Stimulation of Secretion of Disaturated Phosphatidylcholine from Isolated Alveolar Type II Cells by 12-O-Tetradecanoyl-13-Phorbol Acetate1,2

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Abstract
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. We studied the secretion of disaturated phosphatidylcholine, the principal component of surfactant, from a purified population of type II cells. We 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; our preparations were 93 ± 5 per cent (mean ± SD; n =10) type II cells. Basal secretion was 2.9 ± 1.0 per cent (n = 16) of total cellular carbon-14 [14C]-disaturated phosphatidylcholine in 3 hours. We found that 10−8 M 12-O-tetradecanoyl-13-phorbol-acetate (TPA), a substance that has 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 phosphatidylcholine than did any other substance that we have tested. Low temperature (4° C) inhibited the basal release by 85 per cent and the TPA-stimulated release by 98 per cent. The effect of TPA was also inhibited 25 per cent by 10−6 M colchicine and 33 per cent by 10−5 M vinblastine. Medium from control cells contained 6.3 ± 1.3 per cent (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 per cent (n = 5). We 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.