D1 dopamine receptor hyperphosphorylation in renal proximal tubules in hypertension

A defect in the coupling of the D(1) receptor (D(1)R) to its G protein/effector complex in renal proximal tubules plays a role in the pathogenesis of spontaneous hypertension. As there is no mutation of the D(1)R gene in the spontaneously hypertensive rat (SHR), we tested the hypothesis that the coupling defect is associated with constitutive desensitization/phosphorylation of the D(1)R. The following experiments were performed: (1) Cell culture and membrane preparations from rat kidneys and immortalized rat renal proximal tubule cells (RPTCs); (2) immunoprecipitation and immunoblotting; (3) cyclic adenosine 3',5' monophosphate and adenylyl cyclase assays; (4) immunofluorescence and confocal microscopy; (5) biotinylation of cell surface proteins; and (6) in vitro enzyme dephosphorylation. Basal serine-phosphorylated D(1)Rs in renal proximal tubules, brush border membranes, and membranes from immortalized RPTCs were greater in SHRs (21.0+/-1.5 density units, DU) than in normotensive rats (7.4+/-2.9 DU). The increased basal serine phosphorylation of D(1)Rs in SHRs was accompanied by decreased expression of D(1)R at the cell surface, and decreased ability of a D(1)-like receptor agonist (fenoldopam) to stimulate cyclic adenosine 3',5' monophosphate (cAMP) production. Increasing protein phosphatase 2A activity with protamine enhanced the ability of fenoldopam to stimulate cAMP accumulation (17+/-4%) and alter D(1)R cell surface expression in intact cells from SHRs. Alkaline phosphatase treatment of RPTC membranes decreased D(1)R phosphorylation and enhanced fenoldopam stimulation of adenylyl cyclase activity (26+/-6%) in SHRs. Uncoupling of the D(1)R from its G protein/effector complex in renal proximal tubules in SHRs is caused, in part, by increased D(1)R serine phosphorylation.

Dopamine receptors in hypertension

There is increased awareness of the role of dopamine in cardiovascular function, renal function and systemic blood pressure regulation. Growing evidence indicates that each of the five dopamine receptor subtypes participates in the regulation of blood pressure by mechanisms distinct for that particular subtype. Some dopamine receptors regulate blood pressure by influencing the central and peripheral nervous system, while others influence renal function and release of renin, aldosterone and vasopressin. This review summarizes the physiology and pathophysiology of the peripheral dopaminergic system and our current understanding of the role of individual dopamine receptors in the pathophysiology of human essential hypertension.

Impaired renal D(1)-like and D(2)-like dopamine receptor interaction in the spontaneously hypertensive rat

D(1)-like (D(1), D(5)) and D(2)-like (D(2), D(3), D(4)) dopamine receptors interact in the kidney to produce a natriuresis and a diuresis. Disruption of D(1) or D(3) receptors in mice results in hypertension that is caused, in part, by a decreased ability to excrete an acute saline load. We studied D(1)-like and D(2)-like receptor interaction in anesthetized spontaneously hypertensive rats (SHR) by the intrarenal infusion of Z-1046 (a novel dopamine receptor agonist with rank order potency of D(3)> or =D(4)>D(2)>D(5)>D(1)). Z-1046 increased glomerular filtration rate (GFR), urine flow, and sodium excretion in normotensive Wistar-Kyoto rats but not in SHRs. The lack of responsiveness to Z-1046 in SHRs was not an epiphenomenon, because intrarenal cholecystokinin infusion increased GFR, urine flow, and sodium excretion to a similar extent in the two rat strains. We conclude that renal D(1)-like and D(2)-like receptor interaction is impaired in SHRs. The impaired D(1)-like and D(2)-like receptor interaction in SHRs is not caused by alterations in the coding sequence of the D(3) receptor, the D(2)-like receptor expressed in rat renal tubules that has been shown to be involved in sodium transport. Because the diuretic and natriuretic effects of D(1)-like receptors are, in part, caused by an interaction with D(2)-like receptors, it is possible that the decreased Z-1046 action in SHRs is secondary to the renal D(1)-like receptor dysfunction in this rat strain.

Adrenergic and endothelin B receptor-dependent hypertension in dopamine receptor type-2 knockout mice

Polymorphism of the dopamine receptor type-2 (D(2)) gene is associated with essential hypertension. To assess whether D(2) receptors participate in regulation of blood pressure (BP), we studied mice in which the D(2) receptor was disrupted. In anesthetized mice, systolic and diastolic BPs (in millimeters of mercury) were higher in D(2) homozygous and heterozygous mutant mice than in D(2)+/+ littermates. BP after alpha-adrenergic blockade decreased to a greater extent in D(2)-/- mice than in D(2)+/+ mice. Epinephrine excretion was greater in D(2)-/- mice than in D(2)+/+ mice, and acute adrenalectomy decreased BP to a similar level in D(2)-/- and D(2)+/+ mice. An endothelin B (ET[B]) receptor blocker for both ET(B1) and ET(B2) receptors decreased, whereas a selective ET(B1) blocker increased, BP in D(2)-/- mice but not D(2)+/+ mice. ET(B) receptor expression was greater in D(2)-/- mice than in D(2)+/+ mice. In contrast, blockade of ET(A) and V(1) vasopressin receptors had no effect on BP in either D(2)-/- or D(2)+/+ mice. The hypotensive effect of an AT(1) antagonist was also similar in D(2)-/- and D(2)+/+ mice. Basal Na(+),K(+)-ATPase activities in renal cortex and medulla were higher in D(2)+/+ mice than in D(2)-/- mice. Urine flow and sodium excretion were higher in D(2)-/- mice than in D(2)+/+ mice before and after acute saline loading. Thus, complete loss of the D(2) receptor results in hypertension that is not due to impairment of sodium excretion. Instead, enhanced vascular reactivity in the D(2) mutant mice may be caused by increased sympathetic and ET(B) receptor activities.

D1 dopamine receptor regulation of NHE3 during development in spontaneously hypertensive rats

To determine if the defective interactions among D1-like receptors, G proteins, and Na+/H+ exchanger 3 (NHE3) are consequences of hypertension, we studied these interactions in rats, before (2–3 wk) and after (12 wk) the establishment of hypertension. To eliminate the confounding influence of second messenger action on D1 receptor-NHE3 interaction, studies were performed in renal brush-border membranes (BBM) devoid of cytoplasmic second messengers. NHE3 activity increased with age in Wistar-Kyoto (WKY) rats (3 wk = 1.48 ± 0.39, n = 13; 12 wk = 2.83 ± 0.15, n = 16, P < 0.05) but not in spontaneously hypertensive rats (SHRs; 3 wk = 2.52 ± 0.37, n = 11; 12 wk = 2.81 ± 0.20, n = 16). D1 receptor protein tended to decrease, whereas NHE3 protein tended to increase with age in both WKY and SHRs. However, the inhibitory effect of a D1-like agonist, SKF-81297, on NHE3 activity increased with age in WKY rats (3 wk = −40.7 ± 5.3%, n = 10, 12 wk = −58.7 ± 4.6%, n = 12, P < 0.05) but not in SHRs (3 wk = −27.6 ± 5.9%, n = 11, 12 wk = −25.1 ± 3.2%, n = 11). The decreased inhibitory effect of another D1-like agonist, fenoldopam, on NHE3 activity in SHRs was not caused by increased activity and binding of Gβγ to NHE3 as has been reported in young WKY rats. Gsα mediates, in part, the inhibitory effect of D1-like agonists on NHE3 activity. In WKY rats, fenoldopam increased Gsα/NHE3 binding to the same extent in 2-wk-old (1.5-fold, n = 4) and adult (1.5-fold, n = 4) rats. In contrast, in SHRs, fenoldopam decreased the amount of Gsα bound to NHE3 in 2-wk-old SHRs and had no effect in 4-wk-old and adult SHRs. These studies indicate that the decreased inhibitory effect of D1-like agonists on NHE3 activity in SHRs (compared with WKY rats) precedes the development of hypertension. This may be caused, in part, by a decreased interaction between Gsα and NHE3 in BBM secondary to impaired D1-like receptor function.

