Mechanisms and functional significance of inhibition of neuronal T-type calcium channels by isoflurane.
Previous data have indicated that T-type calcium channels (low-voltage activated T-channels) are potently inhibited by volatile anesthetics. Although the interactions of T-channels with a number of anesthetics have been described, the mechanisms by which these agents modulate channel activity, and the functional consequences of such interactions, are not well studied. Here, we used patch-clamp recordings to explore the actions of a prototypical volatile anesthetic, isoflurane (Iso), on recombinant human Ca(V)3.1 and Ca(V)3.2 isoforms of T-channels. We also performed behavioral testing of anesthetic endpoints in mice lacking Ca(V)3.2. Iso applied at resting channel states blocked current through both isoforms in a similar manner at clinically relevant concentrations (1 minimum alveolar concentration, MAC). Inhibition was more prominent at depolarized membrane potentials (-65 versus -100 mV) as evidenced by hyperpolarizing shifts in channel availability curves and a 2.5-fold decrease in IC(50) values. Iso slowed recovery from inactivation and enhanced deactivation in both Ca(V)3.1 and Ca(V)3.2 in a comparable manner but caused a depolarizing shift in activation curves and greater use-dependent block of Ca(V)3.2 channels. In behavioral tests, Ca(V)3.2 knockout (KO) mice showed significantly decreased MAC in comparison with wild-type (WT) litter mates. KO and WT mice did not differ in loss of righting reflex, but mutant mice displayed a delayed onset of anesthetic induction. We conclude that state-dependent inhibition of T-channel isoforms in the central and peripheral nervous systems may contribute to isoflurane's important clinical effects.
Orestes
Peihan
P
Department of Anesthesiology, University of Virginia Health System, Charlottesville, Virginia 22908-0710, USA.
Bojadzic
Damir
D
Chow
Robert M
RM
Todorovic
Slobodan M
SM
D000818:Animals; D002121:Calcium Channel Blockers / Q000494:pharmacology*; D020747:Calcium Channels, T-Type / Q000502:physiology*; D002460:Cell Line; D005260:Female; D006801:Humans; D007530:Isoflurane / Q000494:pharmacology*; D008297:Male; D051379:Mice; D018345:Mice, Knockout; D011994:Recombinant Proteins / Q000037:antagonists & inhibitors
D002121:Calcium Channel Blockers; D020747:Calcium Channels, T-Type; D011994:Recombinant Proteins; D007530:Isoflurane