The effects of D600 and verapamil on action potential in the X-organ neuron of the crayfish.

The effects of D600 and verapamil on the membrane activity of the X-organ neuron of the crayfish were investigated. In TTX solution, D600 and verapamil reversibly reduced the amplitude and the maximum rate of rise of the Ca-dependent action potential as well as delayed rectification. The decrease and the recovery in amplitude of the Ca spike were usually accompanied by parallel changes in delayed rectification. The threshold concentration for observable effects on the Ca spike was from 5 X 10(-6) g/ml to 1X 10(-5) g/ml for both drugs. At 1 X 10(-4) g/ml, the Ca spike was almost blocked. The dose-response relation with regard to the maximum rate of rise of the Ca spike showed no noticeable difference in effect between D600 and verapamil. By increasing the extracellular Ca concentration, the Ca spike, which had been reduced by these drugs, was restored by the delayed rectification remained to be reduced. In Mn solution, both D600 and verapamil reduced the amplitude and the maximum rate of rise of the Na-dependent action potential as well as the delayed rectification. At 1 X 10(-4) g/ml, the maximum rate of rise of the Na spike was reduced by 30 to 50% with each drug. From the experiments in the Ca and the Mn solutions, it was inferred that the decrease in K activation by these drugs was not brought about secondarily by the decrease in Ca activation but both activation were inhibited independently. It was concluded that both D600 and verapamil are the inhibitors for Ca activation in the X-organ neuron of the crayfish, although they are not Ca specific.