[Regulation of NHE3 by D1 dopamine receptor during development of spontaneously hypertensive rats]

OBJECTIVE

To determine if spontaneous hypertension is secondary to defective interaction among dopamine receptor, G protein, and Na(+)/H(+) exchanger 3 (NHE3).

METHODS

The inhibitory effect of a D(1) dopamine agonist upon NHE3 activity and its impact upon G(s)alpha/NHE3 binding in renal brush border membrane (BBM) of spontaneously hypertensive rats (SHRs) 2 - 3 weeks before and 12 weeks after the establishment of hypertension were examined. In order to avoid the confounding influence of second messenger on D(1) receptor/NHE3 interaction, study was made in BBM devoid of cytoplasmic component.

RESULTS

NHE3 activity increased with age in Wister-Kyoto (WKY) rats but not in SHRS. D1 receptor expression did change with age in both WKY rats and SHRs. The inhibitory effect of a D(1)-like agonist on NHE3 activity increased with age in WKY rats but not in SHRs. In WKY rats, another D(1)-like agonist, fedoldopam, increased G(s)alpha/NHE3 binding to the same extent in 2 week old and adult rats, but decreased the amount of G(s)(alpha)bound to NHE3 in 2 week old SHRs.

CONCLUSION

The decrease of inhibitory effect of D(1)-like agonist upon NHE3 activity in SHRs precedes the development of hypertension. Spontaneous hypertension may be caused, in part, by a decreased interaction between G(s)alpha and NHE3 in BBM secondary to D(1)-like receptor function.

Renal protein phosphatase 2A activity and spontaneous hypertension in rats

The impaired renal paracrine function of dopamine in spontaneously hypertensive rats (SHR) is caused by hyperphosphorylation and desensitization of the renal D(1) dopamine receptor. Protein phosphatase 2A (PP(2A)) is critical in the regulation of G-protein-coupled receptor function. To determine whether PP(2A) expression and activity in the kidney are differentially regulated in genetic hypertension, we examined the effects of a D(1)-like agonist, fenoldopam, in renal cortical tubules and immortalized renal proximal tubule cells from normotensive Wistar-Kyoto rats (WKY) and SHR. In cortical tubules and immortalized proximal tubule cells, PP(2A) expression and activities were greater in cytosol than in membrane fractions in both WKY and SHR. Although PP(2A) expressions were similar in WKY and SHR, basal PP(2A) activity was greater in immortalized proximal tubule cells of SHR than WKY. In immortalized proximal tubule cells of WKY, fenoldopam increased membrane PP(2A) activity and expression of the regulatory subunit PP(2A)-B56alpha, effects that were blocked by the D(1)-like antagonist SCH23390. Fenoldopam had no effect on cytosolic PP(2A) activity but decreased PP(2A)-B56alpha expression. In contrast, in immortalized proximal tubule cells of SHR, fenoldopam decreased PP(2A) activity in both membranes and cytosol but predominantly in the membrane fraction, without affecting PP(2A)-B56alpha expression; this effect was blocked by the D(1)-like antagonist SCH23390. We conclude that renal PP(2A) activity and expression are differentially regulated in WKY and SHR by D(1)-like receptors. A failure of D(1)-like agonists to increase PP(2A) activity in proximal tubule membranes may be a cause of the increased phosphorylation of the D(1) receptor in the SHR.

Renal dopamine and sodium homeostasis

During the past decade, it has become evident that dopamine plays an important role in the regulation of fluid and electrolyte balance and blood pressure. Dopamine exerts its actions through two families of dopamine receptors, designated D1-like and D2-like, which are identical in the brain and in peripheral tissues. The two D1-like receptors--D1 and D5 receptors--expressed in mammals are linked to stimulation of adenylyl cyclase. The three D2-like receptors--D2, D3, and D4,--are linked to inhibition of adenylyl cyclase. Dopamine affects fluid and electrolyte balance by regulation of renal excretion of electrolytes and water through actions on renal hemodynamics and tubular epithelial transport and by modulation of the secretion and/or action of vasopressin, renin, aldosterone, catecholamines, and endothelin B receptors (ETB) receptors. It also affects fluid and sodium intake by way of "appetite" centers in the brain and alterations of gastrointestinal tract transport. The production of dopamine in neural and non-neural tissues and the presence of receptors in these tissues suggest that dopamine can act in an autocrine or paracrine fashion. This renal autocrine-paracrine function, which becomes most evident during extracellular fluid volume expansion, is lost in essential hypertension and in some animal models of genetic hypertension. This deficit may be caused by abnormalities in renal dopamine production and polymorphisms or abnormal post-translational modification and regulation of dopamine receptor subtypes.

Gbeta regulation of Na/H exchanger-3 activity in rat renal proximal tubules during development

The decreased natriuretic action of dopamine in the young has been attributed to decreased generation of cAMP by the activated renal D(1)-like receptor. However, sodium/hydrogen exchanger (NHE) 3 activity in renal brush-border membrane vesicles (BBMV) can be modulated independent of cytoplasmic second messengers. We therefore studied D(1)-like receptor regulation of NHE activity in BBMVs in 2-, 4-, and 12-wk-old (adult) rats. Basal NHE activity was least in 2-wk-old compared with 4- and 12-wk-old rats. D(1)-like agonist (SKF-81297) inhibition of NHE activity was also least in 2-wk-old (-1 +/- 9%, n = 3) compared with 4 (-15 +/- 5%, n = 6)- and 12 (-65 +/- 4%, n = 6)-wk-old rats. The decreased response to the D(1)-like agonist in BBMV was not caused by decreased D(1) receptors or NHE3 expression in the young. G(s)alpha, which inhibits NHE3 activity by itself, coimmunoprecipitated with NHE3 to the same extent in 2-wk-old and adult rats. G(s)alpha function was also not impaired in the young because guanosine 5'-O-(3-thiotriphosphate) decreased NHE activity to a similar extent in 4-wk-old and adult rats. Galpha(i-3) protein expression in BBMV also did not change with age. In contrast, Gbeta expression and the amount of Gbeta that coimmunoprecipitated with NHE3 in BBMV was greatest in 2-wk-old rats and decreased with age. Gbeta common antibodies did not affect D(1)-like agonist inhibition of NHE activity in adult rats (8%) but markedly increased it (48%)in 4-wk-old rats. We conclude that the decreased inhibitory effect of D(1)-like receptors on NHE activity in BBMV in young rats is caused, in part, by the increased expression and activity of the G protein subunit Gbeta/gamma. The direct regulation of NHE activity by G protein subunits may be an important step in the maturation of renal tubular ion transport.

D1 dopamine receptor signalling defect in spontaneous hypertension

Dopamine modulates cardiovascular function by actions in the central and peripheral nervous system, by altering the secretion/release of prolactin, pro-opiomelanocortin, vasopressin, aldosterone, and renin, and by directly affecting renal function. Dopamine produced by the renal proximal tubule exerts an autocrine/paracrine action via two classes of dopamine receptors, D1-like (D1 and D5) and D2-like (D2, D3, and D4), that are differentially expressed along the nephron. The autocrine/paracrine function of dopamine, manifested by tubular rather than by haemodynamic mechanisms, becomes most evident during extracellular fluid volume expansion. This renal autocrine/paracrine function is lost in essential hypertension and in some animal models of genetic hypertension. The molecular basis for the dopaminergic dysfunction in hypertension may involve an abnormal post-translational modification of dopamine receptors.

