Long-term observation of atrial and ventricular rates in the unanesthetized dog with complete atrioventricular block

Interactions of the cardiac chronotropic effects of rilmenidine with the autonomic nervous system in conscious dogs: comparison with clonidine

1. The cardiac chronotropic effects of rilmenidine (10-100 micrograms kg-1) and clonidine (1-10 micrograms kg-1) were studied in conscious dogs with chronic atrioventricular block. 2. Rilmenidine and clonidine initially (< 3 min) decreased atrial rate, although the effect was not related to dose. More lastingly, ventricular rate was decreased in a dose-related manner (ratio, 1:21). Rilmenidine lowered mean blood pressure only at 100 micrograms kg-1, while clonidine had the same effect at doses of 5 micrograms kg-1 upward (ratio, 1:15). 3. When administered after atropine and pindolol, rilmenidine (50 micrograms kg-1) produced a decrease in atrial rate, with an identical intensity but longer duration than under basal conditions. When clonidine (2.5 micrograms kg-1) was given after atropine, no chronotropic atrial effect was observed. However, when clonidine (2.5 micrograms kg-1) was given after pindolol, it produced a decrease in atrial rate that was more marked, both in intensity and duration, than under basal conditions. After phenoxybenzamine, rilmenidine decreased atrial rate with a more marked and lasting effect than observed under basal conditions. Clonidine produced a bradycardic atrial effect identical to the basal effect. After yohimbine, rilmenidine and clonidine decreased atrial rate with an intensity similar to that under basal conditions, although the time course was totally different. 4. When given after atropine, rilmenidine (50 micrograms kg-1) and clonidine (2.5 micrograms kg-1) decreased ventricular rate as under basal conditions, whereas after pindolol and phenoxybenzamine, both drugs decreased ventricular rate less markedly than under basal conditions, both in intensity and duration. After yohimbine, rilmenidine and clonidine produced no chronotropic ventricular effect. 5. These results show that (a) the initial atrial bradycardia caused by rilmenidine results from both a decrease in sympathetic tone and an increase in cholinergic activity; while the effect of clonidine is caused mainly by the enhancement of cholinergic activity. For both drugs, alpha 2-adrenoceptors are involved at least in the initiation of the effect; (b) the very short duration of atrial bradycardia may result from reflex buffering in response to ventricular bradycardia. This buffering is less effective when heart rate was high; and (c) the ventricular bradycardia caused by both drugs is mainly the result of a decrease in sympathetic tone in response to the stimulation of alpha 2-adrenoceptors. The results also suggest that negative chronotropic postsynaptic alpha 2-adrenoceptors could be involved in the ventricular bradycardia.

Chronotropic cardiac effects of NPY in conscious dogs: interactions with the autonomic nervous system and putative NPY receptors

The chronotropic cardiac effects of neuropeptide Y (NPY) were studied in the conscious dog with chronic atrioventricular block. NPY (0.2-5 micrograms/kg i.v.) produced no effect on atrial cycle length (ACL), and increased ventricular cycle length (VCL) and mean arterial blood pressure (MBP). After atropine, NPY produced no effect on ACL and increased MBP. At 0.2 microgram/kg, it shortened VCL, whereas at 1 and 5 micrograms/kg, it lengthened this parameter. After pindolol, NPY produced no effect on ACL, shortened VCL and increased MBP. These results indicate that in the conscious dog, NPY (0.2-5 microgram/kg i.v.) does not exert any chronotropic effect on the sinoatrial node, most likely because of competition between opposite chronotropic effects and/or absence of specific NPY receptors in the sinoatrial node. They also suggest that the ventricular bradycardic effects produced by NPY result mainly from a reflex withdrawal of beta-adrenergic tone and that its ventricular tachycardic effects result from a direct action of NPY on specific receptors located in the His bundle.

Methods and limitations of an experimental model of long QT syndrome

An experimental model of the long QT syndrome has been developed in conscious dogs. This report discusses the methods used in its preparation and the strengths and weaknesses of the model. This new model is suitable for screening the bradycardia-dependent proarrhythmic effects of drugs and for studying the electrophysiology of "torsades de pointes." Permanent bradycardia (RR: 1558 +/- 83 ms) was obtained in 37 dogs by chemically-induced complete atrioventricular block. A 10% further increase of ventricular repolarization (QT: 306 +/- 7.0 ms to 331 +/- 5.5 ms) was obtained in 28 of these dogs by diuretic-induced hypokalemia. Diuretics, despite saline replacement, induced some degree of functional renal failure and extracellular volume losses. The QT interval increased although ventricular cycle length decreased slightly. These biological and electrophysiological parameters were reproducible except for a slow increase in plasma creatinine. Cardiac failure and sudden death rarely occurred. The most severe, but reversible, renal failure occurred in some dogs given the highest diuretic doses. Hypokalemia resulted in ventricular arrhythmias in only 6 dogs, 2 of them exhibiting runs of ventricular tachycardia and even "torsade de pointes" as their potassium levels fell below 2 mmol/L. The results of studies with several drugs using the model, with or without hypokalemia, or with bradycardia worsened by propranolol are analysed.

