EFFECTS OF DEXAMETHASONE ON (NA-ATPASE AND OTHER PARAMETERS RELATED TO TRANSEPITHELIAL NA+ TRANSPORT BY AMPHIBIAN RENAL DISTAL CELLS (A6) IN CULTURE(+K+))

In the present report, the effects exerted by dexamethasone on transep ithelial, electrogenic Na+ transport across A6 cell monolayers grown o n permeable support were further characterized in terms of time course and relationship to the rate of Na+ transport; furthermore this agoni st was compared to vasopressin and insulin. (Na+ + K+)-ATPase activity and density of ouabain binding sites were measured in cell homogenate s and on dispersed cells, respectively, after documenting transepithel ial electrical parameters of the preparations. Na+ transport, measured by short-circuit current (I-sc), was increased almost five-fold (cont rol: 6.7 +/- 0.1 mu A/cm(2)) after incubation with 10(-7) M dexamethas one 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 mu mol P-i/mg prot.h), and ouabain binding site density almost doubled (control: 236 +/- 10 fmol/10(6) cells). The steroid acted on the Napump 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 con trast, there was no effect on Na+ pump activity in the absence of Natransport by A6 cell monolayers. The increase in (Na+ + K+)-ATPase act ivity observed in A6 cell monolayers treated with dexamethasone is the refore a result of the direct induction of Na+ pump biosynthesis, with an almost proportional insertion of operational Na+ pumps into the ba solateral membrane. In contrast, increased Na+ entry at the apical cel l pole appears to be essential, for insulin and vasopressin action on A6 cell Na+ pump. Copyright (C) 1996 Elsevier Science Ltd.