The metabolic clearance rate of aldosterone in pregnant and nonpregnant subjects estimated by both single-injection and constant-infusion methods.

Jones and colleagues (1) found that the pattern of urinary metabolites of aldosterone was changed in pregnant compared with nonpregnant subjects. The ratio, however, of the secretion rate to the urinary free aldosterone, which was taken to indicate the over-all metabolic rate, was not appreciably altered. Daughaday (2) and Meyer (3) and their associates found that the binding of aldosterone to plasma proteins other than albumin was the same for nonpregnant and pregnant subjects and for subjects treated with small amounts of estrogens. The unaltered binding, unlike that of cortisol, might suggest that the over-all metabolism of aldosterone is unchanged in pregnancy. Tait and co-workers (4) described methods for the calculation of volumes of distribution and rate constants by measurement of the plasma radioactivity, as aldosterone, as a function of time after a single injection of the tritiated hormone. In addition, the ratio of the secretion rate to the plasma steroid concentration, which is a measure of the over-all metabolic process, could be calculated both after a single injection and after constant infusion of radioactive steroid. This ratio was previously termed a "turnover rate" (4), but as it has the dimensions of volume divided by time, it will henceforward be referred to as the "metabolic clearance rate" (MCR) of the steroid. In the present paper, the singleinjection and constant-infusion methods for estimating MCRare compared for both pregnant and nonpregnant subjects. The MCR