Evidence for postsynaptic alpha 1- and alpha 2-adrenoceptors mediating catecholamine-induced negative chronotropic ventricular responses in the conscious dog

1. Adrenaline (0.25-1 microgram/kg), noradrenaline (0.125-0.5 microgram/kg) and dopamine (25-100 micrograms/kg) given in the conscious dog with chronic atrio-ventricular block after beta-adrenoceptor blockade, increased ventricular cycle length (VCL) and mean blood pressure (MBP). 2. Atropine (muscarinic receptor blocker) reduced the catecholamine-induced effects on VCL without modifying their hypertensive effects. 3. Phenoxybenzamine or phentolamine (alpha-adrenoceptor blockers) only decreased the effects of adrenaline on VCL but suppressed those of noradrenaline and dopamine. They only reduced the effects of adrenaline and noradrenaline on MBP, but reversed that of dopamine. 4. Yohimbine (alpha-adrenoceptor blocker) suppressed the catecholamine-induced effects on VCL, and reduced strongly the hypertensive effects of adrenaline and noradrenaline and reversed that of dopamine. 5. Thus, these results show the existence of negative chronotropic postsynaptic alpha-adrenoceptors in the ventricles.

Chronotropic cardiac effects of histamine in the conscious dog with chronic atrioventricular block: interactions with the autonomic nervous system

Chronotropic effects of histamine and dimaprit were studied in the conscious dog with chronic atrioventricular block. Histamine at 0.2-5 micrograms/kg and dimaprit at equimolar doses (i.e. 0.25-6.25 micrograms/kg) increased atrial rate dose-relatedly. Blockade of muscarinic receptors reduced these effects and simultaneous blockade of muscarinic and beta-adrenergic receptors abolished them. Histamine and dimaprit moderately increased ventricular rate. Blockade of muscarinic receptors did not modify these effects, but blockade of beta-adrenoceptors with or without simultaneous blockade of muscarinic receptors suppressed them. After blockade of beta-adrenoceptors, histamine and more rarely dimaprit sometimes decreased atrial and ventricular rates. These effects were prevented by additional muscarinic blockade. Histamine and dimaprit lowered mean blood pressure to the same degree before and after each antagonist. The positive chronotropic effects of histamine and dimaprit, at these doses, are probably reflex responses to their hypotensive effects. The negative chronotropic effects of histamine after pindolol are due to muscarinic receptor activation. No evidence was found to implicate histamine-specific receptors in any of the chronotropic effects of histamine and dimaprit.

Studies on the stereoisomers of beta-adrenoceptor antagonists in conscious A-V blocked dogs

Atrial and ventricular chronotropic effects of the individual stereoisomers of propranolol, pindolol, metoprolol and penbutolol were studied in conscious dogs with chronic atrio-ventricular (A-V) block. Ventricular beta-adrenoceptor blocking activity was assessed for all drugs against isoprenaline under the same experimental conditions. At low doses, the stereoisomers of propranolol and penbutolol decreased atrial rate, whereas those of pindolol and metoprolol produced an increase. At higher doses, all drugs increased atrial rate. All drugs decreased ventricular rate dose-dependently except (+)-pindolol. Relative ventricular beta-blocking potencies of the (-)-isomers of propranolol, pindolol, metoprolol and penbutolol were respectively 38, 21, greater than 43 and 31 times higher than those of their corresponding (+)-isomers. In addition, beta-blocking potencies of (-)- and (+)-pindolol were respectively 60 and 120 times higher, those of (-)- and (+)-penbutolol 7 and 8 times higher and those of (-)- and (+)-metoprolol 4 and greater than 4 times weaker than those of (-)- and (+)-propranolol. At comparable levels of ventricular beta-adrenoceptor blockade, (-)-pindolol and (-)-metoprolol were more potent in producing ventricular bradycardia than their respective (+)-isomers, whereas (-)- and (+)-propranolol and (-)- and (+)-penbutolol were equiactive. In addition, regardless of which isomer was being studied, the order of ventricular bradycardiac potencies, at comparable levels of beta-adrenoceptor blockade, was metoprolol greater than propranolol greater than penbutolol greater than pindolol. In addition, regardless of which isomer was being studied, the order of ventricular bradycardiac potencies, at comparable levels of beta-adrenoceptor blockade, was metoprolol > propranolol > penbutolol >pindolol. 5 These results show that antagonism of beta-adrenoceptors in the ventricle is at least partly responsible for the ventricular bradycardiac effect produced by these drugs, but also that some other factor, apparently distinct from the membrane stabilizing activity, is involved, suggesting the existence of some other as yet unknown pharmacological property of the beta-adrenoceptor blocking drugs, especially evident in metoprolol. Finally, these results demonstrate that the intrinsic sympathomimetic activity exhibited by some of these drugs attenuate their bradycardiac effect.

