It is reported that a defect in dopamine-1 (DA-1) receptor adenylate cyclase coupling in the proximal convoluted tubule in the spontaneously hypertensive rat may contribute to the diminished natriuretic response to DA-1 receptor agonists. Since the tubular DA-1 receptor is also coupled to phospholipase C, and both of these cellular signaling processes are involved in DA-1 receptor-mediated diuresis and natriuresis, it is important to know whether a similar defect is also present in DA-1 receptor-coupled phospholipase C pathway. The present study was therefore designed to determine the functional status of DA-1 receptor-phospholipase C coupling system of adult spontaneously hypertensive rats using a renal cortical slice preparation. In addition, the renal response to exogenously administered dopamine (1 /ig/kg/min i.v.) was also determined. We found that basal phospholipase C activity was significantly higher in hypertensive rats than in age-matched Wistar-Kyoto rats (7J6±0J2% versus 5.61±0.27%, p<0.05). However, compared with the normotensive controls, dopamine-induced increases in phospholipase C activity were significantly attenuated in the preparations of hypertensive rats in a concentration-dependent manner (13±6% versus 38±6% for 1 mM dopamine, p<0.05; 49±6% versus 71±9% for 3 mM dopamine, p<0.05; 50±16% versus 106±22%, p<0.05 for 10 mM dopamine). The diminished dopamine-induced phospholipase C activation was due to a deficiency in dopamine receptor-phospholipase coupling since the DA-1 receptor antagonist SCH 23390 (30 /iM), which blocked 50% of dopamine-induced inositol phosphate production in the Wistar-Kyoto rats, did not exert such an effect in the spontaneously hypertensive rats. The in vivo functional study showed that the diuretic and natriuretic responses to intravenous administration of dopamine were significantly diminished in spontaneously hypertensive rats compared with Wistar-Kyoto rats (urine output, 34± 1 versus 63±10 fil/30 min,p<0.05; urinary sodium excretion, 1.8±0.1 versus 3.2±0.2 ^teq/30 min,/x0.05). These results suggest that there is a defect in the tubular DA-1 receptor-phospholipase C coupling process in spontaneously hypertensive rats that may contribute to a diminished natiiuretic response to DA-1 receptor activation. (Hypertension 1992;19:102-108) I t is generally accepted that an abnormality in renal handling of sodium is one of the factors in the pathogenesis and/or maintenance of high blood pressure in genetic hypertension. Central to this assumption are the observations that spontaneously hypertensive rats (SHR), as opposed to their normotensive Wistar-Kyoto (WKY) counterparts, retain sodium avidly before the development of hypertension 1 and that urinary sodium excretion (U Na V) in SHR is normal or subnormal despite the presence of elevated blood pressure.2 Furthermore, dietary sodium restriction retards the development of hypertension in SHR.

Activation of phospholipase C (PLC) is considered to be one of the cellular signaling events involved in dopamine (DA)-mediated natriuresis. In the present study we have examined the role of renal cortical PLC in contributing to the increase in urinary sodium excretion during high sodium intake and its relationship with intrarenal DA synthesis. Rats were given either 1% NaCl (high sodium intake) or tap water (normal sodium intake) to drink for 24 h, and urine was collected over this time period. PLC activity in the renal cortex from these rats was measured by prelabeling cortical slices with myo-[2-3H]inositol and was expressed as fractional release (FR) of inositol (mono-, bis-, and tris-) phosphates. Acute increase in sodium intake produced 93 +/- 8% increase over control in urinary DA excretion. These changes were accompanied by significant increases (30 +/- 8%) in basal FR of inositol phosphates and 243 +/- 40 and 76 +/- 14% increases in urinary sodium and water excretion, respectively. The elevated basal PLC activity in rats with high sodium intake was significantly reduced in the presence of Sch 23390, a selective DA-1 receptor antagonist. Exogenously added DA (3 mM) also produced significant increases in PLC activity, although the magnitudes of increases were different in rats with high (37 +/- 8%) and normal (66 +/- 9%) sodium intake. However, Sch 23390 alone or carbidopa pretreatment did not affect the basal PLC activity in rats maintained on normal sodium intake.(ABSTRACT TRUNCATED AT 250 WORDS)

