Effect of aldosterone on ribonucleic acid polymerase activity in rat kidney cortical and medullary mitochondria

Abstract

The involvement of mitochondria in the mechanism of action of aldosterone is suggested by the ability of this hormone to alter the activity of certain mitochondrial enzymes. Since the mitochondrion is able to synthesize some of its own macromolecules, we investigated whether the mineralocorticoid effects mitochondrial RNA synthesis in isolated kidney mitochondria. Rat kidney mitochondria were purified and then separated into two populations by sucrose density gradient centrifugation; mitochondria sedimenting at 1.2 M sucrose were predominantly of medullary origin, while mitochondria sedimenting at 1.4 M sucrose were mainly from the cortex. RNA polymerase activity in medullary mitochondria was almost twice that of cortical mitochondria. The activity was characterized as α-amanitin resistant and rifampicin sensitive DNA-dependent RNA polymerase. Adrenalectomy resulted in a 40 per cent decrease in the RNA polymerase activity of cortical mitochondria 5 days later (P < 0.001), but no decrease occurred in medullary mitochondrial activity, compared to sham-operated controls. Three hr after aldosterone administration ( 2 μmg/100 g body wt, i.p.) the RNA polymerase activities of medullary and cortical mitochondria were increased 20 and 50 per cent, respectively, above the adrenalectomized values (P < 0.05, P < 0.001). Spironolactone, an antagonist of aldosterone (7 mg/100 g body wt, i.p., 30 min prior to aldosterone administration), blocked the stimulation of RNA polymerase by aldosterone (P < 0.05). Steroid hormones other than aldosterone did not produce significant enhancement of mitochondrial RNA polymerase activity. No change in RNA polymerase activity occurred after direct addition of aldosterone or spironolactone to the assay medium. The results suggest that the mechanism of mineralocorticoid action is at least in part mediated through its effects on mitochondrially synthesized proteins.