INHIBITION OF NA+ H+ EXCHANGE STIMULATES CCK SECRETION IN STC-1 CELLS/

It has been demonstrated that K+ channel regulation of membrane potent ial is critical for control of CCK secretion. Because certain K+ chann els are pH sensitive, it was postulated that pH affects K+ channel act ivity in the CCK-secreting cell line STC-1 and may participate in regu lating CCK secretion. The present study examines the role of electrone utral Na+/H+ exchange on extracellular acidification and hormone secre tion. Treatment of STC-1 cells with the amiloride analog ethylisopropy l amiloride (EIPA) to inhibit Na+/H+ exchange inhibited Nai-dependent H+ efflux and increased basal CCK secretion. Substituting choline for NaCl in the extracellular medium elevated basal intracellular Ca2+ con centration and stimulated CCK release. Stimulatory effects on hormone secretion were blocked by the L-type Ca2+ channel blocker diltiazem, i ndicating that secretion was dependent on the influx of extracellular Ca2+ To determine whether the effects of EIPA and Na+ depletion were d ue to membrane depolarization, we tested graded KCl concentrations. Th e ability of EIPA to increase CCK secretion was inhibited by depolariz ation induced by 10-50 mM KCl in the bath. Maneuvers to lower intracel lular pH (pH(i)), including reducing extracellular pH (pH(o)) to 7.0 o r treatment with sodium butyrate, significantly increased CCK secretio n. To examine whether pH directly affects membrane K+ permeability, we measured outward currents carried by K+, using whole cell patch techn iques. K+ current was significantly inhibited by lowering pH(o) to 7.0 . These effects appear to be mediated through changes in pH(i), becaus e intracellular dialysis with acidic solutions nearly eliminated curre nt activity. These results suggest that Na+/H+ exchange and membrane p otential may be functionally linked, where inhibition of Na+/H+ exchan ge lowers pH(i) and depolarizes the membrane, perhaps through inhibiti on of pH-sensitive K+ channels. In turn, K+ channel closure and membra ne depolarization open voltagedependent Ca2+ channels, leading to an i ncrease in cytosolic Ca2+ and CCK release. The effects of pH(i) on KC channels may serve as a potent stimulus for hormone secretion, linking cell metabolism and secretory functions.