Persistent defective coupling of dopamine-1 receptors to G proteins after solubilization from kidney proximal tubules of hypertensive rats

Unusually persistent Gαi-signaling of the neuropeptide Y2 receptor depletes cellular Gi/o pools and leads to a Gi-refractory state

BACKGROUND

A sensitive balance between receptor activation and desensitization is crucial for cellular homeostasis. Like many other GPCR, the human neuropeptide Y₂ receptor (hY₂R) undergoes ligand dependent activation and internalization into intracellular compartments, followed by recycling to the plasma membrane. This receptor is involved in the pathophysiology of distinct diseases e.g. epilepsy and cancer progression and conveys anorexigenic signals which makes it an interesting and promising anti-obesity target. However, Y₂R desensitization was observed after daily treatment with a selective PYY_13-36 analog in vivo by a yet unknown mechanism.

MATERIALS

We studied the desensitization and activatability of recycled Y₂R in transiently transfected HEK293 cells as well as in endogenously Y₂R expressing SH-SY5Y and SMS-KAN cells. Results were evaluated by one-way ANOVA and Tukey post test.

RESULTS

We observed strong desensitization of the Y₂R in a second round of stimulation despite its reappearance at the membrane. Already the first activation of the Y₂R leads to depletion of the functional cellular Gα_i/o protein pool and consequently desensitizes the linked signal transduction pathways, independent of receptor internalization. This desensitization also extends to other Gα_i/o-coupled GPCR and can be detected in transfected HEK293 as well as in SH-SY5Y and SMS-KAN cell lines, both expressing the Y₂R endogenously. By overexpression of chimeric Gα_qi proteins in a model system, activation has been rescued, which identifies a critical role of the G protein status for cellular signaling. Furthermore, Y₂R displays strong allosteric coupling to inhibitory G proteins in radioligand binding assays, and loses 10-fold affinity in the G protein-depleted state observed after activation, which can be largely abrogated by overexpression of the Gα_i-subunit.

CONCLUSION

The unusually persistent Gα_i-signaling of the Y₂R leads to a state of cellular desensitization of the inhibitory Gα_i-pathway. The strong allosteric effects of the Y₂R-Gα_i-interaction might be a mechanism that contributes to the burst of Gα_i-signaling, but also serves as a mechanism to limit the Y₂-mediated signaling after recycling. Thus, the cell is left in a refractory state, preventing further Gα_i-signaling of the Y₂R itself but also other Gα_i/o-coupled receptors by simply controlling the repertoire of downstream effectors. Video abstract.

Targeting GPCR-Gβγ-GRK2 signaling as a novel strategy for treating cardiorenal pathologies

The pathologic crosstalk between the heart and kidney is known as cardiorenal syndrome (CRS). While the specific mechanisms underlying this crosstalk remain poorly understood, CRS is associated with exacerbated dysfunction of either or both organs and reduced survival. Maladaptive fibrotic remodeling is a key component of both heart and kidney failure pathogenesis and progression. G-protein coupled receptor (GPCR) signaling is a crucial regulator of cardiovascular and renal function. Chronic/pathologic GPCR signaling elicits the interaction of the G-protein Gβγ subunit with GPCR kinase 2 (GRK2), targeting the receptor for internalization, scaffolding to pathologic signals, and receptor degradation. Targeting this pathologic Gβγ-GRK2 interaction has been suggested as a possible strategy for the treatment of HF. In the current review, we discuss recent updates in understanding the role of GPCR-Gβγ-GRK2 signaling as a crucial mediator of maladaptive organ remodeling detected in HF and kidney dysfunction, with specific attention to small molecule-mediated inhibition of pathologic Gβγ-GRK2 interactions. Further, we explore the potential of GPCR-Gβγ-GRK2 signaling as a possible therapeutic target for cardiorenal pathologies.

G Protein-Coupled Receptor-G-Protein βγ-Subunit Signaling Mediates Renal Dysfunction and Fibrosis in Heart Failure

