The effect of altered thyroid state on atrial intracellular potentials

Recent trends in the management of life-threatening ventricular arrhythmias

An edited summary of an Interdepartmental Conference arranged by the Department of Medicine, UCLA School of Medicine, Los Angeles. William M. Pardridge, MD, Associate Professor of Medicine, is Director of Conferences. This study was supported in part by grants from the Public Health Service; the National Institutes of Health (HL-23970, 1978-1981); the Medical Research Service of the Veterans Administration, and the American Heart Association, the Greater Los Angeles Affiliate.

Arrhythmias in patients with drug toxicity, electrolyte, and endocrine disturbances

The common rhythm disturbances related to electrolyte imbalance are due predominantly to abnormalities of potassium. An understanding of the mechanism underlying these abnormalities is facilitated by a brief review of normal electrical activity during impulse propagation in cardiac tissue. Also discussed are the actions of all cardioactive and antiarrhythmic drugs on membrane permeability to ions. Lastly, the nonspecific arrhythmias associated with endocrine disturbances are outlined.

Does amiodarone affect heart rate by inhibiting the intracellular generation of triiodothyronine from thyroxine?

The hypothesis that the antiarrhythmic drug amiodarone slows down the heart rate by its inhibitory action on the intracellular conversion of thyroxine (T4) to 3,5,3' triiodothyronine (T3) was investigated. For this purpose we compared the effect of amiodarone with that of another potent inhibitor of the T4----T3 conversion, i.e. the radiographic contrast medium iopanoic acid, on the heart rate of unanaesthetized guinea-pigs. Both amiodarone and, to an even greater extent, iopanoic acid induced an increase in serum 3.5',3' triiodothyronine (reverse T3), indicating effective inhibition of T4----T3 conversion. Both amiodarone and iopanoic acid were accumulated in the liver and in the heart (measured as iodine). While amiodarone induced bradycardia, iopanoic acid did not change the heart rate. Supraphysiological amounts of exogenous T3 reverted the amiodarone induced bradycardia to near normal values. A comparable effect was observed with isoprenaline. The intracellular inhibition of the T4----T3 conversion is not the ultimate mode of the action of the amiodarone effect on heart rate. It is thought that amiodarone interacts with T3 at its receptor or somewhere later along the pathway from the T3-receptor interaction to the final effect of T3 on heart rate.

A classification of antiarrhythmic actions reassessed after a decade of new drugs

The past decade has seen the introduction of many new class 1 drugs, restricting fast inward current. Confirmative evidence has been obtained that the antiarrthymic action of lidocaine and diphenylhydantoin is indeed due to their effect as class 1 agents depressing conduction. The original class 3 drug, amiodarone, is increasingly in use as an antiarrhythmic of first choice for WPW and for arrhythmias associated with hypertrophic myopathy, and as a reserve drug in resistant arrhythmias of other types. Other compounds delaying repolarization have proved to be clinically effective as antiarrhythmics. In addition to their class 2 antiarrhythymic action exhibited acutely, on long-term treatment beta blockers have a class 3 action, which might be, at least in part, responsible for the protection of postinfarction patients against sudden death. Recent research suggests that inhibition of slow inward current may lead, as a secondary consequence of lowered [Ca]i, to improved cell-to-cell conduction. Finally, all but one of the new antiarrhythmic drugs, none of which existed in 1972, have turned out to possess one or more of the four classes of action originally described. This can hardly be a coincidence. The single exception, alinidine, a selective bradycardic agent, may restrict anionic currents, which would constitute a fifth class of action, but this is far from proved.

Free thyroxine in myocardial infarction

High values for the free thyroxine fraction were found in the serum of 24 patients during the early phase of myocardial infarction. A strong correlation between the free thyroxine fraction and free fatty acids suggests that they compete for protein binding sites. The increase in free thyroxine may have undesirable effects on myocardial oxygen demand during acute myocardial infarction.

