Negative Dromotropic Response Research Articles

Evidence for the cardiodepressive effects of the plasticizer di-2-ethylhexyl phthalate.

Di-2-ethylhexyl phthalate (DEHP) is commonly used in the manufacturing of plastic materials, including intravenous bags, blood storage bags, and medical-grade tubing. DEHP can leach from plastic medical products, which can result in inadvertent patient exposure. DEHP concentrations were measured in red blood cell units stored between 7 and 42 days (17-119 μg/ml). Using these concentrations as a guide, Langendorff-perfused rat heart preparations were acutely exposed to DEHP. Sinus activity remained stable with lower doses of DEHP (25-50 μg/ml), but sinus rate declined by 43% and sinus node recovery time (SNRT) prolonged by 56.5% following 30-min exposure to 100 μg/ml DEHP. DEHP exposure also exerted a negative dromotropic response, as indicated by a 69.4% longer PR interval, 108.5% longer Wenckebach cycle length (WBCL), and increased incidence of atrioventricular (AV) uncoupling (60-min exposure). Pretreatment with doxycycline partially rescued the effects of DEHP on sinus activity, but did not ameliorate the effects on AV conduction. DEHP exposure also prolonged the ventricular action potential and effective refractory period, but had no measurable effect on intracellular calcium transient duration. Follow-up studies using human-induced pluripotent stem cell-derived cardiomyocytes confirmed that DEHP slows electrical conduction in a time (15 min-3 h) and dose-dependent manner (10-100 μg/ml). Previous studies have suggested that phthalate toxicity is specifically attributed to metabolites of DEHP, including mono-2-ethylhexylphthalate. This study demonstrates that DEHP exposure also contributes to cardiac dysfunction in a dose- and time-dependent manner. Future work is warranted to investigate the impact of DEHP (and its metabolites) on human health, with special consideration for clinical procedures that employ plastic materials.

Age-dependent electrocardiographic changes in Pgc-1β deficient murine hearts.

SummaryIncreasing evidence implicates chronic energetic dysfunction in human cardiac arrhythmias. Mitochondrial impairment through Pgc‐1β knockout is known to produce a murine arrhythmic phenotype. However, the cumulative effect of this with advancing age and its electrocardiographic basis have not been previously studied. Young (12‐16 weeks) and aged (>52 weeks), wild type (WT) (n = 5 and 8) and Pgc‐1β−/− (n = 9 and 6), mice were anaesthetised and used for electrocardiographic (ECG) recordings. Time intervals separating successive ECG deflections were analysed for differences between groups before and after β1‐adrenergic (intraperitoneal dobutamine 3 mg/kg) challenge. Heart rates before dobutamine challenge were indistinguishable between groups. The Pgc‐1β−/− genotype however displayed compromised nodal function in response to adrenergic challenge. This manifested as an impaired heart rate response suggesting a functional defect at the level of the sino‐atrial node, and a negative dromotropic response suggesting an atrioventricular conduction defect. Incidences of the latter were most pronounced in the aged Pgc‐1β−/− mice. Moreover, Pgc‐1β−/− mice displayed electrocardiographic features consistent with the existence of a pro‐arrhythmic substrate. Firstly, ventricular activation was prolonged in these mice consistent with slowed action potential conduction and is reported here for the first time. Additionally, Pgc‐1β−/− mice had shorter repolarisation intervals. These were likely attributable to altered K+ conductance properties, ultimately resulting in a shortened QTc interval, which is also known to be associated with increased arrhythmic risk. ECG analysis thus yielded electrophysiological findings bearing on potential arrhythmogenicity in intact Pgc‐1β−/− systems in widespread cardiac regions.