Development of CKD secondary to chronic heart failure (CHF), known as cardiorenal syndrome type 2 (CRS2), clinically associates with organ failure and reduced survival. Heart and kidney damage in CRS2 results predominantly from chronic stimulation of G protein-coupled receptors (GPCRs), including adrenergic and endothelin (ET) receptors, after elevated neurohormonal signaling of the sympathetic nervous system and the downstream ET system, respectively. Although we and others have shown that chronic GPCR stimulation and the consequent upregulated interaction between the G-protein βγ-subunit (Gβγ), GPCR-kinase 2, and β-arrestin are central to various cardiovascular diseases, the role of such alterations in kidney diseases remains largely unknown. We investigated the possible salutary effect of renal GPCR-Gβγ inhibition in CKD developed in a clinically relevant murine model of nonischemic hypertrophic CHF, transverse aortic constriction (TAC). By 12 weeks after TAC, mice developed CKD secondary to CHF associated with elevated renal GPCR-Gβγ signaling and ET system expression. Notably, systemic pharmacologic Gβγ inhibition by gallein, which we previously showed alleviates CHF in this model, attenuated these pathologic renal changes. To investigate a direct effect of gallein on the kidney, we used a bilateral ischemia-reperfusion AKI mouse model, in which gallein attenuated renal dysfunction, tissue damage, fibrosis, inflammation, and ET system activation. Furthermore, in vitro studies showed a key role for ET receptor-Gβγ signaling in pathologic fibroblast activation. Overall, our data support a direct role for GPCR-Gβγ in AKI and suggest GPCR-Gβγ inhibition as a novel therapeutic approach for treating CRS2 and AKI.

A Polymorphism within the Promoter of the Dopamine Receptor D1 (DRD1 -48A/G) Associates with Impaired Kidney Function in White Hypertensive Patients

Dopamine DRD1 receptor regulates renal function and vascular resistance. It plays a role in the pathogenesis of hypertension in animal models. In humans, the DRD1 gene presents a A-48G polymorphism associated to hypertension in a Japanese population. To explore the role of this polymorphism on blood pressure and renal function in Caucasian hypertensive patients (H), we evaluated the allele frequencies in a populations of 697 H and 100 blood volunteers, and found no difference in the distribution of the alleles between the two groups (AA;AG;GG: 13%;50%;37%; and 12%; 51%;36% respectively). In H, we found a significant difference between AA and GG in serum creatinine (AA: 1.06±.08 mg/dl; GG:0.97±0.02 mg/dl, p<0.03). Treatment restored serum creatinine at levels comparable between genotypes (AA: 0.99±0.03 mg/dl; GG: 0.94±0.02 mg/dl, n.s.). To replicate the finding, in a case control study of 8 AA and 7 GG hypertensive patients matched for age, sex and body mass index, in pharmacological wash out for 30 days, we evaluated serum (Creatinine, Na, Uric Acid, Urea) and urinary (volume/24h, protein/24h, creatinine clearance/24h) biochemistry and renal hemodynamic assessed by ultrasound. Once again, the AA group showed higher serum creatinine, Na, Uric acid and urea, reduced creatinine clearance and a higher level of urinary protein excretion. These changes occurred while no differences were observed in diuresis and renal vascular resistances. In conclusions, the DRD1 A-48G polymorphism identifies a class of H that is prone to hypertension related kidney alterations.

Inflammation compromises renal dopamine D1 receptor function in rats

We tested the effects of inflammation on renal dopamine D1 receptor signaling cascade, a key pathway that maintains sodium homeostasis and blood pressure during increased salt intake. Inflammation was produced by administering lipopolysaccharide (LPS; 4 mg/kg ip) to rats provided without (normal salt) and with 1% NaCl in drinking water for 2 wk (high salt). Control rats had saline injection and received tap water. We found that LPS increased the levels of inflammatory cytokines, interleukin-6, and tumor necrosis factor-alpha in the rats given either normal- or high-salt intake. Also, these rats had higher levels of oxidative stress markers, malondialdehyde and nitrotyrosine, and lower levels of antioxidant enzyme superoxide dismutase in the renal proximal tubules (RPTs). The nuclear levels of transcription factors NF-kappaB increased and Nrf2 decreased in the RPTs in response to LPS in rats given normal and high salt. Furthermore, D1 receptor numbers, D1 receptor proteins, and D1 receptor agonist (SKF38393)-mediated (35)S-GTPgammaS binding decreased in the RPTs in these rats. The basal activities of Na-K-ATPase in the RPTs were similar in control and LPS-treated rats given normal and high salt. SKF38393 caused inhibition of Na-K-ATPase activity in the primary cultures of RPTs treated with vehicle but not in the cultures treated with LPS. Furthermore, LPS caused an increase in blood pressure in the rats given high salt but not in the rats given normal salt. These results suggest that LPS differentially regulates NF-kappaB and Nrf2, produces inflammation, decreases antioxidant enzyme, increases oxidative stress, and causes D1 receptor dysfunction in the RPTs. The LPS-induced dysfunction of renal D1 receptors alters salt handling and causes hypertension in rats during salt overload.