The clinical results of amiodarone in cardiac arrhythmias: optimal dosing

Amiodarone hydrochloride is a relatively new antiarrhythmic agent, the properties of which differ in a significant manner electrophysiologically, pharmacokinetically and structurally from those of conventional as well as other investigational antidysrhythmic compounds. It is also pharmacologically unique in so far as its fundamental action on cardiac muscle following chronic therapy differs markedly from that found during the intravenous administration; its I.V. action is dominated by the lengthening of intranodal (AV) conduction time and the effective refractory period of the AV node, the electrophysiologic basis for which is unclear but accounts for the slowing of the ventricular response in atrial flutter and fibrillation and the variable conversion rate of narrow QRS reentrant paroxysmal supraventricular tachycardia. Intravenous amiodarone is ineffective in most other arrhythmias; it does not lengthen repolarization, nor does it prolong the effective refractory period of atria, ventricle, His-Purkinje system or the accessory pathways of the heart in the WPW syndrome. In contrast, chronically administered amiodarone lengthens repolarization and the effective refractory period of all cardiac tissues as a function of dose and duration of therapy consistent with its wide spectrum of antiarrhythmic activity in the prophylactic control of supraventricular and ventricular tachyarrhythmias. The nature of the slow onset of action of the oral drug is not well-understood; it may be due to the slow formation of active metabolites or the gradual and selective inhibition of T3 action on the myocardium since the effects of amiodarone on cardiac repolarization are identical to those of hypothyroidism and are negated by the concomitant administration of thyroxine. Serum reverse T3 levels increase as a function of dose and duration of amiodarone therapy and tentative data indicate that serial measurements of rT3 levels may provide a reliable index for gauging efficacy and toxicity of amiodarone during chronic therapy. The role of serum drug and metabolite levels appears less reliable in this regard. The exceedingly long and variable elimination half-life of amiodarone necessitates individualized loading and maintenance dosage regimens, and the latency of onset of antiarrhythmic action during oral therapy is not shortened by intravenous bolus injections or sustained infusions. However, the judicious choice of oral dosage as discussed herein permits the development of an effective prophylactic regimen for most patients with supraventricular and ventricular tachyarrhythmias; when the lowest dosage regimen to control a particular arrhythmia is identified, limiting side

Amiodarone: historical development and pharmacologic profile

Although synthesized as a coronary dilator for use as an antianginal agent over 20 years ago, amiodarone hydrochloride has recently drawn much attention as a potent antiarrhythmic compound for the control of a variety of cardiac dysrhythmias. The rapidly expanding clinical and experimental data continue to emphasize the unusual electrophysiologic, pharmacologic, and especially pharmacokinetic properties of this benzofuran derivative. The compound is a potent coronary dilator and has minimal negative inotropic propensity of a direct nature while exhibiting a mild degree of noncompetitive sympathetic antagonism. Pharmacokinetically, it has a long elimination half-life with a correspondingly long and variable latency of onset of therapeutic effect. Electrophysiologically, the drug has the propensity to lengthen the action potential duration and hence the voltage-dependent effective refractory period in all cardiac tissues after long-term, rather than short-term, administration. It has little effect on depolarization, conduction velocity, or the slow response. The precise ionic mechanisms mediating its effects on repolarization are not known. Clinically, the electrophysiologic effects of the drug differ significantly when it is given by mouth over a longer period and when it is given intravenously, a difference that remains to be explained in terms of mechanism. These differences, however, account for the varying spectrum of the drug's action after single intravenous doses (when its antiarrhythmic effects are essentially explained by the drug's action on the atrioventricular node and possibly its antiadrenergic actions) in comparison to long-term oral administration, which predictably suppresses ectopic activity and lengthens the effective refractory period in all cardiac tissues. These features may account for the drug's remarkable efficacy in the control of supraventricular and ventricular tachyarrhythmias. The safe and rational therapeutic uses of amiodarone as an antiarrhythmic agent presuppose detailed understanding of its manifold pharmacodynamic and pharmacokinetic properties.

