Cardiovascular dopamine receptors: role of renal dopamine and dopamine receptors in sodium excretion

The role of dopamine D2, but not D3 or D4, receptor subtypes, in quinpirole-induced inhibition of the cardioaccelerator sympathetic outflow in pithed rats

BACKGROUND AND PURPOSE

Quinpirole (a dopamine D2-like receptor agonist) inhibits the cardioaccelerator sympathetic outflow in pithed rats by sympathoinhibitory D2-like receptors. The present study was designed to identify pharmacologically the specific D2-like receptor subtypes (i.e. D2 , D3 and D4) involved in this sympathoinhibition by quinpirole.

EXPERIMENTAL APPROACH

One hundred fourteen male Wistar rats were pithed, artificially ventilated with room air and prepared for either preganglionic spinal (C7-T1) stimulation of the cardioaccelerator sympathetic outflow (n = 102) or i.v. bolus injections of exogenous noradrenaline (n = 12). This approach resulted in frequency-dependent and dose-dependent tachycardic responses, respectively, as previously reported by our group.

KEY RESULTS

I.v. continuous infusions of quinpirole (0.1-10 μg kg(-1)  min(-1)), but not of saline (0.02 mL min(-1)), dose-dependently inhibited the sympathetically induced tachycardic responses. Moreover, the cardiac sympathoinhibition induced by 3 μg kg(-1)  min(-1) quinpirole (which failed to affect the tachycardic responses to i.v. noradrenaline) was: (i) unchanged after i.v. injections of the antagonists SB-277011-A (D3 ; 100-300 μg kg(-1)) or L-745,870 (D4 ; 30-100 μg kg(-1)); and (ii) markedly blocked and abolished by, respectively, 100 and 300 μg kg(-1) of the D2 preferring receptor subtype antagonist L-741,626. These doses of antagonists, which did not affect per se the sympathetically induced tachycardic responses, were high enough to completely block their respective receptors.

CONCLUSIONS AND IMPLICATIONS

The cardiac sympathoinhibition induced by 3 μg kg(-1)  min(-1) quinpirole involves the dopamine D2 receptor subtype, with no evidence for the involvement of the D3 or D4 subtypes. This provides new evidence for understanding the modulation of the cardioaccelerator sympathetic outflow.

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.

The α1-adrenergic receptor not the DA1-dopaminergic receptor mediates cyclosporine–SKF38393 renovascular interaction

In this study, we investigated the effect of acute exposure to cyclosporine A (CyA) on renal vasodilations evoked by the DA1 dopaminergic agonist SKF38393 and whether dopamine DA1 receptors are directly involved in the interaction. Changes evoked by CyA in SKF38393 vasodilations were evaluated in phenylephrine-preconstricted isolated perfused rat kidneys in the absence and presence of SCH23390, a DA1 receptor antagonist. SKF38393 (3 × 10–8 to 3 × 10–6 mol) produced dose-dependent reductions in the renal perfusion pressure that were significantly attenuated in tissues pretreated with SCH23390 or CyA. Unlike SKF38393, the vasodilatory action of sodium nitroprusside, a nitrovasodilator, was not altered by CyA. The attenuating effect of CyA on SKF38393 vasodilations was preserved in preparations pretreated with SCH23390, suggesting that sites other than DA1 receptors may be involved in CyA–SKF38393 interaction. The study was then extended to investigate the possible involvement of renal α1-adrenoceptors in the interaction. Blockade of α1-adrenoceptors by prazosin (30 nmol/L) significantly reduced the vasodilatory effect of SKF38393 and virtually abolished the CyA-induced attenuation of SKF38393 responses. Further, CyA failed to alter SKF38393 vasodilations when the renal tone was raised with prostaglandin F2α (PGF2α), a vasoconstrictor whose effect is independent of α1-adenoceptors. Together, these findings support earlier reports that both DA1 and α1-receptors mediate the renal vasodilatory action of SKF38393 and suggest that CyA interacts selectively with the α1-receptor component to compromise SKF38393 responses.

Vascular and renal effects of dopamine during extracellular volume expansion: Role of nitric oxide pathway

OBJECTIVE

The aim of the study was to determine the possible role of NO-system activation in vascular and renal effects of the dopaminergic system and the probable interaction between both systems during acute volume expansion in rats.

