/kg intravenous (i.v.) bolus, then 10 microg/kg per h i.v.] did not significantly alter MAP in any group. HOE 140 did not significantly alter MBF or RIHP in the untreated or losartan-treated SHR. MBF in enalapril-treated rats receiving HOE 140 was not significantly different from that of the enalapril-only group; however, the relation between RAP and RIHP was shifted to lower levels of RAP by approximately 45 mmHg.

CONCLUSIONS

Both enalapril and losartan increase MBF in SHR, suggesting that the medullary circulation of SHR is influenced by endogenous levels of angiotensin II. The failure of enalapril to increase RIHP in parallel with MBF appears to be due to an enhanced effect of kinins.

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.

Enhanced renal vascular responsiveness to angiotensin II in hypertensive ren-2 transgenic rats

The present study was performed to evaluate renal vascular responsiveness (RVR) to ANG II in hypertensive transgenic rats [TGR; strain TGR(mRen2)27] harboring the mouse ren-2 renin gene. Renal blood flow (RBF) responses to either intravenous or intrarenal arterial administration of ANG II were assessed in pentobarbital sodium-anesthetized female heterozygous TGR (9-12 wk old) and age-matched transgene-negative Hanover Sprague-Dawley rats (HanSD). Intravenous bolus injections of 15 and 30 ng ANG II elicited dose-dependent increases in mean arterial blood pressure (AP) and decreases in RBF in both TGR and HanSD. However, the magnitude of the increases in AP was greater in TGR than in HanSD (24 +/- 1 vs. 17 +/- 2 mmHg and 33 +/- 2 vs. 25 +/- 1 mmHg, respectively, P < 0.05 in both cases). Similarly, the magnitude of the decrease in RBF elicited by intravenous administration of 15 ng of ANG II was greater in TGR than HanSD (-62 +/- 3 vs. -52 +/- 5%, P < 0.05). Intrarenal arterial administration of 1.5 and 3 ng ANG II did not alter mean AP in either group but elicited larger decreases in RBF in TGR than in HanSD (-24 +/- 2 vs. -13 +/- 1% and -41 +/- 5 vs. -30 +/- 2%, respectively, P < 0.05 in both cases). In contrast, intrarenal arterial administration of norepinephrine (40 and 80 ng) elicited smaller decreases in RBF in TGR than in HanSD (-24 +/- 3 vs. -40 +/- 6% and -51 +/- 9 vs. -71 +/- 8%, respectively, P < 0.05 in both cases), indicating that TGR do not exhibit a generalized increase in RVR to endogenous vasoconstrictors. Furthermore, the enhanced RVR to ANG II does not appear to reflect an impaired RVR to endogenous vasodilator factors since intrarenal administration of bradykinin and acetylcholine elicited larger increases in RBF in TGR than in HanSD. The present findings indicate that hypertensive TGR exhibit exaggerated renal and peripheral vascular responses to ANG II, which likely contributes to an increased renal and peripheral vascular resistance and thereby to the hypertension in TGR.

Kidney aminopeptidase A and hypertension, part I: spontaneously hypertensive rats

Tissue and plasma levels of aminopeptidase A (APA), the principal enzyme that hydrolyzes angiotensin II (Ang II) to angiotensin III, were measured in spontaneously hypertensive rats (SHR) and their normotensive control strain at 3 different ages corresponding to prehypertensive (4 weeks), developing (8 weeks), and established (16 weeks) phases of hypertension. Plasma APA activity was significantly but modestly elevated in SHR at all 3 ages compared with normotensive Wistar-Kyoto rats. Likewise, levels of APA in brain, heart, and adrenal gland were generally, but again only moderately, elevated in SHR at all ages. However, a large increase in APA activity was seen within the kidney in which APA levels were elevated 41%, 51%, and 68% in SHR at 4, 8, and 16 weeks of age, respectively. Kidney APA levels were also significantly increased in immunoblots from 8- and 16-week-old SHR. Glomeruli isolated from 16-week-old SHR had 57% higher APA activity and increased immunoreactivity compared with Wistar-Kyoto rats. To determine whether the increase in kidney APA activity in SHR was related to Ang II levels, SHR were treated for 2 weeks with the angiotensin-converting enzyme inhibitor captopril. Captopril treatment reduced blood pressure to normotensive values and resulted in a 25% reduction in kidney APA activity. These results suggest that APA expression in the kidney may be regulated by activity of the renin-angiotensin system. If so, this would further suggest that upregulation of APA during conditions in which Ang II levels were elevated would have a protective effect against Ang II-mediated cardiovascular diseases, whereas a decrease in APA expression or a failure to upregulate would exacerbate such conditions.