Amiodarone and thyroid function: clinical implications during antiarrhythmic therapy

Amiodarone, an iodinated benzofuran derivative, has electrophysiologic effects on cardiac muscle akin to those of hypothyroidism. It is possible that the drug exerts its salutary effect, at least in part, by selectively inhibiting the action of triiodothyronine (T3) on the myocardium. The drug produces complex changes in thyroid hormones, with significant elevations in thyroxine (T4) and reverse T3 (rT3), with minor decreases in T3, and with minor and transient increases in thyroid-stimulating hormone, but without effect on thyroid-binding globulin. These changes may interfere with the biochemical evaluation of thyroid function. Rarely, hypothyroidism or hyperthyroidism may develop during the course of amiodarone therapy, a complication caused by the iodine contained in the drug rather than by the direct pharmacologic actions of the compound. The incidence of altered thyroid function induced is likely to vary with populations susceptible to iodine-induced goiter. Under the action of amiodarone, serum rT3 levels increase as a function of dose and duration of therapy and therefore provide a basis for judging the magnitude of in vivo drug cumulation. It was found that therapeutic efficacy was usually predictable on the basis of the attainment of a defined range of serum values, established by a correlation of rT3 levels with therapeutic responses both during loading and maintenance phases as well as after withdrawal of treatment of steady-state drug effects. Serious adverse effects occurred nearly always in association with four- to fivefold increases of rT3 above baseline values, and disappeared when such levels fell as a result of dosage reduction or after temporary drug discontinuation. The data suggest that the determination of serum rT3 levels during amiodarone therapy provides a simple and reliable technique for monitoring the drug's antiarrhythmic efficacy and toxicity, thereby enhancing its clinical utility. The use of rT3 levels may permit the development of a safe but optimal therapeutic regimen for the control of a wide spectrum of refractory atrial and ventricular tachyarrhythmias. The use of this technique, however, presupposes the allowance that must be made for variations in the methods for the serum assay of rT3 and of the systemic conditions in which the rT3 levels fluctuate relative to severity of the illness.

The effects of altered thyroid state upon responses mediated by atrial muscarinic receptors in the rat

Untreated rats, and rats treated with methimazole (0.05% w/v in drinking water) or thyroxine (1 mg/kg, s.c., three times weekly) for 4-6 weeks to induce hypo- and hyperthyroidism respectively, were used to study the influence of thyroid hormone upon negative chronotropic and inotropic responses mediated by cardiac muscarinic receptors, and upon the affinity of these receptors for atropine. Negative chronotropic effects of methacholine were investigated by establishing partial concentration-response curves for isolated preparations of right atria. Methacholine was least potent in tissues from thyroxine-treated rats. Isolated left atria paced at 3 Hz, and spontaneously beating right atria, were used in studies of the negative inotropic effects of methacholine. This agonist was least potent in atria from the thyroxine-treated rats in which it also produced the smallest maximal responses. The negative inotropic effects of carbachol were examined on left atria paced at 3, 5 and 5.8 Hz to approximate the basal contraction rates of isolated right atria from methimazole-treated, untreated control and thyroxine-treated rats, respectively. At each of these frequencies, carbachol was most potent in atria from methimazole-treated rats, and least potent in atria from thyroxine-treated rats. Maximal responses were smallest in the latter group. pA2 values for atropine with methacholine as the agonist were obtained by the method of Arunlakshana & Schild (1959) for spontaneously beating right atria (negative chronotropic and inotropic effects) and left atria paced at 3 Hz (negative inotropic effects). Slopes of Schild plots did not differ from minus one in tissues from each of the experimental groups; pA2 values were similar, indicating that thyroid status is without effect upon the affinity of this antagonist for muscarinic receptors mediating both negative inotropic and chronotropic effects. 6 The results are discussed in the light of reports that hypothyroidism increases, and hyperthyroidism decreases the numbers of high affinity muscarinic receptor binding sites in the rat myocardium.

Pharmacokinetic significance of serum reverse T3 levels during amiodarone treatment: a potential method for monitoring chronic drug therapy