Functional genomics of the dopaminergic system in hypertension

Abnormalities in dopamine production and receptor function have been described in human essential hypertension and rodent models of genetic hypertension. Under normal conditions, D(1)-like receptors (D(1) and D(5)) inhibit sodium transport in the kidney and intestine. However, in the Dahl salt-sensitive and spontaneously hypertensive rats (SHRs) and in humans with essential hypertension, the D(1)-like receptor-mediated inhibition of epithelial sodium transport is impaired because of an uncoupling of the D(1)-like receptor from its G protein/effector complex. The uncoupling is receptor specific, organ selective, nephron-segment specific, precedes the onset of hypertension, and cosegregates with the hypertensive phenotype. The defective transduction of the renal dopaminergic signal is caused by activating variants of G protein-coupled receptor kinase type 4 (GRK4: R65L, A142V, A486V). The GRK4 locus is linked to and GRK4 gene variants are associated with human essential hypertension, especially in salt-sensitive hypertensive subjects. Indeed, the presence of three or more GRK4 variants impairs the natriuretic response to dopaminergic stimulation in humans. In genetically hypertensive rats, renal inhibition of GRK4 expression ameliorates the hypertension. In mice, overexpression of GRK4 variants causes hypertension either with or without salt sensitivity according to the variant. GRK4 gene variants, by preventing the natriuretic function of the dopaminergic system and by allowing the antinatriuretic factors (e.g., angiotensin II type 1 receptor) to predominate, may be responsible for salt sensitivity. Subclasses of hypertension may occur because of additional perturbations caused by variants of other genes, the quantitative interaction of which may vary depending upon the genetic background.

Links between dietary salt intake, renal salt handling, blood pressure, and cardiovascular diseases

Epidemiological, migration, intervention, and genetic studies in humans and animals provide very strong evidence of a causal link between high salt intake and high blood pressure. The mechanisms by which dietary salt increases arterial pressure are not fully understood, but they seem related to the inability of the kidneys to excrete large amounts of salt. From an evolutionary viewpoint, the human species is adapted to ingest and excrete <1 g of salt per day, at least 10 times less than the average values currently observed in industrialized and urbanized countries. Independent of the rise in blood pressure, dietary salt also increases cardiac left ventricular mass, arterial thickness and stiffness, the incidence of strokes, and the severity of cardiac failure. Thus chronic exposure to a high-salt diet appears to be a major factor involved in the frequent occurrence of hypertension and cardiovascular diseases in human populations.

Rosiglitazone restores renal D1A receptor-Gs protein coupling by reducing receptor hyperphosphorylation in obese rats

Dopamine D(1A) receptor function is impaired in obesity-induced insulin resistance, contributing to sodium retention. We showed previously that uncoupling of D(1A) receptors from G proteins is responsible for diminished natriuretic response to dopamine in obese Zucker rats (OZRs). We hypothesized that overexpression of G protein-coupled receptor kinases (GRKs) leads to increased phosphorylation of D(1A) receptors, which in turn causes uncoupling of the receptors from G(s) proteins in proximal tubules of OZRs. We also examined effects of an insulin sensitizer, rosiglitazone, in correcting these defects. We found that basal and agonist (fenoldopam)-induced coupling of D(1A) receptors to G(s) proteins was impaired in proximal tubules of OZRs compared with lean Zucker rats (LZRs). Moreover, basal serine phosphorylation of D(1A) receptors was elevated two- to threefold in proximal tubules of OZRs compared with LZRs. Fenoldopam increased D(1A) receptor phosphorylation in proximal tubules of LZRs but not OZRs. Compared with that in LZRs, GRK4 expression in OZRs was elevated 200-300% in proximal tubule cell lysates and GRK2 expression was approximately 30% higher in plasma membranes isolated from proximal tubules of OZRs. Rosiglitazone treatment restored basal and agonist-induced coupling of D(1A) receptors to G(s) proteins and reduced basal serine phosphorylation of D(1A) receptors, GRK4 expression, and translocation of GRK2 to the plasma membrane in proximal tubules of OZRs. Furthermore, rosiglitazone significantly reduced fasting blood glucose and plasma insulin in OZRs. Collectively, these results suggest that insulin resistance is responsible for GRK4 overexpression and GRK2 translocation leading to hyperphosphorylation of D(1A) receptors and their uncoupling from G(s) proteins as rosiglitazone treatment corrects these defects in OZRs.

