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Cardiovascular diseases are considered the leading cause of mortality globally; even with low mortality in dogs, such diseases are described in the same way in companion animals and humans. This study aimed to devise an effective decellularization protocol for the canine myocardium through the association of physical, chemical, and enzymatic methods, assessing resultant alterations in the myocardial extracellular matrix to obtain a suitable scaffold. Two canine hearts were collected; the samples were sectioned into ±1 cm2 fragments, washed in distilled water and 1× PBS solution, and followed by treatment under four distinct decellularization protocols. Sodium Dodecyl Sulfate (SDS) 1% 7 days + Triton X-100 1% for 48 h (Protocol I); Sodium Dodecyl Sulfate (SDS) 1% 5 days + Triton X-100 1% for 48 h (Protocol II); Trypsin 0.05% for 1 h at 36 °C + freezing −80 °C overnight + Sodium Dodecyl Sulfate (SDS) 1% for 3 days, Triton-X-100 for 48 h hours (Protocol III); 0.05% trypsin for 1 h at 36 °C + freezing at −80 °C overnight + 1% Sodium Dodecyl Sulfate (SDS) for 2 days + 1% Triton-X-100 for 24 h (Protocol IV). After analysis, Protocols I and II showed the removal of cellular content and preservation of extracellular matrix (ECM) contents, unlike Protocols III and IV, which retracted the ECM and removed essential elements of the matrix. In theory, although Protocols I and II have similar results, Protocol II stands out for the preservation of the architecture and components of the extracellular matrix, along with reduced exposure time to reagents, making it the recommended protocol for the development of a canine myocardial scaffold. [ABSTRACT FROM AUTHOR]

Cardiovascular diseases are considered the leading cause of mortality globally; even with low mortality in dogs, such diseases are described in the same way in companion animals and humans. This study aimed to devise an effective decellularization protocol for the canine myocardium through the association of physical, chemical, and enzymatic methods, assessing resultant alterations in the myocardial extracellular matrix to obtain a suitable scaffold. Two canine hearts were collected; the samples were sectioned into ±1 cm2 fragments, washed in distilled water and 1× PBS solution, and followed by treatment under four distinct decellularization protocols. Sodium Dodecyl Sulfate (SDS) 1% 7 days + Triton X-100 1% for 48 h (Protocol I); Sodium Dodecyl Sulfate (SDS) 1% 5 days + Triton X-100 1% for 48 h (Protocol II); Trypsin 0.05% for 1 h at 36 °C + freezing −80 °C overnight + Sodium Dodecyl Sulfate (SDS) 1% for 3 days, Triton-X-100 for 48 h hours (Protocol III); 0.05% trypsin for 1 h at 36 °C + freezing at −80 °C overnight + 1% Sodium Dodecyl Sulfate (SDS) for 2 days + 1% Triton-X-100 for 24 h (Protocol IV). After analysis, Protocols I and II showed the removal of cellular content and preservation of extracellular matrix (ECM) contents, unlike Protocols III and IV, which retracted the ECM and removed essential elements of the matrix. In theory, although Protocols I and II have similar results, Protocol II stands out for the preservation of the architecture and components of the extracellular matrix, along with reduced exposure time to reagents, making it the recommended protocol for the development of a canine myocardial scaffold.

En este reporte se expone el caso de una cachorra de raza Springer Spaniel que presentaba taquipnea, ascitis, intolerancia al ejercicio, leve cianosis y baja condición corporal. A la auscultación cardiaca se evidenció un soplo 5/6 mitral que irradiaba a ambos hemitórax. En la ecocardiografía se apreció una disminución de la función contráctil del ventrículo izquierdo, hipertrofia excéntrica del atrio y ventrículo izquierdo, e insuficiencia mitral.Este relatório descreve o caso de uma cachorra da raça Springer spaniel de baixa condição corporal que tinha taquipnéia, ascite, intolerância ao exercício, cianose leve. Na ausculta cardíaca foi mostrado um fôlego 5/ 6 mitral que irradiava ambos hemitórax. Em ecocardiografia revelou uma diminuição na função contrátil do ventrículo esquerdo, excêntrica hipertrofia do ventrículo direito e átrio esquerdo e insuficiência mitral.This is a case of a Springer spaniel puppy which presented tachypnea, ascitis, exercise intolerance, mild cyanosis and a poor body condition. At cardiac auscultation a 5/6 mitral murmur was detected that irradiated to both hemithorax. Echocardiography revealed diminished left ventricular contractility, eccentric hypertrophy of the left atrium and ventricle and mitral insufficiency.

Bisphenol A (BPA) is a common environmental chemical with a range of potential adverse health effects. The impact of environmentally-relevant low dose of BPA on the electrical properties of the hearts of large animals (e.g., dog, human) is poorly defined. Perturbation of cardiac electrical properties is a key arrhythmogenic mechanism. In particular, delay of ventricular repolarization and prolongation of the QT interval of the electrocardiogram is a marker for the risk of malignant arrhythmias. We examined the acute effect of 10−9 M BPA on the electrical properties of female canine ventricular myocytes and tissues. BPA rapidly delayed action potential repolarization and prolonged action potential duration (APD). The dose response curve of BPA on APD was nonmonotonic. BPA rapidly inhibited the I Kr K+ current and I CaL Ca2+ current. Computational modeling indicated that the effect of BPA on APD can be accounted for by its suppression of I Kr. At the tissue level, BPA acutely prolonged the QT interval in 4 left ventricular wedges. ERβ signaling contributed to the acute effects of BPA on ventricular repolarization. Our results demonstrate that BPA has QT prolongation liability in female canine hearts. These findings have implication for the potential proarrhythmic cardiac toxicity of BPA in large animals. [ABSTRACT FROM AUTHOR]

