Abstract

To investigate the mechanisms that increase ionic currents when Ca2+ channels' ?1 subunits are co-expressed with the ?-subunits, we compared channel activity of CaV1.2 (?1C) co-expressed with ?1a and ?2a in Xenopus oocytes. Normalized by charge movement, ionic currents were near threefold larger with ?2a than with ?1a. At the single-channel level, the open probability (Po) was over threefold larger with ?2a, and traces with high Po were more frequent. Among traces with Po > 0.1, the mean duration of burst of openings (MBD) were nearly twice as long for ?1C?2a (15.1 ± 0.7 ms) than for ?1C?1a (8.4 ± 0.5 ms). Contribution of endogenous ?3xo was ruled out by comparing MBDs with ?1C-cRNA alone (4.7 ± 0.1 ms) with ?3xo (14.3 ± 1.1 ms), and with ?1b (8.2 ± 0.5 ms). Open-channel current amplitude distributions were indistinguishable for ?1C?1a and ?1C?2a, indicating that opening and closing kinetics are similar with both subunits. Simulations with constant opening and closing rates reproduced the microscopic kinetics accurately, and therefore we conclude that the conformational change-limiting MBD is differentially regulated by the ?-subunits and contributes to the larger ionic currents associated with ?2a, whereas closing and opening rates do not change, which should reflect the activity of a separate gate.