Dopamine fails to stimulate protein kinase C activity in renal proximal tubules of spontaneously hypertensive rats

Renal dopamine receptors and hypertension

Dopamine has been recognized as an important modulator of central as well as peripheral physiologic functions in both humans and animals. Dopamine receptors have been identified in a number of organs and tissues, which include several regions within the central nervous system, sympathetic ganglia and postganglionic nerve terminals, various vascular beds, the heart, the gastrointestinal tract, and the kidney. The peripheral dopamine receptors influence cardiovascular and renal function by decreasing afterload and vascular resistance and promoting sodium excretion. Within the kidney, dopamine receptors are present along the nephron, with highest density on proximal tubule epithelial cells. It has been reported that there is a defective dopamine receptor, especially D(1) receptor function, in the proximal tubule of various animal models of hypertension as well as in humans with essential hypertension. Recent reports have revealed the site of and the molecular mechanisms responsible for the defect in D(1) receptors in hypertension. Moreover, recent studies have also demonstrated that the disruption of various dopamine receptor subtypes and their function produces hypertension in rodents. In this review, we present evidence that dopamine and dopamine receptors play an important role in regulating renal sodium excretion and that defective renal dopamine production and/or dopamine receptor function may contribute to the development of various forms of hypertension.

Intestinal dopaminergic activity in obese and lean Zucker rats: response to high salt intake

The present study examined intestinal dopaminergic activity and its response to high salt (HS, 1% NaCl over a period of 24 hours) intake in obese (OZR) and lean Zucker rats (LZR). The basal Na+,K+-ATPase activity (nmol Pi/mg protein/min) in the jejunum of OZR was higher than in LZR on normal salt (NS) (OZR-NS = 111.3 +/- 6.0 vs. LZR-NS = 88.0 +/- 8.3). With the increase in salt intake, the basal Na+,K+-ATPase activity significantly increased in both animals (OZR-HS = 145.9 +/- 11.8; LZR-HS = 108.8 +/- 6.7). SKF 38393 (10 nM), a specific D1-like dopamine receptor agonist, inhibited the jejunal Na+,K+-ATPase activity in OZR on HS intake, but failed to inhibit enzyme activity in OZR on NS intake and LZR on NS and HS intakes. The aromatic L-amino acid decarboxylase (AADC) activity in OZR was lower than in LZR on NS intake. The HS intake increased AADC activity in OZR, but not in LZR. During the NS intake the jejunal monoamine oxidase (MAO) activity in OZR was similar to that in LZR. The HS intake significantly decreased MAO activity in both OZR and LZR. The jejunal COMT activity in OZR was higher than in LZR on NS intake. The HS intake reduced COMT activity in OZR but not LZR. It is concluded that inhibition of jejunal Na+,K+-ATPase activity through D1 dopamine receptors is dependent on salt intake in OZR, whereas in LZR, the enzyme failed to respond to the activation of D1 dopamine receptors irrespective of their salt intake.

The renal dopaminergic system, neurohumoral activation, and sodium handling in heart failure

BACKGROUND

Dopamine of renal origin exerts natriuretic and diuretic actions by activating specific receptors located in the renal proximal tubular epithelial cells. Heart failure (HF) is accompanied by activation of several neurohumoral systems. The interaction of these systems with the renal dopaminergic system and its effect on sodium handling in HF are not clarified.