Role of angiotensin II in hyperinsulinemia-induced hypertension in rats

OBJECTIVE

To investigate the role of angiotensin II in the pathogenesis of hyperinsulinemia-induced hypertension in rats.

MATERIALS AND METHODS

Chronic hyperinsulinemia was achieved by infusing insulin (3 mU/kg per min) subcutaneously by an osmotic minipump for 6 weeks. An angiotensin converting enzyme inhibitor (fosinopril, 10 mg/kg per day) was added in drinking water and the angiotensin II subtype 1 receptor antagonist losartan (3.5 microg/kg per min) was co-infused via the minipump. Control rats were administered the vehicle only. The rats were housed in individual metabolic cages and fed a sodium-controlled diet. Food and water intake and urine output were measured daily. Systolic blood pressure and heart rate were measured by the tail-cuff method twice a week.

RESULTS

By the end of weeks 4 and 6 of the sustained insulin infusion, systolic blood pressure had increased significantly (P < 0.05), from 134+/-1 to 157+/-2 and 158+/-2 mmHg, respectively, and the heart rate had increased significantly (P< 0.05), from 380+/-9 to 423+/-7 and 426+/-6 beats/min, respectively. The plasma insulin concentration increased by 2-2.5 times but no significant changes in plasma glucose and triglyceride levels were noted. Concomitant treatment with fosinopril prevented the rises in systolic blood pressure and heart rate in the insulin-infused rats. When the insulin-induced hypertension had become established (systolic blood pressure increased from 132+/-3 to 155+/-2 mmHg 4 weeks after the infusion, P< 0.05 ), subsequent fosinopril or losartan treatment for 2 weeks reversed the elevated systolic blood pressure and heart rate to the control levels. There were no significant differences in water intake, urine flow, sodium gain and body weight gain between the control and the insulin-infused rats.

CONCLUSIONS

Angiotensin converting enzyme inhibition or angiotensin II type 1 receptor antagonism can prevent and reverse insulin-induced hypertension in rats, suggesting that angiotensin II itself or an angiotensin II-dependent mechanism has an etiological influence in the pathogenesis of this hypertension model.

Angiotensin receptor blockade blunts hyperinsulinemia-induced hypertension in rats

The study was conducted to examine the effects of the angiotensin subtype 1 and 2 receptor antagonists (losartan and PD123319, respectively) on blood pressure (BP) and renal excretory function in chronic hyperinsulinemia-induced hypertension in rats. Hyperinsulinemia was achieved by insulin infusion (21.5 pmol/kg per minute) via osmotic minipump for 6 weeks. Losartan or PD 123319 was coinfused either at the beginning or after 4 weeks of insulin infusion. The results showed that insulin infusion significantly increased the plasma insulin concentration from 259.0+/-22.2 to 646.5+/-33.0 and 713.9+/-26.5 pmol/L (P<0.05) by the end of the fourth and sixth weeks, respectively, after insulin infusion. There were no significant changes in plasma glucose and triglyceride concentrations. Systolic BP increased from 139+/-3 to 156+/-1 and 157+/-2 mm Hg (P<0.05) at the corresponding time points. Combined losartan (3.5 microg/kg per minute) and insulin infusion prevented the rise in BP and improved insulin resistance. When hypertension had been established after 4 weeks of insulin infusion, superimposed infusion of losartan on insulin reversed the elevated BP to control levels within 1 week. In contrast, administration of PD123319 (0.5 and 10 microg/kg per minute) failed to alter insulin-induced hypertension. Combined PD123319 with losartan did not alter the losartan-induced hypotensive effect in insulin-infused rats. There were no significant differences in water intake, urine flow, body weight gain, and sodium gain before and after antagonist administration among groups. These results indicate that angiotensin type 1 receptors play a determinant role in the pathogenesis of insulin-induced hypertension in rats.

