MULTIPLE FACTORS CONTRIBUTE TO ACETYLCHOLINE-INDUCED RENAL AFFERENT ARTERIOLAR VASODILATION DURING MYOGENIC AND NOREPINEPHRINE-INDUCED AND KCL-INDUCED VASOCONSTRICTION - STUDIES IN THE ISOLATED-PERFUSED HYDRONEPHROTIC KIDNEY

Acetylcholine (ACh) elicits vasodilation by releasing a number of endothelium-derived relaxing factors (EDRFs). We used the isolated perfused hydronephrotic rat kidney to examine the characteristics of ACh-induced vasodilation of renal afferent arterioles during different types of underlying vasoconstriction. Basal arteriolar tone was increased by either elevating perfusion pressure to 180 mm Hg (myogenic), administering 0.3 mu mol/L norepinephrine (NE), or elevating medium potassium concentration to 30 mmol/L (KCl). ACh (10 mu mol/L) completely reversed myogenic and NE-induced vasoconstriction and reversed KCl-induced vasoconstriction by 80 +/- 5%. However, whereas ACh produced a sustained vasodilation during KCl- and NE-induced vasoconstriction, only a transient reversal of myogenic vasoconstriction was observed, and myogenic tone recovered within 5 to 10 minutes. ACh-induced vasodilation of arterioles preconstricted with KCl was markedly inhibited by either indomethacin (100 mu mol/L) or nitro-L-arginine (100 mu mol/L) and was completely abolished by pretreatment with both inhibitors. In contrast, indomethacin and nitro-L-arginine had no effect on the transient response to ACh observed during pressure-induced vasoconstriction. In vessels preconstricted with NE, nitro-L-arginine converted the normally sustained response to ACh to a transient vasodilation, which was refractory to both nitric oxide synthase and cyclooxygenase inhibition. Since this component was not observed during KCl-induced vasoconstriction, it may reflect the actions of an, as yet unidentified, endothelium-derived hyperpolarizing factor (EDHF). Our findings thus suggest that prostanoids, nitric oxide, and EDHF all contribute to ACh-induced renal afferent arteriolar vasodilation and that the relative contributions of these individual EDRFs depends on the nature of the underlying renal vascular tone.