METHODS AND RESULTS

We studied 13 patients with decompensated New York Heart Association class III/IV HF and 17 sex- and age-matched patients with mild to moderate stable class I/II HF. We measured plasma catecholamines, aldosterone, type B natriuretic peptide (BNP), sodium, creatinine (UCr), and 24-hour urinary excretion of sodium, UCr, levo-3,4-dihydroxyphenylalanine (L-DOPA), 3-o -methyldopa, dopamine and its metabolites, 3,4-dihydroxyphenylacetic acid and homovallinic acid, and norepinephrine. All patients had HF of ischemic etiology. No statistically significant differences were found between the groups with respect to urine volume (1.79 +/- 0.23 L x d(-1) vs 2.20 +/- 0.18 L x d(-1), P =.18) and urinary sodium (161.3 +/- 27.5 mmol x d(-1) vs 232.9 +/- 28.8 mmol x d(-1), P =.12). Urinary L-DOPA was significantly lower in patients with decompensated class III/IV HF than in the other group (79.0 +/- 13.8 nmol x g UCr(-1) vs 108.4 +/- 10.3 nmol x g UCr(-1), P =.04). Urinary dopamine showed a nonstatistically significant trend to be slightly higher (1294.3 +/- 188.5 nmol x g UCr(-1) vs 953.2 +/- 107.4 nmol x g UCr(-1), P =.14). Consequently, urinary dopamine/L-DOPA ratios were markedly higher in patients with decompensated class III/IV HF than in the other patients (20.6 +/- 3.4 vs 9.0 +/- 0.9, P <.001). Plasma L-DOPA (38.1 +/- 4.4 pmol x mL(-1) vs 40.0 +/- 3.0 pmol x mL(-1), P =.48), dopamine (37.0 +/- 6.3 pmol x mL(-1) vs 41.1 +/- 2.6 pmol x mL(-1), P =.53), 3,4-dihydroxyphenylacetic acid (51.7 +/- 11.7 pmol x mL(-1) vs 56.5 +/- 5.4 pmol x mL(-1), P =.09), and norepinephrine (9.5 +/- 2.4 pmol x mL(-1) vs 5.6 +/- 1.0 pmol x mL(-1), P =.12) did not differ between groups. Plasma aldosterone (180.2 +/- 28.0 pg x mL(-1) vs 69.9 +/- 13.3 pg x mL(-1), P <.001) and BNP (677.5 +/- 133.9 pg x mL(-1) vs 389.4 +/- 88.4 pg x mL(-1), P <.04) levels were higher in the decompensated class III/IV HF group than in the other group, whereas serum sodium was lower (137.3 +/- 1.2 mmol x L(-1) vs 143.2 +/- 1.0 mmol x L(-1), P =.001).

CONCLUSIONS

These results suggest that, in patients with HF, the increased renal utilization of L-DOPA may constitute a compensatory mechanism, activated in response to stimuli leading to sodium reabsorption.

Heart failure, aging, and renal synthesis of dopamine

The present study evaluates renal dopaminergic activity in 23 patients with heart failure (HF), 10 age-matched controls, and 10 young subjects during normal-salt (NS) intake and after 8 days of low-salt (LS) intake (patients with HF and age-matched controls only). LS intake produced a marked reduction in urine volume in patients with HF but failed to affect urine volume in age-matched controls. Urinary sodium and fractional excretion of sodium were markedly reduced by LS intake in patients with HF and age-matched controls. Daily urinary excretion of L-3,4-dihydroxyphenylalanine (L-dopa) and dopamine was lower in patients with HF than in age-matched controls. LS intake failed to alter L-dopa and dopamine urinary excretion in control subjects. In patients with HF, LS intake produced a significant decrease in urinary L-dopa excretion, but failed to alter the urinary excretion of dopamine. No significant differences were observed in urinary L-dopa, dopamine, and dopamine metabolite levels between aged controls and young healthy subjects. Urinary dopamine-L-dopa ratios in patients with HF on LS intake (24.5 +/- 7.1) were significantly greater than those with NS intake (11.6 +/- 1.3). Urinary dopamine-L-dopa ratios in old control subjects (LS, 9.7 +/- 1.3; NS, 9.3 +/- 1.1) did not differ from those in young healthy subjects (9.2 +/- 0.8). LS intake produced a marked increase in plasma aldosterone levels in both patients with HF (84.6 +/- 14.4 to 148.2 +/- 20.4 pg/mL; P = 0.0008) and controls (102.1 +/- 13.4 to 151.6 +/- 15.7 pg/mL; P < 0.04). Plasma norepinephrine levels were not significantly affected by LS intake in controls (5.1 +/- 1.62 to 6.3 +/- 1.6 pmol/mL; P = 0.22), but were significantly increased in patients with HF (5.8 +/- 0.8 to 7.1 +/- 0.9 pmol/mL; P = 0.04). In conclusion, patients with HF are endowed with an enhanced ability to take up (or decarboxylate) filtered L-dopa, which might counterbalance the reduced renal delivery of L-dopa, contributing to a relative preservation of dopamine synthesis. This may result as a compensatory mechanism, activated by stimuli leading to sodium reabsorption. Age seems to have no influence on renal dopamine production.

