Renal responses to prolonged central volume expansion in conscious primates.

Urinary and hormonal responses to prolonged central vascular volume expansion induced by 4 days of continuous lower body positive air pressure (LBPP) were examined in conscious, chair-acclimatized squirrel monkeys. The initial 12 h of exposure to LBPP (20 mmHg) induced a marked natriuresis, diuresis, kaliuresis, and decrease in urine osmolality. During the subsequent 4 days of LBPP stimulation, the acute natriuresis gradually declined by 75%, whereas potassium excretion completely returned to control rates after 24 h. In contrast, the diuresis and decrease in osmolality persisted, although there were significant circadian variations in urine flow (as well as sodium and potassium excretion) produced by a marked nocturnal suppression of renal responses to LBPP. Plasma aldosterone and potassium concentration both decreased acutely following LBPP initiation; however, aldosterone levels subsequently returned to normal range during continued LBPP exposure, whereas plasma potassium remained low until the pressure stimulus was terminated. These results indicated that prolonged central volume expansion induced both a sustained natriuresis and diuresis; however, chronic adaptation limited net fluid and electrolyte losses through the first 24 h of LBPP exposure. The loss of potassium appeared to be a consequence of the initial diuresis and natriuresis induced by LBPP, rather than being directly mediated by aldosterone.