4-Aminopyridine inhibits the neuromuscular effects of nitric oxide and 8-Br-cGMP

Apamin reduces neuromuscular transmission by activating inhibitory muscarinic M(2) receptors on motor nerve terminals

This study was undertaken to investigate the mechanism by which the toxin from the bee venom, apamin, might exert beneficial effects in patients suffering from myotonic dystrophy. The effects of apamin were compared with those produced by another potassium channel blocker, 4-aminopyridine, on rat hemidiaphragm preparations stimulated at a 100 Hz frequency via the phrenic nerve. Apamin and 4-aminopyridine increased nerve-evoked tetanic fade without changing the maximal tetanic tension. The inhibitory effect of apamin was mimicked by acetylcholine. In contrast with apamin, 4-aminopyridine increased the amplitude of muscle contractions induced by nerve stimulation at 0.2 Hz frequency. All these compounds were devoid of effect when diaphragm muscle fibres were stimulated directly in the presence of the neuromuscular blocker, D-tubocurarine. The muscarinic M(2) receptor antagonist, methoctramine, prevented the inhibitory effects of both apamin and acetylcholine. Blockade of presynaptic facilitatory muscarinic M(1) and nicotinic receptors respectively with pirenzepine and hexamethonium increased apamin-induced tetanic fade. Data suggest that apamin inhibits neuromuscular transmission by a mechanism independent of the blockade of Ca(2+)-activated K(+) channels, which might involve the activation of inhibitory muscarinic M(2) receptors on motor nerve terminals. Such a mechanism may be the origin of the beneficial effect of apamin controlling muscle excitability in patients suffering from myotonic diseases.

Gastric emptying effect by 4-aminopyridine in patients with chronic spinal cord injury

BACKGROUND

4-Aminopyridine (4-AP) given to patients with chronic spinal cord injury (SCI) has shown beneficial effects in some somatic and autonomic functions, although patients often develop dyspeptic symptoms. 4-AP is a potassium-channel blocker capable of altering gastro-pyloric functions as demonstrated experimentally. Our objective was to examine the influence of 4-AP treatment on gastric emptying in patients with chronic SCI.

METHODS

Gastric emptying was measured by the acetaminophen absorption test in 18 patients (9 with cervical and 9 with thoracic injury), and 9 healthy volunteers. Patients received increasing oral doses, 5 mg day(-1) of oral 4-AP (5-30 mg day(-1)), for 12 weeks. Patients were studied before and at the end of the last week of 4-AP treatment, whereas healthy volunteers (without 4-AP treatment) were studied only once. Whole blood samples of 2.5 mL were drawn at 0 (before 1 g of oral acetaminophen) and at 15, 30, 45, 60, 75, 90, 105 and 120 min postdose. Acetaminophen concentration in plasma was determined by high-pressure liquid chromatography.

RESULTS

Treatment with 4-AP significantly delayed stomach emptying in patients with chronic SCI, considering the significant decreasing of acetaminophen absorption (t paired test, p <0.05). This effect did not correlate either to the level or ASIA score of the injury (linear regression correlation analysis, r(2) = 0.003 and 0.015, respectively). No significant differences were observed by comparing data of patients before 4-AP treatment with healthy volunteers.

CONCLUSIONS

4-AP intake in patients with chronic spinal cord injury significantly slowed gastric emptying regardless of level and ASIA score of the injury.