Cell shrinkage evoked by Ca 2+ ‐free solution in rat alveolar type II cells: Ca 2+ regulation of Na + –H + exchange

The effects of intracellular Ca 2+ concentration, [Ca 2+ ] i , on the volume of rat alveolar type II cells (AT‐II cells) were examined. Perfusion with a Ca 2+ ‐free solution induced shrinkage of the AT‐II cell volume in the absence or presence of amiloride (1 μ m , an inhibitor of Na + channels); however, it did not in the presence of 5‐( N ‐methyl‐ N ‐isobutyl)‐amiloride (MIA, an inhibitor of Na + –H + exchange). MIA decreased the volume of AT‐II cells. Inhibitors of Cl − –HCO 3 − exchange, 4,4′‐diisothiocyanostilbene‐2,2′‐disulfonic acid (DIDS) and 4‐acetamido‐4′‐isothiocyanatostilbene‐2,2′‐disulfonic acid (SITS) also decreased the volume of AT‐II cells. This indicates that the cell shrinkage induced by a Ca 2+ ‐free solution is caused by a decrease in NaCl influx via Na + –H + exchange and Cl − –HCO 3 − exchange. Addition of ionomycin (1 μ m ), in contrast, induced cell swelling when AT‐II cells were pretreated with quinine and amiloride. This swelling of the AT‐II cells is not detected in the presence of MIA. Intracellular pH (pH i ) measurements demonstrated that the Ca 2+ ‐free solution or MIA decreases pH i , and that ionomycin increases it. Ionomycin stimulated the pH i recovery after an acid loading (NH 4 + pulse method), which was not noted in MIA‐treated AT‐II cells. Ionomycin increased [Ca 2+ ] i in fura‐2‐loaded AT‐II cells. In conclusion, the Na + –H + exchange activities of AT‐II cells, which maintain the volume and pH i , are regulated by [Ca 2+ ] i .