Adaptational responses to prolonged beta-adrenoceptor blockade in adult rabbits

Cardiac adrenergic control and atrial fibrillation

Atrial fibrillation (AF) is the most common cardiac arrhythmia, and it causes substantial mortality. The autonomic nervous system, and particularly the adrenergic/cholinergic balance, has a profound influence on the occurrence of AF. Adrenergic stimulation from catecholamines can cause AF in patients. In human atrium, catecholamines can affect each of the electrophysiological mechanisms of AF initiation and/or maintenance. Catecholamines may produce membrane potential oscillations characteristic of afterdepolarisations, by increasing Ca(2+) current, [Ca(2+)](i) and consequent Na(+)-Ca(2+) exchange, and may also enhance automaticity. Catecholamines might affect reentry, by altering excitability or conduction, rather than action potential terminal repolarisation or refractory period. However, which arrhythmia mechanisms predominate is unclear, and likely depends on cardiac pathology and adrenergic tone. Heart failure (HF), a major cause of AF, causes adrenergic activation and adaptational changes, remodelling, of atrial electrophysiology, Ca(2+) homeostasis, and adrenergic responses. Chronic AF also remodels these, but differently to HF. Myocardial infarction and AF cause neural remodelling that also may promote AF. beta-Adrenoceptor antagonists (beta-blockers) are used in the treatment of AF, mainly to control the ventricular rate, by slowing atrioventricular conduction. beta-Blockers also reduce the incidence of AF, particularly in HF or after cardiac surgery, when adrenergic tone is high. Furthermore, the chronic treatment of patients with beta-blockers remodels the atria, with a potentially antiarrhythmic increase in the refractory period. Therefore, the suppression of AF by beta-blocker treatment may involve an attenuation of arrhythmic activity that is caused by increased [Ca(2+)](i), coupled with effects of adaptation to the treatment. An improved understanding of the involvement of the adrenergic system and its control in basic mechanisms of AF under differing cardiac pathologies might lead to better treatments.

Reduction by beta-adrenoceptor blockade of hypoxia-induced right heart hypertrophy in the rat

1. The study was undertaken to assess the role of beta-adrenoceptors in the induction of compensatory cardiac hypertrophy in an in vivo model. 2. In the rat, exposure to severe hypoxia (6% inspired oxygen for 8 h day) caused a 51% increase in right heart weight and a 75% increase in haematocrit. 3. The hypoxia-induced right ventricular hypertrophic response was reduced by 65% by oral treatment with a high dose of the non-selective beta-adrenoceptor antagonist, propranolol (80 mg kg-1 body weight); the drug treatment caused only a minor reduction (6%) in secondary polycythaemia. 4. With a less severe degree of hypoxia (7% inspired oxygen) there was only minimal secondary polycythaemia (+15%), and a lesser degree of compensatory right ventricular hypertrophy in untreated rats (+33%). 5. Treatment with the beta 1-adrenoceptor antagonist, atenolol, in a dose of 80 mg kg-1 body weight abolished right ventricular hypertrophy in response to 7% inspired oxygen, without affecting haematocrit and caused a small reduction in the ratio of heart weight to body weight in normoxic rats. 6. The results show that the effect of propranolol on hypoxic right ventricular hypertrophy is not secondary to any effect on secondary polycythaemia as has previously been suggested and that a marked reduction of compensatory cardiac hypertrophy can be obtained by a beta 1-selective adrenoceptor antagonist. Thus these findings support the view that noradrenaline released from cardiac sympathetic nerve terminals exerts a trophic effect on myocardial cells and demonstrates that in vivo, this trophic effect can be reduced by beta 1-adrenoceptor blockade.

