Membrane potential modulation of ionomycin-stimulated Ca2+ entry via Ca2+/H+ exchange and SOC in rat submandibular acinar cells

Ionomycin (IM) at 5 μM mediates the Ca2+/H+ exchange, while IM at 1 μM activates the store-operated Ca2+ entry channels (SOCs). In this study, the effects of depolarization on both pathways were examined in rat submandibular acinar cells by increasing extracellular K+ concentration ([K+]o). IM (5 μM, the Ca2+/H+ exchange) increased the intracellular Ca2+ concentration ([Ca2+]i) to an extremely high value at 151 mM [K+]o. However, with increasing [K+]o, the rates of Ca2+ entry decreased in a linear relationship. The reversal potential (E rev) for the Ca2+/H+ exchange was +93 mV, suggesting that IM (5 μM) exchanges 1 Ca2+ for 1 H+. Thus, depolarization decreases the Ca2+ influx via the Ca2+/H+ exchange because of its electrogenicity (1 Ca2+ for 1 H+). On the other hand, IM (1 μM, the SOCs) abolished an increase in [Ca2+]i at 151 mM [K+]o. With increasing [K+]o, the rate of Ca2+ entry immediately decreased linearly. The E rev for the SOC was +3.7 mV, suggesting that the SOCs are nonselective cation channels and less selective for Ca2+ over Na+ (P Ca/P Na = 8.2). Moreover, an increase in extracellular Ca2+ concentration (20 mM) enhanced the Ca2+ entry via the SOCs at 151 mM [K+]o, suggesting depolarization does not inhibit the SOCs and decreases the driving force for the Ca2+ entry. This suggests that membrane potential changes induced by a secretory stimulation finely regulate the [Ca2+]i via the SOCs in rat submandibular acinar cells. In conclusion, IM increases [Ca2+]i via two pathways depending on its concentration, the exchange of 1 Ca2+ for 1 H+ at 5 μM and the SOCs at 1 μM.