Role of dopamine in the pathogenesis of hypertension

1. Dopamine, via different dopamine receptor subtypes, regulates cardiovascular functions by actions on the central and peripheral nervous systems, vascular smooth muscle, the heart and the kidney. The dopaminergic system in the central nervous system (CNS) may participate in the regulation of systemic blood pressure. 2. Dopamine 'D2-like' (D2, D3 and D4) receptors, rather than 'D1-like' (D1 and D5) receptors, are involved in the CNS regulation of blood pressure; post-synaptic D2-like receptors increase blood pressure, while presynaptic D2-like receptors (the predominant action) produce the opposite effect. 3. Outside the CNS, dopamine may regulate blood pressure via pressure controls that act with intermediate rapidity (e.g. stress relaxation, arginine vasopressin and renin-angiotensin vasoconstriction), as well as those systems related to the long-term control of body fluid volume. 4. Dopamine D1- and D2-like receptors have been described in resistance vessels, such as the renal, mesenteric, coronary, pulmonary and cerebral arteries. The ability of D1-like receptors to inhibit renal smooth muscle hypertrophy indicates their importance in longer-term regulation of blood pressure. 5. Aberrant dopaminergic regulation of aldosterone secretion, via D2-like receptors, has been reported to be involved in some forms of hyperaldosteronism and hypertension. Some forms of hypertension may also be caused by an aberrant renal dopaminergic system. Abnormalities of three aspects of the renal dopaminergic system may lead to hypertension: (i) renal production of dopamine; (ii) transduction of the renal vascular dopamine signal; and (iii) transduction of the renal tubular dopamine signal. 6. Thus, increased blood pressure occurs after either blockade of D1-like receptors or of dopamine production in rats or disruption of the D1 receptor or the D3 receptor gene in mice.

Renal dopamine receptors in health and hypertension

During the past decade, it has become evident that dopamine plays an important role in the regulation of renal function and blood pressure. Dopamine exerts its actions via a class of cell-surface receptors coupled to G-proteins that belong to the rhodopsin family. Dopamine receptors have been classified into two families based on pharmacologic and molecular cloning studies. In mammals, two D1-like receptors that have been cloned, the D1 and D5 receptors (known as D1A and D1B, respectively, in rodents), are linked to stimulation of adenylyl cyclase. Three D2-like receptors that have been cloned (D2, D3, and D4) are linked to inhibition of adenylyl cyclase and Ca2+ channels and stimulation of K+ channels. All the mammalian dopamine receptors, initially cloned from the brain, have been found to be expressed outside the central nervous system, in such sites as the adrenal gland, blood vessels, carotid body, intestines, heart, parathyroid gland, and the kidney and urinary tract. Dopamine receptor subtypes are differentially expressed along the nephron, where they regulate renal hemodynamics and electrolyte and water transport, as well as renin secretion. The ability of renal proximal tubules to produce dopamine and the presence of receptors in these tubules suggest that dopamine can act in an autocrine or paracrine fashion; this action becomes most evident during extracellular fluid volume expansion. This renal autocrine/paracrine function is lost in essential hypertension and in some animal models of genetic hypertension; disruption of the D1 or D3 receptor produces hypertension in mice. In humans with essential hypertension, renal dopamine production in response to sodium loading is often impaired and may contribute to the hypertension. The molecular basis for the dopaminergic dysfunction in hypertension is not known, but may involve an abnormal post-translational modification of the dopamine receptor.

Effects of costimulation of dopamine D1- and D2-like receptors on renal function

In vitro studies have suggested that dopamine D1- and D2-like receptors interact to inhibit renal sodium transport. We used Z-1046, a dopamine receptor agonist with the rank-order potency D3 >/= D4 > D2 > D5 > D1, to test the hypothesis that D1- and D2-like receptors interact to inhibit renal sodium transport in vivo in anesthetized rats. Increasing doses of Z-1046, administered via the right renal artery, increased renal blood flow (RBF), urine flow, and absolute and fractional sodium excretion without affecting glomerular filtration rate. For determination of the dopamine receptor involved in the renal functional effects of Z-1046, another group of rats received Z-1046 at 2 microgram . kg-1 . min-1 (n = 10) in the presence or absence of the D2-like receptor antagonist domperidone and/or the D1-like antagonist SCH-23390. Domperidone alone had no effect but blocked the Z-1046-mediated increase in urine flow and sodium excretion; it enhanced the increase in RBF after Z-1046. SCH-23390 by itself decreased urine flow and sodium excretion without affecting RBF and blocked the diuretic, natriuretic, and renal vasodilatory effect of Z-1046. We conclude that the renal vasodilatory effect of Z-1046 is D1-like receptor dependent, whereas the diuretic and natriuretic effects are both D1- and D2-like receptor dependent.

Disruption of the dopamine D3 receptor gene produces renin-dependent hypertension

Since dopamine receptors are important in the regulation of renal and cardiovascular function, we studied the cardiovascular consequences of the disruption of the D3 receptor, a member of the family of D2-like receptors, expressed in renal proximal tubules and juxtaglomerular cells. Systolic and diastolic blood pressures were higher (approximately 20 mmHg) in heterozygous and homozygous than in wild-type mice. An acute saline load increased urine flow rate and sodium excretion to a similar extent in wild-type and heterozygous mice but the increase was attenuated in homozygous mice. Renal renin activity was much greater in homozygous than in wild-type mice; values for heterozygous mice were intermediate. Blockade of angiotensin II subtype-1 receptors decreased systolic blood pressure for a longer duration in mutant than in wild-type mice. Thus, disruption of the D3 receptor increases renal renin production and produces renal sodium retention and renin-dependent hypertension.

Renal nerves and D1-dopamine receptor-mediated natriuresis

The resistance of the spontaneously hypertensive rat (SHR) kidney to the natriuretic effect of dopamine and D1 agonists may be due to increased renal nerve activity. Therefore, we compared the effects of the intrarenal arterial infusion of the D1 agonist, SKF 38383, into the denervated (DNX) kidney of saline-loaded-anesthetized SHR and its control, the Wistar-Kyoto (WKY) rat. In both WKY and SHR, DNX of the left kidney slightly decreased urine flow (UV) and absolute (UNaV) and fractional sodium excretion (FENa) in the innervated right kidney; neither vehicle nor D1 agonist infusion exerted any effect. In the left kidney, denervation increased UV, UNaV, and FENa to a similar degree in WKY and SHR (2-fold), without affecting renal blood flow, glomerular filtration rate, or blood pressure. In WKY but not in SHR, after DNX, the D1 agonist dose-dependently increased UV, UNaV, and FENa in the denervated kidney. We conclude that the decreased natriuretic effect of D1 agonists in the SHR is not due to increased renal nerve activity. These data support our previous studies implicating a defect of the D1 receptor or its regulation in the kidney in genetic hypertension.

Dopamine and diltiazem-induced natriuresis in the spontaneously hypertensive rat

An attenuated natriuretic response to dopamine and D1 agonists in genetic hypertension has been attributed to an uncoupling of the renal D1 dopamine receptor from its G protein-effector protein complex. We have reported that in normotensive Wistar-Kyoto (WKY) rats the natriuresis induced by calcium channel blockers is caused in part by activation of renal D1 dopamine receptors. We tested the interaction between the renal D1 receptor and a calcium channel blocker, diltiazem, infused into a renal artery of anesthetized spontaneously hypertensive rats (SHR) acutely loaded with 5% saline. Diltiazem produced a 50% increase in renal blood flow and nearly tripled absolute and fractional sodium excretion; urine flow rate more than doubled, but glomerular filtration rate did not change. However, the D1 receptor antagonist SKF-83742, which had no effect by itself, did not diminish the response to diltiazem. In a separate group of concurrent experiments, we found that the diltiazem-induced natriuresis was associated with a decrease in Na(+)-K(+)-adenosinetriphosphatase activity in the renal medulla of SHR. In contrast, in WKY rats, no changes were noted in the renal medulla but a decrease in Na(+)-K(+)-adenosinetriphosphatase activity was noted in the renal cortex. Diltiazem had no effect on urinary dopamine excretion in either rat strain. We conclude that diltiazem induces natriuresis differently in SHR and WKY rats; it is independent of D1 receptors in SHR and is in great part mediated by renal hemodynamic, rather than by cortical tubular, effects. These studies support previous findings of a defective renal cortical tubular D1 mechanism in SHR.

