Response of isolated rat descending vasa recta to bradykinin.

Outer medullary descending vasa recta (OMDVR) were dissected from the outer medullary vascular bundles of young rats, perfused in vitro, and loaded with fura 2 for measurement of intracellular calcium concentration ([Ca2+]i) by fluorescent ratio imaging. Fluorescent video images revealed that fura 2 selectively loads into endothelial cells but not pericytes. Bradykinin (BK), at concentrations >10-11 M, elicited an increase in [Ca2+]ifrom baseline values in the range from 50 to 100 nM to peak values of 600-800 nM followed by a sustained plateau of 150-250 nM. The vasopressin V1-receptor agonist [Phe2,Ile3,Orn8]vasopressin constricted OMDVR but yielded no observable [Ca2+]iresponse, a finding that is consistent with an endothelial cell origin for the fura 2 fluorescent signal. The BK [Ca2+]iresponse was blocked by the selective BK B2-receptor antagonistsd-Arg-[Hyp3,Thi5,8, d-Phe7]BK andd-Arg-[Hyp3,d-Phe7,Leu8]BK but not the B1 antagonist des-Arg9-[Leu8]BK. BK vasodilated microperfused OMDVR that had been preconstricted with 10-8 M angiotensin II. We conclude that the [Ca2+]iresponse of OMDVR endothelia can be selectively studied with fura 2, that BK increases endothelial [Ca2+]ivia the B2 receptor, and that BK can vasodilate descending vasa recta.