D5 dopamine receptor knockout mice and hypertension

Abnormalities in dopamine production and receptor function have been described in human essential hypertension and rodent models of genetic hypertension. All of the five dopamine receptor genes (D1, D2, D3, D4, and D5) expressed in mammals and some of their regulators are in loci linked to hypertension in humans and in rodents. Under normal conditions, D1-like receptors (D1 and D5) inhibit sodium transport in the kidney and the intestine. However, in the Dahl salt-sensitive and spontaneously hypertensive rats, and humans with essential hypertension, the D1-like receptor-mediated inhibition of sodium transport is impaired because of an uncoupling of the D1-like receptor from its G protein/effector complex. The uncoupling is genetic, and receptor-, organ-, and nephron segment-specific. In human essential hypertension, the uncoupling of the D1 receptor from its G protein/effector complex is caused by an agonist-independent serine phosphorylation/desensitization by constitutively active variants of the G protein-coupled receptor kinase type 4. The D5 receptor is also important in blood pressure regulation. Disruption of the D5 or the D1 receptor gene in mice increases blood pressure. However, unlike the D1 receptor, the hypertension in D5 receptor null mice is caused by increased activity of the sympathetic nervous system, apparently due to activation of oxytocin, V1 vasopressin, and non-N-methyl D-aspartate receptors in the central nervous system. The cause of the activation of these receptors remains to be determined.

Renal dopamine receptors and hypertension

Dopamine has been recognized as an important modulator of central as well as peripheral physiologic functions in both humans and animals. Dopamine receptors have been identified in a number of organs and tissues, which include several regions within the central nervous system, sympathetic ganglia and postganglionic nerve terminals, various vascular beds, the heart, the gastrointestinal tract, and the kidney. The peripheral dopamine receptors influence cardiovascular and renal function by decreasing afterload and vascular resistance and promoting sodium excretion. Within the kidney, dopamine receptors are present along the nephron, with highest density on proximal tubule epithelial cells. It has been reported that there is a defective dopamine receptor, especially D(1) receptor function, in the proximal tubule of various animal models of hypertension as well as in humans with essential hypertension. Recent reports have revealed the site of and the molecular mechanisms responsible for the defect in D(1) receptors in hypertension. Moreover, recent studies have also demonstrated that the disruption of various dopamine receptor subtypes and their function produces hypertension in rodents. In this review, we present evidence that dopamine and dopamine receptors play an important role in regulating renal sodium excretion and that defective renal dopamine production and/or dopamine receptor function may contribute to the development of various forms of hypertension.

Differential expression of D2-like dopamine receptors in the kidney of the spontaneously hypertensive rat

OBJECTIVE

To compare the expression and cellular distribution of D(2)-like dopamine receptors in the kidney of the spontaneously hypertensive rat (SHR) and normotensive Wistar-Kyoto (WKY) rat.

DESIGN

Renal D(2)-like receptor protein expression and distribution has not been studied in the SHR. Since changes in D(2)-like receptor expression and/or distribution may contribute to the dysregulation of renal dopamine and D(1A) receptor function, we examined the expression of the three subtypes of D(2)-like receptors (D(2), D(3) and D(4)) in SHR and WKY rat kidneys.

METHODS

Western blot analysis and confocal immunocytochemistry with specific polyclonal antipeptide antibodies directed against the receptor subtypes, were used to assess protein expression.

RESULTS

There were no differences in protein expression and cellular immunolocalization of the D(2) receptor subtypes between SHR and WKY rats. Expression of the 50 kDa D(3) receptor was reduced in the cortex of the SHR; no differences in D(3) receptor levels were seen in the inner medulla of SHR and WKY rats. The D(4) receptor polypeptides were overexpressed in the cortex of SHR, while in the inner medulla no difference in expression of the D(4) receptor proteins was observed between SHR and WKY rats. Immunocytochemistry also showed increased immunostaining of D(4) receptors in tubular structures in the cortex, but diminished staining in the SHR inner medulla.

CONCLUSION

The observed differences in expression and distribution of D(3) and D(4) dopamine receptors between cortex and inner medulla of the kidneys of SHR and WKY rats may contribute to the aberrant state of dopaminergic-mediated natriuresis in SHR.

Dopamine acutely stimulates Na+/H+ exchanger (NHE3) endocytosis via clathrin-coated vesicles: dependence on protein kinase A-mediated NHE3 phosphorylation

Dopamine (DA) is a key hormone in mammalian sodium homeostasis. DA induces natriuresis via acute inhibition of the renal proximal tubule apical membrane Na(+)/H(+) exchanger NHE3. We examined the mechanism by which DA inhibits NHE3 in a renal cell line. DA acutely decreases surface NHE3 antigen in dose- and time-dependent fashion without altering total cellular NHE3. Although DA(1) receptor agonist alone decreases surface NHE3, simultaneous DA(2) agonist synergistically enhances the effect of DA(1). Decreased surface NHE3 antigen, caused by stimulation of NHE3 endocytosis, is dependent on intact functioning of the GTPase dynamin and involves increased binding of NHE3 to the adaptor protein AP2. DA-stimulated NHE3 endocytosis can be blocked by pharmacologic or genetic protein kinase A inhibition or by mutation of two protein kinase A target serines (Ser-560 and Ser-613) on NHE3. We conclude that one mechanism by which DA induces natriuresis is via protein kinase A-mediated phosphorylation of proximal tubule NHE3 leading to endocytosis of NHE3 via clathrin-coated vesicles.

