Preferential metabolism of (—)-3H-norepinephrine through the deaminated glycol in the rat vas deferens

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.

Section: Generalmentioning

confidence: 99%

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

Beattie

Cunnane

Muir

1986

“…After removing I ml of the fluid to determine the total tritium content, the [3H]-noradrenaline content of the samples was determined (Graefe et al, 1973;Fuder et al, 1982a). Noradrenaline values were corrected for minor cross contaminations (Fuder et al, 1982a (Fuder & Jung, 1985) 100 ± 0%, n = 3).…”

Section: Methodsmentioning

confidence: 99%

Cholinesterase activity and exposure time to acetylcholine as factors influencing the muscarinic inhibition of [³H]‐noradrenaline overflow from guinea‐ pig isolated atria

Fuder

Muscholl

Wolf

1985

1 Guinea-pig isolated atria were incubated and loaded with [3H]-noradrenaline. The release of 3H and of [3H]-noradrenaline was induced by field stimulation (6-9 trains of 150 pulses at 5 Hz). The stimulation-evoked overflows of 3H and of [3H]-noradrenaline were determined. 2 In the absence of an inhibitor of acetylcholinesterase, acetylcholine (12 min preincubation before nerve stimulation, up to 10pM) failed to inhibit the evoked [3H]-noradrenaline overflow. In the presence of atropine, an increase by acetylcholine of evoked release was observed in the same atria. In contrast, the selective muscarinic agonist methacholine significantly decreased the evoked overflow. 6 It is concluded that a muscarinic inhibition by acetylcholine (upon prolonged exposure time) may be masked by a concomitant facilitation of release and/or desensitization of the muscarinic inhibitory mechanism. Furthermore, degradation by acetylcholinesterase contributes in part to the ineffectiveness of acetylcholine as a presynaptic inhibitor. When a distortion of the overflow/release ratio was excluded, adrenergic and nicotinic effects were prevented, and acetylcholinesterase was inhibited, the fading of muscarinic inhibition by acetylcholine may have been exclusively due to a slow and moderate desensitization of the presynaptic muscarinic mechanism.

“…Square wave direct current impulses (I Hz, 10 V, 2 ms) were triggered by a Grass S44 stimulator at 60-65, 90-95, 120-125 and 150-155min Column chromatography (Graefe et al, 1973) was used to separate NA from its metabolites in 3H-a overflow. In these experiments, one sample (3.4 ml) collected at 55-60 min (i.e.…”

Section: Ratsmentioning

confidence: 99%

The effect of diltiazem on noradrenaline release

Wolchinsky

Zsoter

1985

1 The effects of diltiazem in rat tail arteries and guinea-pig vasa deferentia have been investigated. 2 Superfusion of the rat tail artery with diltiazem (10-6 10-M) resulted in a dose-related increase in 3H-overflow (P < 0.001) both in Wistar Kyoto (WKY) and in spontaneously hypertensive (SHR) rats. Release of 3H by transmural stimulation (I Hz, 2 ms, 10 V) was also much greater in vessels perfused with diltiazem; this effect was dose-dependent. 3 Diltiazem did not significantly alter the proportion of noradrenaline and its metabolites in 3H-overflow, as analysed by column chromatography. 4 In the vasa deferentia of guinea-pigs, diltiazem (10--10-M) increased spontaneous 3H-release. 5 The results indicate that diltiazem acts on sympathetic nerves and causes the release of noradrenaline.