Serial evaluation of the renin-angiotensin-aldosterone system in caval dogs.

The blood pressure (BP) response to a competitive inhibitor of angiotensin II (saralasin), was evaluated serially along with renal hemodynamics and sodium clearance in dogs with constriction of the thoracic inferior vena cava which were undergoing chronic sodium and water balance studies. Plasma renin activity (PRA), plasma and urinary aldosterone, and plasma volume were measured periodically. In 14 normal dogs under anesthesia, saralasin infusion, 6ptg/kg per min, produced a transient agonistic pressor response. In 14 dogs after caval constriction, sodium balance became markedly positive, PRA and plasma and urine aldosterone increased, and sodium clearance decreased. Now saralasin infusion caused a decrease in BP. In five dogs, avid sodium retention persisted. In nine, after a variable period of time, sodium balance became less positive, PRA and plasma and urine aldosterone decreased, and sodium clearance increased. Saralasin infusion at this time again produced a transient agonistic pressor response. Renal hemodynamics remained stable throughout. Plasma volume increased following caval constriction. These data suggest that initially, following caval constriction, the renin-angiotensin-aldosterone system is responsible for BP maintenance. Following a period of salt and water retention and extracellular volume expansion, sodium excretion increases, the renin-angiotensin-aldosterone system is suppressed, and BP becomes sodium-volume dependent. These dynamic physiological adjustments which lead to great variability in caval dogs may help to reconcile some of the conflicting findings reported for this model of salt and water retention, as well as for its clinical counterparts.