R56865 AND FLUNARIZINE AS NA-CHANNEL BLOCKERS IN ISOLATED PURKINJE NEURONS OF RAT CEREBELLUM()

Dose-related blocking effects of R56865, flunarizine and nimodipine on voltage-activated Na+ currents recorded in the whole-cell voltage cla mp mode were studied in acutely isolated Purkinje neurons of rat cereb ellum. The dose-dependences of blocking action were obtained for all d rugs at a holding potential of -110 mV and rare stimulation. At stimul ation frequencies 5 and 15 Hz the block produced by R56865 was increas ed showing a shift of dose-dependence to lower concentrations of antag onist. This shift was less pronounced for flunarizine, practically abs ent for nimodipine, and increased for all drugs with an increase in th e amplitude of stimulating voltage pulse. With the change in holding p otential to -80 mV the block produced by R56865 and flunarizine increa sed showing a dose-dependence shift to lower concentrations of antagon ists. All the drugs tested induced parallel shifts of the steady-state voltage-dependence of inactivation of Na+ channels to more negative m embrane potentials. R56865, and to a lesser extent flunarizine, slowed down the recovery of Na+ channels from steady-state inactivation incr easing the relative number of channels which showed slow recovery. In the absence of Na+ current inactivation (treatment by intracellular pr onase) R56865 at a concentration of 1 muM blocked modified channels pr eferentially in the open state, while the block produced by flunarizin e showed no dependence on voltage pulse protocol. R56865 was shown to decrease the cell leakage while other drugs produced little or no effe ct. It is concluded that R56865 and flunarizine block Na+ currents pre dominantly by interacting with inactivated Na+ channels. The higher ab ility of R56865 to block open channels and to increase slow inactivati on underlies its higher frequency-dependence, These characteristics su ggest the use of R56865 and flunarizine in the treatment of cerebral i schemia.