Glucocorticoids stimulate Na⁺/H⁺ antiporter in OKP cells

Previous studies have demonstrated that systemic administration of glucocorticoids stimulates proximal tubule acidification in part by increasing Na⁺/H⁺ antiporter activity; however, these studies could not exclude the possibility that changes in Na⁺/H⁺ antiporter activity were secondary to glucocorticoid-induced hemodynamic changes. The present study examined the effect of dexamethasone on Na⁺/H⁺ antiporter activity in quiescent OKP cells. Na⁺/H⁺ antiporter activity was assayed as the initial rate of Na⁺-dependent pH recovery from an acid load. Intracellular pH was measured using the pH-sensitive dye 2',7'-bis(carboxyethyl)-5(6)- carboxyfluorescein (BCECF). Dexamethasone produced a dose- and time-dependent stimulation of Na⁺/H⁺ antiporter activity in OKP cells. Dexamethasone produced a 24% stimulation in Na⁺/H⁺ antiporter activity at 10^-9 M and an ~40% stimulation of Na⁺/H⁺ antiporter activity at both 10^-8 and 10^-6 M. The effect of 10^-6 M dexamethasone was seen within 4 h of incubation and was due to an increase in maximal velocity (V(max), 3.03 vs. 1.79 pH units/min) with no change in the affinity constant for sodium (K(Na), 47.2 vs. 42.0 mM). The stimulatory effect of dexamethasone on Na⁺/H⁺ antiporter activity was blocked by cycloheximide and was not observed with 10^-8 M aldosterone. These data demonstrate a direct effect of glucocorticoids to stimulate Na⁺/H⁺ antiporter activity in OKP cells.