Mineralcorticoid receptors along the nephron: [3H]aldosterone binding in rabbit tubules

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Abstract
To identify the site of mineralocorticoid action along the nephron, the specific binding of [3H]aldosterone to nephron segments microdissected from aldosterone-deficient rabbits was measured. Specific binding was defined as the difference between binding measured in the absence or in the presence of 2000-fold excess of unlabeled hormone (in 10-18 mol .cntdot. cm tubule length-1 .+-. SE). High specific binding capacity was found in the branched collecting tubule (108 .+-. 4), the cortical collecting tubule (119 .+-. 9), and the outer medullary collecting tubule (115 .+-. 16); specific binding was negligible in the proximal convoluted tubule (8 .+-. 9), pars recta (2 .+-. 6), medullary thick ascending limb (4 .+-. 6), cortical thick ascending limb (6 .+-. 2) and distal convoluted tubule (6 .+-. 6). In cortical collecting tubules, Scatchard analysis of the specific [3H]aldosterone binding indicated a KD of 2.2 .times. 10-9 M and a maximum number of binding sites of 157 .times. 10-18 mol .cntdot. cm tubule length-1. The steroid specificity was assessed from the competition of various steroids for [3H]aldosterone binding sites. Receptors from the cortical collecting tubule revealed the following sequence of affinities: aldosterone > deoxycorticosterone acetate > spironolactone > dexamethasone > 5.alpha.-dihydrotestosterone = progesterone = 17.beta.-estradiol, indicating that the binding sites in the collecting tubule are mineralocorticoid receptors. Significant [3H]aldosterone binding to receptors of high affinity and mineralocorticoid specificity were seen only in the collecting tubule and suggest that this nephron segment is the target site of mineralocorticoid action in the rabbit kidney.

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