Antagonism of calmodulin-stimulated adenylate cyclase by trifluoperazine, calmidazolium and W-7 in rat cerebellar membranes

Abstract

Ca++-calmodulin (CaM)-dependent stimulation of adenylate cyclase in rat cerebellar plasma membranes was demonstrated by removal of endogenous Ca++ and CaM and addition of exogenous Ca++ and CaM to the membranes. This CaM-dependent adenylate cyclase activity could be inhibited by calmidazolium and trifluoperazine in an apparently competitive manner, whereas the inhibition produced by W-7 was not competitive. The potency of the antagonists was strictly dependent upon the concentration of exogenous CaM present in the assay. Preincubation of membranes with exogenous CaM, followed by addition of anti-CaM agents, greatly reduced the inhibition of CaM-dependent adenylate cyclase activity. The potency of the anti-CaM agents was further decreased in membranes that had not been depleted of endogenous Ca++ and CaM (native membranes). The results suggest that optimal inhibition of CaM-dependent adenylate cyclase activity occurs upon simultaneous addition of exogenous CaM and anti-CaM agents to membranes depleted of endogenous Ca++ and CaM. Association of CaM with the catalytic unit of adenylate cyclase before introduction of CaM antagonists results in a CaM-C complex that is relatively refractory to inhibition by these anti-CaM agents. This CaM-catalytic unit complex probably exists in native membranes, rendering the antagonism of adenylate cyclase activity supported by endogenous Ca++-CaM essentially insensitive to low concentrations of CaM antagonists.