Steroid regulation of Na+-K+-ATPase: differential sensitivities along the nephron

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Abstract
Steroid hormonal activation of the Na+-K+-ATPase enzyme was examined in enriched preparations [rat] of outer medullary collecting tubules (MCT) and outer medullary thick ascending limbs of the loop of Henle (MAL), prepared by sedimentation through a discontinuous Ficoll gradient. Using morphological criteria, there was a 2.9-fold enrichment of MCT in fraction 1 when compared with fraction 2 and a 2.2-fold enrichment of MAL in fraction 2 when compared with fraction 1. This separation was further defined using biochemical markers. Na+-K+-ATPase activity, Mg2+-ATPase activity and the adenylate cyclase response to a number of hormones each supported the morphologic definition of separation. The 2 preparations were challenged in vitro with both aldosterone and dexamethasone. In fraction 1, the fraction enriched in the MCT, 10-8 M aldosterone stimulated Na+-K+-ATPase activity by 37%. The same concentration of dexamethasone was without effect. In contrast, 10-8 M dexamethasone stimulated Na+-K+-ATPase activity by 27% in fraction 2, the fraction enriched in the MAL. In this fraction an equimolar concentration of aldosterone was without effect. The regulation of Na+-K+-ATPase activity by mineralocorticoids on the 1 hand and by glucocorticoids on the other appears to be discontinuously localized along the length of the outer medullary distal nephron.