Dopamine fails to inhibit renal tubular sodium pump in hypertensive rats

Diminished natriuretic response to dopamine in old rats is due to an impaired D1-like receptor-signaling pathway

BACKGROUND

Dopamine (DA) causes natriuresis and diuresis, which results from activation of D1-like receptor (D1R) located on proximal tubules. Earlier, we reported that DA failed to inhibit Na,K-ATPase in proximal tubules of old Fischer 344 rats. The present study was designed to investigate the functional consequence of this phenomenon.

METHODS

Measurements of the functional (natriuretic and diuretic) response to intravenously infused DA and SKF 38393 (D1R agonist) in adult (6 month) and old (24 month) Fischer 344 rats were taken. Biochemical measurements were carried out to determine the potential defects in D1R and its signaling pathway in proximal tubules of old rats.

RESULTS

We found that intravenous infusion of DA and SKF 38393 caused natriuresis and diuresis in adult rats, but this response was blunted in old rats. In the isolated proximal tubules, DA and SKF 38393 inhibited Na,H-exchanger (NHE) in adult rats; however, this inhibition was attenuated in old rats. Radioligand binding revealed approximately 46% reduction in D1R binding sites in brush border membranes (BBMs) in old compared with adult rats. SKF 38393 stimulated [35S]GTPgammaS binding in BBM in adult rats, but not in old rats, suggesting an impaired D1R-G protein coupling. DA and SKF 38393 stimulated adenylyl cyclase (AC) activity in adult but not in the old rats. Forskolin and NaF stimulated AC activity in a comparable manner in adult and old rats, indicating no defect in AC and G proteins. DA and SKF 38393 failed to stimulate protein kinase A (PKA) activity in proximal tubules of old rats. Dibutyryl-cAMP-mediated PKA activation was also absent in old rats.

CONCLUSIONS

A decrease in D1R binding sites, a coupling defect with G proteins, and a defect in PKA activation lead to diminished DA-mediated inhibition of NHE in old rats, which may contribute to the blunted natriuretic response to DA in these animals.

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

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

Phosphoinositide-3 kinase binds to a proline-rich motif in the Na+, K+-ATPase alpha subunit and regulates its trafficking

Endocytosis of Na(+),K(+)-ATPase molecules in response to G protein-coupled receptor stimulation requires activation of class I(A) phosphoinositide-3 kinase (PI3K-I(A)) in a protein kinase C-dependent manner. In this paper, we report that PI3K-I(A), through its p85alpha subunit-SH3 domain, binds to a proline-rich region in the Na(+),K(+)-ATPase catalytic alpha subunit. This interaction is enhanced by protein kinase C-dependent phosphorylation of a serine residue that flanks the proline-rich motif in the Na(+),K(+)-ATPase alpha subunit and results in increased PI3K-I(A) activity, an effect necessary for adaptor protein 2 binding and clathrin recruitment. Thus, Ser-phosphorylation of the Na(+),K(+)-ATPase catalytic subunit serves as an anchor signal for regulating the location of PI3K-I(A) and its activation during Na(+),K(+)-ATPase endocytosis in response to G protein-coupled receptor signals.

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

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

Renal dopaminergic mechanisms and hypertension: a chronology of advances

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

Biphasic effects of dopamine on 86rubidium uptake in rat renal proximal tubules

The mechanism(s) by which dopamine inhibits Na+-K+-ATPase activity in the renal proximal tubule is still controversial. We studied the short-term effects of dopamine on the sodium pump in rat renal proximal tubule suspensions with the 86Rb uptake method. Dopamine and the D1-like agonist, SKF81297, initially stimulated Na+-K+-ATPase activity at 5 min and subsequently inhibited it at 10 min and 20 min; the inhibition by 10 microM dopamine at 20 min was 21.3 +/- 4.5%. The inhibitory effect of dopamine on Na+-K+-ATPase activity was mimicked by thymeleatoxin (a classical protein kinase C [PKC] agonist) while Sp-8-CPT-cAMPS (a protein kinase A [PKA] agonist) had no effect. However, the combination of the PKC and PKA agonists mimicked the biphasic effects of dopamine and SKF81297. Rp-8-CPT-cAMPS (a PKA inhibitor), U-73122 (a phospholipase C inhibitor), or calphostin C (a PKC inhibitor), blocked the dopamine-mediated biphasic effects on Na+-K+-ATPase activity. It is suggested that the biphasic effects of dopamine on Na+-K+-ATPase activity (an initial stimulation and a subsequent inhibition) are transduced by activating both PKA and PKC through a D1-like receptor.

Dopamine inhibits renal Na+:HCO3- cotransporter in rabbits and normotensive rats but not in spontaneously hypertensive rats

BACKGROUND

Dopamine (DA) is thought to regulate renal proximal transport through the inhibition of the Na+,K+-ATPase and/or Na+/H+ exchanger. Defects in this dopaminergic system are proposed to be a pathogenic factor of genetic hypertension. However, microperfusion studies have not consistently confirmed direct tubular effects of DA.

