Calcium/Calmodulin Inhibits the Binding of Specific [125I]Omega-Conotoxin GVIA to Chick Brain Membranes

The effect of Ca²⁺/calmodulin (CaM) on the specific binding of [¹²⁵I]omega-conotoxin GVIA (¹²⁵I--CTX) to crude membranes from chick brain was investigated. When we examined the effects of the activation of various endogenous protein kinases on specific [¹²⁵I]-CTX binding to crude membranes, we observed that Ca²⁺/CaM had an inhibitory effect regardless of whether or not the standard medium contained ATP (0.5 mM). Ca²⁺/CaM also had an inhibitory effect in a simple binding-assay medium containing HEPES-HCl buffer, BSA, Ca²⁺ and CaM, and this effect was dependent on the concentration of Ca²⁺. The effect of Ca²⁺/CaM was attenuated by the CaM antagonists W-7 and CaM-kinase II fragment (290–309). An experiment with modified ELISA using purified anti -CTX antibody indicated that Ca²⁺/CaM did not affect the direct binding of [¹²⁵I]-CTX and CaM. These results suggest that Ca²⁺/CaM either directly or indirectly affects specific [¹²⁵I]-CTX binding sites, probably N-type Ca²⁺ channels in crude membranes from chick whole brain.