Differential-effects of Nifedipine on nerve-mediated and noradrenaline-evoked contractions of rat anococcygeus muscle

In rat anococcygeus muscle the inhibitory effect of nifedipine (0.01. 0.1, 1.0 and 10 muM) was determined on adrenergic twitches in response to electrical field stimulation (trains of 4 pulses, 0.1 ms pulse duration. 10 Hz) and on twitch-matching contractions evoked by noradrenaline. Nifedipine concentration-dependently reduced the neurogenic twitch with an IC50 of 0.083 muM. Nifedipine reduced the noradrenaline-evoked contraction to a markedly lesser degree (IC50 > 10 muM). The difference in the magnitude of inhibition of electrically evoked twitch and twitch-matching noradrenaline-evoked contraction was statistically significant at every concentration of nifedipine. It is concluded that inhibition of the twitch by nifedipine involves some other mechanism(s) in addition to its Ca2+ Channel blocking property in smooth muscle.