Prepulse facilitation of the honeybee CaV4 channel is produced by a shift in channel activation and requires an intact inactivation sequence

The recently characterized honeybee CaV4 channel is a high-voltage-activated Ca2+ channel ortholog to the DSC1 channel identified in Drosophila. While sequence similarities to NaV channels are obvious, permeation properties and current kinetics are more closely aligned with those of CaV channels. CaV4 exhibits a distinctive cation-dependent inactivation pattern, a hallmark of Ca2+ channel behavior, and nonetheless displays sensitivity to a Na+ channel-specific regulator, veratrine. Calcium channel facilitation is a phenomenon whereby the probability of calcium channel opening increases with successive depolarization pulses, resulting in an enhanced Ca2+ influx during repetitive or sustained electrical activity. In this study, we have identified an additional specific property of CaV4 in the form of an atypical voltage-dependent facilitation of the Ca2+ or Ba2+ currents by strong pre-depolarizations or prepulses (pPs). This physiologically relevant phenomenon, known as pP-induced facilitation (PiF), is subject to positive regulation by the amplitude of the pP but to negative regulation by its duration. It produces a hyperpolarizing shift of the I-V curve without any change in the reversal potential and macroscopic or single channel conductance. PiF is thus more pronounced for small depolarizations and almost absent when channels reach their maximal open probability. A mutation that affects the inactivation of the CaV4 channel prevents the occurrence of PiF. This previously undocumented form of facilitation appears exclusive to CaV4 channels. A strong pP may lock CaV4 channels in a pre-open state, rendering them more susceptible to activation and thereby shifting the activation curve toward more negative potentials. This, in turn, would accelerate channel opening and increase current amplitude. Lastly, we show that the inactivation particle of CaV4 (MFLT sequence, equivalent to the IFMT motif in human NaV, or MFMT in Apis NaV channel), in addition to its role in the initiation of the voltage-dependent inactivation, also modulates PiF.