Voltage-dependent acceleration of Cav1.2 channel current decay by (+)- and (-)-isradipine

Abstract

1. Inhibition of Cav1.2 by antagonist 1,4 dihydropyridines (DHPs) is associated with a drug-induced acceleration of the calcium (Ca2+) channel current decay. This feature is contradictorily interpreted as open channel block or as drug-induced inactivation. To elucidate the underlying molecular mechanism we investigated the effects of (+)- and (-)-isradipine on Cav1.2 inactivation gating at different membrane potentials. 2. α11.2 Constructs were expressed together with α2-δ- and β1a- subunits in Xenopus oocytes and drug-induced changes in barium current (IBa) kinetics analysed with the two microelectrode voltage clamp technique. To study isradipine effects on IBa decay without contamination by intrinsic inactivation we expressed a mutant (V1504A) lacking fast voltage-dependent inactivation. 3. At a subthreshold potential of -30 mV a 200-times higher concentration of (-)-isradipine was required to induce a comparable amount of inactivation as by (+)-isradipine. At +20 mV the two enantiomers were equally efficient in accelerating the IBa decay. 4. Faster recovery from (-)- than from (+)-isradipine-induced inactivation at -80 mV in a Cav1.2 construct (τ(-)-isr.(Cav1.2) = 0.74 s < τ(+)-isr.(Cav1.2) = 2.85 s) and even more rapid recovery of V1504A (τ(-)-isr.(V1504A) = 0.39 s < τ(+)-isr.(V1504A) = 1.98 s) indicated that drug-induced determinants and determinants of intrinsic inactivation (V1504) stabilize the DHP-induced channel conformation in an additive manner. 5. In the voltage range between -25 and 20 mV where the channels inactivate predominantly from the open state the (+)- and (-)-isradipine-induced acceleration of the IBa decay in V1504A displayed similar voltage-dependence as intrinsic fast inactivation of Cav1.2. 6. Our data suggest that the isradipine-induced acceleration of the Cav1.2 current decay reflects enhanced fast voltage-dependent inactivation and not open channel block.