Aldosterone binding in isolated tubules. III. Autoradiography along the rat nephron

Aldosterone binding was investigated along the nephron of adrenalectomized Wistar rats. Autoradiographs of microdissected tubular segments were performed on dry film after incubation with [3H]aldosterone [( 3H]A) at 2 X 10(-10), 2 X 10(-9), and 2 X 10(-8) M in the presence or absence of an excess unlabeled hormone. Competition studies were done with a 10 times excess A, dexamethasone (D), and estradiol in the presence of 2 X 10(-8) [3H]A. Specific nuclear labeling predominated in the cortical collecting tubule (CCT) and A specificity of the binding was assessed by the much better displacing potency of unlabeled A as compared with D. Labeling in the distal tubule (DCT) resembled that of CCT, but D was nearly as good a competitor as A. In the loop and medullary collecting tubule, labeling was lower, with mixed steroid specificity. No specific nuclear labeling was detectable in the proximal tubule. We conclude that the CCT and DCT present the highest binding capacity for aldosterone. Competition studies revealed that the CCT is probably the more specific mineralocorticoid target segment. In other segments beyond the pars recta (loop of Henle, medullary collecting tubule), aldosterone probably binds to both mineralocorticoid and glucocorticoid binding sites. No major difference appeared between the profile of aldosterone binding along the rat tubule and the one previously described in the rabbit.