Cardiac afferents attenuate renal sympathetic baroreceptor reflexes during acute hypertension

We have studied the effect of acute hypertensive episodes on the renal sympathetic baroreceptor reflex in conscious rabbits and the role played by cardiac afferents and endogenous opiate mechanisms. Renal sympathetic nerve activity was recorded during brief perivascular balloon-induced ramp changes in mean arterial pressure before and during 40-minute elevations in resting pressure. Methoxamine infusion was adjusted to increase pressure by +30 and +45 mm Hg in the presence of autonomic blockade of the heart with atenolol and methscopolamine. Experiments were repeated in other rabbits after blocking cardiac afferents with 5% intrapericardial procaine or during intravenous naloxone (4-6 mg/kg, then 0.12 mg/kg/min). We found a progressively severe attenuation of the renal sympathetic baroreceptor reflex during increasing elevations in resting pressure. The upper plateau and range of the reflex curve were both reduced by one third and two thirds during moderate and severe hypertension, respectively. The average gain fell by 64% and 87%, and the range-independent gain and hypotensive reversal response were also reduced. There was no resetting of the reflex to higher pressures as would be expected. One third of the reflex inhibition was prevented by blocking cardiac afferents; none of it was affected by intravenous naloxone, which had previously been shown to reverse the renal baroreceptor reflex depression elicited by hemorrhagic hypotension. Factors possibly responsible for the remaining two thirds of the hypertension-induced sympathoinhibition are suggested to be either central depression of sympathetic tone after elevation of arterial baroreceptor discharge during the hypertensive episode or additional inhibitory afferent input arising from the pulmonary circulation.