Characterization of acute inhibition of Na/H exchanger NHE-3 by dopamine in opossum kidney cells

BACKGROUND

Dopamine (DA) is a principal natriuretic hormone that defends extracellular fluid volume from a Na load. Natriuresis is effected partly through inhibiting the proximal tubule Na/H exchanger NHE-3. Changes in NHE-3 phosphorylation is one mechanism by which NHE-3 activity is regulated.

METHODS

We used opossum kidney (OK) cells to characterize the differential and synergistic effects of DA receptor subtype-1 (DA1) and -2 (DA2) agonists and the effect of blockade of protein kinase A (PKA) or protein kinase C (PKC) on NHE-3 activity and phosphorylation.

RESULTS

DA and DA1 agonists inhibited NHE-3 activity, and DA1 antagonist blocked the effect of either DA or DA1 agonist. DA2 agonist alone had no effect, but DA2 antagonist reduced the DA effect on NHE-3 activity. DA1 and DA2 agonists together were more potent than DA1 alone. PKA inhibition eliminated the effect of DA1 agonist and partially blocked the effect of DA on NHE-3 activity. PKC inhibition did not block the DA effect. DA1 agonist and PKA activation phosphorylated NHE-3 on identical sites. Despite lack of effect on NHE-3 activity, DA2 agonists increased NHE-3 phosphorylation. DA-induced NHE-3 phosphorylation was distinct from DA1 and PKA but closely resembled DA2.

CONCLUSION

We postulate the following: (1) DA modifies NHE-3 phosphorylation by activating PKA through DA1 and by other kinases/phosphatases via DA2. (2) DA1 is sufficient to inhibit NHE-3, while DA2 is insufficient but plays a synergistic role by altering NHE-3 phosphorylation.

Intrarenal dopamine: a key signal in the interactive regulation of sodium metabolism

The kidney regulates sodium metabolism with extraordinary precision and sensitivity. This is accomplished by an intricate interaction between signals from extrarenal and intrarenal sources and between anti-natriuretic and natriuretic factors. Dopamine, produced in renal proximal tubule cells, plays a central role in this interactive network. Natriuretic hormones that are released from extrarenal sources, such as atrial natriuretic peptide, mediate some of their effects via renal dopamine receptors. On the level of the tubules, dopamine acts by opposing the effects of anti-natriuretic factors, such as angiotensin II and alpha-adrenergic receptors. Sodium retention leads to an increase in renal dopamine tonus, and the natriuretic effects of dopamine are more prominent under this condition. Inhibition or down-regulation of dopamine receptors significantly attenuates the natriuretic response to salt loading. Renal dopamine is modulated by the supply of filtered L-DOPA and the metabolism of dopamine via catechol-O-methyldopamine. The importance of dopamine as a natriuretic hormone is reflected by its capacity to inhibit the majority of renal tubule sodium transporters. Notably, the activity of Na+, K+ ATPase is inhibited in most tubule segments by dopamine. Recent studies have elucidated many of the signaling pathways for renal dopamine receptors. Novel principles for homologous and heterologous sensitization of dopamine receptors have been detected that may explain some of the interaction between dopamine and other first messengers that modulate renal tubule sodium transport. A broad understanding of the renal dopamine system has become increasingly important, since there is now strong evidence from both clinical and experimental studies that dysregulation of the renal dopamine system plays a role in many forms of multigenetic hypertension.

