Sodium-independent in vitro induction of Na+,K+-ATPase by aldosterone in renal target cells: permissive effect of triiodothyronine.

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Abstract
The aim of this study was to develop an in vitro system in which we could study the causal relationship between short-term stimulation of Na+,K+-ATPase in the collecting tubule by aldosterone on the one hand and protein synthesis and changes in intracellular Na+ concentration on the other hand. Previous in vivo studies suggested that triiodothyronine might facilitate aldosterone-induced stimulation of Na+,K+-ATPase. Results show that when segments of cortical collecting tubules microdissected from collagenase-treated kidneys of adrenalectomized rats were incubated for 3 hr in the presence of either 10-8 M aldosterone or 10-8 M triiodothyronine alone Na+,K+-ATPase activity was not altered, whereas the addition of both hormones markedly stimulated the activity and the number of catalytic sites of Na+,K+-ATPase. This stimulation was abolished by actinomycin D and cycloheximide, whereas it was not altered in the absence of extracellular sodium or in the presence of the luminal Na+-channel blocker amiloride. Thus, triiodothyronine facilitates the in vitro induction of Na+,K+-ATPase synthesis by aldosterone. Aldosterone action on Na+,K+-ATPase is independent of Na+ availability.