Facilitation by the β2a subunit of pore openings in cardiac Ca2+ channels

1. Single channel recordings were performed on the cardiac calcium channel (alpha(1C)) in order to study the effect of coexpression of the accessory beta(2a) subunit. On-cell patch damp recordings were performed after expression of these channels in Xenopus oocytes. 2. The alpha(1C) subunit, when expressed done, had similar single channel properties to native cardiac channels. Slow transitions between low and high open probability (P-o) gating modes were found as well as fast gating transitions between the open and closed states. 3. Coexpression of the beta(2a) subunit caused changes in the fast gating during high P-o mode. In this mode, open time distributions reveal at least three open states and the beta(2a) subunit favours the occupancy of the longest, 10-15 ms open state. No effect of the beta(2a) subunit was found when the channel was gating in the low P-o mode. 4. Slow gating transitions were also affected by the beta(2a) subunit. The high P-o mode was maintained for the duration of the depolarizing pulse in the presence of the beta(2a) subunit; while the alpha(1C) channel when expressed alone, frequently switched into and out of the high P-o mode during the course of a sweep. 5. The beta(2a) subunit also affected mode switching that occurred between sweeps. Runs analysis revealed that the alpha(1C) subunit has a tendency toward non-random mode switching. The beta(2a) subunit increased this tendency. A chi(2) analysis of contingency tables indicated that the beta(2a) subunit caused the alpha(1C) channel to gain 'intrinsic memory', meaning that the mode of a given sweep can be non-independent of the mode of the previous sweep. 6. We conclude that the beta(2a) subunit causes changes to the alpha(1C) channel in both its fast and slow gating behaviour. The beta(2a) subunit alters fast gating by facilitating movement of the channel into an existing open state. Additionally, the beta(2a) subunit decreases the slow switching between low and high P-o modes.