ALKALOSIS-INDUCED AND ATP-INDUCED INCREASES IN THE DIACYLGLYCEROL POOL IN ALVEOLAR TYPE-II CELLS ARE DERIVED FROM PHOSPHATIDYLCHOLINE AND PHOSPHATIDYLINOSITOL

Alkalosis and ATP increase surfactant secretion in alveolar type II ce lls, possibly via non-receptor- and receptor-mediated mechanisms respe ctively. We compared the effects of these two agonists on phosphatidyl inositol (PI) and 1,2-diacylglycerol (DAG) pools and on phosphatidylch oline (PC) hydrolysis in alveolar type II cells. Alkalosis, caused by transfer of cells from 5 0. (control) to 0% CO2 in air, and ATP increa sed the secretion of surfactant compared with the controls. The stimul ated secretion was inhibited by staurosporine, a protein kinase C inhi bitor. DAG and PI contents of control cells were 50+/-1.1 (means+/-S.E .M., n=8) and 14+/-0.8 nmol/mg phospholipid (n = 7) respectively. The DAG content increased by approximately 50 nmol (100%) within 5 s of tr eatment with both alkalosis and ATP, returned to control levels by 1 m in, and increased again at 5 min by approximately 20 nmol. The PI cont ent decreased maximally by approximately 6 nmol (40%) at 5 s and retur ned to control levels by 30 s with both alkalosis and ATP, but was unc hanged thereafter. Mass-balance analysis of net changes in DAG and PI pools suggests that additional sources, possibly PC, must also contrib ute to the DAG increase. ATP or alkalosis also increased the hydrolysi s of PC. The labelling of phosphocholine was increased (approximately 60%) at as early as 5 s and remained elevated at subsequent time point s, whereas labeling of choline was higher only with ATP at 50 s and la ter, suggesting activation of phospholipase C by both agonists, and of phospholipase D by only ATP. Our studies demonstrate that ATP and alk alosis stimulate rapid hydrolysis of inositol and choline phospholipid s to increase the DAG mass in type II cells, and that phospholipase C- stimulated PC hydrolysis is the major pathway for DAG formation.