Pressure-Independent Inhibition of Sympathetic Activity by Noradrenaline: Role of Baroreceptor C Fibres*

Abstract

The effect of i.v. infusion of noradrenaline [noreprnephrine, NE] on activity in the renal sympathetic nerve was studied in rabbits anesthetized with chloralose and urethane. NE (3-8 .mu.g/kg .cntdot. min) initially increased mean arterial pressure 20-40 mmHg and consequently reduced renal nerve activity. Studies over a wide range of pressures-obtained by changing the blood volume, revealed that NE after a few minutes had induced a presure-independent reduction of sympathetic discharge. The effect disappeared with baroreceptor denervation. An unchanged relationship between arterial pressure and integrated activity in the whole left aortic nerve (which is largely a measure of activity in A fibers) suggested that the sympathetic depression was due to excitation of aortic nerve C fibers. This conclusion was supported by studies of sympathetic responses to selective stimulation of aortic nerve A and C fibers at equal pressures before and during NE infusion. Compared to the reflex activity from A fibers, C fiber stimulation was invariably less effective in suppressing renal nerve activity during the infusion. Studies indicate that NE may effect a negative feedback control of sympathetic discahrge through activation of baroreceptor C fibers.