METHODS

Isolated proximal straight tubules were perfused peritubularly with Dulbecco's modified Eagle's tissue culture medium (DMEM) containing norepinephrine (NE) to improve incubation conditions. Intracellular Na+ concentrations ([Na+]i) and cell pH (pHi) were measured with fluorescence probes.

RESULTS

When incubated in DMEM plus NE, DA increased [Na+]i in rabbit tubules. Inhibition of Na+,K+-ATPase could not explain this response, as it was not suppressed by ouabain. An analysis of pHi responses to bath HCO3- reduction revealed that DA, SKF 38393 (a DA1 agonist), and adenosine 3',5'-cyclic monophosphate (cAMP) inhibited the basolateral Na+:HCO3- cotransporter in rabbit and Wistar-Kyoto rat (WKY), if its transport stoichiometry was converted to 3 HCO3-:1 Na+ by DMEM plus NE incubation. The inhibitory effect of DA was abolished by SCH 23390, a DA1 antagonist, but not by (-)-sulpiride, a DA2 antagonist. In spontaneously hypertensive rats (SHRs), however, DA and SKF 38393 failed to inhibit the cotransporter, although the inhibitory effects of cAMP and parathyroid hormone were comparable to those in WKY.

CONCLUSION

These results indicate that DA inhibits the Na+:HCO3- cotransporter in renal proximal tubules and also suggest that dysregulation of the cotransporter, possibly through the defect in DA1 receptor signaling, could play an important role in development of hypertension in SHRs.

Defective renal dopamine D1-like receptor signal transduction in obese hypertensive rats

It is reported that dopamine promotes renal sodium excretion via activation of D1-like dopamine receptors located on the proximal tubules. In spontaneously hypertensive rats the natriuretic and diuretic response to exogenously administered and endogenously produced dopamine is reduced, which results from a diminished dopamine-induced inhibition of the enzyme, Na+,K+-ATPase. The present study was designed to examine dopamine-receptor mediated inhibition of Na+,K+-ATPase and its associated signal transduction pathway in the proximal tubules of Zucker obese and lean control rats. The obese animals were hypertensive, hyperinsulinaemic and hyperglycaemic compared with the lean rats. While dopamine caused inhibition of Na+,K+-ATPase activity in lean rats, this effect was significantly attenuated in the obese animals. There was significant reduction in D1-like receptor numbers in the basolateral membranes of obese rats compared with lean rats with no change in the affinity to the ligand [3H]SCH 23390 between the two groups of rats. Dopamine failed to stimulate G proteins as measured by [35S]GTPgammaS binding in the obese rats. Also, dopamine was unable to cause phospholipase-C activation in obese rats, but it did activate phospholipase-C in lean rats. These results show that reduction in D1-like receptor numbers and a defect in receptor-G protein coupling may account for the inability of dopamine to activate the D1-like receptor-coupled signal transduction pathway and cause inhibition of Na+,K+-ATPase in the obese hypertensive rats.

Intrarenal dopamine coordinates the effect of antinatriuretic and natriuretic factors

The precision by which sodium balance is regulated suggests an intricate interaction between modulatory factors released from intra- and extrarenal sources. Intrarenally produced dopamine has a central role in this interactive network. Dopamine, produced in renal tubular cells acts as an autocrine and paracrine factor to inhibit the activity of Na+,K+-ATPase as well as of a number of sodium influx pathways. The natriuretic effect of dopamine is most prominent under high salt diet. The antinatriuretic effects of noradrenaline, acting on alpha-adrenoceptors and angiotensin II are opposed by dopamine as well as by atrial natriuretic peptide (ANP). Several lines of evidence have suggested that ANP acts via the renal dopamine system and recent studies from our laboratory have shown that this effect is attributed to recruitment of silent D1 receptors from the interior of the cell towards the plasma membrane. Taken together, the observations suggest that dopamine coordinates the effects of antinatriuretic and natriuretic factors and indicate that an intact renal dopamine system is of major importance for the maintenance of sodium homeostasis and normal blood pressure.

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.

Defective dopamine receptor function in proximal tubules of obese zucker rats

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

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

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

Ontogenetic aspects of hypertension development: analysis in the rat

In this review, we attempt to outline the age-dependent interactions of principal systems controlling the structure and function of the cardiovascular system in immature rats developing hypertension. We focus our attention on the cardiovascular effects of various pharmacological, nutritional, and behavioral interventions applied at different stages of ontogeny. Several distinct critical periods (developmental windows), in which particular stimuli affect the further development of the cardiovascular phenotype, are specified in the rat. It is evident that short-term transient treatment of genetically hypertensive rats with certain antihypertensive drugs in prepuberty and puberty (at the age of 4-10 wk) has long-term beneficial effects on further development of their cardiovascular apparatus. This juvenile critical period coincides with the period of high susceptibility to the hypertensive effects of increased salt intake. If the hypertensive process develops after this critical period (due to early antihypertensive treatment or late administration of certain hypertensive stimuli, e.g., high salt intake), blood pressure elevation, cardiovascular hypertrophy, connective tissue accumulation, and end-organ damage are considerably attenuated compared with rats developing hypertension during the juvenile critical period. As far as the role of various electrolytes in blood pressure modulation is concerned, prohypertensive effects of dietary Na+ and antihypertensive effects of dietary Ca2+ are enhanced in immature animals, whereas vascular protective and antihypertensive effects of dietary K+ are almost independent of age. At a given level of dietary electrolyte intake, the balance between dietary carbohydrate and fat intake can modify blood pressure even in rats with established hypertension, but dietary protein intake affects the blood pressure development in immature animals only. Dietary protein restriction during gestation, as well as altered mother-offspring interactions in the suckling period, might have important long-term hypertensive consequences. The critical periods (developmental windows) should be respected in the future pharmacological or gene therapy of human hypertension.