Renal vascular responses to angiotensin II in conscious spontaneously hypertensive and normotensive rats

It has been postulated that exaggerated renal sensitivity to angiotensin II may be involved in the development and maintenance of hypertension in the spontaneously hypertensive rat (SHR). The purpose of this study was to compare the renal vascular responses to short-term angiotensin II infusions (50 ng/kg/min, i.v.) in conscious SHRs and Wistar-Kyoto (WKY) rats. Renal cortical blood flow was measured in conscious rats by using quantitative renal perfusion imaging by magnetic resonance, and blood pressure was measured by an indwelling carotid catheter attached to a digital blood pressure analyzer. Renal vascular responses to angiotensin II were similar in control SHRs and WKY rats. Pretreatment with captopril to block endogenous production of angiotensin II significantly augmented the renal vascular response to exogenous angiotensin II in the SHRs but not in the WKY rats. The renal vascular responses to angiotensin II were significantly greater in captopril-pretreated SHRs than in WKY rats (cortical blood flow decreased by 1.66 +/- 0.13 ml/min/g cortex in WKY rats compared with 2.15 +/- 0.14 ml/min/g cortex in SHR; cortical vascular resistance increased by 10.5 +/- 1.4 mm Hg/ml/min/g cortex in WKY rats compared with 15.6 +/- 1.7 mm Hg/ml/min/g cortex in SHRs). Responses to angiotensin II were completely blocked in both strains by pretreatment with the angiotensin II AT1-receptor antagonist losartan. Results from this study in conscious rats confirm previous findings in anesthetized rats that (a) the short-term pressor and renal vascular responses to angiotensin II are mediated by the AT1 receptor in both SHRs and WKY rats, and (b) the renal vascular responses to angiotensin II are enhanced in SHRs compared with WKY rats when endogenous production of angiotensin II is inhibited by captopril pretreatment.

Effects of AT1 receptor blockade on blood pressure and the renin-angiotensin system in spontaneously hypertensive rats of the stroke prone strain

The aim of the study was to assess the effects of chronic angiotensin I receptor blockade on blood pressure, the renin-angiotensin system in plasma and kidney and the extent of renal damage in spontaneously hypertensive rats of the stroke prone strain (SHRsp). Four months old male SHRsp rats were orally treated with a high (10 mg/kg b.w. per day) or a low dose (1 mg/kg b.w. per day) of the AT1 receptor antagonist Telmisartan and compared to Losartan- (20 mg/kg b.w. per day), Captopril-treated (50 mg/kg b.w. per day) or untreated control groups for 38 days. Despite a similar extent of blood pressure reduction in all groups (except low dose Telmisartan), high dose Telmisartan but not Losartan or Captopril significantly reduced left ventricular weight by 24% compared to controls (p<0.05). Renal damage as assessed by urinary albumin or glomerulosclerosis index was significantly reduced in all treatment groups (p<0.02). Plasma renin concentration was significantly elevated (p<0.02) and plasma angiotensinogen significantly lowered (p<0.05) in all pharmacologically treated group compared to controls. In the kidney, renin-mRNA as well as AT1 receptor gene expression were elevated in all treatment groups, but no significant changes were found for renal angiotensinogen-mRNA. Chronic oral treatment of genetically hypertensive rats by the AT1 receptor antagonist Telmisartan reveals a blood pressure lowering and reno-protective effect of this drug comparable to other AT1 receptor antagonists or converting enzyme inhibitors, and demonstrates a marked reduction of cardiac hypertrophy by Telmisartan in this model.

G protein mRNA expression in renal microvessels from spontaneously hypertensive and Wistar-Kyoto rats

