Oxyhemoglobin enhancement of vasopressin-induced constriction in rat cerebral arterioles

This study was undertaken to determine the effects of oxyhemoglobin on vasopressin-induced responses in cerebral arterioles. Rat intracerebral arterioles about 60 microns in diameter were isolated and cannulated using pipettes. Changes in diameter secondary to the extraluminal application of drugs were monitored through a video micro-scaler. Vasopressin produced a triphasic, dose-dependent response consisting of vasodilation (10(-11) M), vasoconstriction (10(-9)-10(-8) M) and a decrease in vasoconstriction (10(-7)-10(-6) M). Pretreatment with oxyhemoglobin (10(-5) M) abolished the vasodilation induced by the lower dose vasopressin and doubled the vasoconstriction induced by the higher dose. The combination of L-arginine (10(-4) M) and superoxide dismutase (600 U) restored low-dose vasopressin vasodilation and suppressed high-dose vasoconstriction in oxyhemoglobin-pretreated arterioles, while they showed little effect when used singly. This study indicates that oxyhemoglobin enhances vasopressin-induced constriction of intracerebral arterioles and these effects can be inhibited by the combination of L-arginine and superoxide dismutase.