T-type CaV3.3 calcium channels produce spontaneous low-threshold action potentials and intracellular calcium oscillations.

The precise contribution of T-type Ca2+ channels in generating action potentials (APs), burst firing and intracellular Ca2+ signals needs further elucidation. Here, we show that CaV3.3 channels can trigger repetitive APs, generating spontaneous membrane potential oscillations (MPOs), and a concomitant increase in the intracellular Ca2+ concentration ([Ca2+]i) when overexpressed in NG108-15 cells. MPOs were dependent on CaV3.3 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 Ca2+ 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 CaV3.3 channels constrained the AP duration and the plateau potential was supported by sustained calcium current through CaV3.3 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 [Ca2+]i oscillations was also closely related to the shape of APs. The CaV3.3 window current was the oscillation trigger as shown by shifting the CaV3.3 window current potential range as a result of increasing the external Ca2+ concentration, which resulted in a corresponding shift of the AP threshold. Overall, the data demonstrate that the CaV3.3 window current is critical in triggering intrinsic electrical and [Ca2+]i oscillations. The functional expression of CaV3.3 channels can generate spontaneous low-threshold calcium APs through its window current, indicating that CaV3.3 channels can play a primary role in pacemaker activity.