Hyperphosphorylation of Na-pump contributes to defective renal dopamine response in old rats

Dopamine D1-like receptor activation causes phosphorylation and inhibition of Na,K-ATPase (Na-pump) activity in the proximal tubules, which is associated with an increase in sodium excretion. It has been shown that dopamine and SKF 38393, a D1-like receptor agonist, caused inhibition of Na,K-ATPase activity in the proximal tubules of adult (6 mo) but not of old (24 mo) Fischer 344 rats. The present study demonstrated that SKF 38393 and PDBu, a phorbol ester and protein kinase C (PKC) activator, increased phosphorylation of the alpha(1)-subunit of Na,K-ATPase in adult but not in old rats. In adult rats, SKF 38393-mediated phosphorylation was antagonized by SCH 23390, a D1-like receptor antagonist. Similarly, Na,K-ATPase activity was inhibited by SKF 38393 and PDBu in adult but not in old rats. The basal activity of Na,K-ATPase was decreased and the basal phosphorylation state of the enzyme was increased in old compared with adult rats. Basal activity of PKC was higher in old compared with adult rats, and SKF 38393 and PDBu stimulated PKC activity in adult but not in old rats. The conclusion is that the failure of D1-like receptor agonist and phorbol ester to stimulate PKC and inhibit Na,K-ATPase activity in old rats is due, at least in part, to the higher basal PKC activity and Na,K-ATPase phosphorylation in old compared with adult rats.

Dopamine-induced translocation of protein kinase C isoforms visualized in renal epithelial cells

Short-term regulation of sodium metabolism is dependent on the modulation of the activity of sodium transporters by first and second messengers. In understanding diseases associated with sodium retention, it is necessary to identify the coupling between these messengers. We have examined whether dopamine, an important first messenger in tubular cells, activates and translocates various protein kinase C (PKC) isoforms. We used a proximal tubular-like cell line, LLCPK-1 cells, in which dopamine was found to inhibit Na(+)-K(+)-ATPase in a PKC-dependent manner. Translocation of PKC isoforms was studied with both subcellular fractionation and confocal microscopy. Both techniques revealed a dopamine-induced translocation from cytosol to plasma membrane of PKC-alpha and -epsilon, but not of PKC-delta, -gamma, and -zeta. The process of subcellular fractionation resulted in partial translocation of PKC-epsilon. This artifact was eliminated in confocal studies. Confocal imaging permitted detection of translocation within 20 s. Translocation was abolished by a phospholipase C inhibitor and by an antagonist against the dopamine 1 subtype (D(1)) but not the 2 subtype of receptor (D(2)). In conclusion, this study visualizes in renal epithelial cells a very rapid activation of the PKC-alpha and -epsilon isoforms by the D(1) receptor subtype.

Mechanisms of sodium pump regulation

The Na(+)-K(+)-ATPase, or sodium pump, is the membrane-bound enzyme that maintains the Na(+) and K(+) gradients across the plasma membrane of animal cells. Because of its importance in many basic and specialized cellular functions, this enzyme must be able to adapt to changing cellular and physiological stimuli. This review presents an overview of the many mechanisms in place to regulate sodium pump activity in a tissue-specific manner. These mechanisms include regulation by substrates, membrane-associated components such as cytoskeletal elements and the gamma-subunit, and circulating endogenous inhibitors as well as a variety of hormones, including corticosteroids, peptide hormones, and catecholamines. In addition, the review considers the effects of a range of specific intracellular signaling pathways involved in the regulation of pump activity and subcellular distribution, with particular consideration given to the effects of protein kinases and phosphatases.

