Effect of norepinephrine and acetylcholine on outer medullary descending vasa recta

To examine their responsiveness to norepinephrine (NE) and acetylcholine (ACh), outer medullary descending vasa recta (OMDVR) have been dissected from vascular bundles of the rat and perfused in vitro. Abluminal application of NE produced graded vasoconstriction in a concentration range of 10(-9)-10(-6) M. When applied with NE, ACh at concentrations of 10(-8)-10(-5) M dilated NE-preconstricted OMDVR. In contrast, ACh applied in the absence of NE caused vasoconstriction. ACh-induced vasodilation was blocked by addition of the nitric oxide synthase inhibitor N omega-nitro-L-arginine (L-NNA, 2 x 10(-4) M). L-NNA in the absence of ACh enhanced NE-induced vasoconstriction. Supraphysiological (10(-3) M) L-arginine (L-Arg) reversed the effects of L-NNA, and abluminal application of L-NNA alone resulted in OMDVR vasoconstriction. At concentrations of 10(-6)-10(-3) M, abluminal application of L-Arg produced graded vasodilation of NE-constricted OMDVR. These results suggest that adrenergic and cholinergic innervation could influence OMDVR vasomotor tone to modulate total and regional blood flow to the renal medulla. The data also favor a role for the activity of constitutively expressed nitric oxide synthase to modulate OMDVR vasoactivity.