Calcium mobilization and response recovery following P2-purinoceptor stimulation of rat isolated alveolar type II cells.

1. The effect of adenosine 5'-triphosphate (ATP) on surfactant phospholipid secretion, calcium mobilization, and the time course for recovery of the response system was studied in isolated alveolar Type II cells of the rat. 2. ATP (10 microM) stimulated a biphasic intracellular Ca2+ transient monitored by changes in Fura-2 fluorescence, from a basal level of 126 +/- 9 nM, to a rapid peak of 391 +/- 1 nM, followed by a prolonged plateau 26 +/- 4 nM above baseline (mean +/- s.e.mean, n = 26). 3. ATP-stimulated surfactant phospholipid secretion and peak Ca2+ levels had similar EC50s (1 x 10(-6) M), and were unaffected by chelation of extracellular Ca2+. However, the prolonged plateau phase was abolished by chelation of extracellular Ca2+. 4. There was a 15 min refractory period before full recovery of the Ca2+-response to ATP. Recovery was dependent on extracellular Ca2+, was accelerated by removing extracellular agonist and was prolonged following stimulation with the poorly hydrolyzed ATP analogue, ATP-gamma-S. 5. While the Type II cell was capable of multiple ATP-induced Ca2+ transients following recovery, no additional surfactant phospholipid was released with sequential stimulation. 6. These findings suggest initial exposure of Type II cells to ATP mobilizes intracellular Ca2+, stimulates phospholipid secretion and rapidly desensitizes the cell to further stimulation by ATP. Recovery of the ATP-induced Ca2+-response depends on presence of extracellular Ca2+ and removal of agonist.

P2-purinoceptor regulation of surfactant phosphatidylcholine secretion. Relative roles of calcium and protein kinase C.

Ca2+ and protein kinase C have both been proposed as intracellular signals for subsequent phosphatidylcholine secretion by alveolar Type II cells. We have determined the relative roles of Ca2+ and protein kinase C in regulating surfactant phosphatidylcholine secretion by utilizing exogenous ATP and the phorbol ester TPA (12-O-tetradecanoylphorbol 13-acetate) as secretagogues, along with MAPTAM to chelate intracellular Ca2+ and sphingosine to inhibit endogenous protein kinase C. Exposure of Type II cells to the P2-purinoceptor agonist, ATP, results in a dose-dependent increase in surfactant phosphatidylcholine secretion from isolated alveolar Type II cells with an EC50 (concn. producing 50% of maximal response) of 2 microM. Administration of exogenous ATP to Type II cells also results in a dose-dependent increase in inositol trisphosphate production, Ca2+ mobilization and [3H]phorbol 12,13-dibutyrate ([3H]PDBu) binding as a measure of protein kinase C translocation. The EC50 in each case is 1-5 microM, indicating association of these events with surfactant phosphatidylcholine secretion. Loading Type II cells with non-hydrolysable GTP analogue (GTP[S]) inhibited ATP-induced Ca2+ mobilization, supporting the hypothesis that Type II cell P2-purinoceptors are coupled to phospholipase C via a GTP-binding protein. The ATP-induced elevation of cytosolic Ca2+ was also inhibited by MAPTAM (a cell-permeant EGTA analogue) by 90%, but MAPTAM was without effect on surfactant phosphatidylcholine secretion induced by ATP. Sphingosine inhibited both ATP- and TPA-induced surfactant phosphatidylcholine secretion as well as [3H]PDBu binding with a similar IC50 (concn. producing 50% of maximal inhibition) (10 microM). Sphingosine did not affect surfactant phosphatidylcholine secretion induced by terbutaline and did not have a significant effect on Ca2+ mobilization induced by exogenous ATP. These results are consistent with a prominent role for protein kinase C in regulation of P2-purinoceptor-induced surfactant phosphatidylcholine secretion, and indicate that Ca2+ mobilization is not a necessary step for ATP-induced surfactant phosphatidylcholine secretion.