Received August 17, 2004
Revised September 22, 2004
Accepted after revision November 10, 2004

¹ Osaka Medical College

^* To whom correspondence should be addressed. E-mail: takan{at}art.osaka-med.ac.jp.

	Abstract

The effects of intracellular Ca²⁺ concentration, [Ca²⁺]_i, on the volume of rat alveolar type II cells (AT-II cells) were examined. Perfusion with a Ca²⁺-free solution induced shrinkage of the AT-II cell volume in the presence or the absence of amiloride (1 µM, an inhibitor of Na⁺ channels), however, it did not in the presence of methyl-isobutyl-amiloride (MIA, an inhibitor of Na⁺/H⁺ exchange). MIA decreased the volume of AT-II cells. Inhibitors of Cl^-/HCO₃^- exchange, diisothiocyanostilbene disulfonic acid (DIDS) and acetamide isothiocyanatostilben disulfonic acid (SITS) also decreased the volume of AT-II cells. This indicates that the cell shrinkage induced by a Ca²⁺- free solution is caused by a decrease in NaCl influx via Na⁺/H⁺ exchange and Cl^- /HCO₃^- exchange. Addition of ionomycin (1 µM), 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 Ca²⁺-free solution or MIA decreases pH_i, and that ionomycin increases it. Ionomycin stimulated the pH_i recovery after an acid loading (NH₄⁺ pulse method), which was not noted in MIA-treated AT-II cells. Ionomycin increased [Ca²⁺]_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 [Ca²⁺]_i.

Key Words: Cell volume, Intracellular pH, Sodium-hydrogen exchange