Responses of plasma atrial natriuretic polypeptide to isotonic volume expansion in conscious spontaneously hypertensive and chronically guanethidine-treated rats.

Responses of plasma immunoreactive atrial natriuretic polypeptide (IR-ANP) to isotonic volume expansion (100 ml/kg per h for 60 min) were investigated in conscious, unrestrained, chronically guanethidine-treated Sprague-Dawley rats and spontaneously hypertensive rats (SHR). Plasma IR-ANP increased during volume expansion and stabilized with continued infusion of isotonic solution. Guanethidine treatment did not alter resting and maximum responses of plasma IR-ANP. Spontaneously hypertensive rats and WKY showed similar plasma IR-ANP levels before volume expansion. Spontaneously hypertensive rats, however, exhibited earlier rises in plasma IR-ANP and larger increases in blood pressure, while maximum plasma IR-ANP response and total fluid retention (as judged from the weight gain) during volume expansion were similar in both groups. The present study suggests that: ANP release is regulated by the extent of filling of the circulation, sympathetic nervous system does not modulate release of ANP, and SHR have altered responses of ANP to volume overload, which may represent a compensatory mechanism that enables the SHR to eliminate excess sodium and fluid out of the circulation and thereby blunt further increases in blood pressure. In separate studies, we found that ether anaesthesia increases, while pentobarbital decreases the levels of plasma IR-ANP.