Single-channel properties of four calcium channel types in rat motoneurons.

Previous studies have demonstrated multiple components of whole-cell calcium currents in hypoglossal motoneurons (HMs); HMs possess a low-voltage-activated (LVA) current and three types of high-voltage-activated (HVA) calcium currents based on sensitivity to omega-Aga IVA, omega-Conotoxin GVIA (omega-CgTx) and dihydropyridine analogs (DHPs). In the present study, we recorded single-calcium channel activities from HMs using a cell-attached patch-clamp method and found four types of channels that could be discriminated based on kinetics, voltage dependency, DHP sensitivity, and single-channel conductances. The average single-channel conductances with 110 mM barium as a charge carrier were 7, 14, 20, and 28 pS. T-type channels had a single-channel conductance of 7 pS, activated at the most negative potentials for the calcium channels and inactivated during depolarization. L-type channels (DHP-sensitive channels) did not inactivate during depolarization and had a 28-pS single-channel conductance. Based on kinetics and sensitivity to holding potential, it is likely that the channels with conductances of 14 pS and 20 pS represent N-type and P-type channels, respectively. The N-type channel (14 pS) was sensitive to holding potential, showed modal gating, and inactivated during maintained depolarizations, whereas the P-type channel (20 pS) was rather insensitive to holding potential and did not inactivate during depolarization.