G-proteins in guinea pig airway smooth muscle: identification and functional involvement

In airway smooth muscle, G-proteins have not been identified directly as yet. This study was an attempt to detect various types of conventional Gi- and Gs-proteins in purified membranes of guinea pig airway smooth muscle and to assess the involvement of the G-proteins in agonist-induced contractile response of the smooth muscle. Immunoblotting using AS/7 antibody which recognizes Gi-1/2 demonstrated the presence of polypeptides of M(r) 34, 41 and 75 kDa. Polypeptides of M(r) 43, 46 and 48 kDa were identified with RM/1 antibody that detects Gs-type G-proteins. The AG/1 antibody that recognizes alpha-subunits common to all Gi- and Gs-proteins detected two polypeptides of M(r) 41 and 75 kDa. Heterotrimeric structure of the G-proteins was confirmed by the identification of a single dense beta-subunit band at M(r) 39 kDa with SW/1 antibody. Pertussis toxin (PT) ADP-ribosylated three Gi alpha polypeptides of M(r) 41, 43, and 62 kDa. On the other hand, cholera toxin (CT) catalysed the ADP-ribosylation of two Gs alpha polypeptides of M(r) 46 and 62 kDa. Both PT and CT attenuated the maximum contractile responses of the airway smooth muscle to the muscarinic agonist, methacholine. Pretreatment of the tissues with the sulphydryl alkylating G-protein inhibitor, N-ethylmaleimide, also inhibited the maximum contractions to methacholine. These data suggest that plasma membranes of guinea pig airway smooth muscle contain a variety of conventional, including Gs and Gi, and other types of G-proteins, and at least a portion of the proteins present may be involved in mediating the contractile responses of the smooth muscle to an agonist such as methacholine.