We studied the antiarrhythmic effects of amiodarone, 600-1400 mg/day, in 18 patients with refractory arrhythmias, and related to drug efficacy and side effects to serum levels of T4, reverse T3 (rT3) and the QTc interval. In the 11 patients with ventricular arrhythmias, premature complexes were reduced by 90-98%, and complex ectopy and runs of ventricular tachycardia were abolished; in the seven patients with paroxysmal atrial flutter, there were no recurrences on stable drug therapy. The QTc lengthened by 11.6% (p less than 0.01), T4 increased by 31.6-63.3% (p less than 0.001) and rT3 increased by 82.9-176.8% (p less than 0.001) as a function of dose and duration of amiodarone therapy. A close correlation was found between rT3 (normal up to 50 ng/dl) and drug efficacy and some of the drug side effects; arrhythmia suppression occurred at levels of 55-100 ng/dl, and some of the known side effects at levels of 100-110 ng/dl. When amiodarone was stopped in nine patients, the changes in QTc, T4 and rT3 regressed toward normal and arrhythmia recurred in eight 2-20 weeks (mean 7.4 weeks) and when rT3 levels fell below 55 ng/dl; arrhythmia resuppression was achieved 3-28 days (mean 11 days) after resumption of amiodarone therapy. The indirect therapeutic half-life of amiodarone in seven patients, computed from the semilogarithmic plots of plasma rT3 after cessation of amiodarone therapy, ranged from 25 to 55 days (mean 35 days). The data suggest that rT3 levels may be useful in monitoring the efficacy and certain side effects of amiodarone.

Effects on rabbit nodal, atrial, ventricular and Purkinje cell potentials of a new antiarrhythmic drug, cibenzoline, which protects against action potential shortening in hypoxia

1 The effects of cibenzoline (UP 339-01), a new anti-arrhythmic drug, have been investigated in various cardiac tissues. 2 UP 339-01 produced a bradycardia, due partly to prolongation of the intracellularly recorded sinus node action potential duration (APD) and partly to depression of the maximum rate of depolarization (MRD). The slope of the slow diastolic depolarization was not significantly reduced. 3 UP 339-01 was not a beta-adrenoceptor antagonist. 4 UP 339-01 was negatively inotropic, and shifted the relation between [Ca2+]o and force of contractions to the right, and increased A-H conduction time. It was concluded that UP 339-01 restricted slow inward current. 5 In all cardiac tissues depolarized by fast inward current, UP 339-01 caused a reduction in MRD and conduction velocity. The reduction was similar in atrial muscle, His and terminal Purkinje fibres, but in papillary muscle the effect was about half as great. On desheathed frog nerve UP 339-01 had a local anaesthetic potency slightly greater than that of procaine. 6 APD was significantly prolonged in a dose-related manner in ventricular muscle but to a lesser extent in the bundle of His and atrial tissue. In terminal Purkinje fibres APD50 and APD90 were unaltered, but the transient outward current ("notch') was abolished, resulting in a lengthening of APD20. 7 The effective and functional refractory periods of the A-V node and right bundle branch were both lengthened by UP 339-01 in a dose-related manner, and the difference between them was greatly increased. 8 UP 339-01 (2.63 microM) completely prevented the shortening of APD90 induced by hypoxia, and the shortening of APD50 and APD20 was much attenuated. There was no protection against hypoxic depression of contractions. 9 It was concluded that UP 339-01 is a highly active class 1 anti-arrhythmic agent with additional class 3 and 4 properties.

Atrial fibrillation and isolated suppression of the pituitary-thyroid axis: response to specific antithyroid therapy

Four patients are described with persistent atrial fibrillation associated with normal plasma total thyroxine (T4) and triiodothyronine (T3) but an absent plasma thyrotrophin (TSH) response to intravenous thyrotrophin releasing hormone (TRH). Initial cardioversion failed to establish sinus rhythm in three of the four patients. Following specific antithyroid therapy to lower thyroid hormone levels sufficient to allow a normal TSH response to TRH sinus rhythm was established in all four patients, one spontaneously and three after cardioversion. Stable sinus rhythm has persisted in three patients over a 2-yr follow-up period. In the presence of atrial fibrillation, an absent plasma TSH response to TRH should be considered sufficient grounds for antithyroid therapy even if plasma total T4 and T3 are within the expected normal range.

