Chapter 10 Involvement of Na+,K+-ATPase in Antinatriuretic Action of Mineralocorticoids in Mammalian Kidney

Increased synthesis and avp unresponsiveness of Na,K-ATPase in collecting duct from nephrotic rats

Renal sodium retention is responsible for ascites and edema in nephrotic syndrome. In puromycin aminonucleoside (PAN)-induced nephrosis, sodium retention originates in part from the collecting duct, and it is associated with increased Na,K-ATPase activity in the cortical collecting duct (CCD). The aims of this study were to evaluate whether the outer medullary collecting duct (OMCD) also participates to sodium retention and to determine the mechanisms responsible for stimulation of Na,K-ATPase in CCD. PAN nephrosis increased Na,K-ATPase activity in the CCD but not in OMCD. The two-fold increase of Na,K-ATPase activity in CCD was associated with two-fold increases in the number of alpha and beta Na,K-ATPase subunits mRNA determined by quantitative RT-PCR and of the total amount of Na,K-ATPase alpha subunits estimated by Western blotting. PAN nephrosis also increased two-fold the amount of Na,K-ATPase alpha subunit at the basolateral membrane of CCD principal cells, as determined by Western blotting after biotinylation and streptavidin precipitation and by immunofluorescence. The intracellular pool of latent Na,K-ATPase units also increased in size and was no longer recruitable by vasopressin and cAMP. This unresponsiveness of the intracellular pool of Na,K-ATPase to vasopressin was not the result of any alteration of the molecular and functional expression of the vasopressin V(2) receptor/adenylyl cyclase (AC) complex. It is concluded that PAN nephrosis (1) does not alter sodium reabsorption in OMCD, (2) is associated with increased synthesis and membrane expression of Na,K-ATPase in the CCD, and (3) alters the normal trafficking of intracellular Na,K-ATPase units to the basolateral membrane.

Signals and Mechanisms of Sorting in Epithelial Polarity

This chapter discusses epithelial-membrane polarity, sorting pathways in polarized cells, and the sorting-signal paradigm. Polarized epithelial cells have long captured the attention of cell biologists and cell physiologists. At the electron-microscopic level, one of the most apparent and fundamental features of this cell type is its polarized organization of intracellular organelles and its structurally and compositionally distinct lumenal (apical) and serosal (basolateral) plasma-membrane domains. The polarized epithelial phenotype is an absolute necessity for organ-system function. In the most general sense, these cells organize to form a continuous, single layer of cells, or epithelium, which serves as a semi-permeable barrier between apposing and biologically distinct compartments. Within the tubules of the nephron, these cells orchestrate complex ion-transporting processes that ultimately control the overall fluid balance of the organism. At the surface of the gastrointestinal tract, specialized versions of this cell type control the digestion, absorption, and immuno-protection of the organism.

Effects of corticosteroids on parameters related to Na+ transport by amphibian renal distal cells (A6) in culture

The present study addresses the effects of the hormones aldosterone and corticosterone, as well as those of dexamethasone, on cultured renal amphibian cells, focusing on parameters thought relevant for the further understanding of the regulation by these steroids of Na+ reabsorption along the renal tubule. Exposure to these steroids of A6 cell monolayers grown on a permeable support produced a motor, dose-dependent, increase in Na+ transport, reflected by the short-circuit current, Isc. (Na+ + K+)-ATPase activity and ouabain binding, both of which are linearly correlated with Isc in control tissue, also increased significantly after steroid treatment. Dexamethasone was consistently more active than corticosterone and aldosterone on the parameters studied. The increase in Isc and (Na+ + K+)-ATPase activity elicited by dexamethasone could be blocked by the glucocorticoid antagonist RU 486, whereas it was only slightly reduced by the mineralocorticoid antagonist, spironolactone. In contrast, the latter strikingly reduced the effects of aldosterone on these parameters, unlike RU 486. Furthermore, the effects of large doses of dexamethasone and aldosterone combined were not additive. Taken together, the data presented appear compatible with the view that the effects of aldosterone on Na+ transport by A6 cells are mediated by a fraction of the receptors involved in the response to dexamethasone; they furthermore raise the question of whether, in lower vertebrates, it is relevant to make a distinction between "gluco" and "mineralo"corticoids.

Aldosterone induces early activation and late accumulation of Na-K-ATPase at surface of A6 cells

In A6 cells cultured on filters, aldosterone (10(-6) M) induces an early increase in the initial rate of ouabain binding to intact monolayers (in K-free buffer) that parallels an early increase in Na transport. This effect is independent of apical Na influx and precedes an increase in the synthesis of Na-K-adenosinetriphosphatase (ATPase) subunits. In the present study we observed in addition a late aldosterone-induced increase in the rate of ouabain binding (2.5 times from 3 to 120 h aldosterone). The role of Na-K-ATPase accumulation and/or translocation to the cell surface was tested by Western blotting, saturation ouabain binding, and cell-surface labeling using sulfosuccinimidobiotin or enzyme-mediated radioiodination. Only cell-surface beta 1-subunit was detected by blotting with streptavidin or autoradiography, because the alpha 1-subunit was not efficiently labeled. Three hours after hormone addition, none of the three parameters had increased significantly, whereas after 20 and 120 h similar increases (approximately 1.6 and 2.3 times, respectively) were detected by all three methods. In addition, it was shown that increasing intracellular Na with amphotericin B or a K-free preincubation also stimulated the rate of ouabain binding without increasing the surface labeling of beta 1-subunits. Taken together, these results suggest that a short aldosterone treatment or an increase in intracellular Na leads to an increase in the rate of ouabain binding that is due to an in situ activation of cell-surface Na-K-ATPase molecules. In contrast, the late increase in the rate of ouabain binding parallels an increase in the number of pumps.