Phenobarbital induces slow recovery from sodium inactivation at the nodal membrane

Effects of phenobarbital and levetiracetam on PR and QTc intervals in patients with post-stroke seizure

BACKGROUND AND OBJECTIVES

Sudden unexplained/unexpected death (SUDEP) is related to high mortality in patients with epilepsy. The prolongation of QT interval, involved in cardiac arrhythmia-related SUDEP, may be precipitated by antiepileptic drugs (AEDs). In this study, we evaluated the effects of phenobarbital and levetiracetam on PR-QTc intervals in patients with post-stroke seizures.

METHODS

We performed an open-label, parallel group, prospective, multicenter study between June 2009 and December 2013 in patients older than 18 years of age with a clinical diagnosis of post-stroke seizure and treated with phenobarbital or levetiracetam. In order to exclude a role of cerebral post-stroke injury on modulation of PR and QTc intervals, patients with cerebral post-stroke injury and without seizures were also enrolled as controls.

RESULTS

Interictal electrocardiography analysis revealed no significant difference in PR interval between patients treated with an AED (n = 49) and control patients (n = 50) (181.25 ± 12.05 vs. 182.4 ± 10.3 ms; p > 0.05). In contrast, a significantly longer QTc interval was recorded in patients treated with an AED compared with control patients (441.2 ± 56.6 vs. 396.8 ± 49.3 ms; p < 0.01). Patients treated with phenobarbital showed a significantly longer QTc interval than patients treated with levetiracetam (460.0 ± 57.2 vs. 421.5 ± 50.1 ms; p < 0.05).

CONCLUSIONS

The study reported that in patients with late post-stroke seizures, phenobarbital prolonged QTc interval more so than levetiracetam.

Phenytoin and carbamazepine: potential- and frequency-dependent block of Na currents in mammalian myelinated nerve fibers

Voltage clamp experiments were performed in single myelinated nerve fibers of the rat and the effects of phenytoin (PHT) and carbamazepine (CBZ) on the ionic membrane currents were studied. PHT and CBZ are almost selective blockers of Na channels. The main part of this inhibition is a potential-dependent block which can be removed by hyperpolarization. The dose-response curve of PHT was described as a first-order reaction with Kd = 37 microM; 100 microM CBZ was equally as effective 100 microM PHT. Both drugs shift the steady-state Na inactivation curve (h infinity (V] to more negative membrane potentials and decrease its steepness. PHT and CBZ have equimolar effects on the shift and decrease in steepness of h infinity (V). All of the drug-induced effects depend on drug concentration. Both drugs induce a slowing of recovery from Na inactivation. PHT has a stronger slowing effect than CBZ. From this ensues a frequency-dependent block, which is more pronounced in the presence of PHT than of CBZ. The effects of both drugs can be interpreted in the framework of the modulated receptor hypothesis.

The effect of temperature on Na currents in rat myelinated nerve fibres

Voltage clamp experiments were performed in single myelinated nerve fibres of the rat and the effect of temperature on Na currents was investigated between 0 degrees C and 40 degrees C. The amplitude of the peak Na current changed with a Q10 = 1.1 between 40 degrees and 20 degrees C and with a Q10 = 1.3 between 20 degrees and 10 degrees C. Below 10 degrees C the peak Na current changed with a Q10 = 1.9. The temperature coefficient for time-to-peak (tp), the measure for Na activation, and tau h1 and tau h2, the time constants for Na inactivation changed throughout the temperature range. Q10 for all of these kinetic parameters increased from 1.8-2.1 between 40 degrees and 20 degrees C to 2.6-2.7 between 20 degrees and 10 degrees C. Below 10 degrees C Q10 increased to 3.7 for tau h1 and tp, and to 2.9 for tau h2. When the series resistance artifacts were minimized by addition of 6 nM TTX, the Q10's at T less than 10 degrees C were 2.9-3.0. When the temperature was decreased from 20 degrees to 0 degrees C, both the curve relating Na permeability to potential, PNa(V), and the steady state Na inactivation curve, h infinity (V), were reversibly shifted towards more negative potentials by 6 mV and 11 mV, respectively. When the temperature was increased from 20 degrees to 37 degrees C no shifts occurred. The Hodgkin-Huxley rate constants alpha h(V) and beta h(V) were calculated from h infinity (V) and tau h (or tau h1) at 20 degrees and 4 degrees C.(ABSTRACT TRUNCATED AT 250 WORDS)

Modulated anticonvulsant block of sodium channels in nerve and muscle

We have looked at several anticonvulsant drugs regarding their potency for basal block of sodium channels in both skeletal muscle and myelinated nerve preparations under voltage clamp conditions. There is an inverse relationship observed between the half-blocking concentration of each drug and its lipid distribution coefficient. Furthermore the anticonvulsants which are cyclic amides show a systematically weaker potency for blocking channels than do local anesthetics (linear amides) of comparable solubility. Additional kinetic characterizations of drug block are described that may contribute to accumulation of frequency-dependent block and to the appearance of 'inactivation shift' phenomena. Finally, evidence is provided showing that phenytoin and carbamazepine (as well as phenobarbital) selectively block the inactive form of closed sodium channels, thus giving these anticonvulsants a capability for selective action in depolarized tissue.