Transgenic mice to study the role of dopamine receptors in cardiovascular function

Dopamine, an intrarenal regulator of sodium transport, is important in the pathogenesis of hypertension. The transduction of D1-like receptors in renal proximal tubules is defective in animal models of genetic hypertension. The defect is associated with an impaired regulation of proximal tubular sodium transport and cosegregates with hypertension in rats. Moreover, mice lacking one or both D1A receptor alleles develop hypertension. Extrasynaptic D3 receptors in renal tubules and juxtaglomerular cells may also regulate renal sodium transport and renin secretion while presynaptic D3 receptors may act as autoreceptors to inhibit neural norepinephrine release. Mice lacking one or both D3 alleles have elevated systolic blood pressure and developed diastolic hypertension. Although basal urine flow, sodium excretion, and glomerular filtration rate are similar, mice homozygous to the D3 receptor have an impaired ability to excrete an acute saline load compared to heterozygous and wild type mice. These studies suggest that abnormalities in dopamine receptor genes or their regulation may lead to the development of hypertension via different pathogenetic mechanisms.

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.

Role of the D1A dopamine receptor in the pathogenesis of genetic hypertension

Since dopamine produced by the kidney is an intrarenal regulator of sodium transport, an abnormality of the dopaminergic system may be important in the pathogenesis of hypertension. In the spontaneously hypertensive rat (SHR), in spite of normal renal production of dopamine and receptor density, there is defective transduction of the D1 receptor signal in renal proximal tubules, resulting in decreased inhibition of sodium transport (Na+/H+ exchanger [NHE] and Na+/K+ATPase activity) by dopamine. To determine if impaired D1 receptor regulation of NHE in proximal tubules is related to hypertension, studies were performed in a F2 generation from female Wistar Kyoto (WKY) and male SHR crosses. A D1 agonist, SKF 81297, inhibited (37.6 +/- 4.7%) NHE activity in brush border membranes of normotensive F2s (systolic blood pressure < 140 mm Hg, n = 7) but not in hypertensive F2s (n = 21). Furthermore, a D1 agonist, SKF 38393, when infused into the renal artery, dose dependently increased sodium excretion in normotensive F2s (n = 3) without altering renal blood flow but was inactive in hypertensive F2s (n = 21). Since the major D1 receptor gene expressed in renal proximal tubules is the D1A subtype, we determined the importance of this gene in the control of blood pressure in mice lacking functional D1A receptors. Systolic blood pressure was greater in homozygous (n = 6) and heterozygous (n = 5) mice compared to normal sex matched litter mate controls (n = 12); moreover, the mice lacking one or both D1A alleles developed diastolic hypertension. The cosegregation with hypertension of an impaired D1 receptor regulation of renal sodium transport and the development of elevated systolic and diastolic pressure in mice lacking one or both D1A alleles suggest a causal relationship of the D1A receptor gene with hypertension.

Dopamine receptor signaling defects in spontaneous hypertension

Dopamine produced by renal proximal tubules acts as an intrarenal natriuretic factor by direct tubular action; this paracrine effect is influenced by the state of sodium balance. Up to 60% of sodium excretion with volume (2%-10%) expansion may be mediated by D1-like receptors. The renal paracrine effect of dopamine is impaired in genetic hypertension; this is due to defects in renal dopamine production or transduction of the dopamine signal. The Dahl salt sensitive rat and the spontaneously hypertensive rat (SHR), which have normal renal dopamine production and expression of dopamine receptors, have a defect in the coupling of a D1-like receptor to G-protein/effector enzyme complex. A consequence of the defective D1-like receptor/effector enzyme coupling in SHR is a decreased ability of D1 agonists to inhibit Na+/H+ exchange and Na+/K+-ATPase activity. The defect is 1) genetic, since it precedes the onset of and cosegregates with the hypertension; 2) receptor specific, since it is not shared by other humoral agents; and 3) confined to the renal proximal tubule. Two of the cloned dopamine receptors in mammals are D1-like (D1A and D1B). The D1A receptor gene is expressed to a greater extent in renal proximal tubules than the D1B receptor gene. The D1-like receptor is important in the pathogenesis of hypertension. Chronic blockade of dopamine receptors accelerates the development of hypertension in normotensive and hypertensive rats. Moreover, disruption of the D1A receptor gene in mice increases systolic blood pressure and results in diastolic hypertension. The abnormal D1-like receptor in SHR may be the D1A receptor; its uncoupling from the G-protein/effector enzyme complex in renal proximal tubules of SHR may be due to mistargeting. The mechanism for this "mistargeting" of the D1A receptor is not due to a mutation in the primary sequence and remains to be determined.

Dopamine D1 receptor regulation of phospholipase C

Dopamine is an endogenous catecholamine which exerts its actions by occupancy of specific receptors. Dopamine receptors are classified into two main groups: the two cloned D1-like receptors (D1A and D1B in rats; D1B is also known as D5 in humans) are linked to stimulation of adenylyl cyclase, while the three cloned D2-like receptors (D2 or D2A, D3 or D2B, D4 or D2C) are linked to inhibition of adenylyl cyclase. All these dopamine receptors originally cloned from the brain are expressed in tissues outside the central nervous system including the kidney. Dopamine regulates many cellular activities, including transmembrane ion transport. Activation of D1-like receptor decreases sodium transport by cAMP dependent and cAMP independent mechanisms. Dopamine, via D1-like receptors, may inhibit Na+/H+ exchange activity in renal brush border membranes by a cAMP independent/Gs alpha-linked mechanism. Another cAMP independent pathway of sodium transport inhibition is mediated by phospholipase C, which has several isoforms (PLC beta, PLC gamma, and PLC delta with several members in each). Catecholamines stimulate expression and activity of phospholipase C isoforms in a concentration, time, and receptor-dependent as well as regional and subcellular compartmental-specific manner. In renal cortical membranes, intrarenal administration of norepinephrine for 3-4 h increases PLC beta expression and activity but has no effect on PLC gamma activity. In contrast, intrarenal administration of a D1 agonist for 3-4 h increases PLC beta 1 but decreases PLC gamma expression and activity.(ABSTRACT TRUNCATED AT 250 WORDS)

A multicenter comparison of adverse reaction profiles of isradipine and enalapril at equipotent doses in patients with essential hypertension

A multicenter, randomized, double-blind trial compared the safety and efficacy of the dihydropyridine isradipine with the angiotensin-converting enzyme (ACE) inhibitor enalapril given twice daily for mild hypertension. 160 patients received either isradipine (starting at 1.25 mg twice daily) or enalapril (starting at 2.5 mg twice daily) for 10 weeks. The dosage was increased if the average sitting diastolic blood pressure was > 90 mm Hg. Significantly greater mean reductions in systolic blood pressure were seen after 2, 6, and 8 weeks of isradipine. However, by the end of the trial, 83% of patients receiving isradipine and 78% receiving enalapril showed a decrease of at least 5 mm Hg in sitting diastolic blood pressure to a level below 96 mm Hg. Possible or probable drug-related adverse effects were reported in 36% of patients showing a good antihypertensive response to isradipine, and in 30% of those who responded to enalapril. There was a trend for a lower frequency of adverse effects in isradipine non-responders (25%) and a higher frequency in enalapril non-responders (43%). Pruritus, dizziness, edema, and fatigue were reported more often with isradipine, and cough and changed bowel habits were more common with enalapril. The relationship between the pattern of adverse effects and the extent of blood pressure reduction may be dependent on the mechanism of action of a drug. In responders, isradipine and enalapril showed differing patterns, but a similar overall incidence of adverse effects.

Differential regulation of renal phospholipase C isoforms by catecholamines

Dopamine and D1 agonists and NE all increase phosphatidyl inositol-specific phospholipase C (PLC) activity, but whereas dopamine produces a natriuresis, NE has an antinatriuretic effect. To determine if catecholamines differentially regulate the expression of PLC isoforms, we infused fenoldopam, a D1 agonist, or pramipexole, a D1/D2 agonist, intravenously or infused fenoldopam or NE into the renal artery of anesthetized rats. After 3-4 h of infusion, when the expected natriuresis (fenoldopam or pramipexole) or antinatriuresis (NE) occurred, the kidneys were removed for analysis of PLC isoform protein expression activity. Western blot analysis revealed that in renal cortical membranes, fenoldopam and pramipexole increased expression of PLC beta 1 and decreased expression of PLC gamma 1; PLC delta was unchanged. In the cytosol, pramipexole and fenoldopam increased expression of both PLC beta 1 and PLC gamma 1. No effects were noted in the medulla. A preferential D1 antagonist, SKF 83742, which by itself had no effect, blocked the effects of pramipexole, thus confirming the involvement of the D1 receptor. In contrast, NE also increased PLC beta 1 but did not affect PLC gamma 1 protein expression in membranes. The changes in PLC isoform expression were accompanied by similar changes in PLC isoform activity. These studies demonstrate for the first time differential regulation of PLC isoforms by catecholamines.

