Canine renal responses to atrial stretch or intravenous saline are not attenuated at night

Primates develop a diuresis and natriuresis in response to stimuli that increase central blood volume. However, the renal response is markedly attenuated when these same experimental stimuli are applied at night. The purpose of the present study was to determine whether a quadruped, specifically the dog, also demonstrates attenuated renal responses to similar stimuli given at night. Two experimental protocols were performed on conscious dogs at night: 1) left atrial distension by partial mitral valve obstruction and 2) intravenous saline administration. Results from these experiments were compared with results from comparable experiments performed on the same dogs during the day. Each experimental intervention produced increases in urine flow and sodium excretion and decreases in urine osmolality during both day and night experiments (all P less than 0.05). The responses obtained during the night did not differ significantly from those obtained during the day. Thus the dog does not demonstrate an attenuated renal response to central volume expansion during the night. This contrasts with the adaptive response in primates that serves to minimize fluid loss when the intrathoracic blood pool is increased during prolonged recumbency at night. Presumably quadrupeds, with trunks horizontal both day and night, do not require nocturnal adaptive mechanisms to conserve body fluids.