Regulation of ATP-dependent surfactant secretion and activation of second-messenger systems in alveolar type II cells

¹ Institute of Experimental Cardiology, 121552 Moscow, USSR; and Cardiovascular Research Institute and Department of Medicine, University of California, San Francisco, California 94143-0130

Several different classes of agonists are known to stimulate exocytosis in type II cells. These agonists cause increases in second messengers, such as adenosine 3',5'-cyclic monophosphate (cAMP) or cytosolic Ca²⁺, and/or stimulate protein kinase C. We studied generation of cAMP and phosphoinositide (PI) turnover in monolayer cultures of type II cells and measured [Ca²⁺]_i in single cultured cells. ATP [10^–4 M], which stimulates secretion of phosphatidylcholine (PC) and increases cellular cAMP, also stimulated PI turnover and increased [Ca²⁺]_i. 12-O-tetradecanoylphorbol-13-acetate (TPA), which stimulates PC secretion and activates protein kinase C, did not increase [Ca²⁺]_i. Pretreatment of type II cells with the calmodulin antagonist N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide (W-7) inhibited the PC secretion induced by ATP and TPA and blocked the increase in PI turnover caused by ATP. ATP-dependent surfactant secretion and stimulation of PI turnover could also be inhibited by pretreatment of the cells with pertussis toxin. We used the fluorescent probe indo-1 to measure [Ca²⁺]_i in single cultured type II cells. ATP produced rapid transient increases in [Ca²⁺]_i, which could be prevented by pretreatment of the cells with either TPA or W-7. Our data suggest that pertussis toxin-sensitive G protein(s) are involved in ATP-dependent activation of PI turnover and in secretion of surfactant in type II cells. Activation of protein kinase C blocks the ATP-stimulated increase in [Ca²⁺]_i. Finally, calmodulin may be involved in the regulation of ATP-dependent increase in [Ca²⁺]_i, the activation of PI turnover, and the secretion of surfactant in type II cells.