Role of renal dopamine D1 receptors in natriuresis induced by calcium channel blockers

The direct tubular natriuretic effect of calcium channel blockers (CCBs) may be due to an interaction between CCBs and a renal tubular dopamine receptor. We therefore studied the effects of two chemically unrelated CCBs, diltiazem and isradipine, infused into the right renal artery of 5% saline-loaded anesthetized rats alone or in the presence of a D1 antagonist, SKF-83742. Isradipine (0.03 microgram.kg-1.min-1) or diltiazem (20 but not 10 micrograms.kg-1.min-1) alone produced an increase in urine flow and an approximate doubling of absolute and fractional sodium excretion, which was not seen in the left kidney or in the control animals (analysis of variance, Scheffé's test, P < 0.05). SKF-83742 alone given systemically or into the right renal artery did not affect these parameters but did block the actions of diltiazem or isradipine. There was no change in mean arterial pressure, renal blood flow, or glomerular filtration rate in any of the experiments. In additional studies, we found that a combined infusion of dopamine (0.1 microgram.kg-1.min-1) and diltiazem (10 micrograms.kg-1.min-1) (doses that by themselves did not alter renal function) produced a twofold or greater increase in urine flow and absolute and fractional sodium excretion; glomerular filtration rate was not significantly changed. Intrarenal arterial CCBs, without a change in renal hemodynamics, produce a natriuresis that is blocked by a D1 antagonist. Concomitant administration of diltiazem and dopamine (each in subeffective doses when used alone) produces a synergistic effect.(ABSTRACT TRUNCATED AT 250 WORDS)

Dopaminergic defect in hypertension

Reverse genetics and the candidate gene approach have been utilized to identify the genetic defect(s) in hypertension. We have proposed the dopamine receptor gene as one candidate in the pathogenesis of hypertension. Because some forms of hypertension are sodium dependent or aggravated by sodium loading and because dopamine is important in aiding the organism to eliminate "excess" sodium, an abnormality in the renal dopaminergic system may be responsible for the sodium retention in hypertension. Both human and animal models of hypertension are associated with renal dopamine production and/or post first messenger defects. The Dahl salt-sensitive rat, which has a decreased ability to generate renal dopamine, and the spontaneously hypertensive rat (SHR), which has no such limitation, have a defective coupling of a D1 receptor to a G protein/adenylyl cyclase complex. This coupling defect is: (1) genetic, since it precedes the onset of hypertension and co-segregates with the hypertensive phenotype, (2) receptor specific, since it is not shared by other humoral agents, and (3) organ and nephron segment selective, since it occurs in proximal tubules but not in cortical collecting ducts or the brain striatum. A consequence of the defective dopamine receptor/adenylyl cyclase coupling in the SHR is a decreased ability of D1 agonists to inhibit Na+/H+ exchange activity. A resistance to the natriuretic effect of dopamine and D1 agonists in the SHR is due mainly to decreased cyclic AMP production, although with maturation a post cyclic AMP defect is acquired. Radioligand binding studies suggest a "loss" of the high-affinity D1 binding site in the SHR.(ABSTRACT TRUNCATED AT 250 WORDS)

cAMP-independent, G protein-linked inhibition of Na+/H+ exchange in renal brush border by D1 dopamine agonists

When D1 dopamine agonists are incubated with renal cortical tissue, Na+/H+ exchange activity is inhibited, presumably due to D1 receptor-mediated stimulation of adenylyl cyclase and subsequent increase in protein kinase A activity. Although the role of adenosine 3',5'-cyclic monophosphate (cAMP) and cAMP-dependent protein kinase in the regulation of Na+/H+ exchange activity is well established, receptors functionally coupled to adenylyl cyclase can regulate Na+/H+ exchange activity independently of changes of cAMP accumulation. The current studies were designed to determine whether D1 agonists can inhibit Na+/H+ exchange activity independently of changes of cAMP accumulation and also to determine the role of G proteins in this process. The D1 agonist, fenoldopam, inhibited Na+/H+ exchange activity in a time-related and concentration-dependent manner. The 50% inhibitory concentration was 5-34 microM. Occupation of the renal D1 receptor mediates this action, since the D1 antagonist, SKF 83742, partially blocks the effect. This action, however, was independent of adenylyl cyclase, protein kinase A, and protein kinase C activity. Inhibition of adenylyl cyclase with dideoxyadenosine or inhibition of protein kinase A and C with the isoquinolines N-(2-guanidinoethyl)-5-isoquinolinesulfonamide hydrochloride (H-4) and 1-(5-isoquinolinesfulfonyl)-2-methylpiperazine (H-7) did not block the effect of fenoldopam on the exchanger. The action of fenoldopam is not due to an amiloride-like action on the exchanger, because kinetic analysis of the inhibitory action was noncompetitive and the effect of fenoldopam was time dependent. The process involved G proteins, since guanosine 5'-O-(2-thiodiphosphate) prevented while guanosine 5'-O-(3-thiotriphosphate) increased the inhibitory effect of fenoldopam.

Renal dopamine receptors and pre- and post-cAMP-mediated Na+ transport defect in spontaneously hypertensive rats

We have reported defective coupling of the renal tubular DA1 dopamine receptor to adenylyl cyclase in both the spontaneously hypertensive rat (SHR) and the Dahl salt-sensitive rat. Since Na+, 5'-guanyl imidodiphosphate [Gpp(NH)p], and N-ethylmaleimide (NEM) reduce agonist affinity for brain D1 dopamine receptors, we compared the effects of these agents on agonist affinity in proximal tubules from SHR and its normotensive control, the Wistar-Kyoto rat (WKY), to delineate further the site of the DA1-adenylyl cyclase coupling defect. In WKY, the D1/DA1 agonist, fenoldopam, competed for 125I-Sch 23982 at a high-affinity site (KiH = 1.8 +/- 0.8 x 10(-8) M) and a low-affinity site (KiL = 7.6 +/- 1.1 x 10(-5) M, n = 6). Na+ (150 mM) or Gpp(NH)p (10(-4) M) converted KiH to KiL. NEM, which alkylates sulfhydryl groups, also converted all the binding to KiL; this effect could be prevented by prior treatment with 10(-4) M fenoldopam. In contrast, in SHR, fenoldopam detected only a KiL (7.8 +/- 1.4 x 10(-5) M, n = 6). Neither Na+, Gpp(NH)p, nor NEM had any effect on KiL. To study a functional expression of these binding sites, the effect of 5 x 10(-5) M fenoldopam or 8-(chlorophenylthio)-adenosine 3',5'-cyclic monophosphate (8-CPT-cAMP) on Na+/H+ exchange activity in proximal tubular brush-border membrane vesicles was tested. In WKY, the inhibitory effects of these agents on the exchanger increased with the age of the rat.(ABSTRACT TRUNCATED AT 250 WORDS)

Ontogeny of DA1 receptor-mediated natriuresis in the rat: in vivo and in vitro correlations

The natriuretic and diuretic effects of dopamine are attenuated in the young. Because dopamine has actions on receptors (e.g., adrenergic, serotonin) other than dopamine, we studied a novel dopamine agonist, pramipexole, which has a selectivity to both DA1 and DA2-receptor subtypes. Intravenous administration of pramipexole resulted in a dose-related (1, 10, and 100 micrograms.kg-1.min-1) increase in urine flow and absolute and fractional sodium excretion and a decrease in mean arterial pressure (MAP) in three groups of rats studied. Pramipexole induced a greater decrease in MAP in 6- to 7- (n = 5) and 9- to 16- (n = 6) than in 3- to 4-wk-old (n = 8) rats; the natriuresis and diuresis were greatest in 12- to 16- and least in 3- to 4-wk-old rats. The renal effects of pramipexole were mainly due to actions at the DA1 receptor, since these effects were completely blocked by the coinfusion of a DA1 antagonist, SKF 83742. To explore further a cause of the attenuated natriuretic effect of pramipexole in the young, we studied the effect of a selective DA1-receptor agonist, fenoldopam, on amiloride-sensitive 22Na+ uptake in renal brush-border membrane vesicles. The 3-s amiloride-sensitive uptake was inhibited (45%) by fenoldopam (5 x 10(-5)M) in 9- to 16- (n = 6) but not in 3- to 4-wk-old (n = 5) rats. These studies suggest that the attenuated natriuretic effect of dopamine in the young is in part due to decreased DA1 action on the brush-border membrane Na(+)-H+ exchanger.

