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

Abstract

Steroid hormonal activation of the Na+-K+-ATPase enzyme was examined in enriched preparations 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 two 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. Thus, the regulation of Na+-K+-ATPase activity by mineralocorticoids on the one hand and by glucocorticoids on the other would appear to be discontinuously localized along the length of the outer medullary distal nephron.

Ultrastructure of rat initial collecting tubule. Effect of adrenal corticosteroid treatment.

“…There is considerable evidence that glucocorticoids increase renal Na-K ATPase activity (25)(26)(27)) and membrane area (1). However, in all of these studies (1,(25)(26)(27), pharmacological doses of glucocorticoids were employed. It is likely, therefore, that the alterations in ultrastructure and ATPase activity in these previous studies resulted, in large part, from nonspecific binding to mineralocorticoid receptors in the cortical collecting tubule.…”

“…This conclusion is substantiated by the fact that spironolactone, a competitive inhibitor of mineralocorticoids, interferes with the dexamethasone-induced stimulation of ATPase (24). Furthermore, low physiological doses of glucocorticoid have been shown not to have an effect on ATPase activity in the cortical collecting tubule (9,23,26). It is also likely that the effects of dexamethasone on ATPase in renal homogenates (25)(26)(27) represent an effect on the proximal tubule and loop of Henle, nephron segments that have glucocorticoid receptors (24,26,28).…”

“…Furthermore, low physiological doses of glucocorticoid have been shown not to have an effect on ATPase activity in the cortical collecting tubule (9,23,26). It is also likely that the effects of dexamethasone on ATPase in renal homogenates (25)(26)(27) represent an effect on the proximal tubule and loop of Henle, nephron segments that have glucocorticoid receptors (24,26,28). For example, Rayson and Lowther (26) and El Mernissi and Doucet (24) have shown that dexamethasone stimulates ATPase activity in the thick ascending limb of Henle's loop, an effect that is insensitive to spironolactone (24).…”

Modulation of Na-K-ATPase activity in the mouse medullary thick ascending limb of Henle. Effects of mineralocorticoids and sodium.

“…First, both aldosterone-specific (37) and dexamethasone-specific (36,38,39) binding sites have been identified in the mTALH using autoradiography and classic binding techniques. In addition, a number of laboratories (13,(40)(41)(42) have reported that Na-K-ATPase activity in the rat mTALH is reduced by 25-50% after adrenalectomy. However, it is unclear whether this effect of adrenalectomy is due to loss of type I receptor effects or type II receptor effects, since either aldosterone (42) or dexamethasone (40,41) has been shown to restore Na-K-ATPase activity to normal or near normal in the mTALH.…”

“…In addition, a number of laboratories (13,(40)(41)(42) have reported that Na-K-ATPase activity in the rat mTALH is reduced by 25-50% after adrenalectomy. However, it is unclear whether this effect of adrenalectomy is due to loss of type I receptor effects or type II receptor effects, since either aldosterone (42) or dexamethasone (40,41) has been shown to restore Na-K-ATPase activity to normal or near normal in the mTALH. Moreover, Marver and co-workers (16, 36) observed a 42% reduction in the activity of another enzyme, citrate synthase (a putative marker of mineralocorticoid action), in the rabbit mTALH following adrenalectomy, and demonstrated that the activity of this enzyme could be restored to normal with aldosterone, but not dexamethasone, administration.…”