Dopamine D1 receptor gene polymorphism is associated with essential hypertension

Dopamine has been shown to influence renal sodium excretion through a direct interaction with the dopamine receptor (DR). The dopamine D1 receptor (DRD1) has been localized to the proximal tubules and is known to increase sodium excretion by inhibiting Na-H exchanger and Na,K-ATPase activity. Defective renal dopamine production and/or DR function have been reported in essential hypertension (EH) as well as in genetic models of animal hypertension. With a restriction fragment length polymorphism of the DRD1 gene, we performed an association study in patients with EH. One hundred thirty-one subjects with EH and 136 age-matched normotensive (NT) controls were studied. Polymerase chain reaction was used to amplify the A-48G polymorphic site in the DRD1 gene, and restriction analysis of the polymerase chain reaction product was used to score the A and G alleles. The allele frequencies in the EH group and NT group were then compared. The G allele was observed more frequently in the EH group than in the NT group, and the allele frequencies in the 2 groups differed significantly (chi(2)=6.5, P=0.01). Multiple logistic linear regression analysis revealed that the genotype frequencies of A/A, A/G, and G/G differed significantly (odds ratio=2.1; 95% CI=1.19 to 3.66) between the EH and NT groups. EH patients who possess the G allele had a higher diastolic blood pressure than those lacking the G allele (P<0.01). Thus, the alleles detected by this restriction fragment length polymorphism in the DRD1 gene are associated with EH, and they appear to influence the diastolic blood pressure of Japanese EH patients.

G-proteins in kidneys of spontaneously hypertensive rats

G(s alpha)-, total G(i alpha)- and G(q/11alpha)-protein concentrations were investigated by quantitative immunoblotting in membranes of total kidney, renal cortex and medulla as well as in cortical tubules and glomeruli of Spontaneously Hypertensive Rats (SHR) and normotensive Wistar Kyoto rats (WKY), aged 5 weeks, 3 or 8 months. We found that total kidney of 5 week old SHR possess less G(s alpha)-, G(i alpha)- and G(q/11alpha)-proteins than controls. For G(s alpha)-proteins, differences found in total kidney were mirrored both in cortex (tubules and glomeruli) and in medulla. Decreased G(i alpha)-concentrations were accompanied by lower tubular but higher glomerular levels, while medullar levels were also increased. Decreased G(q/11alpha)-concentrations were reflected in decreased glomerular and medullary concentrations. Kidneys of 3 month old SHR and WKY possessed similar concentrations of all G(alpha)-species. In 8 month old SHR similar G(i alpha)-, but decreased G(s alpha)-and G(q/11alpha)-concentrations were observed. The G(s alpha)-decrease was reflected in cortex and medulla, the G(q/11alpha)-decrease in the medulla. We conclude that the main strain-related differences in G(alpha)-concentrations are seen in prehypertensive SHR.

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.

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.

Increased oxidative stress in renal proximal tubules of the spontaneously hypertensive rat: a mechanism for defective dopamine D1A receptor/G-protein coupling

AIM

Defective dopamine D1A dopamine receptor/G-protein coupling has been demonstrated in renal proximal tubules of the spontaneously hypertensive rat (SHR). In the present study, we aimed to analyze the underlying mechanisms through which such defects are introduced into the D1A receptor protein of SHR.

MATERIALS AND METHODS

The oxidative state of SHR proximal tubules was analyzed by measuring lipid peroxidation. D1A receptor/G-protein coupling was measured following the induction of oxidative stress in normotensive Wistar-Kyoto (WKY) rats.

RESULTS

For the first time, an increased state of oxidative stress was demonstrated in SHR proximal tubules compared with those of normotensive controls, WKY and Sprague-Dawley rats. Lipid peroxidation levels in SHR were significantly higher by 66 and 79%, relative to WKY or Sprague-Dawley rats, respectively. Hydrogen peroxide treatment of proximal tubules from SHR, WKY and Sprague-Dawley rats induced an additional increase in lipid peroxidation in a dose-dependent manner, although the percentage induction was lower in SHR than in WKY and Sprague-Dawley rats. This induction of lipid peroxidation in WKY rats resulted in a loss of D1A/G-protein coupling, with no decrease in receptor protein. Treatment of WKY rat proximal tubules with an antioxidant, ascorbic acid, or a reducing agent, dithiothreitol, induced D1A receptor/G-protein coupling.

CONCLUSIONS

These data indicate that D1A receptor/G-protein coupling is modulated by changes in redox states. Therefore, the D1A receptor/G-protein coupling in SHR may have been damaged by reactive oxygen species released as a result of the elevated oxidative stress seen in the proximal tubules.

Alteration of association of agonist-activated renal D1(A) dopamine receptors with G proteins in proximal tubules of the spontaneously hypertensive rat

BACKGROUND

Defective D1A dopamine receptor-G protein coupling has been identified in renal proximal tubules of the spontaneously hypertensive rat (SHR).

OBJECTIVE

To determine whether association of D1A dopamine receptors with the alpha subunits of G proteins in kidney of SHR is normal.

METHODS

We analyzed the association of agonist-activated [1251]-labeled D1A dopamine receptors in kidneys of SHR and the normotensive Wistar-Kyoto (WKY) rat through immunoprecipitation, using highly specific antipeptide antibodies directed against alpha subunits of G proteins.