Chronic Augmentation of the Parasympathetic Tone to the Atrioventricular Node: A Nonthoracotomy Neurostimulation Technique for Ventricular Rate Control During Atrial Fibrillation

The right inferior ganglionated plexus (RIGP) selectively innervates the atrioventricular node. Temporary electrical stimulation of this plexus reduces the ventricular rate during atrial fibrillation (AF). We sought to assess the feasibility of chronic parasympathetic stimulation for ventricular rate control during AF with a nonthoracotomy intracardiac neurostimulation approach. In 9 mongrel dogs, the small endocardial area inside the right atrium, which overlies the RIGP, was identified by 20 Hz stimulation over a guiding catheter with integrated electrodes. Once identified, an active-fixation lead was implanted. The lead was connected to a subcutaneous neurostimulator. An additional dual-chamber pacemaker was implanted for AF induction by rapid atrial pacing and ventricular rate monitoring. Continuous neurostimulation was delivered for 1-2 years to decrease the ventricular rate during AF to a range of 100-140 bpm. Implantation of a neurostimulation lead was achieved within 37 +/- 12 min. The latency of the negative dromotropic response after on/offset or modulation of neurostimulation was <1 s. Continuous neurostimulation was effective and well tolerated during a 1-2 year follow-up with a stimulation voltage <5 V. The neurostimulation effect displayed a chronaxie-rheobase behavior (chronaxie time of 0.07 +/- 0.02 ms for a 50% decrease of the ventricular rate during AF). Chronic parasympathetic stimulation can be achieved via a cardiac neurostimulator. The approach is safe, effective, and well tolerated in the long term. The atrioventricular nodal selectivity and the opportunity to adjust the negative dromotropic effect within seconds may represent an advantage over pharmacological rate control.

Interactions within the intrinsic cardiac nervous system contribute to chronotropic regulation.

The objective of this study was to determine how neurons within the right atrial ganglionated plexus (RAGP) and posterior atrial ganglionated plexus (PAGP) interact to modulate right atrial chronotropic, dromotropic, and inotropic function, particularly with respect to their extracardiac vagal and sympathetic efferent neuronal inputs. Surgical ablation of the PAGP (PAGPx) attenuated vagally mediated bradycardia by 26%; it reduced heart rate slowing evoked by vagal stimulation superimposed on sympathetically mediated tachycardia by 36%. RAGP ablation (RAGPx) eliminated vagally mediated bradycardia, while retaining the vagally induced suppression of sympathetic-mediated tachycardia (-83%). After combined RAGPx and PAGPx, vagal stimulation still reduced sympathetic-mediated tachycardia (-47%). After RAGPx alone and after PAGPx alone, stimulation of the vagi still produced negative dromotropic effects, although these changes were attenuated compared with the intact state. Negative dromotropic responses to vagal stimulation were further attenuated after combined ablation, but parasympathetic inhibition of atrioventricular nodal conduction was still demonstrable in most animals. Finally, neither RAGPx nor PAGPx altered autonomic regulation of right atrial inotropic function. These data indicate that multiple aggregates of neurons within the intrinsic cardiac nervous system are involved in sinoatrial nodal regulation. Whereas parasympathetic efferent neurons regulating the right atrium, including the sinoatrial node, are primarily located within the RAGP, prejunctional parasympathetic-sympathetic interactions regulating right atrial function also involve neurons within the PAGP.

Modulation of cardiac A1-adenosine receptors in rats following treatment with agents affecting heart rate.

Effects of chronic treatment affecting heart rate on A1 adenosine receptor levels and their functions were studied. Treatment of rats with isoproterenol for 10 days accelerated heart rate and increased the level of adenosine receptors, in both the atria and ventricles. Negative dromotropic response of isolated heart to adenosine was enhanced in isoproterenol-treated rats. Similar results were obtained following treatment with atropine sulfate, or swimming training but not after treatment with thyroxine. On the other hand, treatment with amiodarone, which normally causes a decrease in heart rate, also increased the level of adenosine receptors in both atria and ventricles. The sensitivity of the isolated heart to the negative dromotropic and chronotropic effects of adenosine was not enhanced in the amiodarone treated rats. Similar results were obtained following treatment with propranolol, while treatment with PTU (6-n-propyl-2-thiouracil) increased adenosine sensitivity in the isolated heart. It was concluded that the levels of A1 adenosine receptors in the heart correspond to heart rate, and to cardiac efficiency. While an increase in heart rate was followed by up-regulation of A1 adenosine receptors, a decrease in heart rate caused a moderate elevation of these receptors.

