Differential effects of Ca antagonists on the noradrenaline release and contraction evoked by nerve stimulation in the presence of 4-aminopyridine.

We previously reported that verapamil, nicardipine and diltiazem inhibited both neurotransmitter release and contraction evoked by transmural nerve stimulation (TNS) in the canine saphenous vein. To evaluate whether the three Ca antagonists act on the nerve endings by inhibiting Ca2+ influx, the effects of the three antagonists were studied in the presence of 4-aminopyridine (4-AP) 3 X 10(-4) M on the TNS-evoked tritium overflow and contraction of canine saphenous veins preloaded with [3H]-noradrenaline. 4-AP increased both tritium overflow and contraction evoked by TNS, but did not enhance the contraction induced by exogenous noradrenaline (10 nmol). In the veins pretreated with 4-AP, verapamil (3 X 10(-5) M) and nicardipine (10(-5) M and 3 X 10(-5) M) caused no significant effects on the TNS-evoked tritium overflow, but they still inhibited the contraction. Diltiazem (10(-5) M and 3 X 10(-5) M) significantly inhibited both responses to TNS in the veins pretreated with 4-AP, the effects being nearly equipotent to those in the absence of 4-AP. The (-)-cis isomer of diltiazem (10(-5) M and 3 X 10(-5) M), which is about 100 times less potent than diltiazem in inhibiting Ca2+ influx, inhibited both responses to TNS in the presence of 4-AP to almost the same degree as diltiazem. When 4-AP was added after the Ca antagonists, it reversed the TNS-evoked tritium overflow inhibiting actions of verapamil (3 X 10(-5) M) and nicardipine (3 X 10(-5) M) much more effectively than that of diltiazem (3 X 10(-5) M). Tetracaine (4 X 10(-6) M) significantly inhibited the TNS-evoked tritium overflow and contraction, which were unaffected by 4-AP. Sodium salicylate (10(-2) M) failed to modify the inhibition of TNS-evoked tritium overflow following diltiazem (3 X 10(-5) M), but it enhanced that of tetracaine (4 X 10(-6) M). Verapamil but not diltiazem and nicardipine significantly increased the spontaneous tritium overflow from veins pretreated with 4-AP. The present study together with previous results suggests that diltiazem but not verapamil and nicardipine may inhibit the TNS-evoked neurotransmitter release through an action other than inhibition of Ca2+ influx into the adrenergic nerve endings, allowing an inhibition of the resulting contraction.