DESIGN AND METHODS

Expanded (10% bw) and non-expanded anaesthetized male Wistar rats were treated with haloperidol, a DA receptor antagonist (3 mg/kg bw, ip). Mean arterial pressure, diuresis, natriuresis, renal plasma flow, glomerular filtration rate, nitrites and nitrates excretion (NOx) were determined. NADPH diaphorase activity was measured using a histochemistry technique in kidney, aorta and renal arteries. NOS activity in kidney and aorta from expanded and non-expanded animals was determined with L-[U14C]-arginine substrate, in basal conditions and after DA (1 microM) administration.

RESULTS

The hypotensive effect of L-arg and hypertension induced by L-NAME were not modified by haloperidol. This blocker reverted the increase in diuresis, natriuresis and RPF induced by L-arg in both groups. Dopaminergic blockade induced a decrease in NOx excretion and in NADPH-diaphorase activity in glomeruli, proximal tubule and medullar collecting duct and in endothelium and vascular smooth muscle of renal arteries. DA induced an increase in NOS activity in renal medulla and cortex in both groups, but no changes in the aorta were observed.

CONCLUSIONS

Our results suggest that renal DA would be associated with the renal response induced by NO during extracellular volume expansion. NO-system activation would be one of the mechanisms involved in renal DA activity during saline load, but NO appears not to be involved in DA vascular effects.

Age-related changes of dopamine receptor protein immunoreactivity in the rat mesenteric vascular tree

Dopamine D1-D5 receptor protein immunoreactivity and tyrosine hydroxylase (TH) immunoreactivity were investigated on the mesenteric arterial tree by immunohistochemistry. The density of various dopamine receptors and TH immunoreactivity was compared between young (6-month-old), adult (15-month-old) and senescent (24-month-old) Fischer 344 rats by computer-assisted microdensitometry. The dopamine D1-like (D1 and D5) receptors were localized on the tunica media of different sized mesenteric artery branches. The D2-like (D2, D3 and D4) receptors as well as TH immunoreactivity were localized only on the adventitia-media transitional zone of mesenteric arterial tree. Expression of the D1 and D5 receptors was decreased in both adult and senescent rats compared to the young rats, suggesting an age-related decline in these receptors. Of the D2-like receptors, the expression of the D2 receptor was decreased as a function of age, while the D3 receptor was unchanged in the senescent rats compared to the young rats. Expression of the D4 receptor was increased in adult, but was unchanged in the senescent rats compared to young animals. TH immunoreactivity was increased as a function of age. The above data suggest that reduction in the D1, D2 and D5 receptor expression may contribute to the deficiency in the dopamine-mediated vasorelaxation and hence blood flow in the mesenteric vascular tree in aging. The different sensitivity to aging of sympathetic neuroeffector junctions labeled by TH and of dopamine D2-like receptors that are known to be prejunctional, suggests that age-related changes of dopamine receptor expression in the mesenteric vasculature reflect more complicated mechanisms than simple up- or down-regulation phenomena.

The dopaminergic system in hypertension

Dopamine exerts cardiovascular and renal actions mediated through interaction with specific dopamine receptors. Dopamine receptors are cell surface receptors coupled to G-proteins and classified into two main super families based on biochemical, pharmacological and molecular characteristics. The dopamine D1-like receptor super family includes D1 and D5 receptors, known also in rodents as D1A and D1B sites. These receptors are linked to stimulation of adenylate cyclase. The dopamine D2-like receptor super family includes D2, D3 and D4 receptors. These receptors are linked to inhibition of adenylate cylase or not related with this enzyme activity. They also interfere with opening of Ca+2 channels and are linked to stimulation of K+ receptors. Dopamine receptor subtypes are expressed in brain as well as in extracerebral structures such as the heart, blood vessels, carotid body, kidney, adrenal gland, parathyroid gland and gastrointestinal tract. In the kidney, which represents the peripheral organ where dopamine receptors were more extensively investigated, dopamine receptors are involved in regulation of hemodynamic, electrolyte and water transport, as well as renin secretion. Hypertension-related dopamine receptor changes were also investigated primarily in the kidney. 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 signalling system in the proximal tubules that lead to reduced dopamine-mediated effects on renal sodium excretion in hypertension. Studies on the influence of hypertension on dopamine D2-like receptors are sparse Disruption of either D1A or D3 receptors at the gene level causes hypertension in mice. Using peripheral blood lymphocytes as possible markers of the status of dopamine receptors in essential hypertension, no changes of dopamine D1-like receptors were noticeable, whereas an increase of dopamine D2-like receptors likely representing an up-regulation mechanism was reported. Available information collectively indicates an involvement of peripheral dopaminergic system in hypertension consisting either in impaired receptor transduction mechanisms and/or in receptor loss. A better knowledge of molecular bases of these changes may contribute to the development of specific therapeutic approaches in the future.