Efficacy of intravenous procainamide infusion in converting atrial fibrillation to sinus rhythm. Relation to left atrial size

The efficacy of intravenous procainamide, infused at 30 mg/min to a maximum dose of 20 mg/kg, in converting atrial fibrillation was evaluated under electrocardiographic and blood pressure control in 21 patients. Nine patients had atherosclerotic heart disease, seven had valvular lesions, five had hypertension, and six had no apparent heart disease. Nine patients (converters), who reverted to sinus rhythm at a mean dose of procainamide of 13.3 +/- 3.6 mg/kg (mean plasma concentration, 7.4 +/- 3.9 micrograms/ml) had normal echocardiographic left atrial diameters. All but one of the remainder, the 12 non-converters, who received a mean drug dose of 13.1 +/- 3.5 mg/kg (mean plasma concentration 13.9 +/- 7.6 micrograms/ml), had atrial diameters exceeding 4.0 cm. QRS and QTc intervals were not altered significantly in converters, but were prolonged significantly in non-converters. No serious side effects from the infusion were encountered. Intravenous procainamide infusion appears to be a safe and rapidly effective method of converting recent-onset atrial fibrillation to sinus rhythm in patients with normal left atrial dimensions.

Adaptation of hypertensives to treatment with cardioselective and non-selective beta-blockers. Absence of correlation between bradycardia and blood pressure control, and reduction in slope of the QT/RR relation

Thirty mild hypertensives were treated for more than two months with either cardioselective (atenolol or metoprolol) or non-selective (propranolol or pindolol) beta-blockers; the patients were assigned to the drugs in a double-blind manner. A procedure was designed to distinguish between the effects of the drugs themselves while treatment continued, and the development of adaptive changes which would persist when the drugs had been eliminated from the body. Though individual responses to treatment varied in both groups, the mean effect of the cardioselective and non-selective drugs in the control of hypertension was similar. There was no evidence of the development of supersensitivity or "rebound". On the contrary, an adaptive bradycardia (that is a fall of not less than 10% in heart rate persisting 52 hours after stopping treatment) was observed at rest in 17/30 patients, and peak heart rates and blood pressures during exercise were lower in both groups than before treatment. Cardioselective drugs induced a significantly greater bradycardia at rest than non-selective, but on exercise increases in heart rate were reduced more by the non-selective drugs, so that the same peak heart rates were reached on exercise in both groups. Adaptation also affected QT. The results suggest that two factors govern the shortening of QT by increases in heart rate, a "metabolic" effect, determined by sympathetic drive, and a "biophysical" effect determined by heart rate. The adrenergic effect is attenuated by acute beta-blockade, or by adaptation to prolonged blockade, leaving a shallow, rate-determined, slope to the QT/RR regression.

Thyroid hormone-dependent interconversion of myocardial alpha- and beta-adrenoceptors in the rat

1 The effects of thyroid state on the properties of adrenoceptors mediating inotropic and chronotropic responses of the rat heart were assessed on the basis of the relative potencies of alpha- and beta-adrenoceptor agonists, the effects of alpha- and beta-adrenoceptor antagonists and the tissue uptake of [3H]-phenoxybenzamine ([3H]-PB). 2 In isolated, electrically driven left atria the ratio of the inotropic potencies of isoprenaline and phenylephrine and the inhibitory potency of propranolol (40nM-4 muM) were significantly reduced after thyroidectomy and were moderately increased after thyroxine treatment of control rats. 3 Block of inotropic responses to noradrenaline and to phenylephrine by PB (7.3 nM-7.3 muM) and the tissue uptake of [3H]-PB were significantly greater in preparations from thyroidectomized than in those from control or from thyroxine treated rats. alpha-Adrenoceptor inhibition by phentolamine (0.26-2.6 muM) also increased after thyroidectomy, and phentolamine effectively protected alpha-adrenoceptors from block by and binding of [3H]-PB. 4 The beta1-receptor antagonist H 93/26 (0.1 muM) significantly potentiated alpha-adrenoceptor blockade by PB in hypothyroid but not in control preparations. 5 In spontaneously beating right atria the chronotropic potency of agonists and the effects of antagonists were altered in the same way as were inotropic responses and the slope of the agonist concentration-response curves were significantly reduced after thyroidectomy. Effects of agonists and antagonists were not significantly influenced by thyroxine treatment. 6 Changes in the effects and tissue uptake of sympathomimetic drugs observed after thyroidectomy were reversed to or beyond control levels by thyroid hormone treatment of thyroidectomized animals. 7 The results presented are interpreted as indicating a thyroid hormone-dependent interconversion of myocardial alpha- and beta-adrenoceptors. It is suggested that this interconversion is similar to that observed earlier in frog hearts at different temperatures, and that both effects may reflect an allosteric transition between two forms of a single basic structure.

