THE EXTRARENAL EFFECTS OF ATRIAL-NATRIURETIC-PEPTIDE ON BODY-FLUID DISTRIBUTION

To study the effects of atrial natriuretic peptide (ANP) on body fluid volume regulation, we estimated the changes in intra- and extravascular fluid volume by measuring hematocrit (Hct), plasma protein concentration and water balance, and the changes in intra- and extracellular fluid volume by the electrical impedance method during intravenous infusion of ANP. We did two studies, as follows: ANP was infused into 18 patients with essential hypertension, 29 with renal parenchymal hypertension and 15 normotensives at 0.025 μg/kg/min for 40 min. Both hypertensive groups showed greater natriuretic responses to ANP than normotensives. ANP infusion into essential hypertensive patients increased the urinary excretion of water by 125%, Na by 205%, Hct by 4.2% and plasma total protein (TP) by 5.2% (each P <. 001). In 9 patients (1 with renal hypertension and 8 normotensives) who did not show a natriuretic response (–2.1%), the infusion of ANP also significantly increased Hct (3.8%) and plasma TP (3.1%, each P <. 01). The electrical impedance method was applied to 12 subjects to simultaneously detect the intracellular (Ri) and extracellular resistivities (Re), of which reciprocals reflect the fluid volume in the extra-and intracellular spaces, respectively. ANP infusion increased Re in all subjects (3.96 ± 0.16 [SE] ν 4.03 ± 0.16 Ω · m, P <. 05), but decreased Ri in 7. Changes in urinary Na excretion correlated positively with those in both Re (r = 0.62, P <. 05) and Ri (r = 0.75, P <. 01). These results suggest that in addition to its natriuretic effects, ANP may regulate plasma volume by modulating the body fluid distribution between extra- and intravascular compartments or between intra- and extracellular compartments. © 1990 by the American Journal of Hypertension, Ltd.