1. Eight normal volunteers were infused with 5% saline (5 g of NaCl/100 ml) at a rate of 0.06 ml min−1 kg−1 for 120 min to increase plasma osmolality and plasma arginine vasopressin. Human atrial natriuretic peptide (α-hANP; 100 μg) or placebo was given in random order in a double-bind cross-over design for the last 20 min of the saline infusion.

2. Compared with the placebo infusion, atrial natriuretic peptide (ANP) produced a 43% greater sodium excretion and a 34% greater urinary volume in the subsequent hour.

3. Mean plasma immunoreactive ANP did not increase in response to changes in osmolality and rose to a peak of 118 pg/ml during the α-hANP infusion. α-hANP produced significant suppression of mean plasma arginine vasopressin over the 60 min after the infusions.

4. We conclude that ANP is not released in response to increased osmolality in vivo, and that it inhibits osmolality-induced arginine vasopressin release in man.

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