Secretagogues increase the expression of surfactant protein A receptors on lung type II cells

Since secretagogues have been shown to increase the internalization of surfactant phospholipid and protein by lung cells, we postulated that their action occurred through a mechanism involving increased surfactant protein A (SP-A) receptor density. Therefore, we evaluated the influence of secretagogues on the binding of iodinated SP-A to alveolar type II cells. Type II cells were isolated from rat lung and maintained in primary culture for 18 h on Transwell membranes. Upon exposure to 8-bromo-cyclic AMP (cAMP, 0.1 mM), phorbol 12-myristate 13-acetate (PMA, 10 nM), terbutaline (0.1 mM), or ATP (1 mM), the binding of SP-A increased 1.5-2-fold. This stimulation was cell substrate-dependent since type II cells plated on plastic dishes did not show this effect. A time course of the stimulation of SP-A binding due to secretagogues showed that both cAMP and PMA increased SP-A binding by 2-fold after 20 min. With cAMP, binding remained elevated for 2 h, while binding in the presence of PMA had returned to control values. The effects of submaximal concentrations of cAMP and PMA on binding were additive. Inhibition of cellular protein synthesis with cycloheximide did not alter the increase of SP-A binding stimulated by the secretagogues. Type II cells pretreated with PMA responded to subsequent treatment with cAMP by increasing SP-A binding, while these cells were refractory to subsequent treatment with PMA. Both constitutive and regulated binding of SP-A to type II cells were sensitive to trypsin. The binding of SP-A to type II cells showed saturation at a concentration of 1 microg/ml SP-A under control and secretagogue-stimulated conditions, with both total and calcium-dependent binding showing a 2-fold increase upon secretagogue exposure. The data are consistent with the hypothesis that secretagogues stimulate surfactant uptake, at least in part, through recruitment of SP-A receptors to the type II cell surface, resulting in an increase in the number of SP-A binding sites.