Abstract

To study the molecular pharmacology of low-voltage–activated calcium channels in biophysical detail, human medullary thyroid carcinoma (hMTC) cells were investigated using the single-channel technique. These cells had been reported to express T-type whole-cell currents and a Ca_v3.2 (or α1H) channel subunit. We observed two types of single-channel activity that were easily distinguished based on single-channel conductance, voltage dependence of activation, time course of inactivation, rapid gating kinetics, and the response to the calcium agonist (S)-Bay K 8644. Type II channels had biophysical properties (activation, inactivation, conductance) typical for high-voltage–activated calcium channels. They were markedly stimulated by 1 μM (S)-Bay K 8644, allowing to identify them as L-type channels. The channel termed type I is a low-voltage–activated, small-conductance (7.2 pS) channel that inactivates rapidly and is not modulated by (S)-Bay K 8644. Type I channels are therefore classified as T-type channels. They were strongly inhibited by 10 μM mibefradil. Mibefradil block was caused by changes in two gating parameters: a pronounced reduction in fraction of active sweeps and a slight shortening of the open-state duration. Single recombinant low-voltage–activated T-type calcium channels were studied in comparison, using human embryonic kidney 293 cells overexpressing the pore-forming Ca_v3.2 subunit. Along all criteria examined (mechanisms of block, extent of block), recombinant Ca_v3.2 interact with mibefradil in the same way as their native counterparts expressed in hMTC cells. In conclusion, the pharmacologic phenotype of these native human T-type channels—as probed by mibefradil—is similar to recombinant human Ca_v3.2.