Article date: August 2001
By: S Berjukow, S Hering in Volume 133, Issue 7, pages 959-966
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
British Journal of Pharmacology (2001) 133, 959–966; doi:10.1038/sj.bjp.0704181
DOI: 10.1038/sj.bjp.0704181
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