Fig. 2.

(A) Mg²⁺ mimics the effect of receptor to accelerate the activation of Gs by GMP-P(NH)P (from Iyengar and Birnbaumer, 1981 [9]). (B) Effect of the hormone–receptor complex to reduce the concentration at which Mg²⁺ facilitates activation of Gs by GMP-P(NH)P. Liver membranes were treated with N-ethyl maleimide to inactivate its adenylyl cyclase and then subjected to a 10-min incubation with GMP-P(NH)P in the absence and presence of glucagon at increasing concentrations of Mg²⁺ shown on the figure. Gs in the incubated membranes was extracted and analyzed for activity in a cyc⁻ reconstitution assay. Note the hormone induced left shift in the Mg²⁺ concentration curve. Inset, expansion of lower range of the Mg²⁺ concentration axis (from Iyengar and Birnbaumer, 1982 [10]). (C) Mg²⁺ increases the rate at which GMP-P(NH)P activates purified Gs. Purified human erythrocyte Gs was preincubated for varying times with GMP-P(NH)P at varying concentrations of Mg²⁺. Progressive activation was then quantified in diluted aliquots using the cyc⁻ reconstitution assay. Note that the rate of activation increased as Mg²⁺ concentration was increased. Top, data are shown with time as the x-axis variable. Bottom, the data are shown with Mg²⁺ concentration as the x-axis variable (adapted from Codina et al., 1984 [11]).