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

Abstract

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.