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 microg/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 microg/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.

Dopamine D3 receptor in peripheral mononuclear cells of essential hypertensives

Dopamine D3 receptor was studied in peripheral mononuclear cells of high-normal, stage 1, stage 2, and stage 3 essential hypertensives using a radioligand binding assay technique with [3H]-7-hydroxy-N,N-di-n-propyl-2-aminotetraline (7-OH-DPAT) as a radioligand. A group of de novo Parkinsonian patients was also examined as a reference group of impaired dopaminergic function. [3H]-7-OH-DPAT was bound specifically to human peripheral mononuclear cells in a manner consistent with the labeling of a dopamine D3 receptor. No changes in free dopamine, norepinephrine, epinephrine and aldosterone levels, renin activity, dissociation constant of [3H]-7-OH-DPAT binding, or the pharmacological profile of [3H]-7-OH-DPAT binding were found between normotensive control subjects and essential hypertensives or Parkinsonians. The density of peripheral mononuclear cell [3H]-7-OH-DPAT binding sites increased in essential hypertensives parallel to blood pressure value augmentation. A higher density of [3H]-7-OH-DPAT binding sites was found in Parkinsonians. In these patients, the density of [3H]-7-OH-DPAT binding sites was similar to that observed in high-normal subjects and in stage 1 essential hypertensives. The increased density of peripheral mononuclear cell dopamine D3 receptor in hypertension as well as in Parkinson's disease may represent an upregulation mechanism consequent to impaired dopaminergic function. In view of the difficulty in identifying markers of peripheral dopamine function, analysis of dopamine D3 receptor in peripheral mononuclear cells may help evaluate whether the dopaminergic system is involved in hypertension.

Dopamine D2-like receptors in the rat kidney: effect of denervation

The effects of monolateral denervation induced by renal artery occlusion on dopamine D2-like receptors were assessed in rat kidney using radioligand binding assay and autoradiographic techniques. [3H]spiperone was used as a ligand. [3H]spiperone was bound specifically to sections of control innervated kidneys with a dissociation constant (Kd) value of 0.07 +/- 0.003 nM and a maximum density of binding sites (Bmax) value of 35.4 +/- 0.16 fmol/mg tissue. Light microscope autoradiography showed the accumulation of silver grains both in the arterial tree and in cortical tubules. At the vascular level, [3H]spiperone binding sites were accumulated primarily in the adventitia and in adventitia-media transitional zone. In cortical tubules, the higher density of [3H]spiperone binding sites was noticeable in proximal convoluted tubules. A few binding sites were also found in the glomerular tuft. In denervated kidneys, noradrenaline and dopamine levels were reduced by about 90% and 60% respectively in comparison with control innervated kidneys. Denervation reduced the density of [3H]spiperone binding sites by more than 85%. In denervated kidneys, light microscope autoradiography showed the disappearance of specific vascular binding sites and a remarkable reduction of tubular [3H]spiperone binding sites. The above results indicate that the largest majority of renal dopamine D2-like receptors labelled by [3H]spiperone is prejunctional in location.

