The structural determinants of mibefradil inhibition were analyzed using wild-type and inactivation-modified CaV1.2 (α1C) and CaV2.3 (α1E) channels. Mibefradil inhibition of peak Ba2+ currents was dose- and voltage-dependent. An increase of holding potentials from −80 to −100 mV significantly shifted dose-response curves toward higher mibefradil concentrations, namely from a concentration of 108 ± 21 μm (n= 7) to 288 ± 17 μm (n= 3) for inhibition of half of the Cav1.2 currents (IC 50) and from IC 50= 8 ± 2 μm (n= 9) to 33 ± 7 μm (n= 4) for CaV2.3 currents. In the presence of mibefradil, CaV1.2 and CaV2.3 experienced significant use-dependent inhibition (0.1 to 1 Hz) and slower recovery from inactivation suggesting mibefradil could promote transition(s) to an absorbing inactivated state. In order to investigate the relationship between inactivation and drug sensitivity, mibefradil inhibition was studied in inactivation-altered CaV1.2 and CaV2.3 mutants. Mibefradil significantly delayed the onset of channel recovery from inactivation in CEEE (Repeat I + part of the I–II linker from CaV1.2 in the CaV2.3 host channel), in EC(AID)EEE (part of the I–II linker from CaV1.2 in the CaV2.3 host channel) as well as in CaV1.2 E462R, and CaV2.3 R378E (point mutation in the β-subunit binding motif) channels. Mibefradil inhibited the faster inactivating chimera EC(IS1-6)EEE with an IC 50= 7 ± 1 μm (n= 3), whereas the slower inactivating chimeras EC(AID)EEE and CEEE were, respectively, inhibited with IC 50= 41 ± 5 μm (n= 4) and IC 50= 68 ± 9 μm (n= 5). Dose-response curves were superimposable for the faster EC(IS1-6)EEE and CaV2.3, whereas intermediate-inactivating channel kinetics (CEEE, CaV1.2 E462R, and CaV1.2 E462K) were inhibited by similar concentrations of mibefradil with IC 50≈ 55–75 μm. The slower CaV1.2 wild-type and CaV1.2 Q473K channels responded to higher doses of mibefradil with IC 50≈ 100–120 μm. Mibefradil was also found to significantly speed up the inactivation kinetics of slower channels (CaV1.2, CEEE) with little effect on the inactivation kinetics of faster-inactivating channels (CaV2.3). A open-channel block model for mibefradil interaction with high-voltage-activated Ca2+ channels is discussed and shown to qualitatively account for our observations. Hence, our data agree reasonably well with a ``receptor guarded mechanism'' where fast inactivation kinetics efficiently trap mibefradil into the channel.
The Journal of Membrane Biology – Springer Journals
Published: Nov 15, 2001
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