Positive correlation between cytosolic free calcium and surfactant secretion in cultured rat alveolar type II cells

To determine whether increases in the cytosolic free Ca2+ concentration ([Ca2+]i) accompany agonist-stimulated surfactant secretion by cultured alveolar type II cells, we measured the [Ca2+]i of quin2-loaded cells isolated from adult rats before and after cells were stimulated with ionomycin, terbutaline or tetradecanoylphorbol acetate (TPA). To determine whether increases in [Ca2+]i are necessary for stimulated surfactant secretion to occur, we measured secretion in cells after [Ca2+]i had been reduced by loading cells with quin2 in medium containing low [Ca2+]. Ionomycin increased [Ca2+]i and stimulated surfactant secretion in a dose-dependent manner. Reductions in [Ca2+]i correlated with reductions in secretion stimulated by ionomycin, terbutaline or TPA. Ionomycin-stimulated secretion was most sensitive to reductions in [Ca2+]i; terbutaline-stimulated secretion was more sensitive than TPA-stimulated secretion. When [Ca2+]i was less than 65 nM, all stimulated secretion was blocked. Restoration of [Ca2+]i to greater than 100 nM restored ionomycin-stimulated secretion. We conclude that ionomycin increases [Ca2+]i and stimulates surfactant secretion in cultured alveolar type II cells, and that increased [Ca2+]i appears to be necessary for ionomycin-stimulated secretion to occur. Terbutaline-stimulated surfactant secretion seems to be more easily inhibited by a reduction in [Ca2+]i than does TPA-stimulated secretion.