PACAP-27 causes negative and positive dromotropic effects in anesthetized dogs

While pituitary adenylate cyclase-activating polypeptide (PACAP) has been identified radioimmunologically in the rat heart, the physiological role of PACAP has not been elucidated in the regulation of the atrioventricular conduction in the heart. We, therefore, determined the dromotropic effects of PACAP-27 injected into the cannulated atrioventricular node artery in the autonomically decentralized heart of the open-chest, anesthetized dog. PACAP-27 caused transient positive followed by negative dromotropic responses in a dose-dependent manner, whereas vasoactive intestinal peptide (VIP) caused only a positive dromotropic response. Atropine and tetrodotoxin blocked the negative dromotropic response to PACAP-27 and after blockade PACAP-27 caused only a positive dromotropic response. Tetrodotoxin and propranolol did not affect the positive dromotropic response to PACAP-27 in atropine treated dogs. PACAP-27 altered the atrio–His bundle interval but did not alter the His–ventricle interval. These results demonstrate that PACAP-27 prolongs the atrio–His bundle interval due to the liberation of acetylcholine from parasympathetic nerves and decreases it by a non-adrenergic mechanism in the dog heart in situ.

Different sympathetic–parasympathetic interactions on sinus rate and AV conduction in dog hearts

We investigated the sympathetic–parasympathetic interactions involved in SA nodal pacemaker activity and AV conductivity in the anesthetized dog heart. Stimulation of the intracardiac parasympathetic nerves to the SA nodal region (SAPS) and stimulation of the intracardiac parasympathetic nerves to the AV nodal region (AVPS) induced negative chronotropic and dromotropic responses, respectively. Cardiac sympathetic stimulation, aminophylline, 3-isobutyl-1-methylxanthine (IBMX, a relatively pure nonselective phosphodiesterase inhibitor) and methyl-1,4-dihydro-2,6-dimethyl-3-nitro-4-(2-trifluoromethylphenyl)-piridine-5-carboxylate (Bay k 8644, a Ca 2+ channel agonist) increased sinus rate and decreased AV conduction time. Sympathetic stimulation augmented the negative chronotropic response to SAPS but not the negative dromotropic response to AVPS, IBMX augmented both responses, Bay k 8644 augmented the chronotropic response and attenuated the dromotropic response, and aminophylline did not affect the chronotropic response to SAPS and inhibited the dromotropic response to AVPS. Additionally, when Bay k 8644 directly given via the AV node artery decreased AV conduction time, it attenuated the negative dromotropic response to AVPS and carbachol injected into the AV node artery. These results suggest that the differential sympathetic–parasympathetic interactions on sinus rate and AV conduction are at least partly induced by an interaction between changes in slow inward Ca 2+ current or intracellular Ca 2+ and the cardiac effects of acetylcholine in the heart in situ.

Reflex cardiac dromotropic responses to stimulation of the carotid and aortic chemoreceptors in the anaesthetized cat.

1. The reflex changes in the dromotropic state of the heart (P-R interval or atrioventricular conduction time) in response to selective stimulation of the carotid and aortic bodies by sodium cyanide were studied in the anaesthetized cat. The heart was paced and the arterial blood pressure was kept constant to minimize secondary effects of changes in arterial baroreceptor activity. 2. Stimulation of the carotid and aortic bodies caused an increase in the atrioventricular conduction time. 3. Evidence is presented to suggest that this negative dromotropic response was due predominantly to a vagal cholinergic mechanism. There is a small sympathetic component but only in so far as the carotid body reflex is concerned. 4. The negative dromotropic responses were enhanced during reflex suppression of the central inspiratory neuronal drive combined with minimal activity of the slowly adapting pulmonary stretch afferents indicating that they are respiratory modulated. 5. The clinical implications of these results are discussed.

NO modulates autonomic effects on sinus discharge rate and AV nodal conduction in open-chest dogs.