Salt intake and sensitivity of intestinal and renal Na+-K+ atpase to inhibition by dopamine in spontaneous hypertensive and Wistar-Kyoto rats

The present study evaluated the activity of jejunal Na+-K+-ATPase and its sensitivity to inhibition by dopamine in spontaneous hypertensive rats (SHR) and Wistar-Kyoto (WKY) rats during low (LS), normal (NS) and high (HS) salt intake. Basal jejunal Na+-K+-ATPase activity in SHR on LS intake was higher than in WKY rats. Jejunal Na+-K+-ATPase activity in WKY rats, but not in SHR, on LS intake was significantly reduced (20% decrease) by dopamine (1 microM) and SKF 38393 (10 nM), but not quinerolane (10 nM), this being antagonized the D1 receptor antagonist (SKF 83566). Changing from LS to NS or HS intake in WKY rats increased basal jejunal Na+-K+-ATPase activity and attenuated the inhibitory effect of dopamine. In SHR, changing from LS to NS or HS intake increased basal jejunal Na+-K+-ATPase activity. Basal renal Na+-K+-ATPase activity in SHR on LS intake was similar to that in WKY rats and was insensitive to inhibition by dopamine. Changing from LS to NS or HS intake in WKY rats increased basal renal Na+-K+-ATPase activity without affecting the inhibitory effect of dopamine. In SHR, changing from LS to NS or HS intake failed to alter basal renal Na+-K+-ATPase activity. It is concluded that inhibition of jejunal Na+-K+ ATPase activity by D1 dopamine receptor activation is dependent on salt intake in WKY rats, and SHR animals fail to respond to dopamine, irrespective of their salt intake.

Renal dopaminergic mechanisms in renal parenchymal diseases, hypertension, and heart failure

The recovery of renal function in renal transplant recipients is accompanied by an enhanced ability to synthesize dopamine (DA), which may contribute to maintain sodium homeostasis. Patients suffering from chronic renal parenchymal disease, a well-recognized form of salt sensitive (SS) hypertension, have a reduced ability to produce DA that correlates well with deterioration of renal function. In patients afflicted with IgA nephropathy, but normal renal function, urinary excretion of DA correlated positively with BP responses to changes from 200 to 20 mmol/day salt intake. In black salt resistant (SR) normotensives (NT) and SR hypertensives, under low salt intake (40 mmol/day), but not SS-NT and SS-HT, the saline infusion induced increments of DA and DOPAC urinary excretion correlated significantly with increments of sodium urinary excretion and sodium fractional excretion. Patients afflicted with heart failure (HF) have a reduced delivery of L-DOPA to the kidney, accompanied by an increase in DA/L-DOPA urinary ratios. This suggests that HF patients have an increased ability to take up or decarboxylate L-DOPA. Sodium restriction resulted in a significant decrease in urinary L-DOPA, DA and DOPAC in HF patients, suggesting that the system responds to sodium. It is concluded that activity of renal dopaminergic system may be altered in SS subjects, despite the level of their BP, and an enhanced delivery of L-DOPA to the kidney may be beneficial in edema formation states.

Renal dopaminergic mechanisms and hypertension: a chronology of advances

Dopamine (DA) has been shown to influence kidney function through endogenous synthesis and subsequent interaction with locally expressed dopamine receptor subtypes (D1, D5 as D1-like and D2, D3, and D4 as D2-like). DA, and DA-receptor specific agonists and antagonists can alter renal water and electrolyte excretion along with renin release when infused systemically or intrarenally. Such effects are brought about by a combination of renal hemodynamic and direct tubular effects evoked along the full length of the nephron. The cellular mechanisms that direct these dopamine-mediated renal electrolyte fluxes have recently been clarified and include alterations in adenylyl cyclase, phospholipase C, and phospholipase A1 activity. The dopaminergic system also interacts directly with the renal kallikrein-kinin, prostaglandin and other neurohumoral systems. Aberrant renal dopamine production and/or dopamine receptor function have been reported in salt-dependent and low-renin forms of human primary hypertension as well as in genetic models of animal hypertension, including the SHR and Dahl SS rat. DA D1 or D3 receptor knockout mice have been shown to develop hypertension.

