Abstract-The ability of dopamine 1 (D 1 ) receptors to inhibit luminal Na ϩ -H ϩ exchanger (NHE) activity in renal proximal tubules and induce a natriuresis is impaired in spontaneously hypertensive rats (SHR). However, it is not clear whether the defect is at the level of the D 1 receptor, G s␣ , or effector proteins. The coupling of the D 1 receptor to G s␣ and NHE3 was studied in renal brush border membranes (BBM), devoid of cytoplasmic second messengers. D 1 receptor, G s␣ , and NHE3 expressions were similar in SHR and their normotensive controls, Wistar-Kyoto rats (WKY). Guanosine-5Ј-O-(3-thiotriphosphate) (GTP␥S) decreased NHE activity and increased NHE3 linked with G s␣ similarly in WKY and SHR, indicating normal G s␣ and NHE3 regulation in SHR. However, D 1 agonists increased NHE3 linked with G s␣ in WKY but not in SHR, and the inhibitory effects of D 1 agonists on NHE activity were less in SHR than in WKY. Moreover, GTP␥S enhanced the inhibitory effect of D 1 agonist on NHE activity in WKY but not in SHR, suggesting an uncoupling of the D 1 receptor from G s␣ /NHE3 in SHR. Similar results were obtained with the use of immortalized renal proximal tubule cells from WKY and SHR. We conclude that the defective D 1 receptor function in renal proximal tubules in SHR is proximal to G s␣ /effectors and presumably at the receptor level. The mechanism(s) responsible for the uncoupling of the D 1 receptor from G proteins remains to be determined. Because the primary structure of the D 1 receptor is not different between normotensive and hypertensive rats, differences in D 1 receptor posttranslational modification are possible. Key Words: dopamine Ⅲ receptors, dopamine Ⅲ G protein Ⅲ rats, inbred SHR R enal dopamine production, dopamine receptors, and dopamine receptor regulation are important in the pathogenesis of hypertension. 1 Dopamine, produced by renal proximal tubules, is an important paracrine/autocrine inhibitor of renal sodium transport under conditions of sodium loading. 1 The inhibition of sodium transport in renal proximal tubules by dopamine is exerted via the Na ϩ -H ϩ exchanger (NHE) at the luminal or brush border membrane (BBM) and via Na ϩ ,K ϩ -ATPase at the basolateral membrane. 1 The major transport of sodium across the luminal membrane of renal proximal tubules is caused by NHE activity. 2 Five of the 6 isoforms of NHE are expressed in the kidney. 3,4 However, the NHE3 isoform predominates in the BBM of rat renal proximal tubules. 2,5 There is an impaired ability of dopamine 1 (D 1) -like receptors to inhibit NHE activity in BBM of the spontaneously hypertensive rat (SHR). 1 The decreased inhibitory effect of D 1 -like receptors on NHE activity in renal proximal tubules and failure to induce a natriuresis cosegregate with hypertension in SHR and normotensive WistarKyoto rat (WKY) crossbreeds. 6 However, it is not clear whether the impaired inhibitory action of D 1 -like receptors on NHE activity is at the receptors, G proteins, signal transducers, or effectors. G proteins can regulate NHE acti...

Key Words: dopamine Ⅲ receptors, angiotensin II Ⅲ mice Ⅲ hypertension Ⅲ angiotensin II Ⅲ endothelin Ⅲ vasopressins E ssential hypertension is a major risk factor for the development of cardiovascular disease. 1 It is a heterogeneous disease in which both genetics and environment influence blood pressure. 2 Dopamine affects cardiovascular regulatory mechanisms by its actions on renal hemodynamics and ion and water transport and by its regulation of hormones and humoral agents, such as aldosterone, catecholamines, endothelin, prolactin, proopiomelanocortin, renin, and vasopressin. In addition, dopamine can control blood pressure by acting on neuronal cardiovascular centers, heart, and arterial and venous vessels. Methods Generation of D 4 Dopamine-Receptor Mutant MiceThe original F 2 hybrid strain (129/SvϫC57BL/6) carrying a mutant form of the D 4 dopamine receptor was initially generated and backcrossed to C57BL/6 mice. Heterozygous (D 4 ϩ/Ϫ ) mice were mated to obtain D 4 Ϫ/Ϫ and D 4 ϩ/ϩ littermates, and the D 4 Ϫ/Ϫ mice were backcrossed with C57BL/6 mice to obtain sixth-and tenthgeneration mice in the Department of Physiology and Pharmacology, Oregon Health and Science University. 21 We used sixth-generation mice for acute studies and tenth-generation mice for chronic and immunoblotting studies. All of the animals were genotyped 21 and treated in accordance with National Institutes of Health guidelines for ethical treatment and handling of animals in research.

Abstract-Polymorphism of the dopamine receptor type-2 (D 2 ) gene is associated with essential hypertension. To assess whether D 2 receptors participate in regulation of blood pressure (BP), we studied mice in which the D 2 receptor was disrupted. In anesthetized mice, systolic and diastolic BPs (in millimeters of mercury) were higher in D 2 homozygous and heterozygous mutant mice than in D 2 ϩ/ϩ littermates. BP after ␣-adrenergic blockade decreased to a greater extent in D 2 Ϫ/Ϫ mice than in D 2 ϩ/ϩ mice. Epinephrine excretion was greater in D 2 Ϫ/Ϫ mice than in D 2 ϩ/ϩ mice, and acute adrenalectomy decreased BP to a similar level in D Disruption of D 3 receptors in mice produces hypertension mediated, at least in part, by activation of the renin-angiotensin system. 7 The D 2 receptor could be involved in D 2 -likemediated hypertension because it is the major D 2 -like dopamine receptor. 3,8 -11 Moreover, BP decreased when a segment of chromosome 8 that contained the D 2 receptor gene was transferred from a normotensive Brown Norway rat to a spontaneously hypertensive rat background. 12 Several variants of the human D 2 dopamine receptor have been reported. 13 Abnormalities of D 2 receptor genes could play a role in the pathogenesis of essential hypertension, because the association of a D 2 dopamine receptor polymorphism with obesity and hypertension has been reported. 14 To determine whether D 2 receptors play a role in the regulation of BP, we measured arterial pressure in congenic B6 mice mutants for the D 2 receptor. 8,9 Because D 2 -like receptors have been shown to interact with vasopressor systems, 1,6,15,16 interactions between D 2 receptors and other vasopressor systems were also studied. Methods D 2 Receptor-Deficient MiceThe original F 2 hybrid strain (129/SvXC57BL/6J, Oregon Health Sciences University, Portland) that contained the mutated D 2 recep-