The exaggerated sensitivity of spontaneously hypertensive rat (SHR) renal microvasculature to angiotensin II (ANG II) may be due to an imbalance between the effectiveness of G alpha s-utilizing vasodilator pathways and vasoconstrictor pathways activated by ANG II (mediated by G alpha i-1, G alpha i-2, G alpha i-3, and G alpha q). Because the alteration appears to be distal to the hormone receptors and proximal to the effector adenylyl cyclase, we hypothesized that SHR have altered amounts of signal-transducing G proteins. This was examined by quantifying the steady-state mRNA levels of specific G alpha subunits in renal microvessels of 12- to 14-wk-old SHR and control Wistar-Kyoto (WKY) rats, using a quantitative-competitive polymerase chain reaction technique coupled to reverse transcription. No significant differences were detected in the absolute levels of G alpha s (0.96 +/- 0.35 vs. 0.74 +/- 0.25 amol/50 ng RNA) or in the relative levels of G alpha i-1 (0.44 +/- 0.05 vs. 0.48 +/- 0.13). G alpha i-2 (40.9 +/- 7.8 vs. 45.2 +/- 8.9), or G alpha i-3 (0.79 +/- 0.05 vs. 0.82 +/- 0.15) normalized to the level of G alpha s for WKY vs. SHR, respectively. The ratio of G alpha q to G alpha s tended to be higher in SHR, but this difference did not achieve statistical significance (0.41 +/- 0.08 vs. 1.04 +/- 0.32, P = 0.08). In conclusion, the steady-state levels of G alpha s, G alpha i-1, G alpha i-2, G alpha i-3, and G alpha q are similar in SHR and WKY renal microvasculature, suggesting that other components of the ANG II signal transduction mechanism are responsible for the enhanced renal vascular responsiveness in SHR.

Renin secretion in Lyon hypertensive rats

In genetically hypertensive rats of Lyon strain (LH), both development and maintenance of hypertension are extremely sensitive to the chronic blockade of the renin-angiotensin system. However, LH rats exhibit a low renin secretory profile as indicated by (1) low basal plasma renin concentration; (2) blunted renin responses to reductions of renal perfusion pressure and beta-adrenoceptor stimulation both in vitro (isolated perfused kidney) and in vivo (conscious rat). None of the latter abnormalities are corrected by chronic sodium deprivation or when hypertension is prevented by hydralazine or perindopril treatment. Future studies will therefore have to elucidate the 'renin paradox' in LH rats.

Role of renal nerves in afferent arteriolar reactivity in angiotensin-induced hypertension

The objective of this study was to determine the contribution of renal nerves to the enhanced afferent arteriolar reactivity observed in angiotensin II (Ang II)-induced hypertension. Uninephrectomized Sprague-Dawley rats were divided into four groups: sham rats, renal-denervated rats, Ang II-infused (at 40 ng/min for 13 days) rats, and Ang II-infused+renal-denervated rats. With the use of an implanted arterial catheter, mean arterial pressure (MAP) was monitored in conscious rats. Ang II infusion resulted in a progressive increase in MAP from 98 +/- 1 (day 0) to 166 +/- 7 mm Hg (day 13). This increase in MAP was attenuated in denervated rats and averaged 136 +/- 3 mm Hg on day 13. Kidneys were harvested on day 13 for microcirculatory experiments or measurement of intrarenal Ang II levels. Basal afferent arteriolar diameter was similar in all groups, and group averages ranged from 19.6 to 20.7 microns. Chronic Ang II infusion increased intrarenal Ang II levels. Renal denervation did not alter this effect. Increasing perfusion pressure from 100 to 160 mm Hg reduced afferent arteriolar diameter significantly by 11.2 +/- 0.6% in the sham group and by a similar degree in the remaining three groups. Superfusion with Ang II (10 nmol/L) reduced afferent arteriolar diameter by 34.3 +/- 2.0% in the sham group. This response was enhanced in Ang II-infused (62.3 +/- 3.4%) but not in renal-denervated or Ang II-infused+renal-denervated rats. Additionally, the enhanced afferent arteriolar reactivity to Ang II was not influenced by adrenergic receptor blockade. The afferent arteriolar response to norepinephrine was enhanced in renal-denervated, Ang II-infused, and Ang II-infused+renal-denervated rats compared with sham controls. Administration of the calcium ionophore A23187 decreased afferent arteriolar diameter similarly in all four groups. These results indicate that renal nerves contribute to the development of hypertension and to the enhanced afferent arteriolar responsiveness to Ang II elicited by chronic Ang II infusion.

