Phenytoin preferentially inhibits L-type calcium currents in whole-cell patch-clamped cardiac and skeletal muscle cells

The effect of phenytoin on embryonic heart rate in Vivo

Phenytoin is a known human teratogen with unknown etiology. Several mechanisms have been proposed including disturbances in folate metabolism, induction of embryonic hypoxia following phenytoin-induced bradycardia, free radical formation following re-oxygenation and phenytoin-induced maternal hyperglycemia. Using high frequency ultrasound, we demonstrated that phenytoin induced a dramatic decrease in the heart rate of embryos. This coincided with a moderate transient decrease in maternal heart rate and blood glucose levels. Embryonic heart rate had not fully recovered 24 h later in some embryos despite normal maternal physiological parameters. In a separate study, extent of hypoxia was measured using the marker pimonidazole. Phenytoin-exposed embryos did not demonstrate increased hypoxia compared to control embryos at 2, 4, 8 or 24 h dosing. Together our results show that phenytoin induces malformations as a result of a combination of insults: embryonic bradycardia, maternal bradycardia and maternal hyperglycemia. However, this does not appear to result in measurable embryonic hypoxia in our animal model.

The effect of drugs with ion channel-blocking activity on the early embryonic rat heart

This study investigated the effects of a range of pharmaceutical drugs with ion channel-blocking activity on the heart of gestation day 13 rat embryos in vitro. The general hypothesis was that the blockade of the I(Kr)/hERG channel, that is highly important for the normal functioning of the embryonic rat heart, would cause bradycardia and arrhythmia. Concomitant blockade of other channels was expected to modify the effects of hERG blockade. Fourteen drugs with varying degrees of specificity and affinity toward potassium, sodium, and calcium channels were tested over a range of concentrations. The rat embryos were maintained for 2 hr in culture, 1 hr to acclimatize, and 1 hr to test the effect of the drug. All the drugs caused a concentration-dependent bradycardia except nifedipine, which primarily caused a negative inotropic effect eventually stopping the heart. A number of drugs induced arrhythmias and these appeared to be related to either sodium channel blockade, which resulted in a double atrial beat for each ventricular beat, or I(Kr)/hERG blockade, which caused irregular atrial and ventricular beats. However, it is difficult to make a precise prediction of the effect of a drug on the embryonic heart just by looking at the polypharmacological action on ion channels. The results indicate that the use of the tested drugs during pregnancy could potentially damage the embryo by causing periods of hypoxia. In general, the effects on the embryonic heart were only seen at concentrations greater than those likely to occur with normal therapeutic dosing.

Use of phenytoin to treat digitalis-induced cardiac arrhythmias in a miniature Shetland pony

Two miniature Shetland ponies showing clinical signs of Digitalis purpurea (foxglove) poisoning were examined. One animal died shortly afterwards, but the second was treated successfully with the anti-arrhythmic agent, phenytoin, and was discharged after 16 days.

Prophylactic efficacy of phenytoin, acetazolamide and hydrochlorothiazide in horses with hyperkalaemic periodic paralysis

Horses with hyperkalaemic periodic paralysis were challenged with an oral dose of potassium chloride, and the prophylactic efficacy of phenytoin, acetazolamide and hydrochlorothiazide was evaluated, with at least three weeks separating the trials of each drug. After the administration of potassium chloride without prophylactic medication the horses' clinical signs ranged from generalised fine muscle fasciculations to gross tremors, and weakness with occassional prolapse of the nictitating membrane; plasma potassium concentration increased significantly (P < 0.01) from 4.0 +/- 0.2 to 6.0 +/- 1.01 mmol litre-1. After treatment with acetazolamide the administration of potassium chloride resulted in a significant (P < 0.02) increase in plasma potassium from 3.7 +/- 0.3 to 4.5 +/- 0.4 mmol litre-1 and two of five horses showed clinical signs unless the dosage was increased from 2.2 to 4.4 mg kg-1 twice daily. Three of the four horses treated with hydrochlorothiazide showed clinical signs but their plasma potassium did not rise significantly (3.6 +/- 0.3 to 4.6 +/- 1.0 mmol litre-1). None of the five horses treated with phenytoin showed clinical signs despite a significant increase in plasma potassium from 3.8 +/- 0.6 to 5.3 +/- 1.1 mmol litre-1 (P < 0.05). In general the clinical signs were not correlated consistently with the plasma levels of potassium, and phenytoin appeared to prevent the clinical signs in spite of the hyperkalaemia.

