Phenytoin reduces calcium current in the cardiac Purkinje fiber

Ventricular Tachycardia: A Rare Case of Myocardial Silicosis

Chronic exposure to silica is a recognized health hazard. Manifestations of pulmonary and extrapulmonary silicosis are well described. Secondary pulmonary arterial hypertension and pericardial involvement are described, but myocardial involvement has not been reported. In this case of newly diagnosed pulmonary silicosis, ventricular tachycardia results are shown from pathological involvement of ventricular myocardium. (Level of Difficulty: Beginner.).

Unusual Cause of Bidirectional Ventricular Rhythm

Bidirectional ventricular tachycardia (BDVT), a rare ventricular arrhythmia, is commonly caused by digitalis toxicity or channelopathies and is rarely caused by aconite toxicity, myocarditis, infarction, or sarcoidosis. This paper describes a patient with BDVT, recurrent syncope, myocardial disarray, and interstitial fibrosis on histology but normal results on echocardiography with variants in the TTN, KCNH2, and GATA4 genes. (Level of Difficulty: Advanced.)

Dissociation between force and intracellular sodium activity with strophanthidin in isolated sheep Purkinje fibres

1. We have recorded membrane potential, intracellular Na activity (aiNa) and force of contraction in sheep Purkinje fibres. Force and aiNa were recorded continuously and simultaneously during exposure to strophanthidin and its subsequent washing off. 2. Exposure to strophanthidin in concentrations of 1.5 X 10(-7), 5 X 10(-7) and 10(-5) M caused an increase in force of contraction which was temporally dissociated from the increase of aiNa. 3. There was hysteresis in the relationship between force and aiNa when the period of increasing force was compared to the period of decreasing force. When force increased on exposure to strophanthidin, the same aiNa was always associated with a higher force than when force was decreasing while washing off the drug. 4. For the same rise of aiNa higher doses of strophanthidin produced larger rises of force than lower doses. 5. When diphenylhydantoin was present in the bathing solution at concentrations of 10(-5) and 10(-4) M, the relation between force and aiNa with 10(-5) M-strophanthidin had a less steep slope, but still displayed hysteresis. 6. The relationship between force and aiNa during changes of the bathing K concentration also displayed a hysteresis, which was in the same direction as that found with strophanthidin. 7. These results are discussed in relation to proposed mechanisms of action of strophanthidin and more generally in relation to the factors linking force of contraction and aiNa. We conclude that in sheep Purkinje fibres the increase of force caused by strophanthidin is not solely due to an increase of aiNa, and that other interventions can also result in hysteresis between force and aiNa.

Effect of phenytoin on [3H]norepinephrine release from synaptosomes

Phenytoin reduces depolarization-linked [3H]norepinephrine release from rat brain synaptosomes. When choline chloride was substituted for NaCl the phenytoin effect was attenuated but still significant. This is consistent with the theory that phenytoin reduces both Na and Ca influx during depolarization.

Delayed rectification in the calf cardiac Purkinje fiber. Evidence for multiple state kinetics

We have investigated the delayed rectifier current (Ix) in the calf cardiac Purkinje fiber using a conventional two-microelectrode voltage clamp arrangement. The deactivation of Ix was monitored by studying decaying current tails after the application of depolarizing voltage prepulses. The reversal potential (Vrev) of these Ix tails was measured as a function of prepulse magnitude and duration to test for possible permeant ion accumulation- or depletion-induced changes in Vrev. We found that prepulse-induced changes in Vrev were less than 5 mV, provided that prepulse durations were less than or equal to 3.5 s and magnitudes were less than or equal to +35 mV. We kept voltage pulse structures within these limits for the remainder of the experiments in this study. We studied the sensitivity of Vrev to variation in extracellular K+. The reversal potential for Ix is well described by a Goldman-Hodgkin-Katz relation for a channel permeable to Na+ and K+ with PNa/PK = 0.02. The deactivation of Ix was always found to be biexponential and the two components shared a common reversal potential. These results suggest that it is not necessary to postulate the existence of two populations of channels to account for the time course of the Ix tails. Rather, our results can quantitatively be reproduced by a model in which the Ix channel can exist in three (two closed, one open) conformational states connected by voltage dependent rate constants.

