Aldosterone Induces a Delayed Parallel Increase in Total and Cell-Surface Na⁺/K⁺-ATPase in A6 Kidney Cells

Abstract

The reabsorption of Na⁺ across tight epithelia is mediated by the apical amiloridesensitive Na⁺ channel and the basolateral Na⁺/K⁺-ATPase which extrudes Na⁺ from the cells and provides the driving force for this transport. Adrenal steroids stimulate this transport in A6 kidney cells cultured on filters as in other tight epithelia. This action can be divided in three phases: (a) a lag period, (b) an early response during which an increase in Na⁺ reabsorption appears to result from a change in the activity of preexisting apical Na⁺ channels, and (c) a late response which appears to be partially supported by quantitative changes at the level of the transporters (4). It has been recently shown that the early increase in Na⁺ transport, which precedes the transcriptionally mediated increase in the rate of Na⁺/K⁺-ATPase subunit synthesis, is paralleled by an increase in the rate of ouabain binding to the intact cell monolayers (in K-free buffer). This effect is independent of apical Na⁺ influx but depends on ongoing transcription and translation (2). Here we test the hypothesis that it is due to an increase in the number of active pump sites which in turn would be mediated by the translocation of Na⁺/K⁺-ATPase molecules to the cell surface (2). Furthermore, we test whether the known induction of Na⁺/K ⁺ -ATPase subunit synthesis leads to a late increase in the number of total and cell-surface expressed Na⁺ pumps.