Impaired renal vascular response to a D1-like receptor agonist but not to an ACE inhibitor in conscious spontaneously hypertensive rats

The natriuretic response to a dopamine 1-like receptor agonist is blunted in spontaneously hypertensive rats (SHRs). Whether the renal vasodilator response to D1-like receptor stimulation in SHRs is defective also is unclear. To determine whether the renal hemodynamic response to a D1-like receptor is impaired in SHR, we examined the effect of a continuous infusion of the D1-like receptor agonist fenoldopam (2 microg/kg/min) on systemic and renal hemodynamics in conscious SHRs and Wistar-Kyoto (WKY) rats. As an active control, we used an equivalent antihypertensive dosage of captopril (10 mg/kg). Fenoldopam significantly increased effective renal plasma flow (ERPF) in WKY rats (+22 +/- 5%; p < 0.01), whereas this response was absent in SHRs (+7 +/- 3%; NS). Mean arterial pressure (MAP) was significantly reduced in SHRs (-11 +/- 2%; p < 0.001), demonstrating a systemic vasodilator response to fenoldopam in SHRs. The reduction in renal vascular resistance (RVR) was more pronounced in WKY rats (-24 +/- 2%) than in SHRs (-13 +/- 4%; p < 0.05). Captopril significantly increased ERPF in SHRs (+16 +/- 3%; p < 0.001), demonstrating a preserved renal vasodilatory capacity in SHRs. The blunting of the renal vasodilatory response to fenoldopam in SHRs is present during a high as well as a low sodium intake. In conscious SHRs, the renal vasodilatory response to a D1-like receptor agonist is impaired, whereas the blood pressure response is more pronounced. The preserved renal vasodilatory response to captopril indicates that the defective vasodilatory response in SHRs is functional rather than due to altered structural properties of the renal vascular bed.

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

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

Bromocriptine regulates angiotensin II response on sodium pump in proximal tubules

-Dopamine and angiotensin II (Ang II) receptors have been reported to exhibit an interaction in renal proximal tubules. The present study was designed to investigate the regulation by a D2-like dopamine receptor of Ang II-mediated stimulation of Na,K-ATPase activity in the renal proximal tubules. Ang II (10(-13) to 10(-9) mol/L) stimulated Na,K-ATPase activity in the proximal tubules that was completely abolished when the tubules were pretreated with the D2-like receptor agonist bromocriptine (1 micromol/L) for 30 minutes. The effect of bromocriptine on Ang II response was prevented by domperidone (1 micromol/L), a D2-like dopamine receptor antagonist. Similarly, the inhibition of forskolin (1 micromol/L)-induced cAMP accumulation caused by Ang II (10 pmol/L) was also abolished in bromocriptine-pretreated tubules. Basal and forskolin-stimulated cAMP was not significantly different in bromocriptine-treated tubules compared with the control. [3H]-Ang II binding sites (angiotensin type 1 [AT1] receptors) were reduced by approximately 65% in bromocriptine-treated proximal tubules, a result that was further substantiated by Western blot analysis revealing a 50% decrease in AT1 receptors in bromocriptine-treated tubules compared with the control. Western blot analysis of G proteins revealed a 2-fold increase in Gsalpha and a 20% decrease in Gialpha1 and Gialpha2 in the bromocriptine-treated proximal tubules. Bromocriptine (1 micromol/L) alone stimulated Na,K-ATPase activity during the first 30 minutes of incubation, and thereafter the stimulation fell to the basal level. Similarly, bromocriptine-mediated inhibition of cAMP lasted only up to 20 minutes. The data suggest that preactivation of D2-like dopamine receptors abolishes Ang II-mediated stimulation of Na,K-ATPase activity and inhibition of cAMP accumulation. This phenomenon may be a consequence of a decrease in AT1 receptors and alterations in G protein levels in the proximal tubules. We propose that such a regulation of Ang II response by bromocriptine is the result of heterologous desensitization of the D2-like receptor system.

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.

