Plasma atrial natriuretic peptide and vasopressin during thermal dehydration in supine posture

After 30 minutes spent in an upright posture six healthy male subjects underwent two 130-minute experiments in a supine posture, the first in thermoneutral conditions (TC) and the second, 15 days later, in a hot environment (HE) in order to obtain a water loss of 2.5% body weight. In thermoneutral conditions, the supine posture induced plasma volume expansion, resulting in slightly lowered plasma vasopressin (AVP) levels and higher plasma atrial natriuretic peptide (ANP) levels, compared to the values obtained in the upright posture (P less than 0.05). During hot environment, the sweating-induced dehydration led to a significant reduction of plasma volume expansion and to an increase in rectal temperature and plasma osmolality (P less than 0.05). Plasma vasopressin levels were higher at the end of the heat exposure (P less than 0.05) but natriuretic peptide levels did not change, compared to the values observed in the upright posture. These data suggest that plasma volume reduction induced by thermal dehydration may limit the natriuretic peptide release, which occurs after changing from the upright to a supine position.