Local Ca2+ rise near store operated Ca2+ channels inhibits cell coupling during capacitative Ca2+ influx

Using a new fluorescence imaging technique, LAMP, we recently reported that Ca2+ influx through store operated Ca2+ channels (SOCs) strongly inhibits cell coupling in primary human fibroblasts (HF) expressing Cx43. To understand the mechanism of inhibition, we studied the involvement of cytosolic pH (pH(i)) and Ca2+ ([Ca2+](i)) in the process by using fluorescence imaging and ion clamping techniques. During the capacitative Ca2+ influx, there was a modest decline of pH(i) measured by BCECF. Decreasing pH(i) below neutral using thioacetate had little effect by itself on cell coupling, and concomitant pHi drop with thioacetate and bulk [Ca2+](i) rise with ionomycin was much less effective in inhibiting cell coupling than Ca2+ influx. Moreover, clamping pHi with a weak acid and a weak base during Ca2+ influx largely suppressed bulk pHi drop, yet the inhibition of cell coupling was not affected. In contrast, buffering [Ca2+](i) with BAPTA, but not EGTA, efficiently prevented cell uncoupling by Ca2+ influx. We concluded that local Ca2+ elevation subjacent to the plasma membrane is the primary cause for closing Cx43 channels during capacitative Ca2+ influx. To assess how Ca2+ influx affects junctional coupling mediated by other types of connexins, we applied the LAMP assay to Hela cells expressing Cx26. Capacitative Ca2+ influx also caused a strong reduction of cell coupling, suggesting that the inhibitory effect by Ca2+ influx may be a more general phenomenon.