Effects of water immersion on sympathoadrenal and dopa-dopamine systems in humans

Water immersion to the neck increases central blood volume and evokes a marked diuresis and natriuresis. The present study examined simultaneously effects of water immersion on activities of three endogenous systems thought to participate in sodium homeostasis: the sympathetic nervous system, the atrial natriuretic peptide system, and the renal dopa-dopamine system. Hourly urine collections and antecubital venous blood samples were obtained from 10 normal subjects before, during, and after sitting in a water-immersion tank for 3 h; four control subjects were studied while seated without immersion. Urine volume was increased by more than threefold after 1 h of immersion (from 1.2 ± 0.2 ml/min at baseline to 5.9 ± 0.7 ml/min, P < 0.001) and peaked during the second hour. Urinary sodium excretion increased by more than twofold (from 103 ± 17 μeq/min at baseline to 196 ± 36 μeq/min at 1 h, P < 0.001) and peaked during the third hour. Plasma levels and urinary excretion of norepinephrine (NE) and epinephrine were suppressed consistently during immersion (P < 0.05). There was a marked, prompt, and sustained increase in plasma levels of immunoreactive atrial natriuretic factor (irANF), from 6.9 ± 1.9 pg/ml at baseline to 17.3 ± 4.3 pg/ml at 1 h (P < 0.001). Urinary excretion of dopa, dopamine, and 3,4-dihydroxyphenylglycol, a neuronal metabolite of NE, changed in a triphasic pattern, with decreased excretion during the first hour of immersion (P < 0.01), small but consistent increases during the next 2 h, and decreased excretion, to below baseline, during recovery (P < 0.01 for dopa and dopamine). Excretion rates of all these compounds were unchanged in the seated control subjects. The findings suggest that central hypervolemia inhibits sympathoadrenal outflow, stimulates release of atrial natriuretic peptide into the bloodstream, and produces a multiphasic effect on renal dopa-dopamine system activity.