Effects of mexiletine on ATP sensitive K+ channel of rat skeletal muscle fibres: a state dependent mechanism of action

Do sodium channel blockers have neuroprotective effect after onset of ischemic insult?

OBJECTIVE

Cerebral ischemia causes a series of pathophysiologic events that may result in cerebral infarct. Some neurons are more vulnerable to ischemia, particularly pyramidal neurons in the hippocampal CA1 region. Pharmacologic intervention for treatment of cerebral ischemia aims to counteract secondary neurotoxic events or to interrupt the progression of this process. In the present study, we compare the neuroprotective effects of sodium channel blockers (mexiletine, riluzole and phenytoin) and investigate whether they have neuroprotective effect when given after ischemic insult.

METHODS

A transient global cerebral ischemia model was performed in this study by clipping bilateral common carotid arteries during 45 minutes. Riluzole (8 mg/kg), mexiletine (80 mg/kg) and phenytoin (200 mg/kg) were injected into the rats intraperitoneally 30 minutes before or after reperfusion. Lipid peroxidation levels and cerebral water contents were evaluated 24 hours after ischemia. Histopathologic assessment of hippocampal region was determined 7 days after ischemia.

RESULTS

Riluzole, mexiletine and phenytoin treatment after global ischemia significantly decreased water content of the ischemic brain (p<0.05 for each). No significant difference was observed in cerebral edema among the drug treatment groups (p>0.05). When pre-treatment and post-treatment groups were compared with each other, only riluzole pre-treatment group revealed better result for cerebral edema (p<0.05). Pre-treatment with these drugs revealed significantly better results for the malonyldialdehyde (MDA) level and the number of survival neuron on the hippocampal region than the post-treatment groups.

CONCLUSION

It is demonstrated that riluzole, mexiletine and phenytoin are potent neuroprotective agents in the rat model of transient global cerebral ischemia, but they are more effective when given before onset of the ischemia.

Comparative neuroprotective effect of sodium channel blockers after experimental spinal cord injury

Spinal cord injury (SCI) results in loss of function below the lesion. Secondary injury following the primary impact includes a number of biochemical and cellular alterations leading to tissue necrosis and cell death. Influx of Na(+) ions into cells has been postulated to be a key early event in the pathogenesis of secondary traumatic and ischemic central nervous system injury. Previous studies have shown that some voltage-sensitive sodium channel blockers provide powerful neuroprotection. The purpose of the present study was to compare the neuroprotective effect of three sodium channel blockers-mexiletine, phenytoin and riluzole--after SCI. Ninety rats were randomly and blindly divided into five groups of 18 rats each: sham-operated group, trauma group (bolus injection of 1 mL physiological saline intraperiteonally [i.p.]), mexiletine treatment group (80 mg/kg, i.p.), phenytoin treatment group (200 mg/kg, i.p.) and riluzole treatment group (8 mg/kg, i.p.). Twenty-four hours after injury, the rats were killed for determination of spinal cord water content and malondialdehyde (MDA) levels. Motor function scores of six rats from each group were evaluated weekly for six weeks. Then the rats were killed for histopathological assessment. Although all the treatment groups revealed significantly lower MDA levels and spinal cord edema than the trauma group (p<0.05), the riluzole and mexiletine treatment groups were better than the phenytoin treatment group. In the chronic stage, riluzole and mexiletine treatment achieved better results for neurobehavioral and histopathological recovery than phenytoin treatment. In conclusion, all the tested Na(+) blockers had a neuroprotective effect after SCI; riluzole and mexiletine were superior to phenytoin.

New potent mexiletine and tocainide analogues evaluated in vivo and in vitro as antimyotonic agents on the myotonic ADR mouse

The antimyotonic activity of chiral derivatives of mexiletine and tocainide, selected as potent use-dependent blockers of skeletal muscle sodium channels, was evaluated in vivo acutely in myotonic ADR mice. The compounds had either aromatic (Me4 and Me6) or branched isopropyl groups (Me5 and To1) on the asymmetric centre, or had this latter one methylene apart from the amino group (Me2). Therapeutic doses of mexiletine (5-10 mg/kg) and tocainide (7-20 mg/kg) significantly reduced the long time of righting reflex (TRR), typical of ADR mice. Me4, Me5 and Me6 were 2-fold more potent than mexiletine. To1 fully normalised the TRR at 7 mg/kg. The electromyographic analysis confirmed a muscle-based activity for drug effectiveness on TRR. All the compounds reduced the myotonic hyperexcitability of intercostal muscle fibres when tested in vitro by current-clamp recordings, with a potency correlated with their action on sodium channels. On stimulus-evoked firing, the isopropyl analogues were 2-4-fold more potent than parent compounds, while the aromatic analogues were about 10-fold more potent than mexiletine. Patch-clamp recordings confirmed a normal-like pharmacological sensitivity of sodium channels of native ADR muscle fibres. Finally, the in vivo antimyotonic activity is due to the block of sodium channels and divergences with in vitro potency can be related to structure-based changes in drug pharmacokinetics.

Taurine blocks ATP-sensitive potassium channels of rat skeletal muscle fibres interfering with the sulphonylurea receptor

Taurine is a sulphonic aminoacid present in high amounts in various tissues including cardiac and skeletal muscles showing different properties such as antioxidative, antimyotonic and anti-schaemic effects. The cellular mechanism of action of taurine is under investigation and appears to involve the interaction of the sulphonic aminoacid with several ion channels. Using the patch-clamp technique we studied the effects of taurine in rat skeletal muscle fibres on ATP-sensitive K(+) channel (K(ATP)) immediately after excision and on channels that underwent rundown. The cytoplasmic application of 20 mM of taurine reduced the K(ATP) current; this effect was reverted by washout of the drug solution. In this experimental condition the IC(50) was 20.1 mM. After rundown, taurine inhibited the K(ATP) current with similar efficacy. Competition experiments showed that taurine shifted the dose-response inhibition curve of glybenclamide to the left on the log-dose axis without significantly affecting those of ATP or Ca(2+) ion. Single channel recording revealed that taurine affects the close state of the channel prolonging it and reducing the bursts duration. Our data indicate that taurine inhibits the muscular K(ATP) channel interfering with the glybenclamide site on the sulphonylurea receptor of the channel or on the site allosterically coupled to it. During ischaemia and hypoxia, the skeletal and heart muscles undergo several changes; for example, the activation of K(ATP) channels and loss of the intracellular taurine content. The depletion of taurine during ischaemia would contribute to the early activation of K(ATP) channels and salvage the intracellular ATP content.