The action of inhibitors of protein kinases on fluid and ion secretion by Malpighian tubules of Locusta migratoria, L

Fluid production in Locusta Malpighian tubules was stimulated by corpora cardiaca extract (c. 100%) and dibutyryl cAMP (c. 50%). Chelerythrine and staurosporine (Protein kinase C, PKC inhibitors) inhibited it in the range 0.07-60&mgr;M (IC(50)3&mgr;M), whereas Rp-cAMP (Protein kinase A, PKA inhibitor) caused inhibition over the concentration range 10-1000&mgr;M (IC(50)264&mgr;M). The protein phosphatase inhibitor, okadaic acid, was also inhibitory over the concentration range 0.1-1000nM (IC(50) 91nM). CC extract stimulation increased fluid [Na(+)] from 41 to 59mM and decreased [K(+)] from 127 to 107mM; stimulation with cAMP had no such effect. The PKC inhibitors reduced the [K(+)] in the secreted fluid from 126 to 107mM but had no effect on the [Na(+)]. Subsequent addition of CC extract stimulated fluid production and caused an increase in [Na(+)] from 41 to about 50mM. The addition of Rp-cAMP reduced fluid production but caused a decrease in [Na(+)] from 37 to 28mM and an increase in its [K(+)] from 124 to 148mM. Fluid production by Rp-cAMP inhibited tubules was not stimulated by corpora cardiaca extract or cAMP, but [Na(+)] rose to 36mM. Protein phosphorylation plays a role in the regulation of fluid production probably via the apical and basal membrane cation transporters.

Renal dopamine receptor function in hypertension

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

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.

Phosphatidylinositol 3-kinase-mediated endocytosis of renal Na+, K+-ATPase alpha subunit in response to dopamine

Dopamine (DA) inhibition of Na+,K+-ATPase in proximal tubule cells is associated with increased endocytosis of its alpha and beta subunits into early and late endosomes via a clathrin vesicle-dependent pathway. In this report we evaluated intracellular signals that could trigger this mechanism, specifically the role of phosphatidylinositol 3-kinase (PI 3-K), the activation of which initiates vesicular trafficking and targeting of proteins to specific cell compartments. DA stimulated PI 3-K activity in a time- and dose-dependent manner, and this effect was markedly blunted by wortmannin and LY 294002. Endocytosis of the Na+,K+-ATPase alpha subunit in response to DA was also inhibited in dose-dependent manner by wortmannin and LY 294002. Activation of PI 3-K generally occurs by association with tyrosine kinase receptors. However, in this study immunoprecipitation with a phosphotyrosine antibody did not reveal PI 3-K activity. DA-stimulated endocytosis of Na+, K+-ATPase alpha subunits required protein kinase C, and the ability of DA to stimulate PI 3-K was blocked by specific protein kinase C inhibitors. Activation of PI 3-K is mediated via the D1 receptor subtype and the sequential activation of phospholipase A2, arachidonic acid, and protein kinase C. The results indicate a key role for activation of PI 3-K in the endocytic sequence that leads to internalization of Na+,K+-ATPase alpha subunits in response to DA, and suggest a mechanism for the participation of protein kinase C in this process.

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

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

Renal substance P-containing neurons and substance P receptors impaired in hypertension

In normotensive rats, increased renal pelvic pressure stimulates the release of prostaglandin E and substance P, which in turn leads to an increase in afferent renal nerve activity (ARNA) and a contralateral natriuresis, a contralateral inhibitory renorenal reflex. In spontaneously hypertensive rats (SHR), increasing renal pelvic pressure failed to increase afferent renal nerve activity. The inhibitory nature of renorenal reflexes indicates that impaired renorenal reflexes could contribute to increased sodium retention in SHR. Phorbol esters, known to activate protein kinase C, increase afferent renal nerve activity in Wistar-Kyoto rats (WKY) but not in SHR. We examined the mechanisms involved in the impaired responses to renal sensory receptor activation in SHR. The phorbol ester 4beta-phorbol 12,13-dibutyrate increased renal pelvic protein kinase C activity similarly in SHR and WKY. Increasing renal pelvic pressure increased afferent renal nerve activity in WKY (27+/-2%) but not in SHR. Renal pelvic release of prostaglandin E increased similarly in WKY and SHR, from 0.8+/-0.1 to 2.0+/-0.4 ng/min and 0.7+/-0.1 to 1.4+/-0.2 ng/min. Renal pelvic release of substance P was greater (P<.01) in WKY, from 16.3+/-3.8 to 41.8+/-7.4 pg/min, than in SHR, from 9.9+/-1.7 to 17.0+/-3.2 pg/min. In WKY, renal pelvic administration of substance P at 0.8, 4, and 20 microg/mL increased ARNA 382+/-69, 750+/-233, and 783+/-124% second (area under the curve of afferent renal nerve activity versus time). In SHR, substance P at 0.8 to 20 microg/mL failed to increase ARNA. These findings demonstrate that the impaired afferent renal nerve activity response to increased renal pelvic pressure is related to decreased release of substance P and/or impaired activation of substance P receptors.