Monophasic action potentials of right atrium and electrophysiological properties of AV conducting system in patients with hypothyroidism

In 12 patients with manifest hypothyroidism right atrial monophasic action potentials showed a significant prolongation in comparison with data from normal or euthyroid patients. Atrial effective refractory periods were also significantly prolonged. After thyroid treatment the monophasic action potential duration and the effective refractory period of the right atrium were within normal ranges. In 6 hypothyroid patients studies of AV conduction with the aid of His bundle electrography and atrial pacing showed a supraHisian conduction delay which was manifest in one case and latent in another two. InfraHisian conduction delay was encountered in 2 cases.

Effects of amiodarone and L8040, novel antianginal and antiarrhythmic drugs, on cardiac and coronary haemodynamics and on cardiac intracellular potentials

1. The effects on the coronary and systemic haemodynamics of intravenous and intracoronary injections of two benzfuran derivatives, amiodarone and its brominated analogue (L8040), were studied in open-chest anaesthetized dogs. The effects of L8040 on cardiac intracellular potentials after 6 weeks of 20 mg/kg intraperitoneal injections in rabbits were also investigated. 2. Both compounds produced dose-related and quantitatively similar decreases in coronary vascular resistance following their intracoronary administration; threshold effects occurred with about 0-25 mg of each drug and maximal effects with 4 mg. Larger intracoronary doses produced measurable systemic effects. 3. Intravenous injections of amiodarone and L8040 (2-5-10 mg/kg) produced dose-related decreases in heart rate and aortic pressure with a fall in total peripheral resistance. The left ventricular output was either unaffected or increased with a consistent augmentation in stroke volume. 4. The bradycardia produced by both drugs was associated with prolongation of the P-R interval of the electrocardiogram with no significant effect on the QRS duration or the Q-T interval. 5. Each drug produced a decrease in the total peripheral vascular resistance with no change in left ventricular end diastolic pressure except after 10 mg/kg doses which led to an increase in this parameter. 6. Cardiac contractile force and peak LV dp/dt were reduced by both drugs in a dose-related manner. 7. Chronic intraperitoneal administration of L8040 in rabbits caused a prolongation of the duration of the atrial and ventricular intracellular potential without an effect on the maximal rate of depolarization. 8. The effect of amiodarone or L8040 on the coronary circulation and arterial pressure may be attributed to their vasodilator properties but their depressant actions on cardiac contractile force and peak LV dp/dt with an increase in left ventricular end diastolic pressure at high doses, also suggest intrinsic negative inotropic propensity for both compounds. 9. It is concluded that the overall effects on coronary and systemic haemodynamics of amiodarone and its brominated analogue are likely to permit a favourable influence on the balance of oxygen supply and demand in myocardial ischaemia; in addition, their actions on sino-atrial and atrio-ventricular conduction as well as those on cardiac repolarization suggest potential antiarrhythmic properties which merit investigation.

Systolic time intervals in thyroid disease

The systolic time intervals; total electro-mechanical systole (QS2), left ventricular ejection time (LVET), pre-ejection period (PEP), and ratio of PEP/LVET were measured from external readings in twenty hyperthyroid and fifteen hypothyroid patients. The hyperthyroid subjects showed shortening of the PEP and reduction in the PEP/LVET. The hypothyroid group showed lengthening of the PEP, shortening of the LVET and increased PEP/LVET. These changes returned towards normal in ten patients rendered euthyroid with appropriate therapy. The value of these measurements to predict the thyroid status was examined in a mixed group of seventy-four patients. There was close agreement between diagnosis predicted from systolic time intervals and that made independently from clinical and biochemical assessment, suggesting that the systolic time intervals may have been a useful place in the clinical assessment of thyroid patients.

Effects of amiodarone on thyroid function in patients with ischaemic heart disease

Thyroid function was evaluated clinically and biochemically in 12 patients with ischaemic heart disease receiving 200 mg oral amiodarone three times daily for periods up to 6 weeks. During drug administration, no patient developed clinical or laboratory evidence of hypothyroidism, but serum levels of T3 tended to fall and those of T4 increased but not to levels outside the normal range. Amiodarone produced a significant reduction in heart rate with prolongation of the QTc interval of the electrocardiogram without altering either the PR interval or the QRS duration. These effects of the drug were still present 4 weeks after cessation of treatment. In spite of the high iodine content, amiodarone does not, therefore, depress thyroid function to any important degree during chronic administration and its antianginal action does not appear to be caused by the production of generalized hypothyroidism.

