The atrial natriuretic factors (ANF) have recently emerged as a novel hormonal system [3, 10] that may be implicated in extracellular volume homeostasis [9]. Recently, employing our highly sensitive radioimmunoassay [1], we have demonstrated elevated A N F plasma levels in patients with diseases involving pressure/volume overload [2, 6]. The physiologic regulation of ANF release in healthy subjects, however, needs further elucidation [7]. Water immersion causes an increase in central venous pressure due to a shift of extracellular volume to the intrathoracic venous bed [4]. This increase in central venous and atrial pressure has been shown to increase diuresis and natriuresis [5, 8]. In the present study, the hypothesis is tested, that water immersion, via the increase in central venous pressure, stimulates an enhanced release of A N F into the circulation that may contribute to the ensuing diuresis.

Abstract. The role of the atrial natriuretic factor (ANF), its second messenger cyclic guanosine monophosphate (cGMP), and the counteracting reninaldosterone system in acute volume regulation was investigated in 25 healthy human subjects. Central volume stimulation by I-h head-out water immersion (WI) into a thermoneutral water-bath increased plasma levels of ANF (mean f SEM) from 6.0 0.6 to 13.6 If: 2.6 fmol ml-I . This was paralleled by a rise of plasma cGMP levels from 1.9f0.2 to 2.8f0.4 pmol ml-I, and an increase of urinary cGMP excretion from 340 _+ 64 to 692 f 103 pmol min -', Water immersion reduced plasma aldosterone concentration (PAC) from 13.0f1.7 to 6.5f0.8 ng 100 ml-' and plasma renin activity (PRA) from 5.3 f 0.9 to 2.4 f 0.3 ng A1 ml-h-]. Volume stimulation markedly increased diuresis and natriuresis. Whereas the plasma cGMP increase correlated with plasma ANF stimulation, neither ANF nor PRAor PAC correlated with basal or stimulated renal parameters. Water immersioninduced changes in natriuresis and urinary cGMP excretion were correlated, These data suggest a role of ANF and cGMP in acute volume regulation of healthy human subjects.

Exercise in water compared to land-based exercise (LE) results in a higher release of natriuretic peptides, which are involved in the regulation of exercise-induced adipose tissue lipolysis. The present study was performed to compare the release of atrial natriuretic peptide (ANP) and free fatty acids (FFA) during prolonged aerobic water-based exercise (WE) with the release after an identical LE. 14 untrained overweight subjects performed 2 steady state workload tests on the same ergometer in water and on land. Before and after exercise, venous blood samples were collected for measuring ANP, FFA, epinephrine, norepinephrine, insulin and glucose. The respiratory exchange ratio (RER) was determined for fat oxidation.The exercises resulted in a significant increase in ANP in LE (61%) and in WE (177%), and FFA increased about 3-fold in LE and WE with no significant difference between the groups. Epinephrine increased, while insulin decreased similarly in both groups. The RER values decreased during the exercises, but there was no significant difference between LE and WE. In conclusion, the higher ANP concentrations in WE had no additional effect on lipid mobilization, FFA release and fat oxidation. Moderate-intensity exercises in water offer no benefit regarding adipose tissue lipolysis in comparison to LE.

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