Pressure-independent inhibition of sympathetic activity by noradrenaline: role of baroreceptor C fibres.

The effect of i.v. infusion of noradrenaline on activity in the renal sympathetic nerve was studied in rabbits anesthetized with chloralose and urethane. Noradrenaline (3--8 microgram/kg-min) initially increased mean arterial pressure 20--40 mmHg and consequently reduced renal nerve activity. However, studies over a wide range of pressures--obtained by changing the blood volume, revealed that noradrenaline after a few minutes had induced a pressure-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 fibres) suggested that the sympathetic depression was due to excitation of aortic nerve C fibres. This conclusion was supported by studies of sympathetic responses to selective stimulation of aortic nerve A and C fibres at equal pressures before and during infusion of noradrenaline. Compared to the reflex activity from A fibres, C fibre stimulation was invariably less effective in suppressing renal nerve activity during the infusion. Our studies indicate that noradrenaline may effect a negative feedback control of sympathetic discharge through activation of baroreceptor C fibres.