Effect of phenytoin on skeletal muscle from quarter horses with hyperkalaemic periodic paralysis

The contractile activity, the threshold for calcium-induced calcium release in fractions of sarcoplasmic reticulum and the potassium concentration were determined in preparations of semimembranosus muscle from normal quarter horses and quarter horses with hyperkalaemic periodic paralysis before and after they were treated with phenytoin. Before the treatment there was no difference in caffeine contracture or electrically elicited twitch response between the two groups. For one week after the treatment, the time to peak tension of caffeine contractures was significantly (P < 0.005) reduced in the horses with hyperkalaemic periodic paralysis but unchanged in the normal horses. The variance but not the mean values for the threshold for Ca(2+)-induced Ca2+ release from the sarcoplasmic reticulum was greater for the horses with hyperkalaemic period paralysis before but not after the treatment with phenytoin.

Damage from oxygen and glucose deprivation in hippocampal slices is prevented by tetrodotoxin, lidocaine and phenytoin without blockade of action potentials

In vitro ischemia (IVI) was simulated with rat hippocampal slices in medium lacking D-glucose, equilibrated with 95% nitrogen, 5% carbon dioxide. Within 5-8 min, synaptic potentials disappeared and a DC negative shift (5-15 mV) occurred. Prolonged application of 95% oxygen and D-glucose 12 min later did not allow synaptic potentials to recover. Slices pretreated with sodium channel blocking drugs allowed synaptic potentials to recover after IVI. Tetrodotoxin (TTX, 100-600 nM), the anticonvulsant phenytoin (5.0 to 100 microM) and the local anesthetic lidocaine (2.0 to 200 microM) each delayed or prevented negative DC shifts from IVI. Histological examination showed that drug treatments also prevented CA1 pyramidal cell damage from IVI. Neuroprotection occurred without blocking synaptic potentials or presynaptic fiber volleys, suggesting relevance for treatment of brain ischemia.

Cardiac T-type calcium current: pharmacology and roles in cardiac tissues

A low threshold, voltage-gated calcium current is reported in most cardiac tissues but rarely in ventricular cells. This article reports some recently described characteristics and discusses their possible pathophysiologic implications. It also reviews the alterations induced in this current by a variety of chemical agents including several neuromediators in cardiac and other tissues.

Phenytoin increases specific triacylglycerol fatty esters in skeletal muscle from horses with hyperkalemic periodic paralysis

Previous studies have demonstrated that phenytoin decreases the levels of triacylglycerols in several tissues other than skeletal muscle. Since phenytoin is clinically effective in several skeletal muscle disorders, triacylglycerol metabolism in skeletal muscle from four normal Quarter horses and four Quarter horses with hyperkalemic periodic paralysis was examined. The horses with hyperkalemic periodic paralysis had low levels of 18:3 in the phospholipids, low levels of 16:0, 16:1 and 18:3 in the free fatty acids and low levels of 20:4 in triacylglycerols. Triacylglycerol levels were increased in skeletal muscle from seven (three controls, four hyperkalemic periodic paralysis) of the eight horses on treatment with oral phenytoin for one week. Instead of an increase in all fatty ester types only 16:0, 16:1, 18:1 and 18:2 were significantly increased. Total lipid phosphorus and the distribution of phospholipid fatty esters and free fatty acids were not significantly altered by phenytoin treatment in most cases. An alteration in triacylglycerol metabolism by phenytoin was also observed in primary cultures of normal equine skeletal muscle radiolabeled with 18:1, but not in those radiolabeled with 18:2. These findings suggest that phenytoin does not just increase the levels of triacylglycerol in skeletal muscle, but alters the utilization and incorporation of fatty esters. These findings suggest a potential involvement of triacylglycerol metabolism in the clinical efficacy of phenytoin in hyperkalemic periodic paralysis.

