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.