Adrenalectomy-induced alterations of calmodulin-dependent hippocampal adenylate cyclase activity: Role of guanine nucleotide-binding proteins

Ca²⁺/calmodulin-dependent processes are altered by manipulations of the hypothalamic-pituitary-adrenal axis. In particular, adrenalectomy (ADX) attenuates hippocampal, but not cortical, calmodulin-dependent adenylate cyclase activity measured during the active (waking) phase of rats. The involvement of calmodulin- and guanine nucleotide (G)-binding proteins in the effects of ADX on the activity of calmodulin-dependent adenylate cyclase were investigated. In hippocampal membranes, inclusion of the GTP antagonist guanosine 5¹-O-(2-thiodiphosphate) (250 μM) caused pronounced inhibition of calmodulin-stimulated adenylate cyclase activity. Guanosine 5¹-O-(2- thiodiphosphate) had much smaller effects on calmodulin-independent (basal and forskolin-stimulated) enzyme activity. Substitution of Mn²⁺ for Mg²⁺ in the assay medium increased basal and forskolin-stimulated adenylate cyclase activity, but abolished calmodulin-dependent activation of this enzyme in both hippocampal and cortical membranes. These treatments blunted ADX-induced attenuation of hippocampal adenylate cyclase. ADX, with or without corticosterone administration (40 mg/kg, sc, once daily), failed to alter either G(iα) or G(sα) membrane protein content in either hippocampus or cortex. The levels of major membrane calmodulin-binding proteins in hippocampus and cortex also were not significantly altered by ADX. These results confirm that hormonal and biochemical regulation of calmodulin- dependent adenylate cyclase is distinct from that of other adenylate cyclase family members. Changes in G(sα) and G(iα) protein content alone cannot account for the effects of ADX on this enzyme. Overall, our studies suggest that the effects of ADX on calmodulin-dependent adenylate cyclase may occur through a reduction in the absolute amount of the catalytic subunit or an alteration(s) in the efficiency of coupling between adenylate cyclase and its modulators.