Abstract

Low-voltage-activated (LVA, T-type, or CaV3), calcium-selective channels open in response to modest depolarizations, just above the resting membrane potential, supporting neuronal burst-firing patterns and spontaneous firing in cardiac pacemaker cells. How LVA-channels open at low voltages is unknown: traditional gating-current experiments suggest that LVA-channel voltage-sensing domains (VSDs) paradoxically require stronger depolarization to activate than pore opening. Using voltage-clamp fluorometry, we found that the activation of all four VSDs in human CaV3.1-channels precedes opening in voltage, solving the longstanding conundrum. We also uncovered confounding effects of La3+ (used for gating-current measurements) on VSD function and clarified the role of unique LVA-channel structure S6Cyto. CaV3.1-VSDs operate within a narrow voltage-range, resembling the VSDs of related NaV-channels more than those of other CaV-channels. Likely, NaV-like VSDs emerged before sodium selectivity.