Contrasting effect of substance P on renal function and dopamine excretion in hydropaenic and volume expanded dogs

1. Substance P (SP) and dopaminergic nerves have been described in the kidney. In the brain, SP increases dopamine production. In the kidney, SP increases sodium excretion. 2. Intrarenal dopamine acts as an endogenous natriuretic hormone. It is possible that dopamine could mediate the natriuretic effect of SP. 3. We therefore studied the effect of the intrarenal arterial infusion of SP (0.1, 1.0, 10 ng kg body wt-1 min-1) on mean arterial pressure (MAP), renal blood flow (RBF), glomerular filtration rate (GFR), urine flow rate (V), absolute (UNaV) and fractional (FENa) sodium excretion as well as dopamine and noradrenaline excretion in dogs. Since dopamine is not natriuretic in hydropaenic states, studies were performed during hydropaenic and saline loaded states. 4. During hydropaenia, SP increased RBF, GFR, and V in a dose-related fashion but did not alter UNaV or FENa. Urinary noradrenaline was not affected but urinary dopamine decreased with increasing doses of SP. MAP was not affected. 5. During saline loading, SP increased RBF, GFR, V, UNaV, and FENa in a dose-related fashion. Both urinary noradrenaline and urinary dopamine increased. The fractional excretion of sodium correlated with dopamine but not noradrenaline excretion. MAP was not affected. 6. The renal haemodynamic and functional effects of SP may be mediated by SP-associated increases in urinary dopamine.

A multicenter comparison of the safety and efficacy of isradipine and enalapril in the treatment of hypertension

This multicenter trial compared the efficacy and safety of isradipine and enalapril in 160 patients with essential hypertension. Patients received isradipine or enalapril for 10 weeks after a placebo wash-out period of three to five weeks. Dosage was titrated for six weeks on the basis of blood pressure (BP) response and was then maintained for the remainder of the study. Isradipine reduced systolic and diastolic BP by 12 and 9 mm Hg, respectively, and enalapril by 10 and 7 mm Hg, respectively (between-treatment difference P less than .05 for diastolic BP). Overall, isradipine resulted in a higher responder rate, particularly among patients who had higher entry BPs. Fifteen enalapril-treated patients and four isradipine-treated patients discontinued treatment (four taking enalapril and none taking isradipine withdrew because of lack of efficacy). The most frequently reported adverse reactions were headache, dizziness, and edema in the isradipine group, and cough, headache, and chest pain in the enalapril group. Both drugs produced significant reductions in BP, but, in this study isradipine was more effective. The drugs were similarly well tolerated.

Effect of pramipexole, a dopamine-1/dopamine-2 receptor agonist, on sodium excretion and blood pressure in spontaneously hypertensive rats

1. Abnormalities of the renal dopaminergic system have been implicated in the pathogenesis of hypertension in the spontaneously hypertensive rat (SHR). 2. Both DA-1 and DA-2 receptors are present in renal tubules and blood vessels. DA-1 receptors mediate the renal vasodilatory and natriuretic effects of DA but the contribution of DA-2 receptors to these effects is not known. 3. We therefore studied the effect of a novel and selective DA-1 and DA-2 agonist, pramipexole, on MAP, glomerular filtration rate (GFR), urine flow (V), absolute (UNaV) and fractional sodium (FeNa) excretion in 9-18-week-old SHR. Wistar-Kyoto rats (WKY) served as control. 4. Pramipexole given intravenously (1, 10, 100 micrograms kg body wt-1 min-1) decreased MAP in a dose-related manner to a greater extent in SHR (n = 5) than WKY (n = 6) such that at the highest dose of pramipexole, MAP was similar in both groups. Pramipexole did not alter GFR in either WKY or SHR. Pramipexole increased V in a dose-related manner in both WKY and SHR. At 100 micrograms pramipexole kg body wt-1 min-1, V increased eightfold in both SHR and WKY. In contrast, pramipexole increased UNaV to a greater extent in WKY (5.1-fold) than SHR (3.7-fold). 5. These studies show a differential effect of pramipexole on renal function and MAP in SHR and WKY. Pramipexole has a more potent blood pressure lowering effect in SHR than in WKY. However, the natriuretic effect of pramipexole was greater in the WKY than in the SHR.(ABSTRACT TRUNCATED AT 250 WORDS)

Hypertension: racial differences

Racial differences in the prevalence, course, and pathophysiologic characteristics of hypertension in black and white populations are reviewed. Accumulated epidemiologic data indicate that the prevalence of hypertension among blacks is greater than that among whites in almost all age- and sex-matched groups. Hypertensive blacks have a higher incidence of left ventricular dysfunction, stroke, and renal damage, but a lower incidence of ischemic heart disease, than do hypertensive whites. A significant pathophysiologic difference between blacks and whites is salt sensitivity; normotensive, as well as hypertensive, blacks tend to be salt sensitive. Blacks also tend to have lower renin levels than do whites, while dopamine response to a salt load is diminished among blacks as compared with whites. These differences and others lead to the recommendation that hypertension among blacks should be managed initially with salt restriction; if dietary control is insufficient, administration of an antihypertensive agent with 24-hour efficacy, which lowers vascular peripheral resistance, promotes sodium excretion, and potentially improves renal hemodynamics, is recommended. A calcium channel blocker may satisfy these requirements.

The signal transducer for the dopamine-1 regulated sodium transport in renal cortical brush border membrane vesicles

We have reported the presence of dopamine-1 (DA-1) and dopamine-2 (DA-2) receptors in renal brush border and basolateral membranes. DA-1 agonists stimulate adenylate cyclase (AC) and phospholipase C (PLC) activity in both membranes. Moreover, the ability of a DA-1 agonist (fenoldopam) to stimulate PLC activity is independent of AC activity. A DA-2 agonist (LY171555) by itself was without effect and did not enhance the ability of the DA-1 agonist to stimulate PLC activity. The DA-1 but not DA-2 agonists inhibit Na+/H+ exchange activity in brush border membrane vesicles (BBMV) and Na+/K(+)-ATPase activity in basolateral membranes. However, cAMP inhibits, while protein kinase C (presumably via PLC activity) stimulates, Na+/H+ exchange activity. We therefore determined the effect of DA-1 agonists on Na+/H+ exchange activity when PLC or AC activity was blocked using neomycin or dideoxyadenosine, respectively. The drugs were incubated with minced renal cortex prior to preparation of BBMV by differential centrifugation and MnCl2 precipitation. Enrichment of BBMV was not affected by drug treatment. The Na+/H+ exchange activity was assessed by measuring amiloride (1 mmol/L) sensitive 22Na+ uptake in BBMV (pHi = 5.5, pHo = 7.5, Nai+ = O, Nao+ = 1 mmol/L). Neomycin inhibited DA and DA-1-stimulated PLC activity in BBMV in a concentration dependent manner (10(-6) to 10(-4) mol/L). Neomycin (10(-4) mol/L) completely blocked the ability of DA and DA-1 agonist to stimulate PLC activity but had no consistent effect on DA-1 inhibited Na+/H+ exchange activity. Dideoxyadenosine inhibited DA and DA-1 simulated AC activity without affecting DA-1 stimulated PLC activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Attenuated renal response to dopaminergic drugs in spontaneously hypertensive rats

