Electrophysiological effects of the salicylates on isolated atrial muscle of the rabbit

Non-fatal ventricular dysrhythmias associated with severe salicylate toxicity

INTRODUCTION

Cardiac dysrhythmias, other than sinus tachycardia, rarely occur with salicylate poisoning. When dysrhythmias do occur, they are typically a terminal event.

CASE REPORT

A 45-year-old woman presented an unknown amount of time after an intentional ingestion of aspirin and acetaminophen. On presentation her vital signs were T 39 degrees C, P 125 beats/minute, R 26 breaths/minute, and BP 153/79 mmHg. She was initially obtunded, but minutes after presentation had a generalized tonic-clonic seizure lasting approximately two minutes, which ceased after 2 mg of intravenous lorazepam. She was sedated, intubated and treated with sodium bicarbonate. Her peak salicylate concentration was 152 mg/dL. Her course was complicated by seizures and dysrhythmias, including monomorphic ventricular tachycardia and Torsades de Pointes. With bicarbonate therapy, hemodialysis, and veno-venous hemofiltration, she survived neurologically intact.

DISCUSSION

The etiology of these dysrhythmias is likely multifactorial. Metabolic derangements typically encountered with severe salicylism, including insensible water losses, respiratory alkalosis, and metabolic acidosis, may contribute. Iatrogenic causes, especially sodium bicarbonate therapy, may cause hypokalemia, hypocalcemia, and hypomagnesemia. Additionally, animal data suggests that high salicylate concentrations may have direct deleterious effects on the electrophysiology of cardiac cells, specifically by its action on the SA node and on the action potential of atria and Purkinje fibers.

CONCLUSIONS

Non-fatal ventricular dysrhythmias associated with salicylate toxicity are rare in patients who survive. The causes of dysrhythmias in salicylate may include electrolyte abnormalities and a direct effect of salicylate on myocardial membrane permeability.

Antiarrhythmic and electrophysiological effects of the novel KATP channel opener, rilmakalim, in rabbit cardiac cells

1. The effects of rilmakalim, a potassium channel opener, were studied on rabbit cardiac Purkinje, ventricular muscle and atrial fibers, with the use of conventional microelectrode techniques. 2. Rilmakalim (0.24-7.2 microM) shortened, in a concentration-dependent manner, the action potential duration (APD) in Purkinje and ventricular muscle without affecting other parameters of the action potential. Pinacidil (30-300 microM) also decreased the APD of Purkinje fibers. 3. Rilmakalim (2.4 microM) and cromakalim (100 microM) hyperpolarized and abolished abnormal automaticity of cardiac Purkinje fibers pretreated with barium (0.2-0.3 mM). Glibenclamide (5 microM) blocked the hyperpolarizing effect. 4. Stable early afterdepolarizations induced in Purkinje fibers by berberine (100 microM) were reversibly blocked by rilmakalim (2.4 microM), which also suppressed late afterdepolarizations induced in Purkinje fibers treated with ouabain (0.3-0.5 microM). 5. The rate of spontaneous discharges of the rabbit sinoatrial node was not affected by rilmakalim (7.2 microM) or by pinacidil (100 microM). Both agents were also unable to affect the APD of atrial muscle fibers. 6. In cardiac Purkinje fibers, tetraethylammonium (TEA; 20 mM) significantly reduced the effects of rilmakalim (2. 4 microM) on the APD. However, neither TEA nor glibenclamide (100 microM) reduced the shortening of the APD induced by dinitrophenol (30 microM) or by salicylate (1 mM).

Transient myocardial dysfunction in a child with salicylate toxicity

Aspirin overdose may result in acid-base disturbances, electrolyte abnormalities, pulmonary edema, chemical hepatitis, seizures, and mental status alteration, but myocardial depression has not been reported following aspirin overdose in children. In addition to these more typical features, the 13-month-old boy reported here developed clinical, radiographic, and echocardiographic evidence of myocardial impairment with pulmonary edema and moderately severe global left ventricular dysfunction (estimated shortening fraction of 23%). Complete resolution of the myocardial dysfunction was demonstrated on follow-up echocardiography as the child recovered from the aspirin intoxication. This case suggests that myocardial dysfunction can occur as a result of toxic aspirin ingestion, and that it may contribute to salicylate-induced pulmonary edema.

