Inhibition of saline-induced diuresis in the rat by sulpiride

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.

Dopamine receptors and hypertension. Physiologic and pharmacologic implications

The endogenous catecholamine dopamine lowers blood pressure by acting on two receptor subtypes: dopamine 1 and dopamine 2. Dopamine 1 receptors subserve vasodilation, especially in the renal, coronary, mesenteric, and cerebral vascular beds. Dopamine 2 receptors have been located at the endings of postganglionic sympathetic nerves and, when activated, inhibit norepinephrine release. Inhibition of emesis and inhibition of prolactin release also appear to be dopamine 2-mediated phenomena. The receptor subtypes have been classified by differences in chemical structure of agonists and by specific antagonists. Dopamine also acts on beta 1 receptors to stimulate the heart and alpha 1 and alpha 2 receptors to cause vasoconstriction. Alpha adrenergic activity and lack of oral availability limit the use of dopamine in the treatment of hypertension. However, studies with the selective dopamine 1 agonist, fenoldopam, and dopamine 2 agonists such as LY 141865 and bromocriptine, indicate that agonists of both receptor subtypes can lower blood pressure in experimental animals and in hypertensive patients. Initial use of dopamine agonists in the treatment of hypertension and its possible involvement in the etiology and maintenance of hypertension are discussed.

Dopamine-induced diuresis in the cat without changes in renal hemodynamics

The effects of intravenous (i.v.) and intraarterial (i.a.) injection and infusion of dopamine (DA) on renal hemodynamics, regional sympathetic activity and kidney function were investigated in anaesthetized cats. In response to the i.v. bolus injection of DA (25 microgram/kg), mean arterial blood pressure (MABP) was increased by 19.7%, renal blood flow (RBF) by 16.6%, and regional sympathetic discharges were inhibited. The principal effect of i.a. bolus injection of DA into the renal artery was vasoconstriction. Vasodilation was observed neither after lower doses of DA nor after pretreatment with phenoxybenzamine. During continuous i.v. infusion of 10 microgram DA kg-1 min-1 MABP, RBF, renal sympathetic discharges and glomerular filtration rate (GFR) did not change, whereas urine volume was increased by 120.5%, sodium excretion by 99.7%, chloride excretion by 143.2%, and potassium excretion by 31.9%. Urine osmolality was decreased and osmolal clearance increased. Raising the DA dose to 25 microgram kg-1 min-1 resulted in a fall of GFR, but the diuretic response was not significantly different from that of the low dose. Bulbocapnine (6 mg/kg i.v.) antagonized the DA-induced diuresis. In conclusion, the diuretic effect of DA in the cat is not dependent on a change in RBF, GFR or renal sympathetic activity. This suggests that a tubular site of action is primarily responsible for DA diuresis.