Renal effect of YM435, a new dopamine D1 receptor agonist, in anesthetized dogs

The renal effects of YM435 ((-)-(S)-4-(3,4-dihydroxyphenyl)-7,8-dihydroxy -1,2,3,4-tetrahydroisoquinoline hydrochloride hydrate), a dopamine D1 receptor agonist, were investigated in anesthetized dogs. Intravenous infusion of YM435 (0.1-3 micrograms/kg per min) increased renal blood flow and decreased mean blood pressure in a dose-dependent manner with little effect on heart rate. Glomerular filtration rate, urine flow and urinary sodium excretion were concomitantly increased. The renal effect of YM435 by intravenous infusion at 0.3 microgram/kg per min was completely blocked by treatment with the selective dopamine D1 receptor antagonist SCH 23390 (7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-(1H)-3-benzazep ine hydrochloride). Furthermore, intravenous infusion of YM435 (0.3 microgram/kg per min) reversed the angiotensin II-induced decreases in renal blood flow, glomerular filtration rate, urine flow and urinary sodium excretion, and prevented the decrease in renal blood flow, glomerular filtration rate and urine flow induced by renal nerve stimulation and platelet-activating factor (PAF). These results suggest that intravenous administration of YM435 produces renal vasodilating and diuretic/natriuretic effects by stimulation of dopamine D1 receptors, and demonstrate that YM435 can inhibit angiotensin II-, renal nerve stimulation- and PAF-induced renal dysfunction.

Dopamine decreases expression of type-1 angiotensin II receptors in renal proximal tubule

Systemic and/or locally produced angiotensin II stimulates salt and water reabsorption in the renal proximal tubule. In vivo, dopamine (DA) may serve as a counterregulatory hormone to angiotensin II's acute actions on the proximal tubule. We examined whether dopamine modulates AT1 receptor expression in cultured proximal tubule cells (RPTC) expressing DA1 receptors. Dopamine decreased basal RPTC AT1 receptor mRNA levels by 67 +/- 7% (n = 10; P < 0.005) and decreased 125I-angiotensin II binding by 41 +/- 7% (n = 4; P < 0.05). The DA1-specific agonist, SKF38393 decreased basal AT1 receptor mRNA levels (65 +/- 5% inhibition; n = 5; P < 0.025), and the DA1-specific antagonist, SCH23390 reversed dopamine's inhibition of AT1 receptor mRNA expression (24 +/- 10% inhibition; n = 8; NS) and angiotensin II binding (5 +/- 15%; n = 4; NS). DA2-specific antagonists were ineffective. In rats given L-DOPA in the drinking water for 5 d, there were decreases in both proximal tubule AT1 receptor mRNA expression (80 +/- 5%; n = 6; P < 0.005) and specific [125I] Ang II binding (control: 0.74 +/- 0.13 fmol/mg pro vs. 0.40 +/- 0.63 fmol/mg pro; n = 5; P < 0.05). In summary, dopamine, acting through DA1 receptors, decreased AT1 receptor expression in proximal tubule, an effect likely mediated by increased intracellular cAMP levels. Local dopamine production also led to decreased AT1 receptor expression, suggesting dopamine may reset sensitivity of the proximal tubule to angiotensin II.

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 microgram/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.(ABSTRACT TRUNCATED AT 250 WORDS)

Chronic manipulation of dietary salt modulates renal physiology and kidney dopamine receptor subtypes: functional and autoradiographic studies

1. Compared to rats maintained on the normal NaCl (0.33%) diet, animals maintained on the low NaCl (0%) diet for 4 weeks exhibited increased plasma aldosterone and chloride and decreased urinary sodium excretion. 2. Rats maintained on the high NaCl (8%) diet for 4 weeks showed increased systolic blood pressure, water intake, urine volume, sodium and dopamine excretion and decreased plasma aldosterone and glomerular filtration rate. 3. Administration of SCH 23390 (10 mg/kg, po), but not domperidone to the high salt diet rats attenuated the diuretic effect, indicating the involvement of DA1 rather than DA2 receptors. The dopamine decarboxylase inhibitor, carbidopa (30 mg/kg, i.p.), also reduced the high salt-induced diuresis. 4. Kidney sections from rats fed the low NaCl diet showed a 63-100% decrease (P < 0.001-0.02) in cortical and medullary DA1 and DA2 binding sites, while rats fed the high NaCl diet demonstrated only a 70% decrease (P < 0.01-0.02) in cortical DA1 binding, without affecting DA2 binding. 5. These data indicate that chronic modification of dietary salt profoundly affects the sodium, water and dopamine excretion and leads to selective modulation of renal dopamine receptor subtypes.

