Biphasic effects of isoflurane on the cardiac action potential: an ionic basis for anesthetic-induced changes in cardiac electrophysiology.
BACKGROUND
The mechanism underlying isoflurane modulation of cardiac electrophysiology is not well understood. In the present study, the authors investigated the effects of isoflurane on the cardiac action potential (AP) characteristics. The results were correlated to modulation of the L-type calcium (I(Ca,L)), the delayed-rectifier potassium (I(Kdr)), and the inward-rectifier potassium (I(Kir)) currents.


METHODS
Single ventricular myocytes were enzymatically isolated from guinea pig hearts. The current clamp and whole cell voltage clamp configurations of the patch clamp technique were used to monitor the cardiac AP and ionic currents, respectively. A dynamic AP voltage protocol that mimicked changes in membrane potential during an AP was used to monitor the I(Ca,L), I(Kdr) and I(Kir).


RESULTS
Isoflurane produced a concentration-dependent, biphasic effect on the AP duration (APD). At 0.6 mm (1.26 vol%), isoflurane significantly increased APD50 and APD90 by 50.0 +/- 7.6% and 48.9 +/- 7.2%, respectively (P < 0.05; n = 6). At 1.0 mm (2.09 vol%), isoflurane had no significant effect on APD (n = 6). In contrast, at 1.8 mm (3.77 vol%), isoflurane decreased APD50 and APD90 by 38.3 +/- 5.4% and 32.2 +/- 5.5%, respectively (P < 0.05; n = 7). The inhibitory effects of isoflurane on I(Kdr) chord conductance were greater than those on I(Ca,L) (P < 0.05; n = 6/group). Both I(Ca,L) inactivation and I(Kdr) activation kinetics were accelerated by isoflurane. Isoflurane had no significant effects on I(Kir) chord conductance (n = 6).


CONCLUSIONS
At the lower anesthetic concentration, the prolongation of the APD may be the result of the dominant inhibitory effects of isoflurane on I(Kdr). At the higher concentration, the shortening of the APD may be caused by the inhibitory effects on I (Ca,L) combined with the isoflurane-induced acceleration of I(Ca,L) inactivation kinetics. Because I(Kdr) is significantly inhibited by isoflurane, I(Kir) appears to be the major repolarizing current, which is minimally affected by isoflurane.
Suzuki
Akihiro
A
Department of Anesthesiology, Medical College of Wisconsin, Milwaukee 53226, USA.
Aizawa
Kei
K
Gassmayr
Susanne
S
Bosnjak
Zeljko J
ZJ
Kwok
Wai-Meng
WM
D000200:Action Potentials / Q000187:drug effects; D018685:Anesthetics, Inhalation / Q000494:pharmacology*; D000818:Animals; D020746:Calcium Channels, L-Type / Q000187:drug effects; D004305:Dose-Response Relationship, Drug; D005260:Female; D006168:Guinea Pigs; D006321:Heart / Q000187:drug effects* / Q000502:physiology; D007530:Isoflurane / Q000494:pharmacology*; D008297:Male; D015221:Potassium Channels / Q000187:drug effects
D018685:Anesthetics, Inhalation; D020746:Calcium Channels, L-Type; D015221:Potassium Channels; D007530:Isoflurane