Increases in circulating atrial natriuretic factor during immersion-induced central hypervolaemia in normal humans

The role of atrial natriuretic factor (ANF) in modulating volume and circulatory homeostasis remains uncertain, and there has been as yet no systematic analysis of the factors promoting ANF release in humans. Since immersion in water to the neck provides a 'volume stimulus' identical to that induced by 2 litres of saline, without plasma compositional change, immersion to the neck was used to assess the ANF response to acute central blood-volume expansion. Using a radio-immunoassay that reliably detected ANF in human plasma extracts, more than 80% of plasma immunoreactive (ir) ANF was shown to elute as a single peak on reverse-phase high performance liquid chromatography, with a retention time identical to that of the synthetic 28-residue α-human (α-h) ANF. The response of plasma irANF to 3 h of immersion in water to the neck was evaluated in four sodium-replete normal subjects; the immersion produced a prompt and marked increase irANF in each subject, and recovery was associated with a prompt return to pre-study levels. Concurrently, there was a marked natriuresis and a profound suppression plasma renin and aldosterone. These findings support the hypothesis that an increase in plasma ANF contributes to the hormonal and renal effects of immersion in water to the neck, suggesting that ANF has an important physiological role in modulating volume homeostasis in humans.