Fig. 2. Tonic inhibition of Na⁺currents by bupivacaine enantiomers. (A ) Na⁺currents in control and in the presence of 100 μm R - or S -bupivacaine were evoked by the test pulses shown to the right, delivered at 20-s intervals. (B ) Concentration dependence of Na⁺current block by R -bupivacaine (solid symbols) and S -bupivacaine (open symbols). Five-second prepulses to −60 mV through 100 mV were applied in the protocol shown in (A ). Solid lines (R -bupivacaine) and dashed lines (S -bupivacaine) represent nonlinear least squares fitting of the function f^b= 1/(1 + (IC⁵⁰/[bupivacaine])) to the data points. (C ) IC⁵⁰values, from the fitting to concentration-inhibition data (B ), graphed as a function of prepulse potentials. Data are presented as fitted values ± SE of the fit. An unpaired Student t  test (SigmaStat, Jandel Scientific Software, San Rafael, CA) was used to evaluate the significance of differences in fitted values between R - and S -enantiomers. P  values < 0.05 (*) were considered to be statistically significant.