The purpose of this study was to investigate the role of nitric oxide (NO) in mediating vagal and sympathetic modulation of spontaneous sinus cycle length (SCL) and atrioventricular (AV) nodal conduction time (A-H interval) in 62 open-chest mongrel dogs anesthetized with alpha-chloralose. Infusion of an NO synthase (NOS) inhibitor, NG-monomethyl-L-arginine (L-NMMA, 4 mg/ ml), into the sinus and AV nodal arteries attenuated significantly (P < 0.01) the negative chronotropic and dromotropic responses to vagal nerve stimulation (VS) and VS during ansae subclaviae stimulation (SS) or isoproterenol (Iso) infusion. Intravenous administration of L-arginine (100 mg/kg) reversed these responses toward control values, whereas D-arginine did not have a significant effect. L-NMMA significantly (P < 0.01) enhanced the effects of SS and Iso on SCL and A-H interval; L-arginine reversed these changes toward baseline. L-NMMA increased the minimum concentration of ACh needed to induce 50 or 100% prolongation of SCL or second-degree or complete AV block during concomitant Iso infusion. L-Arginine reversed these effects. NOS inhibition did not affect the direct cholinergic actions of ACh on SCL and A-H interval but enhanced adrenergic positive chronotropic and dromotropic effects. We conclude that NO plays a stimulatory role in mediating vagal neurotransmission and vagal modulation of sympathetic effects and an inhibitory role in mediating sympathetic neurotransmission.

Cardiovascular Effects of R- and S-Enantiomers of Ro 22–9194, (2/?)-2-Amino-AM2,6-imethylphenyl)- AT-[3-(3-Pyridyl)Propyl]Propionamide D-Tartrate and (2S)-2-Amino-AK2,6-Dimethylphenyl)-N-[3-(3-Pyridyl)Propyl]Propionamide L-Tartrate, in Dog Heart Preparations

A newly synthesized compound, Ro 22-9194, relates in part of to the chemical structure of lidocaine. The cardiac effects of R- and S-enantiomers of Ro 22-9194 were investigated on isolated right atrial and left ventricular (LV) preparations which were cross-perfused with blood from another donor dog and an anesthetized open-chest dog. Each enantiomer (1-1,000 micrograms) decreased dose-dependently the sinus rate and atrial developed tension in the isolated right atrium (RA). The negative chronotropic responses to R- and S-enantiomers were not significantly different, and the negative inotropic responses to R- and S-enantiomers were also generally comparable. Both R- and S-enantiomers (10-3,000 micrograms) also decreased the ventricular developed tension in a dose-related manner similarly. In neurally decentralized, anesthetized, open-chest dogs, R- and S-enantiomers (0.1-3 mg/kg) injected into the femoral vein dose-dependently prolonged atrioventricular (A-V) conduction time and decreased heart rate (HR) and arterial blood pressure (ABP). Each enantiomer (3 mg/kg intravenously, i.v.) prolonged the interval between His bundle and ventricle rather than the interval between atrium and His bundle. There was no significant difference between R- and S-enantiomer-induced negative dromotropic actions. The duration of the negative dromotropic response to each enantiomer (3 mg/kg i.v.) was longer than that of the decrease in BP. These results suggest that the negative chronotropic, inotropic, and dromotropic effects of R- and S-enantiomers of Ro 22-9194 are not stereospecific in dog heart.

Differential Intracardiac Sympathetic and Parasympathetic Innervation to the SA and AV Nodes in Anesthetized Dog Hearts

Stimulation of discrete intracardiac sympathetic nerves to the SA (SAS stimulation) or AV nodal region (AVS stimulation) increased the heart rate or decreased AV conduction time and caused an AV junctional rhythm, respectively, in anesthetized dogs treated with atropine. Topical application of tetrodotoxin (TTX) at the SAS or AVS stimulation locus totally inhibited the response to each stimulation, whereas each TTX treatment slightly attenuated the chronotropic response to the right ansa stimulation by 23 ± 7.7% and the dromotropic response to the left ansa stimulation by 7 ± 7.5%. TTX abolished AVS stimulation-induced one. Before atropine, topical application of hexamethonium at the locus for stimulation of intracardiac parasympathetic nerves to the SA (SAP stimulation) or AV nodal region (AVP stimulation) abolished almost totally negative chronotropic responses to SAP and cervical vagus stimulation or negative dromotropic responses to AVP and cervical vagus stimulation, respectively. These results demonstrate that activation of a very small population of intracardiac sympathetic nerves to target cells is enough to induce positive chronotropic and dromotropic responses in the heart in situ, and that SA and AV nodal pacemaker activity and AV conductivity are controlled multi-directionally by intracardiac sympathetic nerves in contrast with parasympathetic ones.