Effects of stimulation and diphenylhydantoin on the intracellular sodium activity in Purkinje fibres of sheep heart

The effects of stimulus frequency on the intracellular Na+ activity (aiNa) of Purkinje fibres from sheep hearts have been investigated using ion-sensitive micro-electrodes. aiNa increased by approximately 44% at 3 Hz and approximately 6.5% at 0.2 Hz. The size of the increase was very variable between preparations and did not appear to be related to the initial level of aiNa before stimulation. During stimulation, the rise of aiNa was exponential and gave a new steady-state level within about 15 min. The time constant of this rise was independent of the frequency of stimulation. The recovery of aiNa following a period of stimulation also appeared to be exponential but the time constant was dependent upon the extent to which aiNa had been raised during stimulation, with shorter time constants following larger rises. The effects of stimulation on aiNa were compared to inhibition of the Na+-K+ pump using high concentrations of cardioactive steroids. The influence of diphenylhydantoin (DPH) on the Na+ permeability of the cell membrane was assessed by measuring the rate of rise of aiNa following Na+-K+-pump inhibition. DPH decreased the rate of rise of aiNa by approximately 37% at concentration of 10(-4) M. The decrease of Na+ permeability produced by DPH is probably responsible for the observed reduction of aiNa in quiescent preparations. DPH also produced substantial decreases of aiNa in stimulated preparations. These results are discussed in relation to its known anti-arrhythmic effects.

Stimulation by proctolin of the ouabain-insensitive sodium efflux in single barnacle muscle fibers

A study has been made of the action of the neuropeptide proctolin on the radiosodium efflux from single barnacle muscle fibers. Proctolin (10(-8) M) when applied externally causes stimulation of the Na efflux in unpoisoned and ouabain-poisoned fibers. The response is prompt in onset, reaches a peak in 15 min and decays slowly. The response of the ouabain-insensitive Na efflux to external proctolin is dose-dependent, the concentration threshold being less than 10(-10) M. The response to proctolin is dependent on external Ca2+ but not on Na+. (i) The response to proctolin is abolished by high external Mg2+, as well as by verapamil, Co2+, Cd2+ and WB-4101. (ii) The response is also abolished by preinjecting 0.5 M MgCl2 or 0.1 M EGTA. The calmodulin antagonists trifluoperazine and imipramine are without effect on the response to proctolin. (i) Adenylate cyclase agonists, e.g. forskolin, fail to augment the response to proctolin. (ii) Prior injection of the phosphodiesterase inhibitors 1-propyl-3-methyl-7-(5-hydroxyhexyl)-xanthine (PMX) or 1-isoamyl-3-isobutylxanthine (IAX) fails to augment the response to proctolin. (iii) Prior injection of protein kinase inhibitor is ineffective. The response to proctolin is significantly reduced in the presence of tyramine. Taken together, these results support the view that proctolin stimulates the ouabain-insensitive Na efflux by activating Ca2+ channels and that the cAMP-protein kinase system is not involved in this response.

Delayed rectification in the cardiac Purkinje fiber is not activated by intracellular calcium

This study was designed to test the hypothesis that an outward current (Ix) responsible for action potential repolarization in the cardiac Purkinje fiber is activated by intracellular calcium (Cai). Pharmacological probes were combined with the measurement of membrane current and contractile activity under voltage clamp conditions. Experiments were designed to examine properties of Ix that have previously linked activation of this current to changes in Cai. The independence of Ix from Cai was demonstrated for each case tested. Thus, the results of these experiments support the view that Ix is not a calcium-activated current.