Biochemical, autoradiographic and pharmacological evidence for the involvement of tubular DA-1 receptors in the natriuretic response to dopexamine hydrochloride

Abstract

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.

Dopexamine Hydrochloride in the Human Kidney: Localization, Receptor Binding and Effect on 3’-5’-Cyclic Adenosine Monophosphate Generation

“…Dopexamine stimu lated the accumulation of cAMP by interacting with do pamine DA-1 receptors and with (T-adrenoceptors. Similar observations have been made in the rat kidney [12].…”

“…Furthermore, dopexamine treatment pre vents and reverses renal failure produced in a canine model of hemorrhagic shock [20], However, in spite of the poten tial usefulness of dopexamine in the treatment of renal failure, no detailed characterization of the receptor profile of dopexamine has been performed in humans. This prompted us to investigate systematically the binding char acteristics of the compound and the effect of dopexamine on cAMP generation using the same approach recently standardized for the rat kidney [12],…”

“…To assess the pharmacological specificity of [!H]-dopexamine binding to the human kidney, other sections were incubated as described above with 2 nMpH]-dopexamine in the presence of I pM SCH 23390 (to block the binding of [3H]-dopexamine to DA-I recep tors), I pM ICI 118,551 (to block binding to (T-adrenoceptors) or 100 pM domperidone (to block binding to DA-2 receptors), or with the various combinations of displacers [for further details see ref. 12,13]. After incubation, sections were washed in ice-cold incubation buffer (2 x 5 min) to remove excess unbound ['H]-dopexamine.…”

“…Moreover, we have shown that in membrane particles of the rat kidney, simi larly as found in the rat superior mesenteric artery [13], dopexamine stimulates the 3'-5'-cyclic adenosine mono phosphate (cAMP)-generating system by activating both DA-1 and P2-adrenoceptors [12]. In terms of functional correlates, these studies have demonstrated that the diuretic and natriuretic effects of dopexamine are mediated by the selective activation of dopamine DA-1 receptors on renal tubules [12].…”

“…We have recently demonstrated, using combined bio chemical, autoradiographic and functional studies, that in the rat kidney, dopexamine binds predominantly to do pamine DA-1 receptors in renal tubules, to p2-adrenoceptors in the glomerulus and to DA-1, DA-2 and (32-adreno ceptors in the renal vascular tree [12]. Moreover, we have shown that in membrane particles of the rat kidney, simi larly as found in the rat superior mesenteric artery [13], dopexamine stimulates the 3'-5'-cyclic adenosine mono phosphate (cAMP)-generating system by activating both DA-1 and P2-adrenoceptors [12].…”