Sotalol: from "just another beta blocker" to "the prototype of class III antidysrhythmic compound"

Sotalol is a beta-blocking drug devoid of membrane stabilizing properties, as well as intrinsic sympathomimetic actions, or cardioselectivity. In addition, sotalol prolongs atrial and ventricular repolarization (Class III antiarrhythmic activity). It appears to have less myocardial depressant effect than other beta-blocking agents. Given orally, bioavailability of the drug reaches 100%. Sotalol's plasma half-life is 15 hours (range 7-18) and is dependent only on renal function. In clinical practice, it has been found effective in the suppression of nearly all supraventricular and ventricular dysrhythmias except those related to prolonged ventricular repolarization. Most common adverse effects are dyspnea, bradycardia, and fatigue, which results in drug termination in 16% of the cases. Torsades de pointes usually associated with bradycardia and drug induced QTc prolongation has been reported in 1.9%-3.5% of the patients receiving sotalol. This complication may be reduced by limiting the dose (< 640 mg/day) especially in patients with impaired renal function. In addition hypokalemia must be avoided. To sum up, the combination of Class II and Class III effects may carry additional benefits. However, further studies are required to test such hypotheses.

Historical development of the concept of controlling cardiac arrhythmias by lengthening repolarization: particular reference to sotalol

Although numerous pathophysiologic states, such as hypocalcemia and hypothyroidism, lengthen repolarization and are associated with a reduced incidence of cardiac fibrillation, the concept of the pharmacologic control of rhythm disorders by prolonging the action potential duration is relatively new. There is now a great deal of interest in the relative merits and applicability of delaying conduction or prolonging refractoriness as ways to prevent arrhythmias. Prolonging the action potential duration in cardiac tissues lengthens the refractory period without affecting conduction, prolongs the cycle length of the tachycardia, and prevents it from deteriorating into fibrillation. Lengthening the action potential duration is also associated with a positive inotropic effect demonstrated most readily in isolated cardiac tissues, an important feature in antiarrhythmic agents intended for use in life-threatening tachyarrhythmias in patients with reduced ventricular function. This array of properties was first recognized in the beta blocker sotalol and formed the basis for a discrete class of antiarrhythmic mechanism--the so-called class III electrophysiologic effect. Such a series of actions was also recognized early in the case of amiodarone, which has a much more complex pharmacologic profile. Clinical studies with sotalol and amiodarone have done much to establish the clinical use of prolonging the action potential duration in controlling a broad spectrum of cardiac arrhythmias. Both amiodarone and sotalol prolong the action potential duration and attenuate adrenergic stimulation, but they do so by fundamentally different mechanisms. The electrophysiologic properties of sotalol represent the combined effects of beta blockade and lengthening the action potential duration.(ABSTRACT TRUNCATED AT 250 WORDS)

Sotalol. A review of its pharmacodynamic and pharmacokinetic properties, and therapeutic use

Sotalol is a beta-adrenoceptor blocking agent devoid of intrinsic sympathomimetic activity, membrane stabilising actions and cardioselectivity. It lengthens repolarisation and the effective refractory period in all cardiac tissues independently of its antiadrenergic properties. Combining Class II and Class III antiarrhythmic properties, sotalol can be given either intravenously or orally and its pharmacokinetic properties permit long dosing (once or twice daily) intervals. Controlled and uncontrolled studies have established the efficacy of sotalol in mild-to-moderate essential hypertension and in angina of effort. Sotalol reduces anginal frequency and glyceryl trinitrate (nitroglycerin) consumption and increases exercise capacity during treadmill stress tests. In addition, although there is evidence that the drug reduces reinfarction rate in survivors of acute infarction, the data for reduction in sudden death rates in these patients are not as compelling as for other beta-blockers. However, comparative and additional long term studies are required before an accurate assessment of the use of sotalol in these disorders can be made. When used in the treatment of mild-to-moderate hypertension sotalol is more effective than placebo and comparable to other beta-blockers in reducing elevated blood pressures. In addition, a synergistic antihypertensive response is achieved when sotalol is combined with hydrochlorothiazide. Still, additional well-controlled comparative studies are required before the value of sotalol relative to other drug treatment regimens in the management of hypertension can be made. In preliminary studies sotalol appeared effective in most forms of supraventricular tachyarrhythmias with its effects being similar to those of other beta-blockers. However, preliminary data indicate that sotalol is likely to be more effective than than conventional beta-blockers in converting atrial flutter and fibrillation to sinus rhythm and maintaining stability post-conversion. Sotalol also appears to be a promising agent in the control of ventricular arrhythmias. In suppressing premature ventricular contractions it is at least as effective as procainamide. In ventricular tachycardia and fibrillation, intravenous sotalol (1.5 mg/kg), prevents reinduction by programmed electrical stimulation in 40 to 50% of cases if double stimuli are used. Both prevention of reinducible arrhythmia and the suppression of spontaneous arrhythmias on Holter recordings are predictive of a long term favourable clinical outcome. In patients with reduced ejection fractions, sotalol depresses ventricular function less than conventional beta-blockers.(ABSTRACT TRUNCATED AT 400 WORDS)

