Renal response to central volume expansion in humans is attenuated at night

The renal response to central volume expansion induced by head-out water immersion was examined in 10 normal, salt-replete subjects studied during the day (0900-1300) and during the night (0000-0400). Sodium excretion in the hour preceding the study was 155 +/- 21 mueq/min and 120 +/- 14 mueq/min, respectively. During the day, immersion was followed by a natriuresis, which reached a mean peak level of 293 +/- 46 mueq/min during the 2nd h of immersion and which was maintained for the remainder of the immersion. During the night, there was no significant increase in sodium excretion from prestudy values during the first 3 h of immersion. Values rose significantly in the 4th h and reached a mean peak level of 211 +/- 20 mueq/min. Potassium excretion rose during the day (from 61 +/- 12 mueq/min to 126 +/- 16 mueq/min) but was unaltered at night. Neither glomerular filtration rate nor plasma levels of aldosterone differed between day and night. To exclude the possibility that the blunted nocturnal natriuresis could be explained by a lesser degree of central fluid translocation induced by immersion at night six normal salt-replete subjects received a 2-liter infusion of normal saline administered over 4 h during the day and during the night. The blunting of the nocturnal natriuresis was again observed. We conclude that, in addition to well-described diurnal variations in electrolyte excretion, there are diurnal variations in the responsivity of volume regulatory mechanisms.