Young SHR express increased type 1 angiotensin II receptors in renal proximal tubule

A potential role for the renin-angiotensin system (RAS) in the development and/or maintenance of hypertension in the genetic model of rat hypertension, spontaneously hypertensive rats (SHR), has been suggested by studies indicating that treatment of immature animals with angiotensin-converting enzyme (ACE) inhibitors prevents subsequent development of hypertension. Because young SHR also demonstrate RAS-dependent increased sodium retention, we examined proximal tubule type 1 angiotensin II receptor (AT1R) mRNA expression in young (4 wk) or adult (14 wk) SHR compared with age-matched Wistar-Kyoto (WKY) rats. Proximal tubules were isolated by Percoll gradient centrifugation, and AT1R mRNA expression was measured by quantitative reverse transcription-polymerase chain reaction (RT-PCR). At 14 wk, when SHR had established hypertension [mean arterial blood pressure (MAP) of SHR vs. WKY: 145 +/- 6 vs. 85 +/- 5 mmHg, n = 14-15], there were no differences in proximal tubule AT1R mRNA levels [SHR vs. WKY: 79 +/- 14 vs. 72 +/- 14 counts/min (cpm) per cpm mutant AT1R per cpm beta-actin x 10(-6), n = 6; not significant (NS)]. In contrast, in 4 wk SHR, at a time of minimal elevations in blood pressure (MAP: 70 +/- 8 vs. 63 +/- 3), SHR proximal tubule AT1R mRNA levels were 263 +/- 30% that of WKY (143 +/- 18 vs. 60 +/- 11 cpm per cpm of mutant AT1R per cpm beta-actin x 10(-6), n = 8; P < 0.005). We have recently shown that chronic ACE inhibition decreases proximal tubule AT1R expression and have also shown that chronic L-3,4-dihydroxyphenylalamine (L-DOPA) administration inhibits AT1R expression in adult Sprague-Dawley proximal tubule and cultured proximal tubule, and this inhibition is mediated via Gs-coupled DA1 receptors. When 3-wk-old animals were given L-DOPA or captopril for 1 wk, MAP was not altered (70 +/- 8 vs. 60 +/- 4 or 61 +/- 5 mmHg), but proximal tubule AT1R mRNA was no longer significantly different between SHR and WKY (68 +/- 9 vs. 38 +/- 7 or 20 +/- 3 vs. 47 +/- 15 cpm per cpm of mutant AT1R per cpm beta-actin x 10(-6)), due to a significant decrease in proximal tubule AT1R expression in SHR (P < 0.005, compared with untreated SHR). Immunoreactive proximal tubule AT1R expression also was increased in 4 wk SHR and was reversed with captopril or L-DOPA treatment. Therefore, these results indicate that young, but not adult, SHR have increased expression of proximal tubule AT1R and that chronic L-DOPA or captopril treatment decreased the elevated AT1R expression to control levels. These results provide further support for an important role of the RAS in the development of hypertension in SHR.

Dopamine receptors: from structure to function

The diverse physiological actions of dopamine are mediated by at least five distinct G protein-coupled receptor subtypes. Two D1-like receptor subtypes (D1 and D5) couple to the G protein Gs and activate adenylyl cyclase. The other receptor subtypes belong to the D2-like subfamily (D2, D3, and D4) and are prototypic of G protein-coupled receptors that inhibit adenylyl cyclase and activate K+ channels. The genes for the D1 and D5 receptors are intronless, but pseudogenes of the D5 exist. The D2 and D3 receptors vary in certain tissues and species as a result of alternative splicing, and the human D4 receptor gene exhibits extensive polymorphic variation. In the central nervous system, dopamine receptors are widely expressed because they are involved in the control of locomotion, cognition, emotion, and affect as well as neuroendocrine secretion. In the periphery, dopamine receptors are present more prominently in kidney, vasculature, and pituitary, where they affect mainly sodium homeostasis, vascular tone, and hormone secretion. Numerous genetic linkage analysis studies have failed so far to reveal unequivocal evidence for the involvement of one of these receptors in the etiology of various central nervous system disorders. However, targeted deletion of several of these dopamine receptor genes in mice should provide valuable information about their physiological functions.

Kidney extracts from spontaneously hypertensive rats (SHR) have greater dopamine 1A receptor RNA-binding activity than extracts from normotensive Wistar-Kyoto (WKY) rats

Rat kidney extracts contain a 52 kDa protein that binds to the 3' untranslated region of the dopamine 1A (D1A) receptor mRNA at a 243 base-long cis element starting at the stop codon and ending approximately 220 bases upstream of an AUUUA-rich region. The D1A receptor RNA-binding protein (D1A-BP) is redox-sensitive; free sulfhydryl groups on the protein are required for binding. Kidney extracts from SHR have significantly more D1A-BP activity than extracts from WKY rats. When kidney extracts were tested for binding to an 80-base RNA containing four AUUUA repeats, there was also greater binding activity in extracts from SHR. These increases are at least partly specific because there was no difference in catalase RNA-binding protein activity between the two rat strains. These data suggest D1A-BP and AUUUA-binding protein may play a role in posttranscriptional regulation of the D1A receptor in the hypertensive rat.

