Effects of isoflurane on voltage-dependent calcium fluxes in rabbit T-tubule membranes: Comparison with alcohols

The effects of racemic (±) and (+)- and (-)-stereoisomers of isoflurane on depolarization-induced ⁴⁵Ca²⁺ fluxes mediated by voltage-dependent Ca²⁺ channels were investigated in transverse tubule membrane vesicles from rabbit skeletal muscle. In the concentration range 0.5 to 2 mM, (±)-isoflurane inhibited ⁴⁵Ca²⁺ fluxes and functionally modulated the effects of the Ca²⁺ channel antagonist nifedipine (1-10 μM). Isoflurane-induced inhibition of ⁴⁵Ca²⁺ fluxes was not significantly affected by pretreatment with either pertussis toxin (5 μg/ml) or phorbol 12-myristate 13-acetate (50 nM). Further experiments indicated that there were no significant differences between (+)- and (-)-stereoisomers of isoflurane with respect to the extent of inhibition of ⁴⁵Ca²⁺ fluxes. Radioligand binding studies indicated that racemic and (+)- and (-)-isoflurane were equally effective in displacing the specific binding of [³H] PN 200-110 to transverse tubule membranes. There were no apparent differences between the effects of (+)- and (-)-isoflurane on the characteristics of [³H]PN 200-110 binding. Although the concentrations of isoflurane for the inhibitions of ⁴⁵Ca²⁺ fluxes and radioligand bindings were similar, the concentrations of n-alcohols required for the inhibition of ⁴⁵Ca²⁺ fluxes were lower than those for the displacement of radioligand. Comparison of the data for the displacement of [³H]PN 200-110 binding and the inhibition of ⁴⁵Ca²⁺ fluxes by isoflurane and by n-alcohols suggested that both isoflurane and n-alcohols may have more than a single binding site. In conclusion, results indicate that isoflurane, independent of intracellular Ca²⁺ levels, nonstereospecifically inhibits the function of voltage-dependent Ca²⁺ channels and this effect is mediated through multiple binding sites.