Alteration of G protein levels in antigen-challenged guinea pigs.

To elucidate the mechanism(s) of altered responses of various receptors observed in asthma, we examined levels of G proteins in lung membranes isolated from guinea pigs at various stages of experimental asthma. We found that levels of Gi2 alpha and Gq alpha increase after multiple exposures to aerosolized antigen but not after a single exposure. Levels of Gi1 alpha, Gs alpha and G beta did not change under these experimental conditions. Tracheas from these animals exhibited hyperresponsive muscarinic bronchoconstriction and hyporesponsive beta adrenoceptor-mediated relaxation of endothelin-preconstricted tracheas as measured by an ex vivo isometric assay. No changes in numbers or affinities of membrane M3 muscarinic and beta-2 adrenergic receptors were detected as measured by binding of [3H]QNB and [125I]CYP, respectively. The impairment of beta adrenoceptor-mediated relaxation of tracheas isolated from animals treated by multiple challenges was reversed by the pretreatment of tracheas with pertussis toxin. On the other hand, the carbamylcholine-stimulated GTPase activity was higher in lung membranes isolated from animals in the multiple challenge group and was blocked by anti-Gq alpha protein antibody. Taken together, our present observations suggest that an underlying mechanism of altered receptor responses associated with asthma is in part due to increased expression of Gi2 alpha and Gq alpha proteins, leading to altered coupling of receptors to effector systems.