Identification of mineralocorticoid target sites in the isolated rabbit cortical nephron

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Abstract
Previous evidence suggests that the activity of the mitochondrial enzyme citrate synthase [citrate oxaloacetate-lyase (pro-3S-CH2COO .fwdarw. acetyl-CoA), EC 4.1.3.7] is increased in target tissues upon acute administration of aldosterone [ALDO]. An ultramicroassay was established to determine citrate synthase levels in isolated rabbit nephron segments as a means of localizing mineralocorticoid-responsive sites within the renal cortex. The relative citrate synthase activities in normal rabbit segments (per kg of dry tissue) correlated with the metabolic activity of the segments. The order was: distal convoluted tubule > proximal convoluted tubule > cortical thick ascending limb of Henle > cortical collecting duct > pars recta. When these segments were isolated from adrenalectomized rabbits, only the citrate synthase activity in the cortical collecting duct was significantly decreased compared to normal values (3.2 mol of citrate/kg dry wt per h compared to 7.1; P < 0.001). Enzyme activities in segments isolated from adrenalectomized rabbits 90 min after i.v. injection of ALDO (10 .mu.g/kg) were unchanged from normal or adrenalectomized rabbit tubule values for all segments except the cortical collecting duct. In this segment, ALDO significantly increased citrate synthase activity compared to adrenalectomized rabbit values (8.1 mol/kg per h compared to 3.2; P < 0.001), in contrast to the effect of dexamethasone at 10 .mu.g/kg (4.4 mol/kg per h compared to 3.2; P, NS [non significant]). Spirolactone SC 26304 administered 30 min prior to injection of ALDO inhibited the increase in collecting duct citrate synthase activity seen with ALDO alone (3.4 mol/kg per h compared to 8.1; P < 0.001). The collecting duct is apparently the primary target for ALDO in the renal cortex.