Inhibition of Na⁺/H⁺ exchange stimulates CCK secretion in STC-1 cells

It has been demonstrated that K⁺ channel regulation of membrane potential is critical for control of CCK secretion. Because certain K⁺ channels are pH sensitive, it was postulated that pH affects K⁺ channel activity in the CCK-secreting cell line STC-1 and may participate in regulating CCK secretion. The present study examines the role of electroneutral Na⁺/H⁺ exchange on extracellular acidification and hormone secretion. Treatment of STC-1 cells with the amiloride analog ethylisopropyl amiloride (EIPA) to inhibit Na⁺/H⁺ exchange inhibited Na⁺-dependent H⁺ efflux and increased basal CCK secretion. Substituting choline for NaCl in the extracellular medium elevated basal intracellular Ca²⁺ concentration and stimulated CCK release. Stimulatory effects on hormone secretion were blocked by the L-type Ca²⁺ channel blocker diltiazem, indicating that secretion was dependent on the influx of extracellular Ca²⁺. To determine whether the effects of EIPA and Na⁺ depletion were due to membrane depolarization, we tested graded KCl concentrations. The ability of EIPA to increase CCK secretion was inhibited by depolarization induced by 10-50 mM KCl in the bath. Maneuvers to lower intracellular pH (pH(i)), including reducing extracellular pH (pH(o)) to 7.0 or treatment with sodium butyrate, significantly increased CCK secretion. To examine whether pH directly affects membrane K⁺ permeability, we measured outward currents carried by K⁺, using whole cell patch techniques. K⁺ current was significantly inhibited by lowering pH(o) to 7.0. These effects appear to be mediated through changes in pH(i), because intracellular dialysis with acidic solutions nearly eliminated current activity. These results suggest that Na⁺/H⁺ exchange and membrane potential may be functionally linked, where inhibition of Na⁺/H⁺ exchange lowers phi and depolarizes the membrane, perhaps through inhibition of pH- sensitive K⁺ channels. In turn, K⁺ channel closure and membrane depolarization open voltage-dependent Ca²⁺ channels, leading to an increase in cytosolic Ca²⁺ and CCK release. The effects of pH(i) on K⁺ channels may serve as a potent stimulus for hormone secretion, linking cell metabolism and secretor/functions.