Mustafa F. Lokhandwala

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.

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 D1 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 D1 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.

Dopamine fails to inhibit renal tubular sodium pump in hypertensive rats.

&NA; We have previously reported that dopamine‐1 receptor‐mediated activation of phospholipase C is diminished in renal cortical slices of adult spontaneously hypertensive rats. To determine the potential consequences of this phenomenon, we performed the present studies in which renal proximal tubule suspensions obtained from spontaneously hypertensive and Wistar‐Kyoto rats of 10‐12 weeks of age were used. The tubule suspensions were incubated with dopamine in the presence or absence of dopamine receptor antagonists, and sodium, potassium adenosine trisphosphatase (sodium pump) activity was measured as the ouabain‐sensitive adenosine trisphosphate hydrolysis. We found that dopamine produced a concentration‐related inhibition of sodium pump activity in the normotensive rats but not in the hypertensive rats. Dopamine‐induced inhibition of sodium pump activity in the normotensive rats was abolished by the phospholipase C inhibitor U‐73122 or the protein kinase C inhibitor sphingosine, suggesting the involvement of a phospholipase C‐coupled protein kinase C pathway in this response. Dopamine‐induced inhibition in the normotensive rats was attenuated by the dopamine‐1 receptor antagonist SCH 23390 but not by the dopamine‐2 receptor antagonist domperidone. To identify possible sites of defect in dopamine‐1 receptor‐coupled signaling pathways in the hypertensive rats, we incubated the proximal tubules with phorbol 12,13‐dibutyrate or the synthetic diacylglycerol analogue 1‐oleoyl‐2‐acetyl‐rac‐glycerol. The results showed that both compounds inhibited sodium pump activity as effectively in the hypertensive as in the normotensive rats, suggesting that the protein kinase C‐coupled sodium pump pathway was not defective in the hypertensive animals. Failure of dopamine to inhibit sodium pump activity in the hypertensive rats could not be due to a defective dopamine‐1 receptor adenylate cyclase coupling, because dopamine was still unable to inhibit sodium pump activity in the presence of dibutyryl cyclic adenosine monophosphate or forskolin. These results show that dopamine failed to inhibit sodium pump activity in the proximal tubules of adult hypertensive rats, which may be mainly due to a defect in the dopamine‐1 receptor‐mediated signal transduction pathway. The site of this defect is most likely proximal to the activation of protein kinase C and may involve a defect in the dopamine‐1 receptor phospholipase C coupling process. (Hypertension 1993;21:364‐372)

Resveratrol prevents endothelial nitric oxide synthase uncoupling and attenuates development of hypertension in spontaneously hypertensive rats

Endothelial dysfunction is a hallmark of hypertension and vascular oxidative stress can contribute to endothelial dysfunction and hypertension development. Resveratrol is an antioxidant polyphenol which improves endothelium dependent relaxation, the mechanisms of which are unknown. Also, the role of resveratrol in hypertension remains to be established. The purpose of this study was to investigate the mechanisms of resveratrol induced improvement of endothelial function and establish its role in hypertension. SHR and WKY rats, 3-4 weeks old, were treated with resveratrol in drinking water for 10 weeks, untreated SHR and WKY rats served as controls. At the end of the treatment, control SHR exhibited increased blood pressure, oxidative stress and attenuated endothelium dependent relaxation in comparison to WKY rats. The impaired endothelium function in SHR was associated with lower nitrite/nitrate levels, elevated nitrotyrosine content and eNOS uncoupling. Resveratrol treatment attenuated hypertension development in SHR as indicated by lower blood pressure in resveratrol treated SHR (SHR-R) compared to control SHR. SHR-R also exhibited reduced H(2)O(2) content and elevated superoxide dismutase activity. Resveratrol treatment normalized endothelium dependent vasorelaxation in SHR. In parallel, resveratrol restored nitrite/nitrate levels and normalized nitrotyrosine content in SHR. SHR exhibited increased l-arginine dependent superoxide production which was blocked by NOS inhibitor l-NNA, suggesting eNOS uncoupling. eNOS uncoupling was prevented by resveratrol treatment. In conclusion, early treatment with resveratrol lowers oxidative stress, preserves endothelial function and attenuates development of hypertension in SHR. More importantly, prevention of eNOS uncoupling and NO scavenging could represent novel mechanisms for resveratrol-mediated antihypertensive effects.

Role of kidney dopamine in the natriuretic response to volume expansion in rats.

