Existence of a Slowly Exchangeable Pool of Body Sodium in Normal Subjects and Its Diminution in Patients with Primary Aldosteronism

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Abstract
Radiosodium administered to 7 normal young men exchanged with 41.2 mEq/kg body weight within 24 hr, and continued to exchange with an exponentially enlarging pool of Na for up to 28 days. Cutaneous and fecal losses of Na were found to average 4.4 mEq/day, and thus could not explain more than 15 % of the mean increment in exchangeable Na with time (121.8 mEq/4 days). The computed lines relating “log. relative Nae” with time for each subject (where “relative Nae” was defined as the daily value of Nae expressed as a percentage of the Nae at 24 hr) were shown to be parallel. A common regression was derived for the 7 normal subjects, expressing the changes in “log. relative Nae” with time. Of 6 patients with primary aldosteronism, 3 showed no progressive increase in exchangeable Na with time, while 2 showed smaller increments than any of the normal subjects. The group as a whole showed significantly less (P < .005) expansion of Na0 with time than the 7 normal subjects. It is concluded that a slowly exchangeable component of body Na is present in healthy young adults and that its most likely site is bone. Patients with primary aldosteronism exhibit smaller amounts of this slowly exchangeable pool of body Na, probably because aldosterone exerts an inhibitory effect on the exchangeability of bone Na. The latter conclusion is supported by subsequent data upon in vivo uptake of radiosodium by rat femur under the influence of excessive amounts of aldosterone.