Differential intracardiac sympathetic and parasympathetic innervation to the SA and AV nodes in anesthetized dog hearts.

Stimulation of discrete intracardiac sympathetic nerves to the SA (SAS stimulation) or AV nodal region (AVS stimulation) increased the heart rate or decreased AV conduction time and caused an AV junctional rhythm, respectively, in anesthetized dogs treated with atropine. Topical application of tetrodotoxin (TTX) at the SAS or AVS stimulation locus totally inhibited the response to each stimulation, whereas each TTX treatment slightly attenuated the chronotropic response to the right ansa stimulation by 23 +/- 7.7% and the dromotropic response to the left ansa stimulation by 7 +/- 7.5%. TTX abolished AVS stimulation-induced one. Before atropine, topical application of hexamethonium at the locus for stimulation of intracardiac parasympathetic nerves to the SA (SAP stimulation) or AV nodal region (AVP stimulation) abolished almost totally negative chronotropic responses to SAP and cervical vagus stimulation or negative dromotropic responses to AVP and cervical vagus stimulation, respectively. These results demonstrate that activation of a very small population of intracardiac sympathetic nerves to target cells is enough to induced positive chronotropic and dromotropic responses in the heart in situ, and that SA and AV nodal pacemaker activity and AV conductivity are controlled multi-directionally by intracardiac sympathetic nerves in contrast with parasympathetic ones.

Cardiac electrical responses to vagal stimulation of fibers to discrete cardiac regions

We stimulated regional intracardiac parasympathetic nerve fibers to the atrioventricular (AV) nodal area (AVP stimuli) or to the sinoatrial (SA) nodal area (SAP stimuli) in autonomically decentralized, anesthetized dogs. AVP stimuli increased the AV interval (AV conduction time); the magnitude of the response depended directly on the stimulation frequency. AVP stimuli did not alter the atrial interval (heart period) in spontaneously beating hearts. The magnitude of the negative dromotropic response increased when the atrial interval was shortened. SAP stimuli increased the atrial interval, and the magnitude of the response depended directly on the stimulation frequency. SAP stimuli evoked little or no change in the AV interval in the unpaced heart. When the atrium was paced, SAP stimuli did not change the AV interval in about half of the preparations, and there was a small but significant change in the remaining preparations. The negative dromotropic or chronotropic response to AVP or SAP stimuli was potentiated by physostigmine and blocked by atropine. These results suggest that 1) AVP stimuli induce a selective negative dromotropic response, and 2) this negative dromotropic effect is secondarily affected by heart period, by the pacemaking site, and by atrial activation patterns, but it does not affect the sinus cycle length in the dog heart.

Prolongation of cardiac cycle length attenuates negative dromotropic response to selective vagal stimuli

We stimulated intracardiac parasympathetic nerve fibers that selectively innervated the atrioventricular (AV) nodal area (AV parasympathetic stimulation), and the sinoatrial (SA) nodal area (SA parasympathetic stimulation), in autonomically decentralized, anesthetized dogs. We then compared these responses to those elicited by stimulation of the cervical vagus nerves. We investigated the interactions between the dromotropic and chronotropic responses to simultaneous AV and SA parasympathetic stimulation. AV parasympathetic stimulation increased the AV interval (AV conduction time) but did not alter the interval between atrial depolarizations (sinus cycle length). SA parasympathetic stimulation increased the sinus cycle length and evoked small changes in the AV interval. Simultaneous AV and SA parasympathetic stimulation, at different combinations of frequencies, induced negative dromotropic and chronotropic responses that were similar to those evoked by cervical vagal stimulation. The greater the increase in sinus cycle length, the less did a given parasympathetic stimulation prolong the AV interval. The prolongation of the AV interval by parasympathetic stimulation did not affect the sinus cycle length. These results suggest that the direct pure negative dromotropic response to parasympathetic nerve stimulation is attenuated by the prolongation of the sinus cycle length, e.g. a concomitant negative chronotropic effect of the parasympathetic stimulation, in the dog heart. This attenuation reflects a mechanism that does not depend on the relative timing of the stimulus impulses in the cardiac cycle, i.e. a phase-independent, as well as the previously reported phase-dependent, mechanism.