Intrarenal dopamine participation in frusemide diuretic and natriuretic responses to frusemide

1. Dopamine (DA) involvement in the renal response to frusemide was analysed using the isolated perfused rat kidney preparation. Endogenous DA levels were increased through the infusion of its precursor levodopa (LD) (0.5 or 1 microM) whereas benserazide (50 or 100 microM), an inhibitor of the enzyme L-dopa-decarboxylase, was used to decrease DA synthesis. 2. Frusemide administration (0.3-20 nmols) induced a dose-dependent increase in fractional excretion of water (FE H2O), sodium (FE Na+) and glucose (FEG). FE Na+ elicited by the diuretic was enhanced 30-40% by 0.5 microM of LD (n = 5, P < 0.05), and 60-80% by LD 1 microM (n = 5, P < 0.05). FE H2O produced by the diuretic was enlarged 80-100% by 0.5 microM (n = 5, P = 0.05), and 130-170% by 1 microM of LD (n = 5, P < 0.01). The increase produced by both concentrations of LD on FEG was 200% for the lowest dose of the diuretic (n = 5, P < 0.01), and 90% for the highest (n = 5, P < 0.05). 3. Benserazide (BZ) (100 microM) decreased the F.E. Na+ induced by frusemide (n = 5, P < 0.05) by 32-42%, and completely abolished frusemide effects on FE H2O and FEG (n = 5). 4. In conclusion, our results suggest that endogenous dopamine participates in the frusemide-induced diuresis and sodium excretion within the kidney, and that the participation of extrarenal factors is not essential for this effect. Dopamine may be involved in frusemide-induced inhibition of proximal sodium reabsorption.

Loss of dopamine D1-like receptors in the umbilical artery of pre-eclamptic subjects

1. The influence of pre-eclampsia on the density and pattern of dopamine D1-like receptors was studied in frozen samples of the placental end of the umbilical artery by using radioligand binding and autoradiographic techniques in combination. 2. Analysis was performed on normotensive (n = 10) and pre-eclamptic subjects (n = 9) undergoing caesarean delivery, using [3H]-SCH 23390 as a ligand. Pre-eclamptic patients received a low salt diet and were treated with magnesium sulphate and hydralazine. The possibility that this treatment may cause changes in the density of dopamine D1-like receptors was evaluated by treating male Wistar rats in the same way and by determining [3H]-SCH 23390 binding in sections of the kidney which represents an organ containing dopamine D1-like receptors. 3. The density of dopamine D1-like receptors of the umbilical artery, which are probably vasodilatory, was decreased in pre-eclamptic compared with normotensive subjects. In contrast, the affinity of the radioligand for dopamine D1-like receptors was not statistically different between normotensive and pre-eclamptic subjects. Low salt diet, magnesium sulphate and hydralazine treatment did not affect [3H]-SCH 23390 binding to sections of rat kidney. This suggests that changes in the density of dopamine D1-like receptors in pre-eclamptic patients are a specific phenomenon not dependent upon antihypertensive measures. 4. Analysis of the pharmacological profile of [3H]-SCH 23390 binding to sections of the umbilical artery both in normotensive and pre-eclamptic subjects indicates the labelling of dopamine D5 receptors. 5. These findings collectively suggest that the dopaminergic vasodilatory tone in the umbilical artery is impaired in pre-eclampsia. The possible significance of these data should be clarified in future studies.

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.

Pharmacological characterization and autoradiographic localization of dopamine D1 receptors in the human umbilical artery

Combined in vitro radioreceptor binding and autoradiographic techniques with [3H]SCH 23390 as a ligand were used to analyze the pharmacological profile and anatomical localization of dopamine D1 receptor sites in sections of human umbilical artery. The ligand was bound to sections of the artery in a manner consistent with the labelling of D1 receptors. These receptors, which show a fetal-to-maternal gradient with the highest concentration near the placenta, are located within the smooth muscle layer of the umbilical artery. The above findings suggest that endogenous dopamine may be involved in the control of the tone of the umbilical artery through the interaction with D1 receptors.

A pharmacological comparison of [3H]GBR12935 binding to rodent striatal and kidney homogenates: binding to dopamine transporters?

