@article {Gomora96, author = {Juan Carlos Gomora and Lin Xu and Judith A. Enyeart and John J. Enyeart}, title = {Effect of Mibefradil on Voltage-Dependent Gating and Kinetics of T-Type Ca2+ Channels in Cortisol-Secreting Cells}, volume = {292}, number = {1}, pages = {96--103}, year = {2000}, publisher = {American Society for Pharmacology and Experimental Therapeutics}, abstract = {We have studied the effect of the Ca2+ antagonist mibefradil on low voltage-activated T-type Ca2+ channels in whole-cell patch clamp recordings from bovine adrenal zona fasciculata (AZF) cells. AZF cells are distinctive in expressing only T-type Ca2+ channels, allowing the mechanism of pharmacological agents to be explored without interference from other Ca2+channels. The inhibition of T-type Ca2+ channels by mibefradil was voltage- and use-dependent. When Ca2+currents were activated from holding potentials of -80 and -60 mV, mibefradil inhibited currents with IC50 values of 1.0 and 0.17 μM, respectively. When T-type Ca2+ current (IT) was activated from a holding potential of -90 mV in the presence of 2 μM mibefradil, a single voltage step to -10 mV inhibited IT by 16.2\% {\textpm} 2.9\% (n = 10). With subsequent voltage steps, applied at 10-s intervals, block reached a steady-state value of 51.9\% {\textpm} 5.0\% (n= 5). Mibefradil (1 μM) produced a leftward shift of 5.7 mV (n = 4) in the voltage-dependent steady-state availability curve such that T-type Ca2+ channels inactivated at more negative potentials, but this drug did not change the voltage-dependence of T channel opening. Mibefradil failed to alter the kinetics of T channel activation, inactivation, or deactivation, but markedly slowed T channel recovery following an inactivating prepulse. Mibefradil inhibited adrenocorticotropin-stimulated cortisol secretion from AZF cells with an IC50 value of 3.5 μM. These results show that mibefradil is a relatively potent antagonist of T-type Ca2+ channels in cortisol-secreting cells. The enhanced potency of mibefradil with sustained or repetitive depolarizations, its shifting of the steady-state inactivation curve, and its slowing of recovery all indicate that this drug preferentially interacts with Ca2+ channels in the open or inactivated state. The inhibition of cortisol secretion by mibefradil at concentrations similar to those that block IT is consistent with a requirement for these channels in corticosteroidogenesis. The American Society for Pharmacology and Experimental Therapeutics}, issn = {0022-3565}, URL = {https://jpet.aspetjournals.org/content/292/1/96}, eprint = {https://jpet.aspetjournals.org/content/292/1/96.full.pdf}, journal = {Journal of Pharmacology and Experimental Therapeutics} }