Effects of ANF infusion on the renal responses to lower-body negative pressure in humans

To investigate the role of atrial natriuretic factor (ANF) in renal responses to a decrease in central blood volume, we examined the effects of ANF infusion on renal function and hormones during prolonged lower-body negative pressure (LBNP). Ten healthy volunteers participated in two experimental sequences, each comprising a 120-min baseline period followed by the application of -20 mm Hg LBNP for 90 min. During one of the two sequences, ANF was infused throughout LBNP application at the constant rate of 2.5 ng/kg/min. Glomerular filtration rate (GFR) and effective renal plasma flow (ERPF) were measured by using inulin and p-aminohippuric acid clearance techniques. LBNP induced a significant decrease in ERPF (534 +/- 28 to 457 +/- 26 ml/min; p < 0.05), GFR (120 +/- 2.5 to 112 +/- 2.5 ml/min; p less than or equal to 0.01), in urine excretion (12 +/- 0.9 to 5.6 +/- 0.5 ml/min; p < 0.001), in sodium excretion (0.36 +/- 0.03 to 0.30 +/- 0.02 mmol/min; p < 0.05), and in plasma ANF (19 +/- 3 to 11 +/- 2 pg/ml; p = 0.001) concomitant with an increase in plasma renin activity (PRA; 0.48 +/- 0.09 to 0.87 +/- 0.16 ng/ml/h; p = 0.01) and of forearm vascular resistance (FVR; p < 0.05). The combination of ANF infusion with LBNP led to a slight increase in plasma ANF from baseline (from 20 +/- 2 to 28 +/- 3 pg/ml; p < 0.05). Compared with values obtained during LBNP with saline vehicle infusion, values obtained during LBNP with ANF infusion were similar for ERPF (463 +/- 23 vs. 457 +/- 26 ml/min), for GFR (111 +/- 2 vs. 112 +/- 2 ml/min), and for urine excretion (7 +/- 0.6 vs. 5.6 +/- 0.5 ml/min; p = 0.07), but greater for fractional excretion of sodium (2.38 +/- 0.25% vs. 1.91 +/- 0.11%; p < 0.05) and FVR (p < 0.05), and smaller for PRA (0.49 +/- 0.1 vs. 0.87 +/- 0.16 ng/ml/h; p < 0.01). These data show that ANF infusion attenuates the antinatriuretic effect of low-level LBNP and its PRA-increasing effects without altering renal hemodynamic responses to LBNP, although there is a decrease in the LBNP-induced forearm vasoconstriction. These results were obtained with plasma ANF levels slightly higher than those in baseline. They support the hypothesis that a decrease in ANF secretion might contribute to the antinatriuretic effect of LBNP.