Salicylate trapping of .OH as a tool for studying post-ischemic oxidative injury in the isolated rat heart

The use of salicylate as a chemical trap for .OH represents a simple and convenient alternative to the use of spin trapping techniques to study oxidative injury in isolated perfused organs. In these systems, salicylate is included in the perfusion buffer at concentrations ranging from 0.1 to 2 mM depending on the detection apparatus employed. In our studies, we have used a coulometric detector, which has a theoretical efficiency of 100% as compared to 1-5% for the standard glassy carbon electrode. We have been able to generate reproducible results by inclusion of only 100 microM salicylate, a concentration demonstrated not to affect pre- or post-ischemic cardiac function. In initial studies, we observed an increase in perfusate 2,5-dihydroxybenzoic acid consistent with an early post-ischemic burst of .OH, not unlike that reported using spin trapping techniques. Since then we and others have used this technique to examine possible relationships between .OH formation and treatments that alter post-ischemic cardiac functional recovery. For example, preischemic loading of hearts with copper results in increases in post-ischemic dysfunction and LDH release that were associated with an increase in 2,5-dihydroxybenzoate and by inference, .OH formation. Alternatively, we have reported that the nitroxide spin label, TEMPO, reputed to be a superoxide dismutase mimetic, decreased post-ischemic arrhythmias and 2,5-dihydroxybenzoate formation. Most recently, we have observed that preischemic loading of hearts with zinc-bis-histidinate results in improved post-ischemic cardiac function and decreased LDH release; changes that were associated with decreased 2,5-dihydroxybenzoate formation. These studies indicate that under certain conditions, salicylate is a valuable alternative to spin trapping techniques to probe the role of .OH in cardiac oxidative injury, particularly when applied to the isolated perfused heart preparation.

Electropharmacological effects of berberine on canine cardiac Purkinje fibres and ventricular muscle and atrial muscle of the rabbit

1. Conventional microelectrode techniques were used for intracellular recordings of the transmembrane electrical potentials, the effects of berberine were studied on canine cardiac Purkinje and ventricular muscle fibres and on rabbit atrial fibres. 2. Berberine (3-30 microM) increased in a concentration-dependent manner, the action potential duration (APD) in canine Purkinje and ventricular muscle without affecting other parameters of the action potential. 3. The berberine-induced enlargement of the APD showed reverse use-dependence, so that the effect was greater at lower rates of stimulation. 4. Preparations perfused with berberine (30 microM) and driven at rates below 0.5 Hz exhibited early after depolarizations which persisted 3-4 h after washing. 5. The early afterdepolarizations were reversibly abolished by perfusion with lignocaine (3 microM) or by the increase in the rate of stimulation. 6. The effective refractory period (ERP) of Purkinje fibres was greatly increased by berberine (30 microM); however, the ratio ERP/APD was not significantly affected. 7. Berberine (10-100 microM) decreased in a concentration-dependent manner the spontaneous frequency of rabbit sinoatrial cells. The decrease in frequency was accompanied by a depression of the phase 4 depolarization, without significant changes in other parameters of the nodal action potential. 8. Atropine (2.5 microM) did not affect the bradycardic effect of berberine. On the other hand, berberine (30 microM) did not alter the chronotropic effect of isoprenaline. 9. Berberine (30 microM) also increased the duration of slow responses in K-depolarized rabbit atrial muscle fibres, other parameters being unaffected. 10. It is suggested that berberine exerts Class III antiarrhythmic and proarrhythmic actions in cardiac muscle of the dog in vitro.

Cardiac conduction abnormalities and atrial arrhythmias associated with salicylate toxicity

Cardiac side effects from aspirin are uncommon; however, severe acid-base imbalance, pulmonary edema, ventricular ectopic activity and cardiopulmonary arrest have been reported in patients with toxic serum salicylate concentrations. We saw a patient with salicylate toxicity who developed a variety of sinus and atrioventricular nodal conduction disturbances and atrial arrhythmias with a relatively low toxic serum salicylate concentration. The cardiac rhythm returned to normal as the serum salicylate concentration decreased, and results of subsequent electrophysiologic testing and Holter monitoring were normal. A low serum albumin level may have resulted in altered salicylate binding in this patient, thereby increasing the availability of unbound (active) drug for toxic effects.

