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

Abstract

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 Ca2+. 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 Ca2+ 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°C for one hour with 10 μM isoproterenol (IP) in the presence and absence of 10 μM 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 epineprine or Ro 5-4864. Both isoproterenol and epinephrine caused a significant increase in the level of cytosolic free Ca2+. However, Ro 5-4864 caused not only a decrease in the level of cytosolic free Ca2+ but also counteracted the stimulatory effect of IP. This may suggest that while ligands for ARs stimulate Ca2+ release into cytosol, the ligand for PBRs stimulates efflux of Ca2+ 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 Ca2+ efflux.