Acute volume expansion enhances baroreceptor discharge in dogs

By recording single-unit discharge from the carotid sinus nerve of normal dogs in the control and in the volume-expanded state, both with the sinus open to the circulation and with it isolated, the effects of acute volume expansion on baroreceptor discharge sensitivity were investigated. The carotid sinus was vascularly isolated except for the common carotid and external carotid arteries. Inflation of hydraulic occluders on these vessels completely isolated the sinus from the systemic circulation. Flow and diameter changes of the carotid sinus were measured. Plasma volume was expanded with isotonic, isoncotic dextran in normal saline until the left ventricular end-diastolic pressure reached a value of 20-25 mmHg. In the group with the sinus open to the circulation, volume expansion did not depress baroreceptor discharge sensitivity; in fact, it augmented baroreceptor activity (peak discharge = 39.2 +/- 1.6 vs. 49.9 +/- 3.3 spikes/s, P less than 0.05). There was no such effect in the group with the sinus isolated from the circulation during volume expansion. In the group with the sinus open to the circulation, volume expansion significantly increased flow through the common carotid artery and shifted the carotid sinus pressure-diameter curves upward without a change in compliance. These data suggest that acute volume expansion augments baroreceptor discharge, and this phenomenon may be mediated either by some circulating substance(s) or by an increase in flow through the carotid sinus during volume expansion.