Appearance and evolution of calcium currents and contraction during the early post-fusional stages of rat skeletal muscle cells developing in primary culture

Primary cultures from enzymatically dissociated satellite cells of newborn rat skeletal muscles enabled developmental in vitro studies of mechanical and electrical properties during the first steps of myogenesis. The present work focused on the appearance, evolution and roles of two types of calcium currents (ICa,T and ICa,L) and of depolarization-induced contractile activity during the early stages of muscle cell development in primary culture. Prefusional mononucleated cells (myoblasts), young myotubes of 1 day (with less than 10 nuclei) or 2–3 days (more than 9 nuclei) and myoballs from 4–6, 7–9, 10–12 and 13–16 days cultures were patch-clamped (whole-cell configuration), and calcium currents and contraction simultaneously recorded. Sodium but not calcium currents could be recorded at the myoblast stage. In young myotubes (1 day), ICa,L was present with high incidence as compared to ICa,T, which was poorly expressed. Contractile responses appeared at the next stage (2-3 days) while the occurrence of ICa,T progressively increased. This developmental evolution of the calcium currents and contraction expression was accompanied by some changes in their characteristics: the ICa,T/ICa,L amplitudes ratio progressively increased and the time-to-peak of contraction progressively decreased with the age of myoballs. Physiological functions for calcium currents in developing muscle are suggested and discussed: ICa,T, which is transiently expressed, could be involved in the pacemaker-like activity while ICa,L could serve as an early contraction triggering mechanism and/or initially to fill and then to maintain the intracellular calcium stores.

The effect of phenytoin medication on dentin apposition, root length, and caries progression in rat molars

To determine the effect of phenytoin (PHT) on dentin apposition and the rate of caries progression in dentin, 40 Wistar rats were treated daily for 35 days with intraperitoneal injections of 5,5-diphenylhydantoin. Twenty-eight controls received saline with pH adjusted to that of PHT. One PHT and one control group were fed a 43% sucrose diet, and the others a non-cariogenic pellet diet. Streptococcus sobrinus was inoculated to induce caries. Schiff staining was used to determine caries. The areas of dentin apposition and dentinal caries lesions were quantified after tetracycline staining. The root lengths were measured. PHT reduced slightly the dentin apposition and activated significantly the progression of the dentinal carious lesions. No difference in caries initiation was found. The high-sucrose diet reduced dentin apposition and increased the rate of progression of existing caries greatly. Our study suggests that both the high-sucrose diet and PHT have an effect on secreting odontoblasts, which can be seen as an alteration in dentin apposition and caries progression rates in dentin.

Veratridine activates a silent sodium channel in rat isolated aorta

To investigate the existence of silent Na+ channels, isolated rat aorta was treated with veratridine (0.1 mM) and the resulting Ca2+ uptake was determined. After 30-min incubation the total tissue uptake of Ca2+ and Ca2+ uptake increased from 2.325 +/- 0.017 to 2.614 +/- 0.080 nmol/mg wet weight (ww) and from 162.6 +/- 9.7 to 218.1 +/- 13.0 pmol/mg ww, respectively. The veratridine-induced Ca2+ uptake was blocked by tetrodotoxin (1 microM; to 17 +/- 5%) but not altered by amiloride (10 microM-1 mM). Activation of Na+/Ca2+ exchange by Na+ removal increased Ca2+ uptake from 74.2 +/- 4.5 to 97.3 +/- 5.3 pmol/mg ww, which was suppressed by amiloride (10 microM-1 mM). Nifedipine (10 nM) and verapamil (0.1 microM) at concentrations at which depolarization-induced Ca2+ uptake was diminished did not attenuate veratridine-induced Ca2+ uptake. Phenytoin at 0.1 mM reduced the Ca2+ uptake induced by veratridine or by depolarization. R 56865 (0.1 microM) and R 59494 (1 microM), novel anti-ischemic compounds inhibiting slowly inactivating Na+ channels, suppressed the veratridine-induced but not the depolarization-induced Ca2+ uptake. Guanidinium uptake was increased by veratridine (0.1 mM) from 371.2 +/- 7.2 to 574.8 +/- 45.9 pmol/mg ww. These results suggest that the rat aorta possesses a Na+ channel which is electrically silent under normal conditions but could be activated by veratridine.

Effects of phenytoin in two myotonic horses with hyperkalemic periodic paralysis

The effects of phenytoin treatment were evaluated in 2 myotonic horses with hyperkalemic periodic paralysis (HPP). Phenytoin treatment abolished the clinical signs of muscle fasciculations following oral potassium challenge and decreased or abolished repetitive firing and myotonic discharges found on electromyographic examination. In both horses, an abnormally low threshold for calcium-induced calcium release was measured in heavy sarcoplasmic reticulum fractions from skeletal muscle, and this threshold increased with phenytoin treatment. Results suggest phenytoin is useful in modifying disordered ion regulation in the sarcolemma and sarcoplasmic reticulum of skeletal muscle in equine hyperkalemic periodic paralysis.