A comparison of head‐down tilt with low‐dose infusion of atrial natriuretic peptide in man.

1. Procedures that increase atrial pressure, such as head‐down tilt, result in an increase in plasma atrial natriuretic peptide (ANP) and a natriuresis, but a direct cause‐and‐effect relationship between these two responses has not been established. This study was undertaken to compare the effects of head‐down tilt with exogenous ANP on renal function. 2. Eight normal sodium‐replete volunteers underwent a 3 h placebo infusion, a 3 h ANP infusion at 1.2 pmol kg‐1 min‐1 and a 3 h period of head‐down tilt. Each procedure was performed on a separate day, in random order. 3. ANP and head‐down tilt produced similar increases in sodium excretion (65 +/‐ 24 and 68 +/‐ 16%, respectively). ANP did not increase urine flow significantly more than placebo. Head‐down tilt increased urine flow significantly more than placebo and ANP. 4. Plasma ANP rose from 8.1 +/‐ 1.0 to 11.4 +/‐ 2.5 pg ml‐1 during head‐down tilt and from 6.5 +/‐ 1.4 to 32.3 +/‐ 10.7 pg ml‐1 with ANP infusion. 5. ANP infusion had no significant effects on systemic haemodynamics whilst head‐down tilt increased cardiac output and reduced heart rate and an index of systemic vascular resistance. 6. ANP infusion, whilst achieving a natriuretic response similar to that of tilt, was associated with a 3‐fold higher mean plasma ANP level. Although plasma ANP rose during both ANP infusion and tilt, there was a lack of correlation between natriuretic response and plasma ANP. 7. The results are not compatible with a direct cause‐and‐effect relationship between plasma ANP and sodium excretion during head‐down tilt.