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

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

Altered arachidonic acid metabolism contributes to the failure of dopamine to inhibit Na+,K(+)-ATPase in kidney of spontaneously hypertensive rats

Dopamine decreases tubular sodium reabsorption in part by inhibition of Na+,K(+)-ATPase activity in renal proximal tubules. The signaling mechanism involved in dopamine-mediated inhibition of Na+,K(+)-ATPase is known to be defective in spontaneously hypertensive animals. The present study was designed to evaluate the role of phospholipase A2 (PLA2) and its metabolic pathway in dopamine-induced inhibition of Na+,K(+)-ATPase in renal proximal tubules from Wistar-Kyoto (WKY) rats and spontaneously hypertensive rats (SHR). Renal proximal tubular suspensions were prepared and Na+,K(+)-ATPase activity was measured as ouabain-sensitive adenosine triphosphate hydrolysis. Dopamine inhibited Na+,K(+)-ATPase activity in a concentration (1 nM-10 microM)-dependent manner in WKY rats while it failed to inhibit the enzyme activity in SHR. Dopamine (10 microM)-induced inhibition of Na+,K(+)-ATPase activity in WKY rats was significantly blocked by mepacrine (10 microM), a PLA2 inhibitor, suggesting the involvement of PLA2 in dopamine-mediated inhibition of Na+,K(+)-ATPase. Arachidonic acid (a product released by PLA2 action) inhibited Na+,K(+)-ATPase in a concentration-dependent (1-100 microM) manner in WKY rats while the inhibition in SHR was significantly attenuated (IC50: 7.5 and 80 microM in WKY rats and SHR, respectively). Furthermore, lower concentrations of arachidonic acid stimulated (30% at 1 microM) Na+,K(+)-ATPase activity in SHR. This suggests a defect in the metabolism of arachidonic acid in SHR. Proadifen (10 microM), an inhibitor of cytochrome P-450 monoxygenase (an arachidonic acid metabolizing enzyme) significantly blocked the inhibition produced by arachidonic acid in WKY rats and abolished the difference in arachidonic acid inhibition of Na+,K(+)-ATPase between WKY rats and SHR. These data suggest that PLA2 is involved in dopamine-induced inhibition of Na+,K(+)-ATPase and altered arachidonic acid metabolism may contribute to reduced dopaminergic inhibition of Na+,K(+)-ATPase activity in spontaneously hypertensive rats.

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.

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.

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.

Bradykinin and protein kinase C activation fail to stimulate renal sensory neurons in hypertensive rats

In normotensive rats, renal sensory receptor activation by increased ureteral pressure results in increased ipsilateral afferent renal nerve activity, decreased contralateral efferent renal nerve activity, and contralateral diuresis and natriuresis, a contralateral inhibitory renorenal reflex response. In spontaneously hypertensive rats (SHR), increasing ureteral pressure fails to increase afferent renal nerve activity. The nature of the inhibitory renorenal reflexes indicates that an impairment of the renorenal reflexes would contribute to the increased efferent renal nerve activity in SHR. We therefore examined whether there was a general decrease in the responsiveness of renal sensory receptors in SHR by comparing the afferent renal nerve activity responses to bradykinin in SHR and Wistar-Kyoto rats (WKY). In WKY, renal pelvic perfusion with bradykinin at 4, 19, 95, and 475 micromol/L increased afferent renal nerve activity by 1066 +/- 704, 2127 +/- 1121, 3517 +/- 1225, and 4476 +/- 1631% x second (area under the curve of afferent renal nerve activity versus time). In SHR, bradykinin at 4 to 95 micromol/L failed to increase afferent renal nerve activity. Bradykinin at 475 micromol/L increased afferent renal nerve activity in only 6 of 10 SHR. In WKY, renal pelvic perfusion with the phorbol ester 4beta-phorbol 12,13-dibutyrate, known to activate protein kinase C, resulted in a peak afferent renal nerve activity response of 24 +/- 4%. However, 4beta-phorbol 12,13-dibutyrate failed to increase afferent renal nerve activity in SHR. These findings demonstrate decreased responsiveness of renal pelvic sensory receptors to bradykinin in SHR. The impaired afferent renal nerve activity responses to bradykinin in SHR may be due to a lack of protein kinase C activation or a defect in the intracellular signaling mechanisms distal to protein kinase C activation.

Adrenergic, dopaminergic, and muscarinic receptor stimulation leads to PKA phosphorylation of Na-K-ATPase

Na-K-adenosinetriphosphatase (Na-K-ATPase) is a potential target for phosphorylation by protein kinase A (PKA) and C (PKC). We have investigated whether the Na-K-ATPase alpha-subunit becomes phosphorylated at its PKA or PKC phosphorylation sites upon stimulation of G protein-coupled receptors primarily linked either to the PKA or the PKC pathway. COS-7 cells, transiently or stably expressing Bufo marinus Na-K-ATPase wild-type alpha- or mutant alpha-subunits affected in its PKA or PKC phosphorylation site, were transfected with recombinant DNA encoding beta 2- or alpha 1-adrenergic (AR), dopaminergic (D1A-R), or muscarinic cholinergic (M1-AChR) receptor subspecies. Agonist stimulation of beta 2-AR or D1A-R led to phosphorylation of the wild-type alpha-subunit, as well as the PKC mutant, but not of the PKA mutant, indicating that these receptors can phosphorylate the Na-K-ATPase via PKA activation. Surprisingly, stimulation of the alpha 1B-AR, alpha 1C-AR, and M1-AChR also increased the phosphorylation of the wild-type alpha-subunit and its PKC mutant but not of its PKA mutant. Thus the phosphorylation induced by these primarily phospholipase C-linked receptors seems mainly mediated by PKA activation. These data indicate that the Na-K-ATPase alpha-subunit can act as an ultimate target for PKA phosphorylation in a cascade starting with agonist-receptor interaction and leading finally to a phosphorylation-mediated regulation of the enzyme.

