Effects of water immersion on atrial natriuretic peptide release in humans.

Water immersion (WI) to the neck induces prompt increases in central blood volume, central venous pressure, and atrial distension. Since atrial distension is a major stimulus for atrial natriuretic peptide (ANP) release, WI constitutes a unique model to investigate the physiological importance of ANP in humans. All studies to date demonstrate that ANP increases during the 1st h of WI in hydrated subjects, rising 2.5- to 3-fold by the end of the 2nd or 3rd h. During recovery from WI, ANP returns promptly to prestudy levels. Although distension of the atria probably constitutes the major stimulus for ANP release during WI, other mechanisms may coexist. In normal hydrated subjects undergoing WI, the increases in ANP correlate with the magnitude of the natriuresis, suggesting that ANP constitutes an important determinant of renal sodium handling in humans. It is simplistic, however, to consider the WI-induced augmentation of ANP to be the sole, or even the prepotent, mediator of the resultant natriuresis. Rather ANP should be considered as one of several hormonal, neural, and hemodynamic factors acting in concert in an integrated matrix to modulate volume homeostasis. Clearly, the results of several recent studies utilizing WI demonstrate that this maneuver constitutes an important investigative model to delineate further the role of ANP in subserving volume homeostasis in normal humans and in disorders of deranged volume regulation.