Ca2+/Calmodulin Distinguishes Between Guanyl-5'-yl-Imidodiphosphate-and Opiate-Mediated Inhibition of Rat Striatal Adenylate Cyclase

The inhibition of adenylate cyclase from rat striatal plasma membranes by guanyl-5'-yl-imidodiphosphate Gpp(NH)p] and morphine was compared to determine whether Gpp(NH)p-mediated inhibition accurately reflected hormone-mediated inhibition in this system. Inhibition of adenylate cyclase activity by Gpp(NH)p and morphine was examined with respect to temperature, divalent cation concentration, and the presence of Ca2+/calmodulin (Ca2+/CaM). Gpp(NH)p-mediated inhibition was dependent on the presence of Ca2+/CaM at 24 degrees C; the inhibition was independent of Ca2+/CaM at 18 degrees C; and inhibition could not be detected in the presence, or absence, of Ca2+/CaM at 30 degrees C. In contrast, naloxone-reversible, morphine-induced inhibition of adenylate cyclase was independent of both temperature and the presence of Ca2+/CaM. Mg2+ dose-response curves also reinforced the differences in the Ca2+/CaM requirement for Gpp(NH)p- and morphine-induced inhibition. Because Gpp(NH)p-mediated inhibition was independent of Ca2+/CaM at low basal activities (i.e., 18 degrees C, or below 1 mM Mg2+) and dependent on the presence of Ca2+/CaM at higher basal activities (24 degrees C, or above 1 mM Mg2+), the inhibitory effects of Gpp(NH)p were examined at 1 mM Mg2+ in the presence of 100 nM forskolin. Under these conditions, both Gpp(NH)p- and morphine-induced inhibition of adenylate cyclase were independent of Ca2+/CaM. The results demonstrate that the requirement for Ca2+/CaM to observe Gpp(NH)p-mediated inhibition depends on the basal activity of adenylate cyclase, whereas hormone-mediated inhibition is Ca2+/CaM independent under all conditions.(ABSTRACT TRUNCATED AT 250 WORDS)