Activation of renal dopamine-1 receptors decreases sodium transport. However, the spontaneously hypertensive rat retains sodium despite increased renal dopamine concentration. We tested the hypothesis that the abnormal sodium handling in spontaneously hypertensive rats (Okamoto-Aoki strain) is related to a decreased dopaminergic response by studying the effects of the intrarenal infusion of the dopamine-1 agonist SKF-38393 and the dopamine-1 antagonist SCH-23390 in hypertensive and in normotensive Wistar-Kyoto rats. Rats (9-16 weeks old) were studied with renal nerves intact under pentobarbital anesthesia (n = 5-6 in each group). Specificity of dopamine-1 effects of SKF-38393 was verified because its natriuretic effect was blocked in a dose-related manner by the dopamine-1 antagonist SCH-23390 (n = 5). Intrarenal but resulted in a dose-related natriuresis and diuresis in normotensive but not in hypertensive rats. Intrarenal arterial infusion of the dopamine-1 antagonist SCH-23390 alone induced an antinatriuresis, without affecting glomerular filtration rate, in normotensive but not in hypertensive rats. Addition of the dopamine-2 antagonist YM-09151 to the dopamine-1 antagonist infusion did not enhance the effect of the dopamine-1 antagonist. The lack of response to the dopamine-1 agonist or antagonist in hypertensive rats was not due to differences in renal dopamine-1 receptor density (1.3 +/- 0.3 pmol/mg protein for spontaneously hypertensive rats, n = 4; 1 +/- 0.2 for Wistar-Kyoto rats, n = 4) or affinity; distribution determined by autoradiography was also similar. The abnormal renal sodium handling in 9-16-week-old spontaneously hypertensive rats is in part due to decreased response distal to the dopamine-1 receptor.

The dopamine receptor in adult and maturing kidney

Dopamine, like other neurotransmitters, exerts its biological effects by occupation of specific receptor subtypes. The dopamine receptors in the central nervous system and certain endocrine organs are classified into the D1/D2 subtypes. Outside the central nervous system, the dopamine receptors are classified into the DA1/DA2 subtypes. The D1/D2 and DA1/DA2 receptor have marked similarities and some differences, the most notable of which is the lower affinity of the DA dopamine compared with the D dopamine receptor. DA1 receptor activation increases renal blood flow (RBF); stimulation of DA1 and DA2 receptors may also increase glomerular filtration rate (GFR). DA1 agonists inhibit fluid and electrolyte transport indirectly via hemodynamic mechanisms and directly by occupation of DA1 receptors in specific nephron segments. In the proximal tubule, DA1 agonists simulate adenylate cyclase and inhibit Na+-H+ antiport activity. They also increase phospholipase C and inhibit Na+-K+-ATPase activity (presumably as a consequence of protein kinase C activation). The latter effects may be facilitated by DA2 agonists. In cortical collecting ducts, dopamine antagonizes the effects of mineralocorticoids and the hydrosomotic effect of antidiuretic hormone. It has also been suggested that DA1 may also decrease sodium transport by influencing other hormones, such as atrial natriuretic peptide. Studies of dopamine in the young are complicated because of the propensity for dopamine to stimulate alpha-adrenoceptors. Dopamine alone may actually decrease RBF in the perinatal period. In some animals, the renal vasodilatory and natriuretic effects of dopamine increase with age. Renal tubular DA1-stimulated adenylate cyclase activity increases, whereas renal tubular DA1 receptors decrease with age. Renal DA2 receptor density is greater in the fetus; after birth renal DA2 receptors do not change. Endogenous dopamine may regulate sodium excretion in the young differently than in the adult. In the adult, sodium surfeit is associated with an increase in urinary dopamine; the opposite occurs in the young. A decrease in dopamine production or blockade of dopamine receptors results in an antinatriuresis in the adult; dopamine blockade in the young results in a natriuresis. It remains to be determined whether these age-related differences in dopamine effects are due to changes in receptor DA subtype density, second messengers, and/or interaction with other receptors.

Dopamine receptor subtypes in renal brush border and basolateral membranes

Dopamine (DA) modulates renal tubular sodium transport by actions at both brush border (BBM) and basolateral membranes (BLM). DA receptors have been demonstrated in proximal tubule but the subtype of DA receptor in either BBM or BLM has not been determined. DA-1 receptors were quantitated by the specific binding of 125I-SCH 23982, a DA-1 antagonist (defined by 20 microM SCH 23390, a DA-1 antagonist) and DA-2 receptors by the specific binding of 3H-methyl-spiroperidol or 3H-spiroperidol (defined by 30 microM trifluperidol, a predominantly DA-2 antagonist). The specific binding of 125I-SCH 23982 and 3H-methyl-spiroperidol or 3H-spiroperidol were saturable with time and ligand concentration and reversible. Analysis of Rosenthal plots by non-linear regression revealed a high affinity site and a very low affinity site for both BLM and BBM. Maximum receptor density was similar in BBM and BLM. Competition experiments with 125I-SCH 23982 revealed high and low affinity binding sites in both BBM and BLM. The high affinity site was characteristic of a DA-1 receptor. Competition experiments with 3H-spiroperidol were also suggestive of DA-2 receptors. DA-1 but not DA-2 drugs increased adenylate cyclase and phospholipase-C activities in both BBM and BLM. However, their effects were greater in BLM than BBM. We conclude that DA-1 and DA-2 receptors are present in both BBM and BLM in canine kidney. Renal DA-1 receptors are linked to stimulation of both adenylate cyclase and phospholipase-C activity.

Dopamine blockade attenuates the natriuresis of saline loading in the adrenalectomized rat

Dopamine blockade with the dopamine-1/dopamine-2 antagonist cis-flupenthixol (CF) attenuates sodium excretion associated with saline loading in both innervated and denervated kidneys of the rat. Blockade of adrenal dopamine-2 receptors prevents the decrease in aldosterone secretion associated with saline loading and may also be responsible for the reduction in sodium excretion induced by CF. Therefore, to determine the role of adrenal dopamine-2 receptors in the attenuated natriuresis induced by CF, the effect of CF or vehicle treatment was examined in two groups of saline-loaded rats 80 (group I) and 120 min (group II) after adrenalectomy. In both groups, CF decreased glomerular filtration rate and sodium excretion after adrenalectomy. It is concluded that dopamine blockade attenuates the natriuresis associated with sodium loading by a direct effect in the kidney.

Renal hemodynamics and natriuresis induced by the dopamine-1 agonist, SKF 82526

The intrarenal infusion of dopamine (DA) during alpha- and beta-adrenergic blockade has been reported to increase renal blood flow (RBF) and sodium excretion by occupation of DA-1 receptors. In addition, DA may potentially influence renal function by occupation of DA-2 receptor subtypes. This study was designed to examine the hemodynamic and/or tubular mechanisms of the natriuretic effect of DA-1 in dogs anesthetized with pentobarbital. The intrarenal infusion of the DA-1 agonist, SKF 82526 (10(-9), 10(-8), 10(-7) M), resulted in dose related increases in RBF and absolute and fractional sodium excretion. These changes were not associated with alterations in urinary prostaglandin E2, F2 alpha, or kallikrein excretion. To determine the role of RBF in the natriuresis due to SKF 82526 infusion (10(-7) M), the renal artery was constricted to return RBF to control levels during continued SKF 82526 infusion. Although absolute and fractional sodium excretion decreased during this maneuver, they remained higher than control. These studies support both a hemodynamic and a tubular mechanism for the natriuretic effect of the DA-1 agonist, SKF 82526. These effects do not appear to be mediated by the renal prostaglandin or kallikrein systems.

