Na+-H+ exchanger in proximal cells isolated from kidney. II. Short-term regulation by glucocorticoids

The purpose of this study was to investigate the acute regulation by glucocorticoid of the Na+-H+ exchanger in isolated renal proximal cells of the rabbit. The changes of intracellular pH (pHi) were determined in a bicarbonate-free buffer by the use of a fluorescent pH probe that may be trapped intracellularly, 2,7-biscarboxyethyl-5(6)-carboxyfluorescein (BCECF). The activity of the Na+-H+ exchanger was estimated by measuring the Na+-induced H+ efflux in BCECF-loaded cells acid loaded with nigericin in choline medium. The uptake of 1.5 mM 22Na was also studied in Na+-depleted cells. Acute application of dexamethasone (1 h at 37 degrees C) increased the activity of the Na+-H+ exchanger. Evidence for this was the fact that the hormone enhanced the rate of Na+-dependent H+ efflux in acid-loaded cells and increased the Na entry in normal pH, sodium-depleted cells. These actions were blocked by 0.5 mM amiloride and were specific to glucocorticoids, since mineralocorticoids (aldosterone) did not promote modification either of the H+ efflux or the Na influx. The effect on the kinetics of amiloride-sensitive Na+-H+ exchange indicated that dexamethasone (DEX) increased the activity by increasing the Vmax of the carrier for external sodium (Vmax control = 26.5 +/- 1.7, DEX = 33.0 +/- 2.4 mmol H+.1-1.min-1, n = 5, P less than 0.001) and for external H+ (Vmax control = 20.6, DEX = 25.8 mmol H+.1-1.min-1, n = 2).(ABSTRACT TRUNCATED AT 250 WORDS)