Receptor-mediated supra-additive activation of guinea pig superior cervical ganglion adenylate cyclase: Role of Mn2+ ions and calmodulin

The effects of Mn²⁺ and calmodulin were studied on the basal and agonist-modulated adenylate cyclase activity of the guinea pig superior cervical ganglion. The divalent cation strongly stimulates the basal and agonist-modulated enzyme in a concentration-dependent manner. Moreover, in the presence of Mn²⁺ the inhibitory effects of high GTP concentrations and of D-Ala²-Met-enkephalinamide on adenylate cyclase are eliminated, while the stimulation exerted by prostaglandin E₂ and the supra-additive activation of the enzyme by the combination of the two drugs are unaffected. In EGTA-washed, calmodulin-depleted membrane preparations, Mn²⁺ still activates the cyclase but the enkephalin inhibition and the superactivation of the enzyme induced by the combination of opiate and prostaglandin are lost, both in the absence and in the presence of the cation. Reconstituting the depleted membranes with exogenous Ca²⁺/calmodulin fully restored the enzyme responsivity to the combination and, partially, to the enkephalin. The findings suggest the existence in the guinea pig superior cervical ganglion of both the calmodulin-sensitive and differently regulated calmodulin-insensitive adenylate cyclase.