Electrophysiologic effects of beta blockers in ventricular arrhythmias

Beta-adrenergic receptor blocking agents are effective antiarrhythmic drugs in patients with ventricular arrhythmias. However, these agents exert little or no measurable electrophysiologic effect on normal Purkinje and ventricular muscle fibers when administered acutely. They prevent catecholamine-induced increases in Purkinje fiber automaticity and may interfere with catecholamine-dependent slow responses. beta-adrenergic blocking drugs also prevent the decrease in ventricular fibrillation threshold induced by catecholamines. In the acutely ischemic ventricle, some beta blockers selectively depress conduction within the ischemic zone. The long-term administration of some beta blockers has, in contrast to their short-term effects, been shown to prolong action potential duration and effective refractory period in the ventricle. Which of these observed electrophysiologic effects, either alone or in combination, contributes to the ventricular antiarrhythmic effects of beta-blocking drugs in man is at present unknown.

Interaction of calcium antagonists with beta-adrenoceptor blocking agents

Responsiveness to verapamil, the best studied calcium antagonist, was examined in cardiac preparations of rabbits pretreated with beta-adrenoceptor blockers (propranolol 2 mg/kg or oxprenolol 4 mg/kg s.c.) twice daily for either one or six weeks. Using this dose-regimen, the degree of cardiac beta-adrenoceptor blockade in conscious rabbits was substantial and similar for propranolol and oxprenolol. When administered for one week, neither propranolol nor oxprenolol affected to any marked extent the electrical and mechanical response to verapamil, diltiazem or fendiline in tissues isolated from various parts of the heart. In contrast, pretreatment with propranolol for six weeks resulted in a significant aggravation of the negative inotropic effect of verapamil in both atrial and ventricular muscle, and the verapamil-induced delay in atrio-ventricular and intra-ventricular conduction also became more pronounced. The same long-term administration of oxprenolol, one of the beta-blockers with "intrinsic" sympathomimetic activity, did not alter the atrial or ventricular contractile response to verapamil and did not significantly increase the lengthening of atrio-ventricular conduction time occurring in the presence of verapamil. It is concluded that from the point of view of adverse direct cardiac interactions with verapamil prolonged administration of oxprenolol appears to be less dangerous than chronic treatment with propranolol. It is also assumed that in those cases in which acute administration of verapamil may be necessary, concomitant chronic blockade of cardiac beta-adrenoceptors is less dangerous if drugs known to possess not only beta-adrenoceptor blocking properties, but also some "intrinsic" sympathomimetic activity are applied.

