Inhibition of [3H]isradipine binding to L-type calcium channels by the optical isomers of isoflurane. Lack of stereospecificity.

BACKGROUND

The dose-dependent myocardial depression of volatile general anesthetics such as isoflurane has been linked to blockade of L-type Ca2+ channels. The effects of (+)- and (-)-isoflurane on the inhibition of [3H]isradipine binding to L-type Ca2+ channels in membranes prepared from mouse heart were examined. In addition, because there is a stereo-specific effect of these isomers on sleep time in mice, the potential contribution of L-type Ca2+ channels to isoflurane-induced sleep was assessed by determining whether a similar stereoselectivity would be manifested at these sites in cerebral cortical membranes.

METHODS

The effects of isoflurane stereoisomers on the binding of an L-type Ca2+ channel ligand ([3H]isradipine) were studied in cardiac and brain cortical membranes. Their potencies and effects on the Kd and Bmax of [3H]isradipine were measured.

RESULTS

Pharmacologically relevant concentrations of (+)- and (-)-isoflurane inhibited [3H]isradipine binding. The IC50 values for (+)-isoflurane were 0.48 +/- 0.02% and 0.40 +/- 0.01% in heart and brain membranes, respectively. The values for (-)-isoflurane were not significantly different from the respective values for the (+)-isomer. Saturation analysis demonstrated (+)- and (-)-isoflurane inhibited [3H]isradipine binding by significantly reducing Bmax and increasing Kd, but there were no significant differences between these isomers in either tissue.

CONCLUSIONS

The stereoisomers of isoflurane are equipotent as inhibitors of [3H]isradipine binding to L-type Ca2+ channels. This lack of stereoselectivity between (+)- and (-)-isoflurane indicates that the [3H]isradipine site on L-type Ca2+ channels in brain does not contribute to the differences in isoflurane-induced sleep time reported for these stereoisomers. Taken with a lack of stereoselectivity at L-type Ca2+ channels in heart, an optically resolved isomer of isoflurane may have clinical advantages compared to the current racemic mixture.