Dopexamine hydrochloride (DPX) is a dopamine analog and it possesses agonistic action at DA-1 receptors and beta 2-adrenoceptors. It also is a weak agonist at DA-2 receptors. In the present study, we have examined the anatomical localization of DPX binding sites in rat kidney and their functional significance in terms of the renal effects of this compound. In receptor-ligand binding studies, [3H]-DPX was found to bind specifically to sections of rat kidney in a time (maximum binding at 60 min), temperature (optimal temperature 25 degrees C) and concentration (highest specific/non-specific ratio at 2 nmol/l) dependent manner. Autoradiographic studies revealed the presence of [3H]-DPX binding sites in renal tubules, glomerulus and various layers of small and large blood vessels. Inhibition studies with SCH 23390, ICI 118.551 and 1-sulpiride showed that DPX binds primarily to DA-1 receptors in tubules, only to beta 2-adrenoceptors in glomerulus and to beta 2-adrenoceptors, DA-1 and DA-2 receptors in blood vessels. Also, DPX caused concentration related increases in cyclic AMP levels in rat kidney membrane particles, which could be completely abolished by a combined presence of SCH 23390 and propranolol suggesting that both binding sites of DPX are linked to adenylate cyclase. In functional studies DPX (1 microgram/kg.min for 30 min) produced a modest fall in blood pressure, pronounced tachycardia and slight but significant increase in renal blood flow (11%). These responses were accompanied by increases in urine output (97%), urinary sodium excretion (89%), and fractional excretion of sodium (132%). There was no change in glomerular filtration rate. Propranolol pretreatment abolished DPX-induced hypotension and tachycardia but seemed to potentiate the natriuretic responses to DPX. On the other hand, SCH 23390, a DA-1 receptor antagonist completely abolished DPX-induced hypotension, natriuresis and diuresis without affecting tachycardia. These results indicate that (1) DPX binds predominantly to DA-1 receptors in renal tubules, to beta 2-adrenoceptors in glomerulus and to beta 2-adrenoceptors, as well as DA-1 and DA-2 receptors in renal blood vessels (2) DPX stimulates cAMP formation in the kidney by activating both DA-1 and beta 2-adrenoceptors and (3) DPX produces natriuresis and diuresis by selectively activating DA-1 receptors located on renal tubules.

Dopamine receptors of the DA₁ subtype have been identified in mesenteric blood vessels, stimulation of which leads to vasodilation. In this study, we have determined the anatomical localization of dopexamine-hydrochloride-binding sites and carried out functional characterization of responses to this DA₁-receptor and β₂-adrenoceptor agonist in rat mesenteric vasculature. Autoradiographic studies showed the presence of [³H]-dopexamine-binding sites in all the different layers of the mesenteric artery. The DA₁ receptor antagonist, SCH 23390 (IC50 = 4.9 μmol/l), and the β-adrenoceptor antagonist, propranolol (IC50 = 6.0 μmol/l), inhibited the binding of dopexamine. The inhibitory effect of these compounds on dopexamine binding was selective for different regions of the mesenteric artery. Also, dopexamine produced concentration-related increases in cAMP formation in membrane particles from superior mesenteric artery and its main branches. The presence of both SCH 23390 and propranolol was required to completely abolish dopexamine-induced increases in cAMP formation. In functional studies, dopexamine (1 and 3 μg/kg/min) produced dose-related increases in mesenteric blood flow (23 and 38%, respectively) which were accompanied by concomitant decreases in the calculated mesenteric vascular resistance. As seen with increases in cAMP, the vascular responses to dopexamine could be completely abolished only by prior treatment with both SCH 23390 and propranolol. These results suggest that in mesenteric vasculature of rat dopexamine binds primarily to DA₁ receptors and β₂-adrenoceptors. The activation of these receptors by dopexamine leads to vasodilation which is mediated by an increase in the intracellular levels of cAMP.

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