Effects of selective alpha 1-, alpha 2-, beta 1-and beta 2-adrenoceptor stimulation on potentials and contractions in the rabbit heart

Selective adrenoceptor agonists and antagonists have been used to analyse the effects of stimulation of individual types of adrenoceptor in various parts of the rabbit heart. The selective alpha 1- and alpha 2-adrenoceptor agonists used were St 587 and BHT 933 respectively, and the antagonists were prazosin (alpha 1) and WY 25309 (alpha 2). The selective beta 1- and beta 2-adrenoceptor antagonists were atenolol and ICI 118551, respectively. Pirbuterol was a highly selective beta 2-adrenoceptor agonist. The non-selective agonists noradrenaline, adrenaline and isoprenaline were also employed with various combinations of antagonists. Phenylephrine was found to stimulate beta- as well as alpha-adrenoceptors. Rimiterol was a beta-adrenoceptor agonist, partially selective for beta 2-adrenoceptors. In the sinus node beta 1-, but not beta 2-adrenoceptor stimulation increased the fast phase of depolarization (Vmax). Both beta 1- and beta 2-adrenoceptor stimulation increased the slope of slow diastolic depolarization, accelerated repolarization and increased maximum diastolic potential. After blockade of both beta 1- and beta 2-adrenoceptors alpha 1-adrenoceptor stimulation caused bradycardia, due exclusively to delayed repolarization. alpha 2-adrenoceptor stimulation had no effect. In Purkinje cells and papillary muscle both beta 1- and beta 2-adrenoceptor stimulation accelerated repolarization. Stimulation of alpha 2-adrenoceptors had no effect. Beta 1-, not beta 2-adrenoceptor stimulation augmented peak contractions 3-5-fold, and greatly increased rate of development of tension. After beta-blockade alpha 1-adrenoceptor stimulation moderately increased peak contractions (up to 47%), but increased time-to-peak and duration of contractions. These patterns of adrenoceptor-mediated effects were unchanged in animals pre-treated with sufficient 6-hydroxydopamine to eliminate responses to sympathetic nerve stimulation. The results would be consistent with beta 1-, and beta 2-adrenoceptor stimulation increasing inward calcium current, and with stimulation of alpha 1-adrenoceptors delaying its inactivation, rather than increasing its magnitude.

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.

The effects of nadolol on various cardiac tissues in normoxia, and on atrial muscle in simulated ischaemia

Nadolol, a non-selective beta-blocker with a long duration of action, reportedly is devoid of membrane stabilizing action. Since such action is augmented by ischaemic conditions, it was of interest to investigate whether, in simulated ischaemia, a direct membrane effect of nadolol might be revealed. We have confirmed that in normoxia nadolol had no effects on intracellulary recorded potentials in isolated rabbit atrial or ventricular muscle at a concentration of 4.84 microM, but significantly reduced action potential amplitude and maximum rate of depolarisation at 14.5 microM. In Purkinje cells a small reduction of action potential amplitude was produced by 1.61 microM nadolol, but the effect was not increased by nadolol 4.84 microM. There were no drug-induced changes in action potential duration (APD), spontaneous frequency, conduction velocity, contractions, electrical threshold, effective refractory period or the maximum frequency at which a stimulus could be followed. Nadolol 14.5 microM did not reduce the positive inotropic effect of increasing extracellular calcium concentrations. Nadolol was 25 times less potent than procaine as a local anaesthetic on desheathed frog nerve. In a solution simulating ischaemia (8 mM KCl, 10 mM NaHCO3, gassed with 20% O2, pH 7.0) nadolol had a significant class 1 action on atrial muscle, however, even at 4.84 microM and reduced the shortening of APD caused by the solution. It is concluded that nadolol has no class 1, 3 or 4 antiarrhythmic activity in normoxia, but could have an additional protective effect in ischaemic myocardium against the arrhythmogenic factor of shortened APD.

