Rhoden_1994_Lung_172_355

Guanine nucleotide-binding proteins, or G proteins, play an important role in transmitting information from membrane receptors to intracellular effector systems. Activation of G proteins results in the hydrolysis of GTP, and the measurement of GTPase activity represents a means by which the role of G proteins in signal transduction can be investigated. GTPase activity of guinea pig bronchial membranes was measured as the liberation of 32Pi from [gamma-32P]GTP. GTPase activity was divided into two components, one possessing a high affinity and the other a low affinity for GTP. The contribution of high- and low-affinity GTPase to total hydrolysis was dependent on Mg2+. In the presence of submicromolar Mg2+, high-affinity GTPase represented 65-80% of all activity, whereas in the presence of > or = 26 microM Mg2+, all detectable hydrolysis was due to the low-affinity GTPase. High-affinity GTPase was stimulated by Mg2+ in the 0.15-1.1 microM range (2.5-fold maximal stimulation, apparent Km for Mg2+ 0.31 microM). Mastoparan (1-100 microM) caused a concentration-dependent stimulation of high-affinity (but not low-affinity) GTPase (71 +/- 13% maximal stimulation, EC50 0.38 microM), suggesting that high-affinity GTPase may be due to a G protein. Carbachol (10 microM) and fenoterol (10 microM) had no effect on high-affinity GTP hydrolysis, suggesting that under the conditions described, GTPase activity of bronchial membranes is not activated by muscarinic or beta-adrenergic receptors, respectively.