Class 3 antiarrhythmic action in man. Observations from monophasic action potential recordings and amiodarone treatment

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The effects on cardiac muscle and nerve of a fluorinated decahydroquinoline derivative, L7810, rapidly absorbed after oral administration

1. L7810 (4-carbamoyloxy-1-(4-(4-fluorophenyl)-4-oxobutyl) decahydroquinoline, has a local anaesthetic action on frog nerve 1.75 times that of procaine.2. L7810 protected anaesthetized guinea-pigs against ouabain-induced ventricular fibrillation and increased the lethal dose of ouabain.3. L7810 reduced the rate of rise of intracellularly recorded action potentials in rabbit isolated atria; the resting potential was not affected, but the duration of the action potential was prolonged.4. Unlike most drugs with local anaesthetic properties L7810 did not depress contractions in isolated atria but increased them.5. L7810 reduced the spontaneous frequency, maximum follow frequency and conduction velocity of rabbit isolated atria.6. L7810 had no blocking action on the chronotropic or positive inotropic actions of isoprenaline on isolated atrial muscle.7. In anaesthetized dogs L7810 caused a small dose-related bradycardia, and a large dose-related decrease in peripheral vascular resistance. There was no blockade of the effects of isoprenaline on heart rate or peripheral blood flow.

The effect of prolonged treatment with oxyfedrine on intracellular potentials and on other features of cardiac function in rabbits and guinea-pigs

1. Previous work in acute experiments has shown that the main pharmacological action of oxyfedrine is stimulation of beta-adrenoceptors, yet there have been clinical reports that the drug is beneficial in the treatment of angina pectoris.2. In the present experiments rabbits and guinea-pigs were treated for several weeks with daily i.p. injections of oxyfedrine.3. A daily dosage of 15 mg/kg oxyfedrine had no effect on growth rate for 4 weeks, but thereafter the growth rate of treated animals fell below that of controls.4. The heart weights of the treated animals, expressed as a percentage of body weight, were significantly lower than those of controls.5. Measurement of intracellular potentials in hearts taken from treated rabbits showed that the main effects were a reduction in the maximum rate of depolarization and a prolongation of the plateau of the action potential.6. Guinea-pigs treated for 6 weeks with 15 mg/kg oxyfedrine daily i.p. were protected to some extent from the toxic effect of ouabain infused intravenously.

Effects on cardiac muscle of the -adrenoceptor blocking drugs INPEA and LB46 in relation to their local anaesthetic action on nerve

1. As a local anaesthetic on frog nerve, LB46 had 0.4 times the activity of procaine and 2 times that of INPEA.2. In reducing the maximum rate of depolarization (MRD) of intracellularly recorded cardiac action potentials, LB46 was 20 times more potent than INPEA.3. As an antagonist of the chronotropic effect on atrial muscle and of the inotropic effects on ventricular muscle of isoprenaline the pA(2) values for LB46 were 9.05+/-0.15 and 9.30+/-0.06 respectively, and for INPEA 6.00+/-0.16 and 6.10+/-0.12.4. LB46 was 8 times more active than racemic propranolol in blocking the effects of isoprenaline on isolated cardiac muscle, and since its direct action on the cardiac membrane was only 0.1 times that of propranolol, there was a net gain of 80 times in the specificity of LB46 for class 2 (antisympathetic) over class 1 (depression of MRD) antidysrhythmic actions.5. INPEA was 2.5 times less active as a beta-adrenoceptor blocking drug on cardiac muscle than (+)-propranolol, and 1,300 times less active than LB46.6. INPEA, but not LB46, prolonged the duration of both atrial and ventricular intracellular action potentials.