RESULTS

We have shown for the first time that the D1A receptors of renal proximal tubules are associated with the adenylyl cyclase inhibitory G proteins G(i)alpha. The association of WKY rat proximal tubule D1A receptors with Gi1alpha and Gi2alpha in the presence of agonist is significantly (P<0.01) greater (2.4-fold and 3.1-fold greater, respectively) than it is without agonist D1A receptors of WKY rat also exhibit (twofold greater) association with G(s)alpha, consistently with the ability of these receptors to mediate stimulation of adenylyl cyclase. The WKY rat D1A receptors do not associate either with G(o)alpha or with G(q)alpha. The D1A receptors of SHR proximal tubule membranes appear to be resistant to activation by agonist and do not associate with G(s)alpha, G(o)alpha and any of the subunits of G(i)alpha. However, the SHR D1A sites exhibit a modestly (1.7-fold) greater association with G(q)alpha, which was not statistically significant. The differences among associations of the D1A receptors of WKY rat and SHR with these Galpha proteins may be important in understanding renal dopaminergic functions in normal and pathophysiologic states.

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.

Diminished expression of renal dopamine D1A receptors in the kidney inner medulla of the spontaneously hypertensive rat

BACKGROUND

Dysfunctional dopamine neurotransmission and greater than normal retention of salt have been found for renal proximal tubules of the spontaneously hypertensive rat

OBJECTIVE

To determine whether there are differences between kidney D1A dopamine receptor distributions of spontaneously hypertensive rats and Wistar-Kyoto rats.

METHODS

We examined the expression of D1A dopamine receptors in kidneys of spontaneously hypertensive rats and the normotensive Wistar-Kyoto rat through Western blots and immunocytochemistry, using highly specific antipeptide antibodies directed against the receptor.

RESULTS

The specificity of the antisera was demonstrated by Western blot studies, using proximal tubules, from Wistar-Kyoto rats. The antiserum recognized a major polypeptide with Mr of 72 kDa and a minor protein of Mr 66 kDa, which were not detected either by antigen-adsorbed or by preimmune sera. In renal cortex of both Wistar-Kyoto rats and spontaneously hypertensive rats, D1A receptors were expressed at equivalent levels. In the inner medulla of Wistar-Kyoto rat, there was diminished (by 60%) expression of D1A receptors compared with that of the renal cortex. However, the expression of D1A receptors in the inner medulla in the spontaneously hypertensive rat was even more diminished (by 83%) relative to levels found in spontaneously hypertensive rat renal cortex. Immunocytochemical studies localized the D1A receptor protein in renal cortex primarily to epithelia of tubules. Relative to renal cortex, there was an overall decrease in staining intensity in the inner medulla both of Wistar-Kyoto rats and of spontaneously hypertensive rats. Compared with that of Wistar-Kyoto rat, the intensity of staining of D1A receptors in the inner medulla of spontaneously hypertensive rats was greatly diminished, confirming the Western blot analyses. The less than normal expression of D1A receptors in the inner medulla of spontaneously hypertensive rats might be of physiologic importance in the etiology of greater than normal retention of salt and hypertension in spontaneously hypertensive rats.

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.

Dysfunctional D1A receptor-G-protein coupling in proximal tubules of spontaneously hypertensive rats is not due to abnormal G-proteins

BACKGROUND

Dysfunctional dopamine neurotransmission and defective D1A receptor-G protein coupling exist in renal proximal tubules (RPT) of the spontaneously hypertensive rat (SHR).

OBJECTIVE

To determine whether the G proteins in SHR are abnormal, preventing formation of agonist high affinity sites in SHR.

METHODS

We examined the expression levels of the alpha-subunits of G proteins, as well as D1A receptor receptor coupling to exogenously added normal G proteins, in RPT of SHR and the normotensive Wister-Kyoto (WKY) rat.