Effects of lisinopril on the structure of renal arterioles

We investigated the effect of long-term administration of the angiotensin-converting enzyme inhibitor lisinopril on renal arterioles in spontaneously hypertensive rats (SHR) and Wistar-Kyoto rats (WKY) using a morphometric method and vascular cast technique. Rats were treated with lisinopril beginning at 4 weeks of age. At 15 weeks of age, the kidney vessels were fixed when maximally relaxed. Resin was perfused into the right kidney to make a cast of the renal vasculature. The opposite kidney was used for the morphometric study to evaluate structural changes of the vascular wall. The vascular cast study demonstrated a significant reduction in the lumen diameter of the afferent but not the efferent arterioles in SHR compared with those in WKY. In lisinopril-treated rats, the afferent arteriolar lumen diameters were significantly larger than those of the respective control groups in both strains. However, treatment did not affect the lumen diameter of efferent arterioles in either strain. The morphometric study revealed that the cross-sectional area of afferent arteriolar media was significantly smaller in SHR than WKY, suggesting that the impaired growth of the afferent arteriolar media was involved in the narrowed afferent arteriolar lumen in SHR. The presence of significantly smaller media-lumen ratio, greater media cross-sectional area, and larger internal as well as external diameters of the afferent arterioles in treated SHR than in untreated rats suggested that lisinopril treatment normalizes the structure of the afferent arterioles in SHR by vascular reverse remodeling and by inducing media growth.

Regional hemodynamic effects of the AT1 receptor antagonist CV-11974 in conscious renal hypertensive rats

Angiotensin II subtype 1 (AT1) receptor antagonists reduce mean arterial pressure in various experimental models of hypertension, including two-kidney, one clip (2K1C) renal hypertension. However, the regional hemodynamic mechanisms underlying the hypotensive effect of AT1 receptor antagonists in 2K1C rats under dynamic conditions have not been documented. Therefore, in the present study we determined the hemodynamic profile of the AT1 receptor antagonist CV-11974 in conscious 2K1C rats and sham-operated control rats. Approximately 4 weeks after clipping, rats underwent a further two-stage operation for implantation of Doppler flow probes on the contralateral (left) renal artery, superior mesenteric artery, and distal aorta as well as for the implantation of intravascular catheters. At least 24 hours after the last operation continuous recordings were made of mean arterial pressure; heart rate; and renal, mesenteric, and hindquarters flows and conductances (Doppler shift/mean arterial pressure) in response to three doses of CV-11974 (0.01, 0.1, and 1.0 mg/kg i.v.). CV-11974 caused a small hypotensive effect (decrease of approximately 15 mm Hg) in the sham group, but regional flows and vascular conductances did not change. By contrast, in 2K1C rats CV-11974 caused dose-dependent hypotension that was maximal (-19 +/- 6, -41 +/- 4, and -51 +/- 8 mm Hg, respectively) after 6 hours. These changes were associated with generalized vasodilatation (increased conductance) in all three vascular beds, although there were subtle differences with the different CV-11974 doses.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of losartan on blood pressure, metabolic alterations, and vascular reactivity in the fructose-induced hypertensive rat

Fructose feeding induces a moderate increase in blood pressure levels in normal rats that is associated with insulin resistance, hyperinsulinemia, and hypertriglyceridemia. The sympathetic nervous system seems to participate in the alterations of this model. To further explore the mechanisms underlying fructose-induced hypertension, the effects of the AT1 receptor antagonist losartan on blood pressure, insulin resistance, renal function, and vascular reactivity in mesenteric vascular beds were studied. Sprague-Dawley rats were fed for 4 weeks with diets containing 60% fructose or 60% starch (control), and half of each group received losartan (1 mg/kg per day) in the drinking water. Fructose-fed rats showed higher (P < .05) blood pressure levels and plasma concentrations of triglycerides and insulin than those of controls. Losartan treatment prevented both blood pressure elevation and hyperinsulinemia in fructose-fed rats but not elevation of plasma triglycerides. Plasma glucose and insulin levels in response to an oral glucose load were higher (P < .05) in fructose-fed rats than in controls. These exaggerated responses were prevented by losartan treatment. No differences in the constrictor responses of mesenteric vascular beds to KCl (60 mumol), angiotensin II (1 nmol), phenylephrine (10(-5) mol/L), or endothelin-1 (10 pmol) were found between the two groups. Relaxing responses to acetylcholine or sodium nitroprusside in phenylephrine-precontracted mesenteric vascular beds and constrictor response to the nitric oxide synthesis inhibitor NG-nitro-L-arginine methyl ester (100 nmol) were comparable in both groups. Losartan blunted angiotensin II constriction and reduced (P < .05) responses to phenylephrine in all groups.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of low and high doses of fosinopril on the structure and function of resistance arteries

