Effects of dexamethasone on (Na+ + K+)-ATPase and other parameters related to transepithelial Na+ transport by amphibian renal distal cells (A6) in culture.

In the present report, the effects exerted by dexamethasone on transepithelial, electrogenic Na+ transport across A6 cell monolayers grown on permeable support were further characterized in terms of time course and relationship to the rate of Na+ transport; furthermore this agonist was compared to vasopressin and insulin. (Na+ + K+)-ATPase activity and density of ouabain binding sites were measured in cell homogenates and on dispersed cells, respectively, after documenting transepithelial electrical parameters of the preparations. Na+ transport, measured by short-circuit current (Isc), was increased almost five-fold (control: 6.7 +/- 0.1 microA/cm2) after incubation with 10(-7) M dexamethasone for 24 h. Stimulation of Na+ transport rate was associated with a 2.3-fold increase in (Na+ + K+)-ATPase activity (control: 5.5 +/- 0.3 micromol Pi/mg prot.h), and ouabain binding site density almost doubled (control: 236 +/- 10 fmol/10(6) cells). The steroid acted on the Na+ pump of A6 cells in the absence of transepithelial Na+ transport, with intracellular Na+ ion activity playing an additional role in terms of cell Na+ pump numbers. In the case of insulin and vasopressin, in contrast, there was no effect on Na+ pump activity in the absence of Na+ transport by A6 cell monolayers. The increase in (Na+ + K+)-ATPase activity observed in A6 cell monolayers treated with dexamethasone is therefore a result of the direct induction of Na+ pump biosynthesis, with an almost proportional insertion of operational Na+ pumps into the basolateral membrane. In contrast, increased Na+ entry at the apical cell pole appears to be essential for insulin and vasopressin action on A6 cell Na+ pump.