RESULTS

In the presence of 110 mmol/l NaCl, the D1A dopamine receptor-selective agonist SKF R-38393 binds both to high- and to low-affinity sites on solubilized and reconstituted D1A receptors extracted from renal proximal tubules of normotensive Wistar-Kyoto (WKY) rats. In the spontaneously hypertensive rat (SHR), SKF R-38393 bound to a single site on the reconstituted receptor with affinity values corresponding to the low-affinity state of the receptor. Western blot analyses indicated that the alpha-subunit of the guanine nucleotide binding protein (G-protein), Gs, was expressed at similar levels, whereas G(o)alpha was not expressed in proximal tubule membranes from WKY rats and SHR. Pretreatment of proximal tubule membranes with the alkylating agent N-ethylmaleimide in the presence of SKF R-38393 inactivated alpha-subunits of endogenous G-proteins, but not D1A receptors, resulting in loss of high-affinity binding sites in WKY rats. These N-ethylmaleimide-treated D1A receptors from WKY rats, when reconstituted with exogenous sources of G-proteins, were able to couple to these exogenous G-proteins, with complete restoration of high-affinity sites. Moreover, the affinity values and the proportion of these hybrid sites were similar to those of untreated receptors, and these affinity sites were regulated by guanine nucleotide analogs. Reconstitution of D1A receptors from SHR with the same exogenous G-proteins failed to similarly induce formation of the high-affinity binding sites in the hybrid reconstituted systems, and SKF R-38393 continued to bind in a single low-affinity state of the receptor.

CONCLUSION

These results demonstrate that the absence of G-protein coupling in SHR is due to intrinsic defects within the receptor protein, rather than to any abnormalities of the endogenous G-proteins themselves.

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.

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.

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)

Renal dopamine-1 receptors in hypertensive inbred rat strains with and without hyperactivity

Renal dopamine-1 (DA-1) receptors are involved in the regulation of sodium transport in several nephron segments, including the proximal convoluted tubule (PCT). DA-1 receptors in the PCT and cortical collecting duct of normotensive rats are linked to the stimulation of adenylyl cyclase (AC). We have reported a defect in the DA-1 receptor/AC coupling in the PCT of the spontaneously hypertensive rat (SHR) of the Okamoto-Aoki strain. Hyperactivity and hypertension are both expressed in the SHR. To determine if the DA-1 receptor coupling defect is associated with hyperactivity or hypertension, we studied the DA-1 receptor in the PCT of two new inbred rat strains derived from the SHR: the hyperactive WKHA and the hypertensive WKHT rat. Tail-cuff blood pressures taken at 4 weeks indicated that WKHT rats were not hypertensive (86 +/- 3 mm Hg, n = 6), whereas at 12 weeks systolic pressures in both SHR and WKHT rats exceeded 150 mm Hg. Hyperactivity, however, was noted in WKHA rats even at this early age. Basal AC activity was similar in WKHA and WKHT PCT in either age group. In the older rats, the DA-1 agonist fenoldopam (10(-7) mol/L) stimulated AC activity in WKHA (70.6 +/- 16.1 fmol per 3 mm PCT per 20 minutes, n = 3) but not in WKHT PCT (43.3 +/- 5.3 fmol per 3 mm PCT per 20 minutes, n = 4). Gpp(NH)p (10(-5) mol/L), a nonhydrolyzable GTP analogue, stimulated AC activity to a similar extent in WKHA and WKHT PCT.(ABSTRACT TRUNCATED AT 250 WORDS)

Dopamine fails to inhibit renal tubular sodium pump in hypertensive rats

We have previously reported that dopamine-1 receptor-mediated activation of phospholipase C is diminished in renal cortical slices of adult spontaneously hypertensive rats. To determine the potential consequences of this phenomenon, we performed the present studies in which renal proximal tubule suspensions obtained from spontaneously hypertensive and Wistar-Kyoto rats of 10-12 weeks of age were used. The tubule suspensions were incubated with dopamine in the presence or absence of dopamine receptor antagonists, and sodium, potassium adenosine trisphosphatase (sodium pump) activity was measured as the ouabain-sensitive adenosine trisphosphate hydrolysis. We found that dopamine produced a concentration-related inhibition of sodium pump activity in the normotensive rats but not in the hypertensive rats. Dopamine-induced inhibition of sodium pump activity in the normotensive rats was abolished by the phospholipase C inhibitor U-73122 or the protein kinase C inhibitor sphingosine, suggesting the involvement of a phospholipase C-coupled protein kinase C pathway in this response. Dopamine-induced inhibition in the normotensive rats was attenuated by the dopamine-1 receptor antagonist SCH 23390 but not by the dopamine-2 receptor antagonist domperidone. To identify possible sites of defect in dopamine-1 receptor-coupled signaling pathways in the hypertensive rats, we incubated the proximal tubules with phorbol 12,13-dibutyrate or the synthetic diacylglycerol analogue 1-oleoyl-2-acetyl-rac-glycerol. The results showed that both compounds inhibited sodium pump activity as effectively in the hypertensive as in the normotensive rats, suggesting that the protein kinase C-coupled sodium pump pathway was not defective in the hypertensive animals.(ABSTRACT TRUNCATED AT 250 WORDS)