Atrial natriuretic factor during head-out immersion at night

The present investigation was undertaken to test the hypothesis that a circadian variation in the atrial natriuretic factor (ANF) response is responsible for the nocturnal inhibition of renal responses to head-out immersion (HOI). Plasma ANF responses to a 3-h HOI (water temperature 34.5 +/- 0.5 degrees C) were studied during day (1000-1300) and night (2400-0300) in six hydropenic male human subjects. In agreement with the previous observations, the renal responses, especially the diuresis, to HOI were attenuated at night compared with the day; furthermore, plasma renin activity decreased to the same low level during HOI at both day and night. Plasma ANF during time control periods was 30-40 pg/ml without showing any circadian variation. Moreover, plasma ANF showed a similar twofold increase within 1 h of HOI and was maintained at this elevated level throughout the 3-h HOI period in both the daytime and the nighttime series. On termination of HOI, plasma ANF decreased linearly to the pre-HOI level within 1 h. Hematocrit during time control periods was higher during the day compared with the night (P less than 0.05). Although HOI appears to induce a transient increase in plasma volume (as indicated by decreases in hematocrit) during the 1 h of HOI, the magnitude of the decrease in the latter parameters was not different between day and night. It is concluded that nocturnal inhibition of renal responses to HOI cannot be fully accounted for by circadian differences in the ANF and fluid shift response to HOI.