Glucocorticoids stimulate Na+/H+ antiporter in OKP cells

Abstract

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 approximately 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 (Vmax, 3.03 vs. 1.79 pH units/min) with no change in the affinity constant for sodium (KNa, 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.

Glucocorticoids enhance acid activation of the Na+/H+ exchanger 3 (NHE3)

“…Again, 10 -4 M cycloheximide was added only during the last 24 hours of the control or acid incubation to allow hydrocortisone 24 hours to induce gene expression and protein synthesis. The low Na/H antiporter activities seen in these experiments are likely due to inhibition of synthesis of new antiporters and are similar to that which we have reported previously (9,12) (Fig. 10).…”

“…Glucocorticoids increase proximal tubule apical membrane Na/H antiporter activity in vivo and in cell culture (5,9,17). The present studies also demonstrate that glucocorticoids increase NHE3 protein abundance.…”

“…In addition, glucocorticoids and acid incubation, applied separately, increase NHE3 activity (8,9). It is therefore likely that the effect of acid incubation in the presence of glucocorticoids is on NHE3.…”

Glucocorticoid Activation of Na+/H+Exchanger Isoform 3 Revisited

“…2) Dexamethasone failed to activate NHE3 in OK cells even though the NHE3 mRNA level was doubled. The absence of dexamethasone-induced activation of NHE3 in OK cells is in contrast to a previous report that 1 M dexamethasone resulted in 40% stimulation of NHE3 activity in OKP cells (37). The reasons for the discrepancy are not clear, but the studies were done using OKP cells, which are a subset of OK cells, and the difference in effect of dexamethasone may be the result of cell type difference.…”

Pyk2 activation is integral to acid stimulation of sodium/hydrogen exchanger 3

“…Cells were incubated with 10 -7 M dexamethasone or vehicle for 24 hours. In cells transfected with dominant-negative pyk2 K457A , dexamethasone increased NHE3 activity by 48% (P < 0.01, n = 6), a degree of stimulation similar to that seen in wild-type OKP cells (11).…”