1. We studied diurnal patterns of plasma atrial natriuretic factor, plasma guanosine 3′:5′-cyclic monophosphate and urinary sodium excretion in normal subjects after 3 days on a 200 mmol of sodium/60 mmol of potassium diet. On the fourth day blood samples and urine were collected every 3 h.
2. Two studies were performed. In study 1, normal subjects (n = 8) were recumbent for 23 h from 09.00 hours to 08.00 hours the next day. In study 2, normal subjects (n = 10) were permitted to ambulate from 09.00 hours to 23.00 hours and then were recumbent until 08.00 hours the next day.
3. In study 1, assumption of the recumbent posture was associated with increases in plasma atrial natriuretic factor (P < 0.01), plasma guanosine 3′:5′-cyclic monophosphate (P < 0.05) and urinary sodium excretion (P < 0.05).
4. In contrast, in study 2 there were no significant changes in plasma atrial natriuretic factor during the day; instead, plasma atrial natriuretic factor increased overnight, reaching a peak at 24.00 hours after 1 h of recumbency (P < 0.01). A smaller rise in plasma guanosine 3′:5′-cyclic monophosphate (P < 0.05) occurred; urinary sodium excretion decreased markedly (P < 0.01) and there was no change in creatinine clearance.
5. In both studies, recumbency was associated with an initial drop, followed by a rise, in packed cell volume.
6. These data demonstrate that assumption of the supine position induces a rise in plasma atrial natriuretic factor and accounts for most of the observed variation. This is associated with natriuresis during the day, but it does not reverse the pattern of antinatriuresis that occurs at night. Instead, changes in plasma atrial natriuretic factor appear to reflect and, in turn, influence changes in intravascular volume.