Binding of [3H]GBR12935 to homogenates of mouse and rat striatum and kidney was studied. [3H]GBR12935 bound to both tissue preparations with high affinity (mouse striatum Kd = 2.4 +/- 0.4 nM, n = 4; mouse kidney Kd = 3.8 +/- 0.9 nM, n = 4), in a saturable (striatal Bmax = 1.5 +/- 0.4 pmol/mg protein; kidney Bmax = 4.9 +/- 0.5 pmol/mg protein) and reversible manner. Saturation experiments revealed the presence of a single class of high affinity binding sites in both tissues of both species. Mouse kidney appeared to possess a greater density of [3H]GBR12935 binding sites than the striatum while the reverse situation prevailed for the rat. Although two dopamine uptake inhibitors, namely GBR12909 and benztropine, displaced [3H]GBR12935 binding from striatal and kidney homogenates with a similar affinity in both tissues of these species, unlabelled mazindol, (+/-)cocaine, nomifensine and amfonelic acid were significantly (P < 0.001-0.02) more potent inhibitors of [3H]GBR12935 binding in the striatum than in the kidney. While the pharmacological profile of [3H]GBR12935 binding in the rodent striatum compared well with that of the dopamine transporter reported previously, the pharmacology in the kidney was considerably different to that in the striatum. GBR12909 (1-30 mg/kg, i.p.), a close analog of GBR12935, induced significant antidiuretic and antinatriuretic effects in spontaneously hypertensive rats. These data suggest that while [3H]GBR12935 labels the dopamine uptake sites in the brain, it does not appear to label similar sites in the kidney. The mechanism of action of GBR12909 on sodium and water excretion remains to be determined.

Stimulation of renal dopamine production during acute volume expansion requires the presence of intact vagi but not renal nerves

We have previously reported that acute volume expansion (VE) with isotonic saline stimulates the production of renal dopamine (DA) which in turn contributes to the accompanying diuresis and natriuresis via activation of renal tubular DA-1 receptors. The purpose of the present study was to determine whether the presence of vagi and/or renal nerves is essential in order to activate the renal dopaminergic system during acute VE. Acute VE (6% body weight) with isotonic saline was performed in two groups of anesthetized rats, one of which served as sham control whereas the other was subjected to bilateral cervical vagotomy. The diuretic and natriuretic responses to acute VE did not differ between the sham control and vagotomized groups. However, urinary DA excretion (UDAV) was significantly increased in the sham control but not vagotomized group. Pretreatment with SCH 23390, a selective DA-1 receptor antagonist led to significant attenuation of the diuretic and natriuretic response to acute VE in the sham control but not vagotomized group. In another group of animals, the diuretic and natriuretic response to acute VE was studied in rats subjected to acute unilateral renal denervation. Basal UDAV was not significantly different between the denervated (DNX) kidney and the contralateral innervated (INX) kidney. Acute VE evoked diuresis and natriuresis in both kidneys, the response in the DNX kidney being significantly greater when compared to that in the INX kidney. UDAV increased significantly and to similar levels in both kidneys. Pretreatment with SCH 23390 led to attenuation of the diuretic and natriuretic response to acute VE in the DNX but not INX kidney. After DA-1 receptor blockade, the residual renal response to volume expansion in the DNX kidney did not differ significantly from that in the INX kidney. The results of this study suggest that 1) afferent vagal pathways appear to mediate the VE induced stimulation of renal DA production; 2) the increase in UDAV during acute VE occurs primarily through a mechanism which is independent of renal noradrenergic and putative dopaminergic nerves.

Diminished phospholipase C activation by dopamine in spontaneously hypertensive rats

It is reported that a defect in dopamine-1 (DA-1) receptor adenylate cyclase coupling in the proximal convoluted tubule in the spontaneously hypertensive rat may contribute to the diminished natriuretic response to DA-1 receptor agonists. Since the tubular DA-1 receptor is also coupled to phospholipase C, and both of these cellular signaling processes are involved in DA-1 receptor-mediated diuresis and natriuresis, it is important to know whether a similar defect is also present in DA-1 receptor-coupled phospholipase C pathway. The present study was therefore designed to determine the functional status of DA-1 receptor-phospholipase C coupling system of adult spontaneously hypertensive rats using a renal cortical slice preparation. In addition, the renal response to exogenously administered dopamine (1 microgram/kg/min i.v.) was also determined. We found that basal phospholipase C activity was significantly higher in hypertensive rats than in age-matched Wistar-Kyoto rats (7.36 +/- 0.32% versus 5.61 +/- 0.27%, p less than 0.05). However, compared with the normotensive controls, dopamine-induced increases in phospholipase C activity were significantly attenuated in the preparations of hypertensive rats in a concentration-dependent manner (13 +/- 6% versus 38 +/- 6% for 1 mM dopamine, p less than 0.05; 49 +/- 6% versus 71 +/- 9% for 3 mM dopamine, p less than 0.05; 50 +/- 16% versus 106 +/- 22%, p less than 0.05 for 10 mM dopamine).(ABSTRACT TRUNCATED AT 250 WORDS)