Regulation of Na(+)-pump activity by dopamine in rat tail arteries

We investigated the effect of dopamine on the vascular Na+-pump activity in isolated rat tail artery sections. Effect of dopamine on vascular tone was also assessed using a perfused tail artery preparation. Dopamine inhibited the Na+-pump activity in isolated rat tail arteries in a dose-dependent manner. Both SKF-38393 HCl, a selective dopamine D1 receptor agonist, and quinpirole HCl, a selective dopamine D2 receptor agonist inhibited the Na+-pump activity. The inhibition of the Na+-pump activity. The inhibition of the Na+-pump by dopamine was accompanied with a transient increase in the vascular tone. SKF-38393, but not quinpirole produced a sustained increase in the vascular tone. Tissues preincubated simultaneously with SCH-23390 HCl, a selective dopamine D1 receptor antagonist, and sulpiride, a selective dopamine D2 receptor antagonist, prevented the dopamine inhibition of the Na+-pump activity. Pertussis toxin blocked the Na+-pump inhibition produced by the dopamine D1 receptor agonist but not by the dopamine D2 agonist. Similarly, the dopamine D1 receptor but not dopamine D2 agonist increased the rate of phosphoinositide hydrolysis in rat tail artery sections. Our results indicate that dopamine inhibition of the Na+-pump is mediated by a pertussis toxin-sensitive mechanism and may be coupled to the activation of the phospholipase C system in rat tail arteries. The modulation of the Na+-pump by dopamine may contribute to the vascular tone.

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

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

Evidence obtained using single hepatocytes for inhibition by the phospholipase C inhibitor U73122 of store-operated Ca2+ inflow

The ability of 1-[6-[[17 beta-3-methoxyestra-1,3,5(10)-trien-17- yl]amino]hexyl]-1H-pyrrole-2,5-dione (U73122), an inhibitor of phospholipase C (Smith et al., J Pharmacol Exp Ther 253:688-697, 1992), to inhibit agonist-stimulated and store-operated Ca2+ inflow in single hepatocytes was investigated with the aim of testing whether the activation of phospholipase C is a necessary step in the process of agonist-stimulated Ca2+ inflow in this cell type. U73122 inhibited the release of Ca2+ from intracellular stores and plasma membrane Ca2+ inflow induced by vasopressin. An inactive analogue of U73122, 1-[6-[[17 beta-3-methoxyestra-1,3,5(10)-trien-17-yl]amino]hexyl]- 2,5-pyrrolidone-dione (U73433), did not inhibit vasopressin-induced release of Ca2+ from intracellular stores, but did partially inhibit Ca2+ inflow. Neither U73122 nor 'inactive' analogue U73433 inhibited the release of Ca2+ from intracellular stores when this was initiated by the photolysis of 'caged' guanosine (5'-[gamma-thio]triphosphate (GTP gamma S) introduced to the cytoplasmic space by microinjection. However, both compounds inhibited GTP gamma S-stimulated Ca2+ inflow. U73122 also inhibited the actions of glycerophosphoryl-myo-inositol-4,5-diphosphate (GPIP2), a slowly-hydrolysed analogue of inositol 1,4,5-triphosphate (InsP3) which is released by photolysis of 'caged' 1-(alpha-glycerophosphoryl)-myo-inositol-4,5-diphosphate, P4(5)-1-(2-nitrophenyl)ethyl ester, and thapsigargin in stimulating Ca2+ inflow. U73122 did not inhibit GPIP2-stimulated release of Ca2+ from intracellular stores, but did partially inhibit the ability of thapsigargin to induce Ca2+ release. It is concluded that, while U73122 does inhibit phospholipase C beta in hepatocytes, complete inhibition of this enzyme in situ requires an intracellular concentration of U73122 higher than that achieved in the present experiments. Moreover, both U73122 and 'inactive' analogue U73433 have one or possibly two additional sites of action. These are likely to be the hepatocyte plasma membrane Ca2+ inflow channel protein (or a protein involved in the activation of this channel by the InsP3-sensitive intracellular Ca2+ store), and a protein involved in thapsigargin action.

Dopamine causes stimulation of protein kinase C in rat renal proximal tubules by activating dopamine D1 receptors

Although it is suggested that in the renal proximal tubules, dopamine D1 receptor activation causes inhibition of Na+/K+ATPase via a phospholipase C and protein kinase C coupled pathway, the direct stimulation of protein kinase C by dopamine has not been reported. The present study was designed to examine the effects of dopamine and selective dopamine D1 receptor and dopamine D2 receptor agonists on protein kinase C activity. The renal proximal tubule suspensions were obtained from male Sprague-Dawley rats. The tubules were incubated separately with dopamine and fenoldopam in the presence or absence of dopamine D1 receptor antagonist, SCH 23390 ([(R)-(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3- benzazepine]). The protein kinase C activity was measured by using a kinase target peptide, conjugated to a fluorescent molecule in water. The amino acid sequence of this peptide is, Proline-Leucine-Serine-Arginine-Threonine-Leucine-Serine-Valine-Alanine- Alanine-Lysine(PKSRTLSVAAK). We found that dopamine and fenoldopam [6-chloro-2,3,4,5-tetrahydro-1-(4-hydroxyphenyl)-1H-3-benzazepine-7,8-di ol] produced concentration-dependent increases in protein kinase C activity, which was blocked by SCH 23390. However, the dopamine D2 receptor agonist, bromocriptine [(5' alpha)-2-bromo-12'-hydroxy-2'-(1-methyl-ethyl)-5'-(2-methylpropyl)erg o- taman-3',6',18-trione] failed to stimulate protein kinase C activity at all the concentrations tested. These results provide direct evidence that dopamine stimulates protein kinase C activity via activation of dopamine D1 receptors.

