Reduction in the stability of the G_S-catalytic unit complex of adenylate cyclase in isoproterenol-induced heterologous desensitization

Treatment of rat reticulocytes with isoproterenol resulted in the heterologous desensitization of adenylate cyclase, causing a reduction in NaF/AlCl₃- and guanylyl 5′-imidodiphosphate (Gpp(NH)p)-stimulated activities as well as in activity stimulated by β-adrenoceptor agonists. Desensitization was also induced by dibutyryl cyclic AMP and 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine (H-7), an inhibitor of cyclic AMP-dependent protein kinase (protein kinase A), prevented the isoproterenol-induced desensitization, suggesting the involvement of protein kinase A in the desensitization. G_s in the desensitized cell membrane was activated by treatment with Gpp(NH)p or NaF/AlCl₃ in the presence of Mg²⁺. The activated state, the G_s-C complex, was gradually shifted to the basal state, i.e. the G_s-C complex was dissociated by removal of Mg²⁺. The rate constant of the dissociation was increased in the desensitized cell membranes (0.074 min^-1) as compared with that in the control cell membranes (0.022 min^-1) The half life of the G_s-C complex, calculated from the rate constant, was decreased during the process of desensitization. The results indicate that reduction in the stability of the G_s-C complex is related to heterologous desensitization. Agents which increase the level of cyclic AMP in the cell also reduced the stability of the complex. The potency of such an effect was in the following order; isoproterenol ∼ NaF/AlCl₃(k = 0.071 min^-1) > forskolin (0.039 min^-1 > Mn²⁺ ∼ dibytyryl cyclic AMP (0.030 min^-1). The former two activate G_s.and form G_s-C complex, while the latter two cause intracellular cyclic AMP accumulation without activation of G_s. It may be deduced from this order that the reduction of the stability depends not only on the increase in the cyclic AMP level but also on the level of G_s-C complex formed.