Effects of calcium channel antagonists on action potential conduction and transmitter release in the guinea‐pig vas deferens

Abstract

The effects of the Ca2+ channel antagonists amlodipine, cobalt, diltiazem, nifedipine and verapamil and the local anaesthetic lignocaine were investigated on action potential conduction in and on evoked transmitter release from sympathetic nerves in the guinea‐pig isolated vas deferens. Transmitter release was investigated by measurement of (a) evoked (trains of pulses at 1 and 2 Hz, 0.1‐0.5 ms supramaximal voltage) excitatory junction potentials (e.j.ps) using microelectrodes; tension was recorded simultaneously; (b) tritium (3H) overflow from vasa preincubated (37°C, 30 min) in Krebs solution containing either [3H]‐noradrenaline (NA, 25 μCi ml−1, 2 × 10−6 M NA) or [3H]‐adenosine (50 μCi ml−1, 1 × 10−6 M adenosine). Amlodipine (0.5–2 × 10−4 M), verapamil (0.5‐2 × 10−4 M), diltiazem (1–8 × 10−4 M), lignocaine (0.1–2 × 10−3 M) and cobalt (2–6 × 10−2 M) in descending order of potency, but not nifedipine (1–5 × 10−3 M), increased the latency and inhibited, then abolished, the amplitude and number of action potentials in a concentration‐dependent manner. Amlodipine (0.5–1 × 10−4 M), verapamil (1–2 × 10−4 M), diltiazem (1–5 × 10−4 M) and cobalt (1 × 10−3 M), in descending order of potency, but not nifedipine (5 × 10−4 M), inhibited then abolished evoked e.j.ps in a concentration‐dependent manner. Cobalt inhibited e.j.ps at a lower concentration than that (2–6 × 10−2 M) required to block action potential conduction. In unstimulated tissues, the resting 3H overflow following preincubation with [3H]‐NA consisted largely of 4‐hydroxy 3‐methoxymandelic acid (VMA), 4‐hydroxy 3‐methoxy phenylglycol (MOPEG), 3,4 dihydroxyphenylglycol (DOPEG) and NA; stimulated tissues (300 pulses at 20 Hz, 0.5 ms supramaximal voltage) released mainly NA. Verapamil (0.1–1 × 10−4 M), amlodipine (0.05–1 × 10−4 M) and nifedipine (1–5 × 10−4 M), but not cobalt (2 × 10−3 M), increased, significantly, the resting overflow of 3H comprising mainly DOPEG. Cobalt (2 × 10−3 M) inhibited, significantly, the stimulation‐evoked overflow of 3H. Verapamil (1 × 10−4 M) had little effect on the resting overflow of 3H following preincubation with [3H]‐adenosine. Diltiazem (5 × 10−4 M) and cobalt (2 × 10−3 M) both inhibited evoked 3H overflow. Nifedipine (5 × 10−4 M) was ineffective. The effectiveness of Ca2+ channel antagonists at pre‐ and postjunctional sites differ; the results are discussed in terms of the selectivity of these drugs for each site and their differential effects on transmitter release.

The electrical and mechanical responses of the rabbit saphenous artery to nerve stimulation and drugs

“…In the rabbit saphenous artery, the action potentials and the accompanying contractions were inhibited by diltiazem, which inhibits voltage-dependent Ca2+ channels (Beattie et al, 1986). The upstroke of the action potential may therefore be carried by calcium current.…”

Calcium in sympathetic varicosities of mouse vas deferens during facilitation, augmentation and autoinhibition

“…Hence, the concentration of ù_conotoxin GVIA used in the current experiments (100 nÒ) is enough to completely block N-type calcium channels. It has been known for some time that nifedipine (20 ìÒ) does not affect junction potentials in the vas deferens (Surprenant, Neild & Holman, 1983;Beattie, Cunnane & Muir, 1986;Stj arne, Stj arne, Msghina & Bao, 1991) but that ù-conotoxin GVIA blocks these in the mouse (Waterman, 1997) and the guinea-pig vas deferens (Brock, Cunnane, Evans & Ziogas, 1989). The failure of ù-conotoxin GVIA to completely block the change in [Ca¥]v following short trains of impulses, in v following stimulation with a 5-impulse train before the application of (þ) and in the presence of yohimbine (10 ìÒ; ±).…”

ω‐Conotoxin GVIA is a potent inhibitor of sympathetic neurogenic responses in rat small mesenteric arteries

“…However, the mechanism sustaining this process has not yet been established. It is thought that depolarization activates presynaptic VOCCs but these channels are relatively insensitive to classical calcium entry blockers (Beattie et al, 1986;McCleskey et al, 1987;Miller, 1987;Hirning et al, 1988). Nifedipine, diltiazem and verapamil have been shown to inhibit the release of noradrenaline from the rabbit isolated heart and pulmonary artery, but the concentrations of calcium antagonists required were much higher than those needed to block VOCCs in smooth or cardiac muscle (Starke & Schumann, 1973;Gothert et al, 1979;Zelis et al, 1985).…”