Dopamine receptors modulate sodium excretion in denervated kidney

Dopamine (DA) modulates sodium excretion by the innervated kidney. To examine the role of DA in the denervated (DNX) kidney the effects of the DA1/DA2 antagonist cis-flupenthixol (group 2, n = 7) (10 nmol X kg-1 X min-1), given intravenously in saline-loaded Wistar-Kyoto rats after acute unilateral left DNX, were compared with a placebo group (group 1, n = 7) and a group that received the DA1 antagonist SCH 23390 (group 3, n = 7) at 2.5 nmol X kg-1 X min-1. Pentobarbital sodium anesthesia was employed. Adequacy of DNX was assessed by a natriuresis and decrease in renal norepinephrine content in the DNX kidney and an antinatriuresis in the innervated right kidney. Mean arterial pressure slightly decreased in the placebo group (group 1, 106.7 +/- 2.2 vs. 99.3 +/- 2.4 mmHg) and after cis-flupenthixol (group 2, 108.8 +/- 2.7 vs. 92.8 +/- 1.8 mmHg) but not after SCH 23390 (group 3, 105.6 +/- 1.6 vs. 103 +/- 1.1 mmHg). Glomerular filtration rate was not affected by placebo or SCH 23390 in the DNX or innervated kidney but did slightly decrease after cis-flupenthixol in the DNX kidney. Sodium and water excretion after drug administration differed among the groups. In the DNX kidney urine flow decreased only in group 2, whereas fractional sodium excretion decreased modestly (P less than 0.05 paired t test) with SCH 23390 (3.53 +/- 0.34 vs. 2.89 +/- 0.20%) markedly with cis-flupenthixol (3.18 +/- 0.50 vs. 1.21 +/- 0.18%) and was unchanged in the placebo group (3.25 +/- 0.61 vs. 3.45 +/- 0.45%).(ABSTRACT TRUNCATED AT 250 WORDS)

Renal alpha-adrenoceptors and sodium excretion in the dog

The purpose of this study was to characterize the age-dependent role of alpha-adrenergic activity on renal vascular resistance (RVR) and renal blood flow (RBF) and the role of alpha 2-receptors in sodium transport. Intrarenal infusion of phentolamine in hydropenic anesthetized pups and adult dogs increased RBF and glomerular filtration rate (GFR) only in pups. The effect was most marked in the youngest pups (15.67 +/- 0.67 days). At 1.0 micrograms X kg-1 X min-1, phentolamine increased RBF from 1.38 +/- 0.04 to 1.64 +/- 0.04 ml X min-1 X g kidney wt-1 and GFR from 0.19 +/- 0.01 to 0.23 +/- 0.01 ml X min-1 X g kidney wt-1. Absolute (UNaV) and fractional (FENa) sodium excretions increased in all animals, but mean percent increases were greatest in adult dogs. Intrarenal yohimbine infusion in adult dogs (10-100 micrograms X kg-1 X min-1) produced a dose-related increase in UNaV and FENa without affecting RBF and GFR. UNaV increased from 0.22 +/- 0.05 to 0.54 +/- 0.12 mueq X min-1 X g kidney wt-1 and FENa increased from 0.32 +/- 0.05 to 0.63 +/- 0.06% at the dose of 100 micrograms X kg-1 X min-1. These studies confirm a modest role for alpha-adrenoreceptors in the high RVR characteristic of newborn pups and provide evidence for a role of alpha 2-adrenoceptors in the renal transport of sodium; the extent of the contribution of renal alpha-adrenergic system could not be tested in this experiment.

Cortical tubular and glomerular dopamine receptors in the rat kidney

Dopamine receptors in glomeruli and renal cortical tubules were characterized using radioligand binding and adenylate cyclase studies. The binding of [3H]haloperidol to glomeruli and tubules was rapid, saturable with time and ligand concentration, reversible, of high affinity, and demonstrated stereoselectivity and antagonist and agonist rank potency for binding to dopamine receptors. Analysis of kinetic data and Rosenthal plots in glomeruli revealed a single class of [3H]haloperidol binding sites with an apparent dissociation constant (Kd) of 6 nM and maximum receptor density (Bmax) of 0.42 pmol/mg protein. In tubules, at least two binding sites were noted, one with an apparent Kd of 38 nM and Bmax of 1.90 pmol/mg protein and another with an apparent Kd of 183 nM and Bmax of 3.50 pmol/mg protein. Dopamine and apomorphine increased adenylate cyclase in tubular membranes while no increases were noted in glomeruli. These studies suggest that glomeruli have D2 dopamine receptors, while renal cortical tubules contain the D1 dopamine receptor.

The influence of age on acute renal toxicity of uranyl nitrate in the dog

The influence of age upon uranyl nitrate (UN) induced acute renal failure (ARF) was evaluated in 30 canine puppies 1-2 wk and 3-5 wk old. Renal function and morphologic studies were performed 2 h (initiation phase) and 24 h (maintenance phase) after UN administration. Age-matched controls received vehicle alone. Administration of UN to 1-2-wk-old puppies produced no changes in whole kidney glomerular filtration rate (GFR), despite a significant reduction in renal plasma flow (RPF) (P less than 0.01). In contrast, during the maintenance phase, GFR was 60% lower than in the control group (P less than 0.02) whereas values for RPF were nearly identical to control values. In 3-5-wk-old puppies the magnitude of response to the heavy metal was much greater and GFR was nearly completely suppressed during the maintenance phase. This major alteration of GFR was independent of changes in RPF, because RPF remained similar to control values. Morphologic alterations consistent with the nephrotoxic effects of UN were observed in the proximal tubules of the most differentiated nephrons. These age-related morphologic alterations correlated well with the functional response (GFR) observed after UN administration, i.e., a proportionately greater degree of both morphologic and functional alterations followed the administration of the heavy metal in the oldest group of puppies.

Effects of dopamine blockade on renal sodium excretion

The renal responses to a specific dopamine antagonist (cis-flupentixol) and its stereoisomer (trans-flupentixol), a weak dopamine antagonist, were examined during hydropenia and Ringer loading in anesthetized rats. During hydropenia glomerular filtration (GFR), absolute (UNaV), and fractional (FENa) sodium excretion rates were similar as were single-nephron filtration (SNGFR) and proximal tubular flow rate (VTF). After Ringer loading GFR, UNaV, and FENa increased in all groups, but the increments were less in the cis-flupentixol than in the control or trans-flupentixol group. SNGFR and VTF increased similarly in all groups. In another series of experiments Ringer loading was performed prior to drug administration. Perfusion pressure (PP) was decreased in trans-flupentixol rats by aortic constriction to control for cis-flupentixol-induced reduction in PP. UNAV and FENa were lower in the cis-flupentixol- than trans-flupentixol-treated rats at comparable PP and GFR. In conclusion, dopamine blockade attenuated the natriuresis of Ringer loading; the mechanism is uncertain but may be related to a tubular effect at a site beyond the proximal convoluted tubule and/or in deeper nephrons.

Alpha-adrenoceptors in the developing kidney

The maturation of renal alpha-adrenoceptors was investigated in dogs. Alpha-adrenoceptors were characterized by radioligand binding using the alpha-1-adrenergic antagonist, [3H]-WB-4101. In renal plasma membranes obtained from the outer cortex of neonates, the binding of the ligand was rapid, reversible, of high affinity, saturable, and stereoselective. The competition studies with adrenergic agonists and antagonists were indicative of alpha-receptors. In puppies less than 1-w-old, the binding affinity of [3H]-WB-4101 (Kd = 0.638 +/- 0.17 nM) was greater than 3-5 week-old puppies (Kd 1.573 +/- 0.315 nm); (P less than 0.05), but receptor number was similar (63.75 +/- 19.1 versus 79.10 +/- 5.99 fmole/mg protein). No consistent specific binding could be demonstrated in plasma membranes obtained from inner cortex of medulla. In the adult dog, no consistent specific binding could be demonstrated for plasma membranes obtained from any region of the kidney. These studies indicate that during maturation there is a decrease in alpha-adrenoceptor density and affinity in membranes from the outer cortex of the kidney.

Ontogeny of myocardial adrenoceptors II. Alpha adrenoceptors

The maturation of myocardial alpha adrenoceptors was investigated in dogs. Alpha adrenoceptors were characterized by radioligand binding using [3H]-WB4101. In neonatal cardiac membranes, the binding of the ligand was rapid, reversible, of high affinity, saturable and stereoselective. The competition studies with adrenergic agonists and antagonists were indicative of alpha adrenoceptors. In adult dog cardiac membranes, consistent specific binding of the ligand could be demonstrated. These studies indicate that there is a decrease in alpha adrenergic receptor density of canine cardiac membranes during maturation.