Subarachnoid hemorrhage and endothelial L-arginine pathway in small brain stem arteries in dogs

Background and Purpose: Experiments were designed to determine the effect of subarachnoid hemorrhage on endothelium-dependent relaxations in small arteries of the brain stem. A "double-hemorrhage" canine model of the disease was used, and the presence of vasospasm in the basilar artery was confirmed by angiography. Methods: Secondary branches of both untreated basilar arteries (inner diameter, 324±11 μm; n=12) and arteries exposed to subarachnoid hemorrhage for 7 days (inner diameter, 328±12 μm; n=12) were dissected and mounted on glass microcannulas in organ chambers. Changes in the intraluminal diameter of pressurized arteries were measured using a video dimension analyzer. Results: In untreated arteries, 10^-11 to 10^-7 M vasopressin, 10^-10 to 10^-6 M bradykinin, and 10^-9 to 10^-6 M calcium ionophore A23187 caused endothelium-dependent relaxations. At 10^-6 and 3×10^-4 M the nitric oxide synthase inhibitor N^G-nitro-L-arginine methyl ester (L-NAME) abolished relaxations to vasopressin and produced small but significant rightward shifts of the concentration-response curves to bradykinin and A23187. At 10^-3 M L-arginine prevented the inhibitory effect of L-NAME. Subarachnoid hemorrhage abolished relaxations to vasopressin but did not affect relaxations to bradykinin or A23187. Conclusions: These studies suggest that in small arteries of the brain stem vasopressin causes relaxations by activation of the endothelial L-arginine pathway. This mechanism of relaxation is selectively inhibited by subarachnoid hemorrhage. Preservation of endothelium-dependent relaxations to bradykinin and A23187 is consistent with the concept that small arteries are resistant to vasospasm after subarachnoid hemorrhage.