Hypoxic cardiac hypertrophy is not inhibited by cardioselective or non-selective beta-adrenoceptor antagonists

1. Young rabbits, eight at a time, were exposed to more than 170 hr of hypoxia (O2 at 70-80 torr) at atmospheric pressure during 9-11 days. 2. The animals were injected twice daily with doses of beta-blockers up to the highest used in human therapy, or with an equivalent volume of saline. These controls were litter mates of the treated rabbits. 3. In comparison with untreated normoxic rabbits of similar age, the hypoxia induced highly significant mean increases in right ventricular dry weight (+57%, controls; +55% treated). 4. In each group, the same degree of hypertrophy was induced in the treated and control animals, irrespective of whether the drug used was a non-selective beta-blocker (propranolol) or a cardioselective drug (atenolol). 5. The right atria were also hypertrophied, though not as much as the right ventricles. 6. In all the hypertrophied tissues the water content was increased. 7. No significant change was observed in the dry weight or water content of the left ventricles of treated or control animals.

Adaptation to prolonged beta-blockade of rabbit atrial, purkinje, and ventricular potentials, and of papillary muscle contraction. Time-course of development of and recovery from adaptation

Groups of littermate rabbits were treated for various periods up to 6 weeks with twice daily subcutaneous injections of saline, D-propranolol, DL-propranolol, or metoprolol, the latter two at doses equivalent to those used in clinical therapy. Investigations were made at a sufficient time (20-24 hours after the most recent dose) to ensure that the drugs would have been eliminated from the body, so that any observed changes would represent an adaptation to treatment, not effects due to the presence of the drugs. The ECG was recorded in vivo at regular intervals during treatment. After several days, Q-Tc was prolonged by the beta-blockers, reaching a peak effect at about 3 weeks from the start of treatment, and returned to control values at 3 weeks after cessation of treatment. Action potential duration, measured in vitro by intracellular recording, was also prolonged uniformly in atria and ventricles over a similar time-course, unrelated to cardiac frequency, but shortened in distal Purkinje cells. Peak tension was not altered in propranolol-adapted papillary muscles, but the relationship of rate of rise of tension was steeper. It is concluded that these effects represent a myocardial adaptation to prolonged beta-blockade.

The effect of prolonged propranolol administration on myocardial transmural capillary density in young rabbits

1. As a sequel to previous evidence that prolonged beta-adrenoceptor blockade in young rabbits led to a relative increase in the capillarity of ventricular muscle, an attempt has been made to localize and quantify the capillary density from epicardium to endocardium in 12-week old rabbits treated for 6 weeks with propranolol (3 mg kg-1) S.C. twice daily and in litter-mates treated with saline (controls). 2. Full-thickness 0.5 micrometers sections of the left ventricular wall were point-counted in 200 micrometers steps. In the controls the capillary 'volume density' decreased progressively below the epicardial surface from a value of 9% to reach a minimum of 7.1% at a depth of 2.5 mm. Capillary density then increased again towards the endocardium, reaching a maximum of 11.5% in the trabeculae carnae (depth 3.5 mm). 3. In the treated animals the capillary density was increased in the mid wall at 2.3 and 2.5 mm, but significantly decreased in the trabeculae at 3.7 mm (P less than 0.05). 4. A more detailed analysis was undertaken at depths of 0.7 (Epi), 2.3 (Endo) and 3.5 mm (Trab), comprising estimates of capillary number, capillary diameter and intercapillary distance, via the stereological parameters of mean intercept length and mean free distance. 5. In the controls the order was Epi greater than Trab greater than Endo for number, Trab greater than Endo greater than Epi for diameter and Endo greater than Epi greater than Trab for intercapillary distance. 6. In the treated rabbits the mean intercapillary distance was significantly reduced in the epicardial (by 17.5%, P less than 0.05) and endocardial zones (by 32%, P less than 0.005), but was not significantly altered (+3.7%, P greater than 0.05) in the trabeculae. 7. The non-uniform distribution of capillaries in the controls illustrates that detection of changes in transmural capillary density requires that the depth of samples be measured from the epicardial surface and not, owing to the presence of trabeculae carnae, from the endocardium. 8. If similar treatment effects were to occur in man, they would suggest that prolonged beta-blockade could induce an increase in capillary density which would render the ventricular mid wall less susceptible to ischaemia.