Aging, high salt intake, and renal dopaminergic activity in Fischer 344 rats

The present study examined renal dopaminergic activity and its response to high salt (HS) intake in adult (6-month-old) and old (24-month-old) Fischer 344 rats. Daily urinary excretion of L-3, 4-dihydroxyphenylalanine (L-DOPA), dopamine, and its metabolites 3, 4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid was similar in adult and old rats; by contrast, daily urinary excretion of norepinephrine in old rats was almost twice that in adult animals. HS intake (1% NaCl) over a period of 24 hours resulted in a 2-fold increase in the urinary excretion of dopamine, DOPAC, and norepinephrine in adult animals but not in old animals. Norepinephrine and L-DOPA plasma levels did not change during HS intake and were similar in both groups of rats. The natriuretic response to an HS intake in old rats (from 4.7+/-0.4 to 10.7+/-2.0 nmol. kg(-1). d(-1); Delta=6.0+/-0.9 nmol. kg(-1). d(-1)) was less than in adult rats (from 5.2+/-0.4 to 13.5+/-2.5 nmol. kg(-1). d(-1); Delta=8.3+/-0.8 nmol. kg(-1). d(-1)). A diuretic response to HS intake was observed in adult rats (from 20.9+/-2.3 to 37.6+/-2.8 mL. kg(-1). d(-1)) but not in old rats (from 37.7+/-5.7 to 42.3+/-6. 0 mL. kg(-1). d(-1)). Dopamine levels and dopamine/L-DOPA ratios in the renal cortex of old rats were greater than in adult rats. HS intake increased both dopamine levels and dopamine/L-DOPA ratios in the renal cortex of adult rats but not in old rats. Aromatic L-amino acid decarboxylase activity was higher in old rats than in adult rats; HS intake increased L-amino acid decarboxylase activity (nmol. mg protein(-1). l5 min(-1)) in adult rats (from 67+/-1 to 93+/-1) but not in old rats (from 86+/-2 to 87+/-2). Dopamine inhibited Na(+),K(+)-ATPase activity in proximal tubules obtained from adult rats, but it failed to exert such an inhibitory effect in old rats. It is concluded that renal dopaminergic tonus in old rats is higher than in adult rats but fails to respond to HS intake as observed in adult rats. This may be due in part to the inability of dopamine to inhibit Na(+),K(+)-ATPase activity in old rats.

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.

Na+,K+ pump and Na+-coupled ion carriers in isolated mammalian kidney epithelial cells: regulation by protein kinase C