It has been suggested that angiotensin-converting enzyme inhibitors may induce a significant regression of cardiovascular hypertrophy not only through blood pressure reduction but also as a possible consequence of growth factor inhibition. The aim of this study was to evaluate the effects of the angiotensin-converting enzyme inhibitor fosinopril, given either at a hypotensive high dose or a nonhypotensive low dose, on structural and functional alterations of mesenteric resistance arteries and on cardiac mass in spontaneously hypertensive rats (SHR) and control Wistar-Kyoto rats. Fosinopril was administered in the drinking water from 6 to 12 weeks of age. Rats were killed at 12 weeks, and the ratio of heart weight to body weight was measured. Mesenteric arterioles were dissected and mounted on a micromyograph (Mulvany's technique). Vascular morphology (media-lumen ratio, media thickness) and endothelial function (response to acetylcholine) were then assessed. During the 6 weeks of treatment, systolic pressure in SHR treated with high-dose fosinopril was significantly lower compared with that in untreated SHR, whereas no difference was observed with low-dose fosinopril. In SHR treated with both high-dose and low-dose fosinopril, a statistically significant reduction of vascular structural alterations, in terms of both media-lumen ratio and media thickness, was observed. The ratio of heart weight to body weight was reduced only in SHR treated with high-dose fosinopril. An improvement in the endothelium-dependent relaxation to acetylcholine was observed in SHR treated with high-dose fosinopril compared with untreated SHR, whereas in SHR treated with low-dose fosinopril no improvement in endothelial function was detected.(ABSTRACT TRUNCATED AT 250 WORDS)

The genetics of hypertension

In the past few years, a number of key insights have been made concerning the genetic basis of hypertension and blood pressure regulation. The genes responsible for two Mendelian forms of hypertension, glucocorticoid-remediable aldosteronism and Liddle's syndrome, were identified. In addition, research into the role of the renin-angiotensin system in blood pressure regulation has further implicated the angiotensinogen and angiotensin-converting enzyme loci in hypertension and its complications, such as myocardial infarction. Finally, several new candidate genes for hypertension have been identified through the use of genome scanning and contemporary gene expression assays in model organisms.

Angiotensin peptides in spontaneously hypertensive and normotensive Donryu rats

The renin-angiotensin system has been implicated in the pathogenesis of hypertension in spontaneously hypertensive rats (SHR). Given that SHR may have normal or suppressed plasma levels of renin and angiotensin peptides, we examined whether the tissue levels of angiotensin peptides are elevated in these rats. We measured angiotensin-(1-7) [Ang-(1-7)], Ang II, and Ang I in plasma, kidney, adrenal, heart, aorta, brown adipose tissue, lung, and brain of male SHR and normotensive Donryu rats at 6, 10, and 20 weeks of age. SHR had higher blood pressures and ratios of heart weight to body weight at all ages. Plasma renin levels of SHR were 13% to 32% of the levels of Donryu rats. Although plasma angiotensin-converting enzyme activity was lower in SHR than in Donryu rats, lung was the only SHR tissue with a reduced Ang II-Ang I ratio. Ang II levels in SHR adrenal were 24% to 42% of the levels of Donryu adrenal, and for SHR plasma, aorta, brown adipose tissue, and lung, Ang II levels were 38% to 93% of the levels of Donryu rats. For kidney and heart, Ang II levels were similar in SHR and Donryu rats at 6 weeks of age although suppressed in SHR at 10 and 20 weeks. Moreover, brain Ang II levels were higher in SHR than Donryu rats at 6 weeks of age and similar at 10 and 20 weeks of age.(ABSTRACT TRUNCATED AT 250 WORDS)

