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 dur ation. 10 Hz) and on twitch-matching contractions evoked by noradrenal ine. Nifedipine concentration-dependently reduced the neurogenic twitc h with an IC50 of 0.083 muM. Nifedipine reduced the noradrenaline-evok ed contraction to a markedly lesser degree (IC50 > 10 muM). The differ ence in the magnitude of inhibition of electrically evoked twitch and twitch-matching noradrenaline-evoked contraction was statistically sig nificant at every concentration of nifedipine. It is concluded that in hibition of the twitch by nifedipine involves some other mechanism(s) in addition to its Ca2+ Channel blocking property in smooth muscle.