Adenylate cyclase activity from Hirudo medicinalis segmental ganglia: modulation by temperature and Mg2+ ions

The basal, serotonin- and fluoride-stimulated activities of homogenate preparations from Hirudo medicinalis segmental ganglia increase with incubation temperature, reaching a maximum at 35 degrees C. Both basal and fluoride-stimulated activities yield linear Arrhenius plots, whereas serotonin-enhanced activity shows a break at 25 degrees C. Increasing Mg2+ concentrations stimulate the adenylate cyclase both in the absence and in the presence of serotonin. The affinity of the enzymatic system for the cation is unaffected by the amine.

Adenylate cyclase from Hirudo medicinalis segmental ganglia: modulation by physiological and non-physiological agents

1. In Hirudo medicinalis segmental ganglia GTP is essential for the full expression of the stimulatory action of serotonin on the adenylate cyclase activity. The amine, in turn, increases the overall affinity of the enzymatic system for GTP. 2. GTP gamma S and Gpp(NH)p, non-hydrolysable analogues of GTP, dose-dependently enhance the basal enzyme activity, but impair the stimulatory effect of serotonin. 3. Fluoride ions biphasically modulate the leech adenylate cyclase both in the absence and in the presence of GTP. The ion effect is also influenced by non-physiological guanine nucleotides.

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.

RMI 12330A, an inhibitor of adenylate cyclase and cyclic AMP-phosphodiesterase activities in the segmental ganglia of the leech Hirudo medicinalis

The effect of RMI 12330A, an inhibitor of basal as well as serotonin-stimulated adenylate cyclase in the segmental ganglia of the leech Hirudo medicinalis, has been tested on cyclic AMP-phosphodiesterase activity in the same tissue. The drug dose-dependently inhibits high and low affinity enzyme, an effect first evident at an RMI 12330A concentration of 0.5 mM. For both enzyme activities the drug reduces Vmax without substantially influencing Km values. Both basal and serotonin-stimulated adenylate cyclase as well as cyclic AMP-phosphodiesterase activities are inhibited by RMI 12330A in a non-competitive manner.