SELECTIVE BLOCK OF TRANSIENT CA CHANNEL CURRENT IN MOUSE NEUROBLASTOMA-CELLS BY U-88779E

Antioxidants and T-type Ca channel antagonists are neuroprotective dur ing ischemia or other central nervous system traumas. U-88779E -2,4,6- cycloheptatrien-1-one)-2-methyl]piperazine) has been shown to have bot h antioxidant activity and the ability to block Ca fluxes in cardiac m icrosomes. In this study, we examined the effect of U-88779E on Ca, Na and K channels in a neuronal cell line, N1E-115 cells. The drug block ed transient barium current (I(Ba)) through low-threshold Ca channels (T-type) with little effect on other noninactivating I(Ba) including t he nifedipine-sensitive one. The drug at 20 muM reduced transient I(Ba ) at a constant rate, -7.2% of the control per min, and abolished the current within 15 min. This implies a continuous accumulation of the d rug in cell membranes probably because of its high lipophilicity (log P = 7.003). U-88779E also blocked Na and K currents but at a rate abou t 8 times slower than that observed with transient I(Ba). Further stud ies on interactions of the drug with T-channels revealed that the drug had no effect on the shape of current-voltage curve, activation and i nactivation kinetics, and steady-state activation curve. The drug, how ever, induced a hyperpolarizing shift in steady-state inactivation cur ve and became more potent under conditions where the channels in inact ivated states prevail. These observations are consistent with the view that U-88779E has a higher affinity to T-type channels in inactivated states than in resting or open states. The drug also showed a use-dep endent inhibition because of a slow recovery rate of the drug-bound ch annels from inactivated states. U-88779E appears to be a selective T-c hannel blocker with antioxidant activity.