The effect of DDT and dieldrin on skeletal muscle fibres

The effects of the chlorinated hydrocarbon insecticides, DDT and dieldrin, on the electrical activity of skeletal muscle fibres of the clawed toad, Xenopus laevis, were investigated by means of intracellular microelectrodes. Both compounds caused an increase in the amplitude of the negative afterpotential and a slowing-down of the falling phase of the action potential, without affecting the resting potential. Neither DDT nor dieldrin caused repetitive activity. It is concluded that the increase in the duration of the action potential, measured at a potential level of ?50 mV, may be responsible for part of the effects of these insecticides on the mechanical response of skeletal muscle.