Characterization of two kinds of high-voltage-activated Ca-channel currents in chick sensory neurons. Differential sensitivity to dihydropyridines and omega-conotoxin GVIA.

High-voltage-activated (HVA) Ca-channel currents in chick sensory neurons were characterized by dihydropyridine compounds (DHPs) and omega-conotoxin GVIA (omega CTX) using patch-clamp methods. In single-channel recordings, two HVA-currents were identified by their single-channel conductances, 13 pS and 25 pS in 110 mM BaCl2. DHPs selectively affected the large-conductance channel. omega CTX (5 microM), on the other hand, irreversibly eliminated only the small-conductance channel, while the large-conductance channel was either unaffected or only transiently blocked. In whole-cell recordings the macroscopic HVA-current was completely and irreversibly blocked by omega CTX but insensitive to DHPs in 60% of the cells. This current presumably was carried by the 13 pS channel. In the remaining cells, a part of the HVA-current (10%, SD = 11%) was either unaffected or transiently blocked by omega CTX and was sensitive to DHPs. This current presumably was carried by the 25 pS channel. Inactivation of both macroscopic current component was incomplete during a 150 ms long depolarization. Our data suggest that the HVA-currents in chick sensory neurons are carried by two distinct Ca-channels that are differentially affected by omega CTX and DHPs.