Changjian Chen

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)

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.

Inhibition of Na+,K(+)-ATPase in rat renal proximal tubules by dopamine involved DA-1 receptor activation.

SummaryEndogenous kidney dopamine (DA) causes natriuresis and diuresis, at least partly, via inhibition of proximal tubular Na+,K+-ATPase. The present study was done to identify the dopamine receptor subtype(s) involved in dopamine-induced inhibition of Na+,K+-ATPase activity. Suspensions of renal proximal tubules from Sprague-Dawley rats were incubated with dopamine, the DA-1 receptor agonist fenoldopam or the DA-2 receptor agonist SK&F 89124 in the presence or absence of either the DA-1 receptor antagonist SCH 23390 or the DA-2 receptor antagonist domperidone. Dopamine and fenoldopam (10−5 to 10−8 mol/1) produced a concentration-dependent inhibition of Na+,K+-ATPase activity. However, SK&F 89124 failed to produce any significant effect over the same concentration range. Incubation with fenoldopam (10−5 to 10−8 mol/1) in the presence of SK&F 89124 (10−6 mol/l) inhibited Na+,K+-ATPase activity to a degree similar to that with fenoldopam alone. Furthermore, DA-induced inhibition of Na+,K+-ATPase activity was attenuated by SCH 23390, but not by domperidone. Since α-adrenoceptor activation is reported to stimulate Na+,K+-ATPase activity and, at higher concentrations, dopamine also acts as an a-adrenoceptor agonist, the potential opposing effect from α-adrenoceptor activation on DA-induced inhibition of Na+,K+-ATPase activity was investigated by using the α-adrenoceptor blocker phentolamine. We found that, in the lower concentration range (10−5 to 10−7 mol/1), dopamine-induced inhibition of Na+,K+-ATPase activity in the presence of phentolamine was similar in magnitude to that observed with dopamine alone. However, at the highest concentration used (10−4 mol/1), dopamine produced a significantly larger degree of inhibition of Na+,K+-ATPase activity in the presence of phentolamine. These results indicate that the DA-1 dopamine receptor subtype, but not the DA-2 receptor subtype, is involved in dopamine-mediated inhibition of Na+,K+-ATPase. At higher concentrations of dopamine, the DA-1 receptor-mediated inhibitory effect on Na+,K+-ATPase activity may be partly opposed by a simultaneous α-adrenoceptor-mediated stimulation of the activity of this enzyme.

Potentiation by enalaprilat of fenoldopam-evoked natriuresis is due to blockade of intrarenal production of angiotensin-II in rats.

We have previously shown that the natriuretic response to DA-1 receptor agonist fenoldopam is markedly potentiated by angiotensin converting enzyme inhibitor captopril. Since inhibition of angiotensin converting enzyme can lead to decreased production of angiotensin-II and increased levels of kinins (e.g., bradykinin), it is likely that both of these mechanisms might be involved in this phenomenon. However, it is not known whether and to what degree the accumulation of kinins contributes to the overall potentiation of natriuretic response to fenoldopam seen during angiotensin converting enzyme inhibition. In the present study, we have examined the effect of angiotensin converting enzyme inhibitor enalaprilat and angiotensin-II receptor antagonist losartan as well as bradykinin-2 receptor antagonist HOE 140 on fenoldopam-induced natriuresis. Intravenous infusion of fenoldopam (1 μg/kg/min) for 30 min produced significant increases in urine output and urinary sodium excretion without causing any changes in glomerular filtration rate, renal blood flow and mean arterial blood pressure, a phenomenon suggestive of a direct tubular site of action. In animals treated with either the angiotensin converting enzyme inhibitor enalaprilat or angiotensin-II receptor antagonist losartan, the diuretic and natriuretic effects of fenoldopam were potentiated to a similar degree. Whereas no significant changes in glomerular filtration rate occurred when fenoldopam alone was given to control rats, in animals treated with either enalaprilat or losartan, fenoldopam produced a modest but significant increase in glomerular filtration rate. In a separate group of animals, the effects of bradykinin-2 receptor antagonist HOE 140 on potentiation of fenoldopam-induced natriuresis by enalaprilat was examined. It was found that the magnitude of changes in urine output, sodium excretion and glomerular filtration rate in animals receiving pretreatment with both enalaprilat and HOE 140 were similar to those with enalaprilat pretreatment alone. These findings suggest that the potentiation of natriuretic response to fenoldopam by enalaprilat is solely due to blockade of angiotensin-II production and kinins do not appear to contribute to this phenomenon.

