T‐type Cav3.3 calcium channels produce spontaneous low‐threshold action potentials and intracellular calcium oscillations

The precise contribution of T-type Ca 2+ channels in generating action potentials (APs), burst firing and intracellular Ca 2+ signals needs further elucidation. Here, we show that Ca v 3.3 channels can trigger repetitive APs, generating spontaneous membrane potential oscillations (MPOs), and a concomitant increase in the intracellular Ca 2+ concentration ([Ca 2+ ] i ) when overexpressed in NG108-15 cells. MPOs were dependent on Ca v 3.3 channel activity given that they were recorded from a potential range of -55 to -70 mV, blocked by nickel and mibefradil, as well as by low external Ca 2+ concentration. APs of distinct duration were recorded: short APs (sAP) or prolonged APs (pAP) with a plateau potential near -40 mV. The voltage-dependent properties of the Ca v 3.3 channels constrained the AP duration and the plateau potential was supported by sustained calcium current through Ca v 3.3 channels. The sustained current amplitude decreased when the resting holding potential was depolarized, thereby inducing a switch of AP shape from pAP to sAP. Duration of the [Ca 2+ ] i oscillations was also closely related to the shape of APs. The Ca v 3.3 window current was the oscillation trigger as shown by shifting the Ca v 3.3 window current potential range as a result of increasing the external Ca 2+ concentration, which resulted in a corresponding shift of the AP threshold. Overall, the data demonstrate that the Ca v 3.3 window current is critical in triggering intrinsic electrical and [Ca 2+ ] i oscillations. The functional expression of Ca v 3.3 channels can generate spontaneous low-threshold calcium APs through its window current, indicating that Ca v 3.3 channels can play a primary role in pacemaker activity.