To test the proposal that the enzyme 11 beta -hydroxysteroid dehydrogenase (11 beta -HSD) confers aldosterone specificity on mineralocorticoid receptors in the distal nephron by inactivating glucocorticoids, we performed a free-flow micropuncture study of distal tubular function in adrenalectomized rats infused with high-dose corticosterone. One-half of the rats were additionally given intravenous carbenoxolone (CBX; 6 mg/h) to inhibit renal 11 beta -HSD activity. Although this maneuver lowered fractional Na(+) excretion (1.1 +/- 0.2 vs. 1.9 +/- 0.2%, P < 0.01), Na(+) reabsorption within the accessible distal tubule was found to be similar in the two groups of animals. In contrast, distal tubular K(+) secretion was enhanced in CBX-treated rats: fractional K(+) deliveries to the early and late distal collection sites in the corticosterone-alone group were 13 +/- 1 and 20 +/- 3%, respectively (not significant), whereas corresponding data in the CBX-treated group were 9 +/- 1 and 24 +/- 2% (P < 0.01). This stimulation of distal K(+) secretion provides the first direct in vivo evidence that 11 beta -HSD normally prevents corticosterone from exerting a mineralocorticoid-like effect in the distal tubule. The reduction in fractional Na(+) excretion during inhibition of 11 beta -HSD, in the absence of a change in end-distal Na(+) delivery, suggests enhanced Na(+) reabsorption in the collecting ducts.
