Abstract
The possibility that mineralocorticoids have a direct influence on renal Na-K ATPase activity has been the focus of intense research effort and some controversy for a number of years. Early studies were hindered by an inability to differentiate between possible glucocorticoid vs. mineralocorticoid effects on this enzyme within the multitude of cells that comprise the heterogeneous mammalian nephron. This study attempts to circumvent this problem by monitoring Na-K ATPase activity in the rabbit renal cortical collecting tubule (CCT), a proposed target epithelium for mineralocorticoids. Using an ultramicro assay, Na-K ATPase activity was measured in CCT from normal, adrenalectomized (adx), and adx rabbits subjected to one of several corticosteroid treatment protocols. The results indicate that Na-K ATPase activity in the CCT decreased by 86% subsequent to adrenalectomy. Injection of physiological doses of aldosterone (10 micrograms/kg) but not dexamethasone (100 micrograms/kg) restored CCT Na-K ATPase activity in adx rabbits to normal levels within 3 h after injection. An insignificant rise in activity was observed 1.5h after aldosterone treatment. In addition, spirolactone SC 26304, a specific mineralocorticoid antagonist, blocked the action of aldosterone on Na-K ATPase.. Therefore an acute increase in Na-K ATPase activity participates in the action of aldosterone on Na transport in this segment. To differentiate between primary vs. secondary activation of this enzyme, adx animals were treated with amiloride before the injection of aldosterone with the intent of blocking luminal membrane Na entry into CCT. In these animals, pretreatment with amiloride blocked the increase in CCT Na-K ATPase act activity seen with aldosterone alone at 3 h. Thus the increase in activity with aldosterone appears to be a secondary adaptation that is dependent on an aldosterone-enhanced increase in the passive entry of Na across the luminal membrane. The subcellular mechanism by which Na modulates Na-K ATPase activity remains obscure.