This review updates our current knowledge on the regulation of Na+/H+ exchanger, Na+,K+,Cl- cotransporter, Na+,Pi cotransporter, and Na+,K+ pump in isolated epithelial cells from mammalian kidney by protein kinase C (PKC). In cells derived from different tubule segments, an activator of PKC, 4beta-phorbol 12-myristate 13-acetate (PMA), inhibits apical Na+/H+ exchanger (NHE3), Na+,Pi cotransport, and basolateral Na+,K+ cotransport (NKCC1) and augments Na+,K+ pump. In PMA-treated proximal tubules, activation of Na+,K+ pump probably plays a major role in increased reabsorption of salt and osmotically obliged water. In Madin-Darby canine kidney (MDCK) cells, which are highly abundant with intercalated cells from the collecting duct, PMA completely blocks Na+,K+,Cl- cotransport and decreases the activity of Na+,Pi cotransport by 30-40%. In these cells, agonists of P2 purinoceptors inhibit Na+,K+,Cl- and Na+,Pi cotransport by 50-70% via a PKC-independent pathway. In contrast with MDCK cells, in epithelial cells derived from proximal and distal tubules of the rabbit kidney, Na+,K+,Cl- cotransport is inhibited by PMA but is insensitive to P2 receptor activation. In proximal tubules, PKC-induced inhibition of NHE3 and Na+,Pi cotransporter can be triggered by parathyroid hormone. Both PKC and cAMP signaling contribute to dopaminergic inhibition of NHE3 and Na+,K+ pump. The receptors triggering PKC-mediated activation of Na+,K+ pump remain unknown. Recent data suggest that the PKC signaling system is involved in abnormalities of dopaminergic regulation of renal ion transport in hypertension and in the development of diabetic complications. The physiological and pathophysiological implications of PKC-independent regulation of renal ion transporters by P2 purinoceptors has not yet been examined.Key words: Na+/H+ exchanger, Na+,K+,Cl- and Na+,Pi cotransporters, Na+,K+ pump, protein kinase C, P2 purinoceptor.

Renal dopamine receptor signaling mechanisms in spontaneously hypertensive and Fischer 344 old rats

Dopamine plays an important role in the regulation of renal sodium excretion. The activation of D1-like receptors located on the proximal tubules causes inhibition of tubular sodium reabsorption by inhibiting Na,H-exchanger and Na,K-ATPase activity. The D1-like receptors are linked via G proteins to the multiple cellular signaling systems namely adenylyl cyclase and phospholipase C (PLC). A defective renal dopamine receptor function exists in spontaneously hypertensive rats (SHR). In the proximal tubules of SHR, the stimulation of adenylyl cyclase and PLC caused by dopamine was significantly reduced in comparison with Wistar-Kyoto (WKY) rats. Also unlike the effects seen in WKY, D1-like receptor activation did not inhibit Na,K-ATPase and Na,H-exchanger activities in SHR. In addition, reduced quantity of Gq/11alpha proteins was detected in the basolateral membranes of SHR compared to WKY rats. Studies revealed that there may be a primary defect in D1-like receptors leading to an altered signaling system in the proximal tubules and reduced dopamine-mediated effect on renal sodium excretion in SHR. Recently, it has been shown that the disruption of D1A receptors at the gene level causes hypertension in mice. Similar to SHR, dopamine and D1-like receptor agonist failed to inhibit Na,K-ATPase activity in the proximal tubules of old Fischer 344 rats. Unlike the observations in SHR where D1-like receptors were equal to WKY rats, there is a 50% decrease in D1-like receptor number in basolateral membranes of the old rats compared to the adult rats. Dopamine was unable to stimulate G proteins in the basolateral membranes of old rats compared to the adult rats. It is suggested that a defective dopamine receptors/signaling system may contribute to the development and maintenance of hypertension. Also, the inability of dopamine to inhibit Na,K-ATPase may lead to a reduced renal sodium excretion in response to dopamine in old rats.

Dopamine D1 receptor and protein kinase C isoforms in spontaneously hypertensive rats

