Temporal Segregation of Surfactant Secretion and Lamellar Body Biogenesis in Primary Cultures of Rat Alveolar Type II Cells

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Abstract
Pulmonary alveolar type II cells synthesize and secrete the phospholipids of surfactant. However, type II cells isolated from adult rat lungs rapidly lose their characteristic morphology and differentiated functions (such as surfactant-specific phospholipid and protein biosynthesis) when maintained on tissue culture plastic. In this study, phospholipid secretion and its regulation by type II cells grown on tissue culture plastic were examined up to 8 days after isolation. Type II cells were preincubated with [3H]choline for varying 24-h periods during culture prior to examining phosphatidylcholine ([3H]PtdCho) secretion. Type II cells cultured for 4 days and incubated with [3H]choline 24 h before the secretion experiment failed to show significant basal and tetradecanoyl phorbol acetate (TPA, 100 nM)-stimulated [3H]PtdCho secretion (basal, 0.29 +/- 0.01%; TPA, 0.48 +/- 0.04%). In contrast, type II cells incubated with [3H]choline for the first 24 h during culture and then cultured for 3 more days showed significant [3H]PtdCho secretion (basal, 1.27 +/- 0.19%; TPA, 6.24 +/- 0.82%). Subcellular fractionation of type II cells revealed that [3H]choline was incorporated into phosphatidylcholines in a lamellar body-enriched fraction during the first 24 h of culture but that the assimilation of phosphatidylcholine into the lamellar body fraction progressively declined with increasing time in culture. Radiolabel incorporated into the lamellar body fraction labeled during the first 24 h of culture was detectable for up to 8 days in culture. The [3H]PtdCho incorporated into the lamellar body during the first 24 h of culture was lost gradually over 8 days, suggesting the continuous secretion or turnover of the lamellar bodies during culture.(ABSTRACT TRUNCATED AT 250 WORDS)

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