Inactivation determinant in the I‐II loop of the Ca 2+ channel α 1 ‐subunit and β‐subunit interaction affect sensitivity for the phenylalkylamine (−)gallopamil

1 The role of calcium (Ca 2+ ) channel inactivation in the molecular mechanism of channel block by phenylalkylamines (PAAs) was analysed in a PAA‐sensitive rabbit brain class A Ca 2+ channel mutant (α 1A‐PAA ). Use‐dependent barium current ( I Ba ) inhibition of α 1A‐PAA by (−)gallopamil and Ca 2+ channel recovery from inactivation and block were studied with two‐microlectrode voltage clamp after expression of α 1A‐PAA and auxiliary α 2 ‐δ‐ and β 1a ‐ or β 2a ‐subunits in Xenopus oocytes. 2 Mutation Arg387Glu (α 1A numbering) in the intracellular loop connecting domains I and II of α 1A‐PAA slowed the inactivation kinetics and reduced use‐dependent inhibition (100 ms test pulses at 0.2 Hz from ‐80 to 20 mV) of the resulting mutant α 1A‐PAA/R‐E /β 1a channels by 100 μ m (−)gallopamil (53 ± 2 %, α 1A‐PAA /β 1a vs. 31 ± 2 %, α 1A‐PAA/R‐E /β 1a , n ≥ 4). This amino acid substitution simultaneously accelerated the recovery of channels from inactivation and from block by (−)gallopamil. 3 Coexpression of α 1A‐PAA with the β 2a ‐subunit reduced fast I Ba inactivation and induced a substantial reduction in use‐dependent I Ba inhibition by (−)gallopamil (25 ± 4 %, α 1A‐PAA /β 2a ; 13 ± 1 %, α 1A‐PAA/R‐E /β 2a ). The time constant of recovery from block at rest was not significantly affected. 4 These results demonstrate that changes in channel inactivation induced by Arg387Glu or β 2a ‐α 1 ‐subunit interaction affect the drug‐channel interaction.