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

The development of hypertension in the spontaneously hypertensive rat (SHR) is associated with renal dysfunction and vasoconstriction. The kidneys of young SHRs exhibit exaggerated reactivity to angiotensin II (Ang-II) and attenuated responses to vasodilators that normally activate the cAMP signal to buffer hormone-induced vasoconstriction. The present study investigates the mechanism(s) responsible for this abnormality in activation of the cAMP second-messenger pathway in hypertensive animals. Renal vascular reactivity was assessed in 7-week-old anesthetized SHRs and normotensive Wistar-Kyoto rats. The animals were pretreated with indomethacin to block prostanoid production throughout an experiment. Ang-II was injected into the renal artery either alone or mixed with the vasodilator fenoldopam, a dopamine-receptor agonist. These two opposing vasoactive agents were administered before and during intrarenal infusion of NaF or cholera toxin, two activators of G proteins that stimulate cAMP production. The results show that Ang-II reduced renal blood flow by 45% in both strains. In Wistar-Kyoto rats, fenoldopam reduced the Ang-II-induced decrease in renal blood flow from -45% to -30%. This protective effect of fenoldopam was increased further during infusion of NaF or cholera toxin (-18% or -19% decrease in renal blood flow). In SHRs, fenoldopam failed to attenuate Ang II-mediated vasoconstriction (-45% vs. -44%). In contrast, fenoldopam effectively blunted the Ang-II-induced vasoconstriction when it was given concurrently with NaF or cholera toxin (-27 or -31% decrease in renal blood flow). These findings provide evidence for defective interaction between receptor coupling and activation of guanine nucleotide stimulatory factor proteins in the renal microcirculation of 7-week-old SHRs. Such a deficiency could play an important role in renal dysfunction associated with the development of genetic hypertension.

Insulin nonattenuation of vasoactive agent-induced responses in mesangial cells from spontaneously hypertensive rats

We recently found that insulin attenuates intracellular calcium transients and cell contraction caused by vasoactive agents in cultured rat mesangial cells. Because altered glomerular function may be causally related to the evolution of hypertension, we examined in the present study the effects of insulin on the functions of mesangial cells derived from spontaneously hypertensive rats (SHR) of 4- and 8-weeks of age. Age-matched Wistar Kyoto rats (WKY) were used as controls. Intracellular calcium concentration ([Ca2+]i) was measured with Fura-2 method in suspended mesangial cells. Pretreatment of mesangial cells with 5 micrograms/ml insulin for 120 minutes did not affect basal [Ca2+]i in either WKY or SHR mesangial cells. However, insulin pretreatment significantly attenuated [Ca2+]i transients to vasoactive agents in WKY mesangial cells. In contrast, [Ca2+]i transients to these agents were not attenuated by insulin in SHR mesangial cells. Additionally, SHR mesangial cell contraction in response to angiotensin II (Ang II) was not altered by insulin, while WKY mesangial cell contraction to Ang II was, as in normal Wistar rats, significantly reduced by insulin. Since we previously showed the possibility that the attenuation of calcium signal by insulin is via insulin-like growth factor I (IGF-I) receptor, we also examined the effect of IGF-I. In contrast to WKY mesangial cells, IGF-I-induced attenuation of [Ca2+]i responses to platelet activating factor was absent in SHR mesangial cells. [125I]-IGF-I binding in SHR mesangial cells was not significantly different from that in WKY mesangial cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Modification of pressure natriuresis by long-term losartan in spontaneously hypertensive rats

The goal of this study was to determine how long-term treatment of spontaneously hypertensive rats with losartan affects the pressure-natriuresis curve. Rats were treated with losartan (12 to 15 mg.kg-1.d-1 in drinking water) starting at 4 to 5 weeks of age. At 8 to 9 weeks of age, pressure natriuresis was studied in treated and untreated anesthetized rats using a preparation involving volume expansion and fixed neural and hormonal influences on the kidney. In some untreated rats, losartan (10 or 30 mg.kg-1 i.v.) was given acutely. Average initial mean arterial pressure (+/- SEM) for untreated rats was 164 +/- 2 mm Hg (n = 13) and 131 +/- 3 mm Hg (n = 13) for rats treated chronically with losartan (P < .01). Short-term losartan did not alter arterial pressure significantly. Glomerular filtration rate was not altered significantly by losartan, and renal blood flow was increased modestly by long- and short-term (10 mg.kg-1) losartan at several levels of renal artery pressure. At renal artery pressures of 130 to 175 mm Hg, there were no significant differences between untreated and short-term losartan rats for urine flow, total and fractional sodium excretions, and renal interstitial hydrostatic pressure. The relation between renal artery pressure and urine flow, sodium excretion, or fractional sodium excretion was shifted to the left by long-term losartan treatment. At identical renal artery pressures, renal interstitial hydrostatic pressure was not significantly different among losartan-treated (short or long term) and respective control groups.(ABSTRACT TRUNCATED AT 250 WORDS)