Role of Endogenous Dopamine in the Natriuretic Response to Various Degrees of Iso-Osmotic Volume Expansion in Rats

It is recognized that endogenous dopamine (DA) plays an important role in regulation of sodium excretion under certain physiological and experimental conditions. However, the relative contribution of intrarenally produced DA to natriuresis accompanying various degrees of acute volume expansion (VE) still remains to be clarified. In the present study, acute iso-osmotic VEs were performed in pentobarbital-anesthetized rats over a 60 min period. The rats were divided into 3 groups, each received either modest (2.5% body weight), moderate (5% body weight), or large degree of VE (10% body weight), respectively. Acute VE in these three groups evoked pronounced increases in urine output (UV) and urinary sodium excretion (UNaV), which were associated with significant increase in urinary DA excretion (UDAV). Compared to the modest VE (2.5% body weight), moderate VE (5% body weight) produced larger increases in UV (43 +/- 4.7 vs 29.0 +/- 4.7 microliters/min, p less than 0.05) and UNaV (7.8 +/- 0.7 vs 4.7 +/- 1.0 microEq/min, p less than 0.05) with slight but significantly greater increase in UDAV (1.38 +/- 0.06 vs 1.23 +/- 0.02 ng/min, p less than 0.05). Compared to moderate VE (5% body weight), large VE (10% body weight) produced more pronounced increases in UV (91 +/- 14 vs 43 +/- 4.7 microliters/min, p less than 0.01) and UNaV (16 +/- 2.3 vs 7.8 +/- 0.7 microEq/min, p less than 0.01), however the increase in UDAV was similar to that seen during moderate VE. These results suggest that endogenous DA is involved in natriuretic response to various degrees of acute VE. Furthermore, it was discovered that the relative contribution of endogenous DA to overall VE-induced natriuresis is related to some extent to the degree of VE, inasmuch as DA appears to play a greater role in the overall natriuretic response seen during modest to moderate degree of VE.

α1-Adrenoceptor subtypes mediating stimulation of Na+,K+-ATPase activity in rat renal proximal tubules

Abstract Although both α1A- and α1B-adrenoceptors are present in renal proximal tubules, the involvement of these receptor subtypes in the stimulation of Na+,K+-ATPase activity is not known. This study was undertaken to delineate the receptor subtype(s) involved in α1-adrenoceptor-mediated increase in Na+,K+-ATPase activity and to identify the cellular signaling mechanisms such as stimulation of inositol triphosphate formation (IP3) and protein kinase C activation in this phenomenon. It was found that norepinephrine-induced increase in Na+,K+-ATPase activity was attenuated by prazosin, but not by rauwolscine, indicating the involvement of α1-adrenoceptors. Furthermore, this response was selectively inhibited by the α1B-adrenoceptor inactivator, chloroethylclonidine (100 μM), but not by the α1A-adrenoceptor antagonist, WB4101 (0.01 μM). We examined whether these effects on Na+,K+-ATPase activity are mediated via the activation of IP3 and protein kinase C, Phenylephrine-induced increase in IP3 levels was abolished by prazosin, and significantly inhibited by WB4101, but not by chloroethylclonidine. similarly, phenylephrine-induced activation of protein kinase C was sensitive to blockade by WB4101, but not by chloroethylclonidine. These results suggest that whereas both α1A- and α1B-adrenoceptors are present in proximal tubules, α1B-adrenoceptors are involved in stimulating Na+,K+-ATPase activity and α1A-adrenoceptors are predominantly linked to renal tubular IP3 production and protein kinase C activation. Therefore, it appears that norepinephrine-induced stimulation of Na+,K+-ATPase activity does not involve phospholipase-C-coupled protein kinase C pathway.

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

We have previously reported that dopamine-1 receptor-mediated activation of phospholipase C is diminished in renal cortical slices of spontaneously hypertensive rats. The present study was carried out to examine the effect of dopamine on protein kinase C (PKC), which is one of the enzymes involved in the signal-transduction pathway leading to dopamine-induced inhibition of Na+/K(+)-ATPase in the renal proximal tubule. Renal proximal tubule suspensions were obtained from spontaneously hypertensive rats (SHR) and Wistar-Kyoto (WKY) rats of 10-12 weeks old. The tubules were incubated with dopamine in the presence or absence of DA-1 receptor antagonist SCH 23390. The PKC activity was measured by using a specific fluorescent peptide substrate (sequence, PKSRTLSVAAK). We found that dopamine produced a concentration-dependent increase in protein kinase C activity in the WKY rats, however, it failed to stimulate PKC activity in the SHR. Peak stimulation of 3.828 +/- 0.35 (ng/micrograms) protein in the WKY rats was observed at dopamine concentration of 1 microM, which was blocked in a concentration-dependent manner by SCH 23390 (0.25 microM). These results provide evidence that dopamine directly stimulates PKC activity via activation of DA-1 receptors in WKY rats. Furthermore, we discovered that dopamine fails to stimulate PKC activity in the SHR. This phenomenon may be responsible for the failure of dopamine to inhibit Na+/K(+)-ATPase activity in the hypertensive animals.