Effect of altering potassium concentration on the action of lidocaine and diphenylhydantoin on rabbit atrial and ventricular muscle

All previously studied antiarrhythmic drugs which also have local anaesthetic properties reduce the maximum rate of depolarization (MRD) of cardiac muscle. Recent evidence suggested that lidocaine and diphenylhydantoin (DPH) might have a different mode of action, because they did not reduce MRD except at high concentration. The latter evidence, however, was obtained from tissues nourished by solutions containing 2.7-3 m M potassium. In the present experiments, the effects of lidocaine and DPH were studied in solutions with 5.6 and 3 mM KCl. In the former, both drugs reduced MRD at concentrations similar to those found in the blood of treated patients, but in low [KCl] higher concentrations were necessary. It was concluded that there was no reason to suppose that the mode of action of lidocaine and DPH on the cardiac membrane is fundamentally different from that of quinidine, procaine, procainamide and other compounds which interfere directly with depolarization.

Hypertrophic cardiomyopathy. An electrophysiological study

Recordings of the transmembrane action potential of the cardiac myofibril from a patient with hypertrophic obstructive cardiomyopathy showed that the repolarization time was appreciably prolonged and that the maximum rate of follow was grossly reduced. This electrical abnormality is compatible with the late and irregular activation of the ventricular muscle in the disease. Propranolol was found to produce a similar "quinidine-like" effect on the transmembrane actionpotential in both cardiomyopathic and control tissue.

The effect of amiodarone, a new anti-anginal drug, on cardiac muscle

1. Amiodarone (2-butyl, 3-(4-diethylaminoethoxy, 3,5-diiodo, benzoyl) benzofuran hydrochloride), an anti-anginal drug which causes coronary dilatation and depresses myocardial oxygen consumption, was found to protect anaesthetized guinea-pigs against ouabain-induced ventricular fibrillation.2. A 5% (73.4 mM) solution of amiodarone had no local anaesthetic action on guinea-pig skin.3. Amiodarone, 20 mg/kg (29.4 mumol/kg) given daily for 6 weeks intraperitoneally, had no effect on the resting potential or action potential height, and only a small effect on the maximum rate of depolarization, of isolated rabbit atrial or ventricular muscle fibres as shown by intracellular recording. It caused a considerable prolongation of the action potential in both tissues.4. Simultaneous administration of thyroxine (5 mug; 6.26 nmol), given daily for 3 weeks intraperitoneally, prevented the prolongation by amiodarone of the duration of the action potential.5. Treatment of rabbits with 20 mg/kg of amiodarone daily intraperitoneally for 6 weeks had no effect on the weight of the thyroid gland, but was associated with a reduction in body growth rate.6. Treatment of rabbits with 10 mg/kg (60.3 mumol/kg) of potassium iodide (equal in its iodine content to that of 20 mg/kg of amiodarone), given daily for 6 weeks intraperitoneally, had no effect on body growth rate or the duration of cardiac action potentials.7. It was concluded that amiodarone had effects on cardiac action potentials similar to those which occur after thyroidectomy.

A third class of anti-arrhythmic action. Effects on atrial and ventricular intracellular potentials, and other pharmacological actions on cardiac muscle, of MJ 1999 and AH 3474

1. Both MJ 1999 and AH 3474 protected guinea-pigs anaesthetized with urethane against ouabain-induced ventricular fibrillation.2. MJ 1999 had 1/90, and AH 3474 1/30, of the activity of procaine in reducing the height of the action potential of frog sciatic nerve.3. MJ 1999 and AH 3474 reduced the rate of rise of intracellularly recorded action potentials at concentrations in excess of 160 x 10(-6)M (50 mg/l.). It was concluded that direct depression of depolarization could have contributed little to the protection against ouabain-induced fibrillation.4. MJ 1999, but not AH 3474, greatly prolonged the duration of the action potential in acute experiments on isolated atrial and ventricular muscle, and prolonged the Q-Tc interval of the electrocardiogram in anaesthetized guinea-pigs. It is suggested that this effect contributes to anti-arrhythmic activity.5. At concentrations up to 80 x 10(-6)M AH 3474 had positive chronotropic and inotropic effects on isolated rabbit atrial muscle, but at higher concentrations these were superseded by negative effects. MJ 1999 was depressant at all concentrations studied, the threshold concentrations being 19 x 10(-6)M for chronotropic, and 162 x 10(-6)M for inotropic effects.