Role of Peripheral Benzodiazepine Receptors on Secretion of Surfactant in Guinea Pig Alveolar Type II Cells

摘要

Granular type II cells located in the alveolar epithelium synthesize and secrete pulmonary surfactant and have specialized ion transport system. Alveolar type II cells are stimulated to secrete pulmonary surfactant by a variety of agonists. One mechanism by which extracellular signals are perceived by cells is the mobilization of intracellular Ca~(2+). Peripheral benzodiazepine receptors (PBRs) are present in both peripheral tissues and central nervous system. We have previously reported the presence of high density PBRs in lung and alveolar type II cells. It is known that both PBRs and beta-adrenergic receptors (beta-ARs) play an important role in cellular Ca~(2+) transport. Furthermore, we have suggested earlier that PBRs are someway functionally associated with the beta-ARs. The objective of the present study was to determine whether PBRs play any role in the secretion of surfactant by alveolar type II cells. Alveolar type II cells were isolated from normal weanling guinea pigs by panning method and incubated with ~3H-palmitic acid in minimum essential medium to synthesize labelled dipalmitoyl phosphatidylcholine (DPPC). After washing, the cells were treated at 37 deg C for one hour with 10 #muM isoproterenol (IP) in the presence and absence of 10 #mu#M Ro 5-4864, an agonist for PBRs. After one hour, the release of labelled DPPC in the medium was analyzed. The control cells released DPPC without any addition of a ligand, However, the treatment of cells with IP, Ro 5-4864 and IP + Ro 5-4864 caused 24, 52 and 171% increase in the secretion of DPPC, respectively. In another experiment, type II cells were loaded with Fura-2 dye and treated with either IP or epinephrine for Ro 5-4864. Both isoproterenol and epinephrine caused a significant increase in the level of cytosolic free Ca~(2+). However, Ro 5-4864 caused not only a decrease in the level of cytosolic free Ca~(2+) but also counteracted the stimulatory effect of IP. This may suggest that while ligands for ARs stimulate Ca~(2+) release into cytosol, the ligand fro PBRs stimulates efflux of Ca~(2+) in alveolar type cells. Thus, the increased secretion of surfactant by the ligand of PBRs in alveolar type II cells may be mediated through its effects on increased Ca~(2+) efflux.