Effects of salicylate and two benzoates on contractions of the field-stimulated guinea-pig vas deferens and atrium

Na salicylate (3.75-30 mmol/l), Na benzoate (10.5-84 mmol/l) and Na metaiodobenzoate (0.375-0.8 mmol/l) concentration dependently reduced the contractile force of the field-stimulated guinea-pig vas deferens. Contractions elicited by either noradrenaline (0.5 mmol/l) or direct muscle stimulation were also reduced while the maximum twitch response induced by KCl (160 mmol/l) was unaffected. The twitch-inhibiting potency of salicylate and the benzoates correlated to their lipophilicity. The positive inotropic effect of field stimulation of the atrium during the refractory period was not altered by the two benzoates but was reduced by Na salicylate. It is concluded that salicylate and benzoate anions are incorporated into smooth muscle cell membranes of the vas and reduce excitability. No binding seems to occur in membranes of the atrial myocardium or the sympathetic neuron terminals and salicylate or benzoate compounds do not interfere with the liberation of neurotransmitters. In addition to its influence on the cell membrane Na salicylate can diminish contractions by uncoupling oxidative phosphorylation. The use of benzoates can help to distinguish between the two effects.

Comparative evaluation of the effects of lidocaine (lignocaine) hydrochloride and salicylate on nervous and Purkinje fibres

Lidocaine hydrochloride and lidocaine salicylate (lisacaine) have been tested on the frog node of Ranvier, on the sheep cardiac Purkinje fibres and rat phrenic nerve-diaphragm. On the node of Ranvier both drugs produced the same degree of reduction of peak INa and steady-state potassium current and the same degree of shift of the steady-state inactivation curve for INa to more negative potential. Lisacaine took less time to reach the steady-state effect. On the cardiac Purkinje fibres both drugs decreased the action potential duration without any detectable difference; but their effects on V max (i.e. the maximum rate of depolarization) were different that of lisacaine being faster. On the rat phrenic nerve-diaphragm both drugs produced the same percentage of reduction of the contractile response of diaphragm but, the action of lisacaine was faster. Therefore the lidocaine molecule with the salicylate anion while displaying the same anaesthetic effectiveness has a faster action than the hydrochloride.

Effects of cyproheptadine on electrophysiological properties of isolated cardiac muscle of dogs and rabbits

The effects of cyproheptadine were studied on cardiac Purkinje and ventricular muscle fibres of the dog and on cells of the sinoatrial (SA) node region of rabbit hearts, by means of electrophysiological techniques. Cyproheptadine (2-8 microM) decreased, in a dose-dependent manner, the plateau amplitude and the action potential duration to 50% repolarization of Purkinje and ventricular muscle cells. Higher concentrations also depressed the action potential amplitude, the overshoot and the maximum rate of rise of the upstroke. These effects were only partially reversed on washing. A four fold increase in Ca concentration of the standard Tyrode solution antagonized the effects of cyproheptadine on the action potential characteristics. The 'slow response' obtained in K-depolarized isoprenaline-treated fibres was blocked by cyproheptadine (4 microM). Cyproheptadine (10 microM) depolarized and suppressed the automaticity of spontaneously beating Purkinje fibres. The frequency of discharge of the SA node cells was slowed or abolished by cyproheptadine (2-4 microM). Atropine (2.6 microM) did not affect the negative chronotropic effect, whereas adrenaline (5 microM) reversed it. It is suggested that cyproheptadine depresses the slow inward current in all types of myocardial fibres studied. Higher concentrations might also affect the fast inward sodium current system.

Lack of direct antiarrhythmic electrophysiological effects of salicylate on isolated guinea-pig myocardium

Conventional microelectrode techniques were used to study the influence of Na-salicylate, Na-benzoate, Na-2,6-dihydroxybenzoate and 2,4-dinitrophenol (2,4-DNP) on the action potential (AP) of guinea-pig papillary muscles and atria. In papillary muscles, Na-salicylate (0.19-1.87 mmol/l) concentration-dependently shortened the AP duration and the functional refractory period. The AP amplitude decreased slightly with the largest concentration, while the resting potential and the maximum depolarisation velocity (Vmax) were not affected. A concentration-dependent negative inotropic effect was also seen. All drug effects were reversible after washout. In atria, 6.24 mmol/l Na-salicylate induced a slight shortening of the AP duration, a decrease of the AP amplitude and Vmax, but no decrease of the contractile force. The effects of the uncoupling agent, 2,4-DNP (10 mumol/l), were similar to those of the largest concentration of Na-salicylate in papillary muscles and in atria. Na-benzoate and Na-2,6-dihydroxybenzoate had no significant influence on AP duration, AP amplitude, resting potential, Vmax, refractory period or force of contraction of either papillary muscles or atria. These results suggest that Na-salicylate exerts its effects on isolated guinea-pig myocardium by uncoupling the oxidative phosphorylation, whereas two other possible mechanisms of action, namely an increase of membrane surface charge and an inhibition of prostaglandin synthesis, seem to be of minor importance.