Effects of nifedipine on electrical and mechanical responses of rat and guinea pig vas deferens

The mechanical response of the vas deferens to single pulses or short trains of stimuli can be resolved into two distinct phases, an initial twitch response ( I ) attributed to the release of an unknown transmitter and a slower better maintained secondary contraction ( II ) 1,2 , attributed to noradrenaline release. Recently, it has been shown that the calcium antagonist nifedipine 3 eliminates the initial twitch response leaving the secondary component intact 4,5 and we have now examined the electrical responses underlying these mechanical events. We show here that nifedipine abolishes the smooth muscle action potential and the initial twitch response ( I ) without reducing the excitatory junction potential (e.j.p.), suggesting a fundamentally different basis of the two responses and illustrating the pharmacological value of nifedipine.