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‐imidodiphos‐phate [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 Ca 2+ /calmodulin (Ca 2+ /CaM). Gpp(NH)p‐mediated inhibition was dependent on the presence of Ca 2+ /CaM at 24°C; the inhibition was independent of Ca 2+ /CaM at 18°C; and inhibition could not be detected in the presence, or absence, of Ca 2+ / CaM at 30°C. In contrast, naloxone‐reversible, morphine‐induced inhibition of adenylate cyclase was independent of both temperature and the presence of Ca 2+ /CaM. Mg 2+ dose‐response curves also reinforced the differences in the Ca 2+ /CaM requirement for Gpp(NH)p‐and morphine‐induced inhibition. Because Gpp(NH)p‐mediated inhibition was independent of Ca 2+ /CaM at low basal activities (i.e., 18°C, or below 1 m M Mg 2+ ) and dependent on the presence of Ca 2+ /CaM at higher basal activities (24°C, or above 1 m M Mg 2+ ), the inhibitory effects of Gpp(NH)p were examined at 1 m M Mg 2+ in the presence of 100 n M forskolin. Under these conditions, both Gpp(NH)p‐and morphine‐induced inhibition of adenylate cyclase were independent of Ca 2+ /CaM. The results demonstrate that the requirement for Ca 2+ /CaM to observe Gpp(NH)p‐mediated inhibition depends on the basal activity of adenylate cyclase, whereas hormone‐mediated inhibition is Ca 2+ /CaM independent under all conditions. Further, this study shows that, under certain conditions, Gpp(NH)p‐mediated inhibition of adenylate cyclase does not mimic the inhibition of adenylate cyclase produced by a complete interaction between the inhibitory hormone receptor, the guanine nucleotide‐regulatory component that mediates inhibition, and the catalytic unit of the adenylate cyclase complex in striatal plasma membranes.