DIURNAL VARIATION IN THE DIURETIC RESPONSE TO INGESTED WATER

Abstract

The relative importance of increased anti-diuretic hormone (ADH) activity and decreased nocturnal Na excretion in diminishing the diuretic response to ingested water at night was studied in 3 normal individuals. Water loads of 20 ml./ kg. of body wt. were instituted and maintained by day and at night in each subject while receiving a diet containing 35 m.eq. of Na daily and again when the intake was 195 m.eq. daily. While the subjects were on the low salt diet during periods in which similar night and day Na excretion was achieved, definite but slight inhibition of nocturnal water diuresis was demonstrated. While the subjects were on the diet of high salt content, Na excretion during the water diuresis was much less at night than during the day. Under these conditions the difference between night and day rates of urine flow was very large. It seems probable that the diminished diuretic response to water loading at night is related both to increased nocturnal secretion of ADH and to decreased renal excretion of Na. The latter appears to be the more important factor when salt intake is liberal.