STIMULATION OF SECRETION OF DISATURATED PHOSPHATIDYLCHOLINE FROM ISOLATED ALVEOLAR TYPE-2 CELLS BY 12-O-TETRADECANOYL-13-PHORBOL ACETATE

Alveolar type II cells synthesize and secrete pulmonary surface-active material; the stimuli for secretion in vivo and the mechanisms by which secretion occurs are not well understood. The authors studied the secretion of disaturated phosphatidylcholine, the principal component of surfactant, from a purified population of type II cells. The authors isolated type II cells from the lungs of adult male rats by treatment with trypsin, centrifugation over discontinuous density gradients, and adherence in primary culture; their preparations were 93±5 % (mean±SD; n=10) type II cells. Basal secretion was 2.9±1.0 % (n=16) of total cellular carbon-14 [14C]-disaturated phosphatidylcholine in 3 hours. The authors found that 10-8 M 12-O-tetradecanoyl-13-phorbol-acetate (TPA), a substance that thas been shown to stimulate secretion in other cell systems, caused a release of 14C-disaturated phosphatidylcholine that was 8.4 times the basal rate. TPA caused a greater release of disaturated phosphatidylhcoline than did any other substance that the authors have tested. Low temperature (4°C) inhibited the basal release by 85% and the TPA-stimulated release by 98%. The effect of TPA was also inhibited 25% by 10-6 M colchicine and 33% by 10-5 M vinblastine. Medium from control cells contained 6.3±1.3% (mean±SD; n=5) of total cellular lactate dehydrogenase (a marker for cell damage) after a 3-hour incubation period; medium from cells treated with TPA contained a similar amount, 6.7±1.5% (n=5). The authors concluded that the TPA-induced secretion of disaturated phosphatidylcholine is an active process probably mediated by microtubules. Because it has a large stimulatory effect on secretion, TPA may be useful for the study of the mechanisms by which surfactant is secreted.