Volume-homeostatic mechanisms in humans during graded water immersion

The purpose of this experiment was to investigate whether a graded increase in cardiac distension induced by graded water immersion (WI) could be related to endocrine responses and renal sodium excretion (UNaV). On 3 separate days, nine healthy males were investigated in the upright seated position before, during, and after 3 h of WI to the midchest (CI) or to the neck (NI) or during control. Central venous pressure increased twice as much during NI as during CI. UNaV increased to the same extent during NI compared with CI, whereas urine flow rate, solute-free water clearance, and osmotic excretion increased more during the 2nd h of NI than during CI. During NI, the plasma concentration of atrial natriuretic peptide (ANP) increased twice as much as during CI. The plasma concentrations of aldosterone and norepinephrine were decreased in a similar manner during NI compared with CI. In conclusion, graded cardiac distension induced by graded WI and accompanied by a graded release of ANP was not accompanied by a graded increase in UNaV. Thus either a cardiac distension pressure of approximately one-half of that during NI is enough to induce a maximum UNaV during WI or other stimuli are important. Furthermore, aldosterone and norepinephrine are probably more important mediators of the natriuresis of WI in humans than is ANP.