Involvement of endogenous dopamine and DA-1 receptors in the renal effects of atrial natriuretic factor in rats

The precise mechanism of the diuretic and natriuretic effects of Atrial Natriuretic Factor (ANF) is still unclear. The present study was undertaken with the aim of elucidating the mechanism of interaction between ANF, dopamine (DA) and DA receptors in the renal effects of ANF in rats. In pentobarbital anesthetized rats, ANF infusion (10 micrograms/kg/hour) produced marked diuresis and natriuresis which was accompanied by significant hypotension, bradycardia and also a modest increase in glomerular filtration rate. However, there was no accompanying increase in urinary DA excretion during ANF infusion. Pretreatment with SCH 23390, a selective DA-1 receptor antagonist caused significant attenuation of the diuretic and natriuretic effects of ANF whereas the hemodynamic changes produced by ANF were still evident in SCH 23390 treated animals. Plasma ANF levels were elevated to the same magnitude in both the control and SCH 23390 treated groups. Pretreatment with carbidopa, a dopa decarboxylase inhibitor also significantly blunted the diuretic and natriuretic effects of ANF. The antagonism of the ANF-induced diuresis and natriuresis by SCH 23390 and carbidopa was comparable in magnitude. These results suggest that endogenous DA, via activation of DA-1 receptors plays a permissive role in the renal effects of ANF, perhaps by enhancing tubular responsiveness to ANF. However, there does not appear to be a stimulatory effect of ANF on renal DA production or release.

The role of intrarenal nitric oxide in the natriuretic response to dopamine-receptor activation

Dopamine and dopamine-1 receptor agonists produce diuresis and natriuresis by causing changes in renal hemodynamics and by the activation of dopamine-1 receptors located within the various regions of the nephron. Nitric oxide plays an important role in the maintenance of systemic and regional hemodynamics. The present study was undertaken to investigate the effect of locally generated nitric oxide on renal function and its potential influence on the renal responses to dopamine-1 receptor agonists. The intrarenal infusion of a nitric oxide synthase inhibitor, L-NAME, (50 μg/kg min for 90 min) in anesthetized rats produced significant decreases in urine volume, urinary sodium excretion, glomerular filtration rate and fractional sodium excretion. These changes in renal function were associated with a concomitant decrease in urinary nitrate excretion, an indicator of nitric oxide release. However, L-NAME at this dose did not produce any significant changes in mean arterial pressure or heart rate. Intravenous infusion of fenoldopam (1 μg/kg min for 30 min), a selective dopamine-1 receptor agonist, produced diuresis and natriuresis without causing any changes in mean arterial pressure and heart rate. These renal effects of fenoldopam were significantly attenuated in animals that received the simultaneous infusion of L-NAME (intrarenal). Similar results were obtained with dopamine in that the natriuretic and diuretic response to dopamine was also attenuated during simultaneous infusion of dopamine with L-NAME. In addition, the diuresis and natriuresis produced by fenoldopam and dopamine was associated with increases in urinary nitrate excretion. Interestingly, these increases in the nitrate levels seen with fenoldopam and dopamine were also significantly reduced in the presence of L-NAME. These results indicate that intrarenal nitric oxide plays an important role in regulating renal sodium excretion and that an intact renal nitric oxide system is required for the full expression of diuretic and natriuretic response seen during dopamine-1 receptor activation.

Lack of Renal Dopamine Production During Acute Volume Expansion in Dahl Salt-Sensitive Rats

Endogenous kidney dopamine (DA) is reported to contribute to the natriuretic response to acute volume expansion (VE). Several studies suggest that a defect in renal DA-ergic system may play a role in genetic hypertension in humans and rats. The present study was performed to determine the role of renal DA and tubular DA-1 receptors in the natriuretic response to VE in age-matched inbred Dahl salt sensitive (SS/Jr) and salt resistant (SR/Jr) rats of 9-11 weeks of age. In pentobarbital anesthetized rats, VE was carried out by intravenous infusion of isotonic sodium chloride (5% body weight) over a period of 60 min. This maneuver evoked marked increases in urine output and urinary sodium excretion in both SR/Jr and SS/Jr species. However, the natriuretic and diuretic response to VE was significantly reduced in SS/Jr as compared to SR/Jr rats. It was also observed that the urinary excretion of DA was significantly increased during VE only in SR/Jr, but not in SS/Jr rats. In separate group of animals, infusion of DA (1 microgram/kg/min) produced similar increases in urine output and urinary sodium excretion without causing any alterations in blood pressure or heart rate in either SS/Jr or SR/Jr rats. These results suggest that SS/Jr rats are not able to eliminate an acute increase in sodium load as efficiently as SR/Jr, which may be partly due to an impaired endogenous kidney DA production.