-Dopamine, via D1-like receptors, stimulates the activity of both protein kinase A (PKA) and protein kinase C (PKC), which results in inhibition of renal sodium transport. Since D1-like receptors differentially regulate sodium transport in normotensive and hypertensive rats, they may also differentially regulate PKC expression in these rat strains. Thus, 2 different D1-like agonists (fenoldopam or SKF 38393) were infused into the renal artery of anesthetized normotensive Wistar-Kyoto rats (WKY) and spontaneously hypertensive rats (SHR) (n=5 to 6/drug/strain). Ten or 60 minutes after starting the D1-like agonist infusion, both the infused kidney and the noninfused kidney that served as control were prepared for analysis. The D1-like agonists produced a greater diuresis and natriuresis and inhibited Na+,K+-ATPase activity in proximal tubule (PT) and medullary thick ascending limb (mTAL) to a greater extent in WKY (Delta20+/-1%) than in SHR (Delta7+/-1%, P<0.001). D1-like agonists had no effect on PKC-alpha or PKC-lambda expression in either membrane or cytosol but increased PKC-theta expression in PT in both WKY and SHR at 10 minutes but not at 60 minutes. However, membranous PKC-delta expression in PT and mTAL decreased in WKY but increased in SHR with either 10 or 60 minutes of D1-like agonist infusion. D1-like agonists also decreased membranous PKC-zeta expression in PT and mTAL in WKY but increased it in PT but not in mTAL in SHR. We conclude that there is differential regulation of PKC isoform expression by D1-like agonists that inhibits membranous PKC-delta and PKC-zeta in WKY but stimulates them in SHR; this effect in SHR is similar to the stimulatory effect of norepinephrine and angiotensin II and may be a mechanism for their differential effects on sodium transport.

Renal dopamine receptor function in hypertension

Dopamine plays an important role in the regulation of renal sodium excretion. The synthesis of dopamine and the presence of dopamine receptor subtypes (D1A, D1B, as D1-like and D2, and D3 as D2-like) have been shown within the kidney. The activation of D1-like receptors located on the proximal tubules causes inhibition of tubular sodium reabsorption by inhibiting Na,H-exchanger and Na,K-ATPase activity. The D1-like receptors are linked to the multiple cellular signaling systems (namely, adenylyl cyclase, phospholipase C, and phospholipase A2) in the different regions of the nephron. Defective renal dopamine production and/or dopamine receptor function have been reported in human primary hypertension as well as in genetic models of animal hypertension. There may be a primary defect in D1-like receptors and an altered signaling system in the proximal tubules that lead to reduced dopamine-mediated effects on renal sodium excretion in hypertension. Recently, it has been shown in animal models that the disruption of either D1A or D3 receptors at the gene level causes hypertension in mice. Dopamine and dopamine receptor agonists also provide therapeutic potential in treatment of various cardiovascular pathological conditions, including hypertension. However, because of the poor bioavailability of the currently available compounds, the use of D1-like agonists is limited to the management of patients with severe hypertension when a rapid reduction of blood pressure is clinically indicated and in acute management of patients with heart failure. In conclusion, there is convincing evidence that dopamine and dopamine receptors play an important role in regulation of renal function, suggesting that a defective dopamine receptor/signaling system may contribute to the development and maintenance of hypertension. Further studies need to be directed toward establishing a direct correlation between defective dopamine receptor gene in the kidney and development of hypertension. Subsequently, it may be possible to use a therapeutic approach to correct the defect in dopamine receptor gene causing the hypertension.

Dopamine-1 receptor G-protein coupling and the involvement of phospholipase A2 in dopamine-1 receptor mediated cellular signaling mechanisms in the proximal tubules of SHR