Cloning of the alpha-subunit of GS protein from spontaneously hypertensive rats

Enhanced sodium reabsorption by the kidney has a significant role in the development of genetic hypertension. In the spontaneously hypertensive rat (SHR) model of genetic hypertension, the enhanced sodium reabsorption likely arises from abnormal hormonal regulation of tubular transport. Since hormonal signaling pathways are coupled frequently via GTP binding proteins, one explanation for hormonal abnormalities in SHR would be a defect in a GTP binding protein or proteins. Recent work has suggested that the regulation of Na+,K(+)-ATPase activity by cholera toxin-sensitive GTP binding proteins is abnormal in SHR. The purpose of the present studies was to clone the alpha S-subunit, which is the subunit ADP ribosylated by cholera toxin, of GS protein to determine whether it is abnormal in SHR. Reverse transcription-polymerase chain reaction was able to detect mRNA for alpha S in both Wistar-Kyoto (WKY) rats and SHR. Northern analysis indicated that equivalent amounts of alpha S mRNA were present in WKY rats and SHR. S1 nuclease analysis demonstrated that there was no difference in the amount of alpha S short and long forms between WKY rats and SHR. Subcloning and sequencing of polymerase chain reaction products from WKY rats and SHR indicated that the alpha S forms present in renal cortex were identical. ADP ribosylation studies with cholera toxin demonstrated the presence of equivalent amounts of alpha S protein in WKY rats and SHR. Taken together, these results suggest that the abnormal regulation of Na+,K(+)-ATPase activity by a cholera toxin-sensitive pathway in SHR does not arise from a defect in the alpha S subunit.

Identification of alpha 1-adrenoceptor subtypes in rat renal proximal tubules

Although the existence of alpha 1-adrenoceptor subtypes has been suggested in the kidney, their characterization in the proximal tubules has not been reported. This study was undertaken to characterize the alpha 1-adrenoceptor subtypes in the renal proximal tubules. [3H]Prazosin bound to proximal tubules with a KD of 106 +/- 17 pM and a Bmax of 152 +/- 9 fmol/mg protein. Pretreatment of the tubules with chloroethylclonidine, 100 microM, reduced the Bmax of [3H]prazosin binding by about 50%, indicating the presence of the alpha 1B-adrenoceptor subtype. Competition studies performed with the alpha-adrenoceptor antagonists, WB 4101, (+)-niguldipine, 5-methylurapidil and phentolamine were shallow and could be distinguished into a high affinity (alpha 1A) and a low affinity site (alpha 1B) with an approximately equal distribution of both receptor subtypes. These observations clearly demonstrate the existence of alpha 1A- and alpha 1B-adrenoceptor subtypes in the renal proximal tubules.

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

Endogenous kidney dopamine (DA) causes natriuresis and diuresis, at least partly, via inhibition of proximal tubular Na+,K(+)-ATPase. The present study was done to identify the dopamine receptor subtype(s) involved in dopamine-induced inhibition of Na+,K(+)-ATPase activity. Suspensions of renal proximal tubules from Sprague-Dawley rats were incubated with dopamine, the DA-1 receptor agonist fenoldopam or the DA-2 receptor agonist SK&F 89124 in the presence or absence of either the DA-1 receptor antagonist SCH 23390 or the DA-2 receptor antagonist domperidone. Dopamine and fenoldopam (10(-5) to 10(-8) mol/l) produced a concentration-dependent inhibition of Na+,K(+)-ATPase activity. However, SK&F 89124 failed to produce any significant effect over the same concentration range. Incubation with fenoldopam (10(-5) to 10(-8) mol/l) in the presence of SK&F 89124 (10(-6) mol/l) inhibited Na+,K(+)-ATPase activity to a degree similar to that with fenoldopam alone. Furthermore, DA-induced inhibition of Na+,K(+)-ATPase activity was attenuated by SCH 23390, but not by domperidone. Since alpha-adrenoceptor activation is reported to stimulate Na+,K(+)-ATPase activity and, at higher concentrations, dopamine also acts as an alpha-adrenoceptor agonist, the potential opposing effect from alpha-adrenoceptor activation on DA-induced inhibition of Na+,K(+)-ATPase activity was investigated by using the alpha-adrenoceptor blocker phentolamine. We found that, in the lower concentration range (10(-5) to 10(-7) mol/l), dopamine-induced inhibition of Na+,K(+)-ATPase activity in the presence of phentolamine was similar in magnitude to that observed with dopamine alone.(ABSTRACT TRUNCATED AT 250 WORDS)