Methods for producing experimental complete atrioventricular block in dogs

This paper reviews methods for producing experimental complete atrioventricular (A-V) block in dogs, with emphasis on those methods that enable chronic A-V block to be obtained. The dog with chronic complete A-V block, particularly when unanesthetized, is a useful experimental model, and the production of complete A-V block is an approach to the clinical treatment of certain supraventricular arrhythmias. Criteria for the choice of method are discussed, and the results obtained to date in the unanesthetized dog with chronic A-V block are briefly described to illustrate the usefulness of this experimental model.

Pharmacological evidence for differences in the beta-adrenoreceptor populations mediating atrial and ventricular chronotropic responses in the unanaesthetized dog

The beta-adrenoreceptor populations mediating atrial and ventricular chronotropic responses were studied comparatively in the unanaesthetized dog with chronic atrio-ventricular block. With this aim, we first compared the increases in atrial and ventricular rate induced by isoprenaline. We then compared the reductions of this isoprenaline-induced atrial and ventricular cardio-acceleration, observed after administration of cumulative doses of propranolol and alprenolol, both non-selective beta-adrenoreceptor blocking agents, and metoprolol, a blocking agent selective for beta 1-adrenoreceptors. Isoprenaline induced a much more intense stimulation of atrial chronotropic beta-adrenoreceptors than of ventricular chronotropic beta-adrenoreceptors, whether or not the cholinoreceptors were blocked. Similarly, all three beta-blocking agents induced a more intense blockade of atrial chronotropic beta-adrenoreceptors than of ventricular chronotropic beta-adrenoreceptors, whatever the dose of isoprenaline considered and whether or not the cholinoreceptors were blocked. These results cannot be readily explained by quantitative differences in the distribution of beta 1- and beta 2-adrenoreceptors in the heart, given the identical effects obtained with metoprolol, propranolol and alprenolol. Therefore, beta-adrenoreceptor populations are not identical throughout the nodal tissue, and in particular, differences occur between beta-adrenoreceptors in the sinus node mediating atrial chronotropic responses and those in the His bundle mediating ventricular chronotropic responses, which might be linked to differing sensitivities of these beta-adrenoreceptor populations.

Dopamine in the conscious dog with chronic heart-block. Mechanisms of chronotropic cardiac effects

The chronotropic effects of dopamine were studied in the conscious dog with chronic A-V block. Dopamine at 12.5-200 micrograms/kg and 12.5-50 micrograms/kg/min lowered atrial rate independently of dose. After blockade of muscarine receptors or alpha-adrenoceptors, it raised atrial rate. After blockade of dopamine receptors, dopamine still lowered atrial rate, and did so dose-relatedly after blockade of beta-adrenoceptors. It raised ventricular rate, and at high doses also induced ventricular rhythm disorders. Blockade of muscarine receptors enhanced the ventricular cardioaccelerator effect of dopamine (P less than 0.025) at 100 micrograms/kg, while blockade of alpha-adrenoceptors reduced it (P less than 0.05). Blockade of dopamine receptors did not modify this effect, but blockade of beta-adrenoceptors reversed it. Dopamine at 25-200 micrograms/kg raised mean blood pressure. This effect was enhanced by blockade of muscarine receptors, reversed by blockade of alpha-adrenoceptors, and was unaffected by blockade of beta-adrenoceptors or dopamine receptors. These results show that the atrial cardiomoderator effect of dopamine is a vagal reflex response to its hypertensive action, and that it is limited by its direct beta-adrenergic stimulating action. They also show that the ventricular cardioaccelerator effect of dopamine is attenuated by a reflex vagal depressor effect consequent to the induced hypertension. No evidence was found for the existence of positive chronotropic dopamine receptors in either atria or ventricles.

Chronotropic effects of pindolol. Relation between ventricular effects and control resting ventricular rate values in conscious dogs with chronic A-V block

The chronotropic effects of pindolol were studied in the conscious dog with chronic A-V block. Pindolol significantly increased atrial rate, probably through both its strong intrinsic sympathomimetic activity and reflex withdrawal of atrial vagal tone in response to its hypotensive effect. Pindolol did not alter ventricular rate overall. However, in individual dogs, pindolol caused ventricular chronotropic effects that were inversely related to resting ventricular rate: it increased ventricular rate when the resting ventricular rate was low and reduced it when it was high. In view of the fact that pindolol is a beta-adrenoceptor antagonist endowed with a strong intrinsic sympathomimetic activity, this finding contributes to a better understanding of how pindolol affects heart rate.