Heterogeneity of presynaptic calcium channels revealed by species differences in the sensitivity of synaptosomal 45Ca entry to ω-conotoxin

Abstract

The inhibition of K +-depolarization dependent Ca influx by ω-conotoxin GVIA was compared in the frog, chick, and rat brain synaptosomes. The toxin at concentrations ≥ 0.3 μM completely inhibited Ca entry in the frog and chick preparations, but was only partly effective inblocking Ca influx in the rat brain synaptosomes. In chick synaptosomes the toxin's effect was biphasic: a small component (⋍ 15%) of total Ca influx was inhibited by the toxin with high affinity (I 50 < 0.002 μM); a major component (⋍ 80 %) of Ca influx was inhibited with a moderate affinity (I 50 ⋍ 0.05 μM). In rat brain synaptosomes 40% of Ca influx was inhibited by the toxin with low affinity (I 50 ⋍ 0.3 μM), and 60% of Ca influx was unaffected by the toxin concentration of up to 10 μM. These data suggest a heterogeneity of voltage-sensitive Ca channels in vertebrate brain synaptosomes.