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A reliable, simple radioimmunoassay for measurement of the urinary excretion rate of the acid-labile conjugate of aldosterone is described, in which aldosterone, reconstituted by acid hydrolysis of a urinary metabolite, is quantified. The results were compared with those obtained by double-isotope dilution. Using both methods, aldosterone excretion rates were characterized in normal subjects over a wide range of dietary sodium intakes. Identity of results from the two methods was demonstrated. Aldosterone excretion ranged from 18 to 85 µg/day at sodium excretion rates of less than 25 mEq/day, from 5 to 26 µg/day at sodium excretion rates between 75 and 125 mEq/day, and from 1.5 to 12.5 µg/day at excretion rates above 200 mEq/day. However, a more precise index was derived with limits inscribed by the whole curve used as a nomogram. During sodium depletion of normal subjects, aldosterone excretion reacted slowly and inadequately until a critical level of sodium or volume depletion was reached. During maintained sodium loading for up to 11 days, initial sodium retention was followed by natriuresis and then by slight sodium retention. Mean urinary aldosterone during these three periods was 23.4, 3.6, and 4.5 µg/day, respectively. Urinary sodium excretion exhibited a more consistent relationship to aldosterone activity than did either sodium intake or cumulative sodium balance, even when constant dietary regimens were used. These data suggest a dynamic role for aldosterone in governing renal sodium excretion, even at very high levels of sodium excretion.
Sealey et al. (Fri,) studied this question.