Dopamine-induced natriuretic response which results from the activation of tubular dopamine1 (DA1) receptors is diminished in spontaneously hypertensive rats (SHR). This may be a result of alterations occurring at the receptor level and within the cellular signaling pathway which ultimately causes inhibition of Na+, K(+)-ATPase. There have been reports showing that DA1 receptor induced inhibition of Na+, K(+)-ATPase is abolished in SHR which is due to a decreased activation of PLC and PKC by dopamine. Of the mechanisms, adenylyl cyclase and phospholipase C are two known enzymes linked to DA1 receptors via G proteins. Furthermore, the involvement of phospholipase A2 (PLA2) has also been reported in this process. However, the site of defect in DA1 receptor signaling pathway in SHR is still not well understood. This report will (i) review the coupling of DA1 receptor with G proteins and their levels in Wistar Kyoto (WKY) rats and SHR and (ii) discuss studies dealing with the role of PLA2 in dopamine-induced inhibition of Na+, K(+)-ATPase in WKY rat and SHR kidneys. Fenoldopam, DA1 receptor selective agonist stimulated [35S]GTP gamma S binding in a concentration (10(-9)-10(-4) M)-dependent manner in WKY rats which was attenuated in SHR. Fenoldopam (10 microM)-induced stimulation of [35S]GTP gamma S binding was significantly reduced by a DA1 receptor selective antagonist, SCH 23390 suggesting the involvement of DA1 receptor. Furthermore, the specific antipeptides Gs alpha, and Gq/11 alpha significantly blocked fenoldopam-stimulation of [35S]GTP gamma S binding suggesting the coupling of DA1 receptor with both the G proteins. Western analysis revealed a significant decrease in Gq/11 alpha but no changes in Gs alpha in SHR compared to WKY rats. Dopamine inhibited Na+, K(+)-ATPase activity in a concentration (10(-9)-10(-5) M)-dependent manner in WKY rats while it failed to inhibit the enzyme activity in SHR. Dopamine (10 microM)-induced inhibition in Na+, K(+)-ATPase activity was significantly blocked by mepacrine (a PLA2 inhibitor) suggesting the involvement of PLA2 in dopamine-mediated inhibition of Na+, K(+)-ATPase. Arachidonic acid (AA), a PLA2 product, inhibited Na+, K(+)-ATPase in a concentration (1-100 microM)-dependent manner in WKY rats while the inhibition in SHR was significantly attenuated (IC50: 7.5 microM in WKY and 80 microM in SHR). Furthermore, lower concentration (1 microM) of AA stimulated the enzyme activity in SHR. This suggests a defect in the metabolism of AA in SHR. Proadifen (10 microM), an inhibitor of cytochrome P-450 monoxygenase (an arachidonic acid metabolizing enzyme) significantly blocked the inhibition produced by arachidonic acid in WKY rats and abolished the difference in arachidonic acid inhibition of Na+, K(+)-ATPase between WKY rats and SHR. These data suggest that (i) the reduced activation of G proteins following DA1 receptor stimulation, (ii) reduced amount of Gq/11 alpha and (iii) a defect in the AA metabolism may be responsible for the reduced dopaminergic inhibition of sodium pump activity and a diminished natriuretic response to dopamine in SHR.

Dopamine D1A receptor regulation of phospholipase C isoform

In LTK- cells stably transfected with rat D1A receptor cDNA, fenoldopam, a D1 agonist, increased phosphatidylinositol 4, 5-bisphosphate hydrolysis in a time-dependent manner. In the cytosol, phospholipase C (PLC) activity increased (50 +/- 7%) in 30 s, returned to basal level at 4 h, and decreased below basal values by 24 h; in the membrane, PLC activity also increased (36 +/- 13%) in 30 s, returned to basal level at 10 min, and decreased below basal value at 4 and 24 h. Fenoldopam also increased PLC-gamma protein in a time-dependent manner. The latter was blocked by the D1 antagonist SKF83742 and by a D1A antisense oligodeoxynucleotide, indicating involvement of the D1A receptor. The fenoldopam-induced increase in PLC-gamma and activity was mediated by protein kinase A (PKA) since it was blocked by the PKA antagonist Rp-8-CTP-adenosine cyclic 3':5'-monophosphorothioate (Rp-8-CTP-cAMP-S) and mimicked by direct stimulation of adenylyl cyclase with forskolin or by a PKA agonist, Sp-cAMP-S. Protein kinase C (PKC) was also involved, since the fenoldopam-induced increase in PLC-gamma protein was blocked by two different PKC inhibitors, calphostin C and chelerythrine; calphostin C also blocked the fenoldopam-induced increase in PLC activity. In addition, forskolin and a PKA agonist, Sp-8-CTP-cAMP-S, increased PKC activity, and direct stimulation of PKC with phorbol 12-myristate 13-acetate increased PLC-gamma protein and activity, effects that were blocked by calphostin C. We suggest that the D1A-mediated stimulation of PLC occurs as a result of PKA activation. PKA then stimulates PLC-gamma in cytosol and membrane via activation of PKC.