The effects of ropivacaine on sodium currents in dorsal horn neurons of neonatal rats

We used a whole cell patch clamp technique to study the effects of ropivaca ine on rat dorsal horn neurons. Under voltage clamp ropivacaine (10-400 mu M) produced a dose-dependent inhibition of sodium current. From a holding p otential (V-h) of -80 mV, sodium currents evoked by test pulses to 0 mV wer e inhibited by ropivacaine with a mean drug concentration required to produ ce 50% current inhibition (IC50) value of 117.3 mu M, which was more than t he value of the bupivacaine (IC50 53.7 mu M). The inhibition effect of ropi vacaine was also voltage-dependent. Current evoked from a V-h of -60 mV was inhibited by ropivacaine with a mean. IC50, value of 74.3 mu M, which was less than that obtained at the V-h of -80 mV. The inhibition effect of ropi vacaine on sodium current was use dependent. Repeated activation by a train of depolarizing pulses (5 Hz, 20 ms) increased the inhibitory effects of r opivacaine. The ratio amplitudes of the 20th to the first pulse were 91.2% and 71.1%, respectively, in the absence and presence of ropivacaine (50 mu M). Ropivacaine also produced a significant hyperpolarizing shift of 11 mV in the steady-state inactivation curve of sodium current. The inhibition of ropivacaine on the sodium channel may contribute to the mechanism of actio n of local anesthetics during epidural and spinal anesthesia.