Effects of Noradrenaline on Activity in Single Aortic Baroreceptor Fibres

Abstract The possibility of sympathetic control of the sensitivity of arterial baroreceptors has been examined by recording activity of single baroreceptor fibres in the rabbit's aortic nerve before and during intravenous infusion of noradrenaline (4–6 μg/kg × min). Activity was related to blood pressure — which could be changed by bleeding and reinfusion of blood — and to diameter of ascending aorta measured with ultrasonic technique inside the intact thorax. At the same blood pressures as in control runs, activity of 34 single myelinated fibres (conduction velocities 10–35 m/s) was lowered by noradrenaline, on average by 1 spike per pulse beat, and diastolic aortic diameter was reduced by about 7%. When compared at equal diastolic diameters, pulsatile diameter variations were smaller with noradrenaline than in control runs, but even so, receptor activity was found to have risen by 2–3 spikes per pulse beat, i.e. , up to 50 %. Thinner myelinated fibres (conduction velocities about 5 m/s) occasionally showed higher than normal activity also at comparable blood pressures. The results demonstrate that the sensitivity of aortic baroreceptors to distension of the receptor area can be increased by noradrenaline. However, as aortic diameter is simultaneously decreased, activity in most single baroreceptor fibres was slightly reduced at any given blood pressure during intravenous infusion of noradrenaline.