Role of peripheral adrenoreceptors and vasopressin in the suppression of plasma renin activity by L-dopa in carbidopa-treated dogs.

When extracerebral dopa decarboxylase is inhibited by carbidopa, L-dopa lowers plasma renin activity (PRA). The present study was designed to determine whether this suppression of PRA is mediated by the sympathetic nerves, and to identify the peripheral adrenergic receptor types involved. All experiments were performed in pentobarbital-anesthetized dogs in which changes in renal perfusion pressure were minimized by means of a suprarenal aortic clamp. Neither alpha adrenoreceptor blockage with phenoxybenzamine nor beta adrenoreceptor blockade with propranolol was by itself sufficient to block the suppression of PRA by L-dopa with carbidopa. However, combined alpha and beta adrenoreceptor blockade lowered PRA and completely prevented any further suppression of PRA by L-dopa with carbidopa. It was also observed that phenoxybenzamine decreased PRA by 48% when administered to propranolol-treated animals. Taken together, these data indicate that L-dopa with carbidopa suppresses PRA by decreasing sympathetic nerve stimulation of both alpha and beta adrenoreceptors. Plasma vasopressin concnetration was significantly decreased by L-dopa with carbidopa both in the control group and in animals with combined alpha and beta adrenoreceptor blockade. Because plasma vasopressin levels decreased after L-dopa, vasopressin is unlikely to play a causative role in the suppression of PRA.