Effect of posture on the plasma levels of atrial natriuretic factor.

Atrial natriuretic factor (ANF) is a peptide with potent natriuretic, diuretic and vasorelaxant activities. Stretching of the right atria causes release of ANF into the circulation. Therefore, changes in central blood volume or acute volume expansion are likely to change the plasma levels of ANF. In this study we investigated the effects of changes in posture on the plasma levels of ANF, plasma renin activity (PRA) and plasma aldosterone (aldo). Eight male and five female volunteers ranging in age from 23 to 26 years were placed on a normal sodium intake and on the experimental day blood was obtained for ANF, PRA, and aldo after 30 minutes of lying supine, 30 minutes of 10 or 20 degrees head-down tilt, and 30 minutes of standing. Plasma ANF increased significantly after 30 minutes of head-down tilt from the supine value of 33.7 +/- 5.2 pg/ml to 47.7 +/- 7.7 pg/ml (p less than 0.02) and suppressed to 14.1 +/- 0.02) after 30 minutes of standing. PRA did not change significantly with head-down tilt, (supine 1.64 +/- 0.44 ngAI/ml/h vs. 30 minutes tilt 1.28 +/- 0.32 ngAI/ml/h (p = NS). Plasma aldosterone decreased by head-down tilt from 11.2 +/- 1.2 ng/ml to 8.4 +/- 0.8 ng/dl (p less than 0.02) and returned to the supine level after standing. In conclusion ANF levels change significantly with posture. Increase in central blood volume by head-down tilt increases ANF levels and suppresses plasma aldosterone with no effect on PRA. Standing decreases ANF significantly. These results suggest that for proper interpretation of plasma levels of ANF, posture at the time of sampling has to be standardized.