It has been postulated that endogenously produced dopamine (DA) may play a role in the regulation of renal sodium excretion. In the present study, experiments were designed to test the hypothesis that acute volume expansion with isotonic sodium chloride stimulates the production of DA within the kidney, which in turn acts on specific DA, receptors to promote sodium excretion. In pentobarbital-anesthetized rats, acute volume expansion over a period of 1 hour evoked a pronounced increase in urine output and urinary sodium excretion. These diuretic and natriuretic effects were not accompanied by any significant changes in blood pressure or heart rate. However, there was a significant elevation in central venous pressure and a transient rise in glomerular filtration rate. The natriuretic and diuretic response was accompanied by a significant increase in urinary DA excretion, and this effect was clearly dissociated from the rise in glomerular filtration rate. In a separate group of rats, the effects of acute volume expansion were studied in the presence of selective DA, receptor antagonist SCH-23390 (50 μ/kg i.v. bolus; 10 μ/kg/min). During DA1 receptor blockade, there was a marked attenuation in the diuretic and natriuretic response throughout the period of volume expansion, when compared with that in the control group. The changes in central venous pressure and glomerular filtration rate were identical in the two groups. In another group of rats, the renal effects of exogenously administered DA were studied. DA (0.5 μ/kg/min) produced significant increases in urine output and urinary sodium excretion, without causing any alterations in blood pressure or glomerular filtration rate, suggesting a tubular site of action. SCH-23390, in a dose that had previously attenuated the natriuretic response to volume expansion, blocked the diuretic response and attenuated the natriuretic response to DA. The DA1 receptor antagonist produced no significant hemodynamic and renal effects by itself in normally hydrated animals. These results suggest that endogenously produced DA contributes, at least in part, to the natriuretic and diuretic response to acute volume expansion by activation of DA, receptors located on renal tubules.

An Impairment of Renal Tubular DA-1 Receptor Function as the Causative Factor For Diminished Natriuresis to Volume Expansion in Spontaneously Hypertensive Rats

It has been demonstrated that endogenous kidney dopamine (DA) contributes to the natriuretic response to acute volume expansion (VE). Several studies suggest that a defect in renal DA-ergic mechanism may play a role in genetic hypertension in humans and rats. The present study was designed to determine the role of renal DA and tubular DA-1 receptors in the natriuretic response to VE in age-matched spontaneously hypertensive rats (SHR) and Wistar-Kyoto (WKY) rats of 10-12 weeks of age. In pentobarbital-anesthetized rats, VE was carried out by intravenously infusing isotonic sodium chloride (5% body weight) over a period of 60 min. This maneuver evoked pronounced increases in urine output, urinary sodium excretion and urinary DA excretion. However, the natriuretic and diuretic response to VE was significantly reduced in SHR, although the increase in urinary DA excretion was similar in both SHR and WKY rats. During VE no significant changes in glomerular filtration rate or blood pressure were noted in either strain of animals, indicating the involvement of renal tubular mechanisms in the natriuretic response. In a separate group of SHR and WKY rats, pretreatment with DA-1 receptor antagonist SCH 23390 caused significant attenuation of the natriuretic and diuretic response to VE in WKY rats but not in SHR, suggesting that unlike WKY rats kidney DA was not contributing to the natriuretic response to VE in SHR. In another group of animals, the renal effects of exogenously administered DA-1 receptor agonist fenoldopam were examined. Fenoldopam (1 microgram/kg/min) produced significant increases in urine output and urinary sodium excretion without causing any alterations in blood pressure or glomerular filtration rate in both SHR and WKY rats. However, the interesting observation was that fenoldopam-induced diuresis and natriuresis were significantly attenuated in SHR compared to the WKY rats. These results show that SHR are not able to eliminate an acute increase in sodium load as efficiently as WKY rats, which may be at least in part due to a defect in renal tubular DA-1 receptor function.

Oxidative stress-induced renal angiotensin AT1 receptor upregulation causes increased stimulation of sodium transporters and hypertension

Reactive oxygen species have emerged as important molecules in cardiovascular dysfunction such as diabetes and hypertension. Recent work has shown that oxidative stress and angiotensin II signaling mutually regulate each other by multiple mechanisms and contribute to the development of hypertension. Most of the known biological actions of angiotensin II can be attributed to AT1 receptors. The present study was carried out to investigate the role of renal AT1 receptor signaling in oxidative stress-mediated hypertension. Male Sprague-Dawley rats received tap water (control) or 30 mM L-buthionine sulfoximine (BSO), an oxidant, with and without 1 mM tempol (an antioxidant) for 2 wk. Compared with control rats, BSO-treated rats exhibited increased oxidative stress and reduced antioxidant levels and developed hypertension. BSO treatment also caused increased renal proximal tubular AT1 receptor protein abundance, message levels, and ligand binding. In these rats, angiotensin II caused significantly higher accumulation of inositol trisphosphate (IP3) and phospholipase C (PLC) activation which was sensitive to blockade by AT1 but not to AT2 antagonist. Also, angiotensin II-mediated, AT1-dependent MAP kinase, Na-K-ATPase, and Na/H exchanger 3 activation was higher in BSO-treated rats than in control rats. Tempol supplementation of BSO-treated rats restored redox status, normalized AT1 receptor expression, and decreased blood pressure. Tempol also normalized the angiotensin II-mediated, AT1-dependent IP3 accumulation and PLC, MAP kinase, Na-K-ATPase, and Na/H exchanger 3 stimulation. These data suggest that oxidative stress leads to AT1 receptor upregulation, which in turn causes overstimulation of sodium transporters and subsequently contributes to sodium retention and hypertension. Tempol, while reducing oxidative stress, normalizes AT1 receptor signaling and decreases blood pressure.

Angiotensin II AT1 receptor/signaling mechanisms in the biphasic effect of the peptide on proximal tubular Na+,K+-ATPase.

