The role of posture on the changes in plasma atrial natriuretic factor and arginine vasopressin levels during immersion

In seven healthy male volunteers we investigated changes in plasma atrial natriuretic factor ( ANF]), arginine vasopressin ( AVP]) and plasma volume (PV) during supine immersion. Twenty minutes head-out water immersion in a supine position in a thermo-neutral water bath attenuated the increase in PV induced by 20 min in a supine position in air, but increased the mean plasma ANF] from 32.0 pg.ml-1, SEM 5.1 to 53.3 pg.ml-1, SEM 3.6 and decreased the mean plasma AVP] from 1.4 pg.ml-1, SEM 0.1 to 0.9 pg.ml-1, SEM 0.04. Simultaneously, diuresis and natriuresis increased markedly. During a 20-min control period in the supine posture without immersion, PV, plasma ANF] and AVP] remained unaffected while diuresis and natriuresis did not increase to the same extent. These data suggest that an increase in the central blood volume induced by a weak external hydrostatic pressure during supine immersion triggered the changes in plasma ANF] and AVP] and that the increase was probably due to a shift of blood volume from peripheral to central vessels. The changes in plasma ANF] contributed to the changes in natriuresis.