Molecular mechanism of use-dependent calcium channel block by phenylalkylamines: Role of inactivation

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Abstract
The role of channel inactivation in the molecular mechanism of calcium (Ca 2+ ) channel block by phenylalkylamines (PAA) was analyzed by designing mutant Ca 2+ channels that carry the high affinity determinants of the PAA receptor site [Hockerman, G. H., Johnson, B. D., Scheuer, T., and Catterall, W. A. (1995) J. Biol. Chem. 270, 22119–22122] but inactivate at different rates. Use-dependent block by PAAs was studied after expressing the mutant Ca 2+ channels in Xenopus oocytes. Substitution of single putative pore-orientated amino acids in segment IIIS6 by alanine (F-1499-A, F-1500-A, F-1510-A, I-1514-A, and F-1515-A) gradually slowed channel inactivation and simultaneously reduced inhibition of barium currents (I Ba ) by (−)D600 upon depolarization by 100 ms steps at 0.1 Hz. This apparent reduction in drug sensitivity was only evident if test pulses were applied at a low frequency of 0.1 Hz and almost disappeared at the frequency of 1 Hz. (−)D600 slowed I Ba recovery after maintained membrane depolarization (1–3 sec) to a comparable extent in all channel constructs. A drug-induced delay in the onset of I Ba recovery from inactivation suggests that PAAs promote the transition to a deep inactivated channel conformation. These findings indicate that apparent PAA sensitivity of Ca 2+ channels is not only defined by drug interaction with its receptor site but also crucially dependent on intrinsic gating properties of the channel molecule. A molecular model for PAA-Ca 2+ channel interaction that accounts for the relationship between drug induced inactivation and channel block by PAA is proposed.