The present study was designed to determine the cellular signaling mechanisms responsible for mediating the effects of angiotensin II on proximal tubular Na+,K+-ATPase activity. Angiotensin II produced a biphasic effect on Na+,K+-ATPase activity: stimulation at 10(-13) - 10(-10) M followed by inhibition at 10(-7) - 10(-5) M of angiotensin II. The stimulatory and inhibitory effects of angiotensin II were antagonized by losartan (1nM) suggesting the involvement of AT1 receptor. Angiotensin II produced inhibition of forskolin-stimulated cAMP accumulation at 10(-13) - 10(-10) M followed by a stimulation in basal cAMP levels at 10(-7) - 10(-5) M. Pretreatment of proximal tubules with losartan (1nM) antagonized both the stimulatory and inhibitory effects of angiotensin II on cAMP accumulation. Pretreatment of the proximal tubules with pertussis toxin (PTx) abolished the stimulation of Na+,K+-ATPase activity but did not affect the inhibition of Na+,K+-ATPase activity produced by angiotensin II. Pretreatment of the tubules with cholera toxin did not alter the biphasic effect of angiotensin II on Na+,K+-ATPase activity. Mepacrine (10microM), a phospholipase A2 (PLA2) inhibitor, reduced only the inhibitory effect of angiotensin II on Na+,K+-ATPase activity. These results suggest that the activation of AT1 angiotensin II receptors stimulates Na+,K+-ATPase activity via a PTx-sensitive G protein-linked inhibition of adenylyl cyclase pathway, whereas the inhibition of Na+,K+-ATPase activity following AT1 receptor activation involves multiple signaling pathways which may include stimulation of adenylyl cyclase and PLA2.

Mechanisms of Oxidative Stress-Induced Increase in Salt Sensitivity and Development of Hypertension in Sprague-Dawley Rats

High salt intake produces vascular changes that contribute to the development of hypertension in salt-sensitive individuals. Because reactive oxygen species play a role in the pathogenesis of cardiovascular diseases, we investigated whether oxidative stress contributes to salt-sensitive hypertension. Sprague–Dawley rats were divided in different groups and received tap water (vehicle), 30 mmol/L of l-buthionine sulfoximine ([BSO] an oxidant), high salt ([HS] 1% NaCl), and BSO plus HS without and with antioxidant tempol (1 mmol/L) in drinking water for 12 days. Compared with vehicle, BSO treatment caused oxidative stress and mild increase in blood pressure. Thoracic aortic rings from BSO–treated rats exhibited decreased response to endothelium–independent vasorelaxants. In HS–treated rats, the response to vasoactive agents, as well as blood pressure, was unaffected. Concomitant treatment of rats with BSO and HS produced a marked increase in blood pressure and a decreased response to both endothelium-dependent and endothelium-independent vasorelaxants with an increase in EC50. Incubation of aortic tissue from BSO-treated rats with sodium nitroprusside showed decreased cGMP accumulation, whereas HS rats had decreased basal NO synthase activity. Tempol decreased oxidative stress, normalized blood pressure, and restored NO signaling and responses to vasoactive compounds in BSO and BSO plus HS rats. We conclude that BSO increases oxidative stress and reduces NO signaling, whereas HS reduces NO levels by decreasing the NO synthase activity. These phenomena collectively result in reduced responsiveness to both endothelium -dependent and endothelium- independent vasorelaxants and may contribute to salt-sensitive hypertension.

Defective Dopamine Receptor Function in Proximal Tubules of Obese Zucker Rats

Some of the pathophysiological consequences of obesity include insulin resistance, increased renal sodium reabsorption, and the development of hypertension. Dopamine promotes renal sodium excretion via activation of D(1)-like receptors present on the proximal tubules. Reduced dopamine-induced natriuresis and a defect in D(1)-like receptor function have been reported in the proximal tubules of hypertensive animals. The present study investigated D(1)-like dopamine receptors and associated G proteins as the initial signaling components in the proximal tubular basolateral membranes of obese Zucker and control lean Zucker rats. We found that the obese rats were hyperinsulinemic, hyperglycemic, and hypertensive compared with the lean rats. Dopamine produced concentration-dependent inhibition of Na,K-ATPase activity in the proximal tubules of lean rats, whereas the inhibitory effect of dopamine was reduced in obese rats. The D(1)-like receptors measured by [(3)H]SCH 23390 binding revealed an approximately 45% decrease in B(max) without a change in K(d) in the basolateral membranes of obese rats compared with lean rats. Although we found an increase in G(q)/11alpha and no change in G(s)alpha in the basolateral membranes of obese rats, dopamine and SKF 38393 failed to stimulate G proteins as measured by [(35)S]GTPgammaS binding in obese rats, suggesting a receptor-G protein coupling defect. We conclude that decrease in D(1)-like dopamine receptor binding sites and diminished activation of G proteins, resulting perhaps from defective coupling, led to the reduced inhibition by dopamine of Na,K-ATPase activity in the proximal tubules of obese Zucker rats. Such a defect in renal dopamine receptor function may contribute to sodium retention and development of hypertension in obese rats.