Peripheral dopamine in pathophysiology of hypertension. Interaction with aging and lifestyle

Dopamine, an ancestral catecholamine, is physiologically natriuretic and vasodilating, thus essentially protecting against hypertension. Its actions are overshadowed by the opposite effects of its main biological partner, norepinephrine, and this is accentuated with aging. Clinical observations combined with molecular biology approaches to catecholamine-synthesizing and catecholamine-metabolizing enzymes and receptors permit the identification of some inborn defects. Subtle changes in the dopamine-norepinephrine balance may account for the enhanced peripheral noradrenergic activity seen in the setting of decreased dopaminergic activity in advanced age. These changes may contribute to the diminished ability of the aged kidney to excrete a salt load, as well as to the finding that systolic blood pressure increases with age in populations with a high, but not in those with a low, intake of salt. The attainment of advanced age in Western societies with adverse lifestyle changes (mental rather than physical stress, excess salt intake, overeating, sedentarism) appears to facilitate the development of hypertension. The adaptation to all the preceding lifestyle changes necessitates an increased dopamine generation, which may initially compensate to maintain appropriate natriuresis and vasodilation since many patients with initial borderline essential hypertension express their sympathetic hyperfunction, in addition to increased norepinephrine release, by excessive dopamine release. However, the progression of hypertension is accompanied by a peripheral dopaminergic deficiency and diminished ability to excrete salt. This may represent an eventual inadequacy of a phylogenetically redundant system resulting in decreased natriuresis and vasodilation and may account for the responsiveness of established chronic hypertension to salt restriction, diuretics, and dopaminomimetic medication.

Dopamine DA-2 receptor sites in the rat renal cortex: a light microscope autoradiographic study

The binding of the DA-2 receptor antagonist [3H]-spiroperidol, in the presence of appropriate concentrations of ketanserin to avoid the possible labelling of 5-HT-2 sites, was studied in sections of rat kidney using combined radioreceptor assay and light microscope autoradiography. [3H]-Spiroperidol was bound by sections of rat kidney in a manner consistent with the labelling of a single class of high affinity of DA-2 sites. Scatchard analysis of the binding isotherms showed a Kd value of 2.5 +/- 0.2 nmol/l and a Bmax value of 129.5 +/- 6.8 fmol/mg protein. Light microscope autoradiography, centered on the renal cortex, revealed the development of silver grains within cortical convoluted tubules. Silver grains were rather homogeneously distributed to both proximal and distal convoluted tubules, with no specific glomerular localization. These findings do not support the assumption of the existence of glomerular DA-2 receptors in the rat kidney.

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.

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.

Renal dopamine and tubular DA-1 receptors in the regulation of sodium excretion

1. We have performed studies in rats with selective DA-1 receptor agonists fenoldopam and dopexamine which show that activation of tubular DA-1 receptors by these agents results in natriuresis and diuresis. 2. In pentobarbital-anaesthetized rats, an acute increase in sodium intake produced by volume expansion (5% body weight) with isotonic sodium chloride led to pronounced increases in sodium and water excretion. These natriuretic and diuretic responses were accompanied by significant increases in urinary dopamine excretion and could be attenuated by the selective DA-1 receptor antagonist, SCH 23390. 3. Intravenous infusion of atrial natriuretic factor produced hypotension, bradycardia and an increase in sodium and water excretion. The natriuretic and diuretic response to the peptide was not accompanied by any changes in urinary dopamine excretion but it was attenuated by SCH 23390 and the dopa decarboxylate inhibitor, carbidopa. 4. These results show that renal tubular DA-1 receptors can be activated by selective agonists, which subsequently leads to natriuresis and diuresis. During acute volume expansion, there is an increased production of renal dopamine, which contributes to the natriuretic response via activation of tubular DA-1 receptors. Finally, we discovered that endogenous dopamine plays a permissive role in the full expression of the renal effects of the atrial natriuretic factor.