Controlled Cryothermal Injury to the AV Node: Feasibility for AV Nodal Modification

Elective subtotal injury to the AV node-His bundle region may create a negative dromotropic effect to provide a therapeutic advantage in some patients with supraventricular tachycardia without creating complete AV block. We examined the effects of cryosurgery to the AV nodal region, varying temperature and time using a 15 mm circular cryoprobe applied directly to the canine AV node-His bundle region. Twelve dogs were anesthetized and the heart was exposed through a right thoracotomy. Electrophysiological data obtained included conduction intervals, incremental pacing, and extrastimulus testing. Under inflow occlusion, the cryoprobe was positioned over the AV node-His bundle region using anatomical landmarks and a single freeze was applied (-15 degrees C to -60 degrees C, 15 to 60 seconds). Dogs were allowed to recover for 1 month, after which time electrophysiological testing was repeated under similar conditions; then the animals were sacrificed. With probe temperatures of -60 degrees C for 15 to 60 seconds, five of six dogs experienced complete heart block with dense fibrosis observed in the AV nodal-His bundle region. After freezing with higher temperatures, the remaining seven dogs had return of atrioventricular conduction postoperatively with prolongation of AH time observed in five and marked prolongation of the Wenckebach cycle length in three of the five. We conclude that controlled cryothermal injury to the AV node-His bundle region may be useful to create a desirable negative dromotropic response without creating complete AV block.

Secondary AV conduction responses during tonic vagal stimulation.

We gave trains of supramaximal stimuli at frequencies of 2, 4, or 8 Hz to both vagi of anesthetized, open-chest dogs and studied the AV conduction and atrial contractility responses while the atria were paced at different frequencies. The vagal stimulation quickly induced a maximum negative inotropic effect on the atrium. This "initial" response was followed by a "secondary" fading back toward the control level. The negative dromotropic responses to vagal stimulation also rapidly reached an initial value. When this initial response was large, there was then a secondary, relatively rapid fading back toward control. However, when the initial dromotropic response was small, the secondary response instead consisted of a gradual increase in conduction time. At intermediate levels of the initial dromotropic response, there was no appreciable secondary change in AV response. We hypothesize that two opposing mechanisms account for the variable secondary changes in AV conduction time and that the stimulus intensity determines which mechanism will be prepotent. The mechanism responsible for the secondary decline appears to be related to that which causes the fade of the inotropic response; muscarinic receptor desensitization probably plays an important role. The secondary augmentation of the dromotropic response may be related either to a slow diffusion of acetylcholine from the surrounding tissue or to a slow cumulative change in AV nodal refractoriness.

Localized myocardial responses to stimulation of small cardiac branches of the vagus.

Direct electrical excitation of small cardiac branches from the thoracic vagus elicited highly localized and differential responses from individualized segments of the myocardium. For example, small nerves from the vagus at the level of the superior pulmonary veins frequently induced moderate inhibition in contractile force of the ipsilateral atrium with little or no influences elsewhere. Branches from more rostral levels of the thoracic vagus induced changes in atrial contractility, with or without changes in sinoauricular (SA) nodal discharge rates, and often with partial or complete artioventricular (AV) nodal blockade.Excitation of individual, small vagal branches sometimes initiated acceleration in artrial rate and augmentation in atrial contractile force concurrently with complete AV nodal blockade. The negative dromotropic response was eliminated by atropine, leaving only the positive chronotropic and inotropic changes, thus revealing the intermingling of both sympathetic and parasympathetic components even in these small branches. There are frequently in excess of 20 small branches from the vagal trunk between the level of the caudal cervical ganglion and the superior pulmonary vein on each side which will induce highly selective changes in cardiac function upon stimulation. Inhibitory branches are particularly concentrated in the region of the recurrent laryngeal nerve on either side.

Negative dromotropic response to phenylephrine blocked by phenoxybenzamine.

In most canine excised and cross-circulated A-V node preparations phenylephrine induced a monophasic long-lasting positive dromotropic effect, and in some a triphasic response, i.e., positive-negativepositive dromotropic one. The negative dromotropic response was abolished by phenoxybenzamine and also blocked by l-hyoscyamine. Then, the triphasic response was converted to a monophasic positive one which was blocked by 1-alprenolol.