G-PROTEIN SUBUNITS IN LUNG-CELLS

Many hormones and neurotransmitters bind to membrane-bound receptors t hat are coupled to signal generating enzymes or ion channels via signa l transducing GTP-binding proteins termed G proteins. Although recepto rs and second messengers have been extensively studied in cells of the respiratory system, the G proteins responsible for the coupling of th ese proteins have not been well-characterized. Therefore, we used immu noblot analysis to determine expression of G protein a and p subunits in membranes prepared from cells and tissues of the respiratory system , including cultured canine tracheal epithelium, cleanly dissected can ine tracheal smooth muscle, canine large conducting airways, and canin e and human lung parenchyma. The two isoforms of G(s) alpha (45 and 52 kDa) were present in all tissues, with a predominant expression of th e 45 kDa isoform. Plasma membranes prepared from canine tracheal epith elium and muscle, and human lung parenchyma, contained greater amounts of G(s) alpha than membranes prepared from canine bronchus and lung. Relative levels of immunoreactive G(i) alpha(2), G(i) alpha(3), G(q)/G (11)alpha, beta(1) and beta(2) were similar in all of the tissues stud ied. By contrast, G(o) alpha was absent in cultured tracheal epitheliu m, and tracheal smooth muscle expressed greater amounts of G(i) alpha( 2) compared to G(i) alpha(3). Specificity of G protein expression can provide one regulatory mechanism for functional biochemical pathways w ithin cells. The demonstration of specific G protein subunits is the f irst step in the molecular characterization of the regulation of these pathways, both in normal tissues and in disease states.