VOLUME-HOMEOSTATIC MECHANISMS IN HUMANS DURING GRADED WATER IMMERSION

The purpose of this experiment was to investigate whether a graded inc rease in cardiac distension induced by graded water immersion (WI) cou ld be related to endocrine responses and renal sodium excretion (UNaV) . On 3 separate days, nine healthy males were investigated in the upri ght seated position before, during, and after 3 h of WI to the midches t (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, solut e-free water clearance, and osmotic excretion increased more during th e 2nd h of NI than during CI. During NI, the plasma concentration of a trial natriuretic peptide (ANP) increased twice as much as during CI. The plasma concentrations of aldosterone and norepinephrine were decre ased in a similar manner during NI compared with CI. In conclusion, gr aded cardiac distension induced by graded WI and accompanied by a grad ed release of ANP was not accompanied by a graded increase in UNaV. Th us either a cardiac distension pressure of approximately one-half of t hat during NI is enough to induce a maximum UNaV during WI or other st imuli are important. Furthermore, aldosterone and norepinephrine are p robably more important mediators of the natriuresis of WI in humans th an is ANP.