Regulation of SP-B and SP-C secretion in rat type II cells in primary culture

Secretion of lung surfactant phospholipids is a highly regulated process. A variety of physiological and pharmacological agents stimulate surfactant p hospholipid secretion in isolated type II cells. Although the lipid and hyd rophobic protein components of surfactant are believed to be secreted toget her by exocytosis of lamellar body contents, regulation of surfactant prote in (SP) B and SP-C secretion has not previously been examined. To address t he question of whether secretion of SP-B and SP-C is stimulated by the same agonists that stimulate phospholipid secretion, we measured secretion of a ll four SPs under the same conditions used to measure phosphatidylcholine s ecretion. Freshly isolated rat type II cells were cultured overnight and ex posed to known surfactant phospholipid secretagogues for 2.5 h, after which the amounts of SP-A, SP-B, SP-C, and SP-D in the medium were measured with immunoblotting. Secretion of SP- B and SP-C was stimulated three- to fivef old by terbutaline, 5'-(N-ethylcarboxyamido)adenosine, ATP, 12-O-tetradecan oylphorbol 13-acetate, and ionomycin. Similar to their effects on phospholi pid secretion, the stimulatory effects of the agonists were abolished by Ro 31-8220. Secretion of SP- A and SP-D was not stimulated by the secretagogu es tested. We conclude that secretion of the phospholipid and hydrophobic p rotein components of surfactant is similarly regulated, whereas secretion o f the hydrophilic proteins is regulated differently.