Mechanoelectric feedback (MEF) in the heart operates through several mechanisms which serve to regulate cardiac function. Stretch activated channels (SACs) in the myocyte membrane open in response to cell lengthening, while tension generation depends on stretch, shortening velocity, and calcium concentration. How all of these mechanisms interact and their effect on cardiac output is still not fully understood. We sought to gauge the acute importance of the different MEF mechanisms on heart function. An electromechanical computer model of a dog heart was constructed, using a biventricular geometry of 500K tetrahedral elements. To describe cellular behavior, we used a detailed ionic model to which a SAC model and an active tension model, dependent on stretch and shortening velocity and with calcium sensitivity, were added. Ventricular inflow and outflow were connected to the CircAdapt model of cardiovascular circulation. Pressure-volume loops and activation times were used for model validation. Simulations showed that SACs did not affect acute mechanical response, although if their trigger level was decreased sufficiently, they could cause premature excitations. The stretch dependence of tension had a modest effect in reducing the maximum stretch, and stroke volume, while shortening velocity had a much bigger effect on both. MEF served to reduce the heterogeneity in stretch while increasing tension heterogeneity. In the context of left bundle branch block, a decreased SAC trigger level could restore cardiac output by reducing the maximal stretch when compared to cardiac resynchronization therapy. MEF is an important aspect of cardiac function and could potentially mitigate activation problems., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 The Authors. Published by Elsevier Ltd.. All rights reserved.)

Organized activity of the myocardium manifested in dominant frequency structure of EСG was typical of the first 10 min of ventricular fibrillation in canine heart. The first minute of fibrillation was characterized by the most pronounced changes in the structure of oscillation frequency and transition from domination of high frequency oscillations (13-17 Hz) to domination of medium (8-12 Hz) and then low frequency (4-7 Hz) oscillations. The second minute was characterized by transition from domination of low frequency oscillations to domination of low and medium frequency oscillations; minutes 3-10 were characterized by domination of low and medium frequency oscillations; after 10 min, non-dominant ventricular oscillations were recorded. [ABSTRACT FROM AUTHOR]

We previously demonstrated a transmural gradient in Na/K pump current (I (P)) and [Na(+)]( i ), with the highest maximum I (P) and lowest [Na(+)]( i ) in epicardium. The present study examines the relationship between the transmural gradient in I (P) and Na/Ca exchange (NCX). Myocytes were isolated from canine left ventricle. Whole-cell patch clamp was used to measure current generated by NCX (I (NCX)) and inward background calcium current (I (ibCa)), defined as inward current through Ca(2+) channels less outward current through Ca(2+)-ATPase. When resting myocytes from endocardium (Endo), midmyocardium (Mid) or epicardium (Epi) were studied in the same conditions, I (NCX) was the same and I (ibCa) was zero. Moreover, Western blots were consistent with NCX protein being uniform across the wall. However, the gradient in [Na(+)]( i ), with I (ibCa) = 0, should create a gradient in [Ca(2+)]( i ). To test this hypothesis, we measured resting [Ca(2+)]( i ) using two methods, based on either transport or the Ca(2+)-sensitive dye Fura2. Both methods demonstrated a significant transmural gradient in resting [Ca(2+)]( i ), with Endo > Mid > Epi. This gradient was eliminated by exposing Epi to sufficient ouabain to partially inhibit Na/K pumps, thus increasing [Na(+)]( i ) to values similar to those in Endo. These data support the existence of a transmural gradient for Ca(2+) removal by NCX. This gradient is not due to differences in expression of NCX; rather, it is generated by a transmural gradient in [Na(+)]( i ), which is due to a transmural gradient in plasma membrane expression of the Na/K pump. [ABSTRACT FROM AUTHOR]

Abstract: Volume overload frequently caused in dogs by chronic degenerative valvular disease (CDVD), eventually leads to cardiac failure. Experimental and clinical evidences demonstrate that increased interleukin-1β serum level in patients with heart insufficiency correlates with the severity of failure irrespective of its etiology. Very little is known about the IL-1β expression in failing vs. non-failing myocardium. IL-1β transcript level was determined in the CDVD dogs (n =17) and control animals (n =9) without cardiac insufficiency by real-time PCR. IL-1β transcript level in failing hearts was higher than in the control. In both groups the highest IL-1β level was detected in the left ventricles. Although IL-1β is a major pro-inflammatory cytokine most of the CDVD dogs displayed no inflammatory infiltrates into the myocardium. Massive fibrosis was observed in the control group, unlike the failing hearts, in which cardiomyocyte hypertrophy and atrophy dominated. The alternative IL-1β transcript identified here (IL-1βsv1) was significantly elevated in the failing myocardium compared with the control group. Increased IL-1β expression seems to be associated with mechanical heart overload. Its endogenous origin, and certain histopathological findings attributed to IL-1β indicate its importance in cardiac hypertrophy and failure. The lack of some typical IL-1β actions, i.e. inflammatory, pyrogenic and fibrotic, may suggest a different role of this cytokine in myocardium. It appears that the canine IL-1β gene can be transcribed in two ways in heart tissue, with the IL-1βsv1 form present mainly in failing hearts. [Copyright &y& Elsevier]

A heart in fibrillation can be entrained by long-lasting alternating current (AC) stimuli, leading to defibrillation. To investigate the role entrainment plays in defibrillation, computer simulations of AC cardioversion in a three-dimensional slice of the canine heart were performed. Methods and Results: A bidomain finite element model of a 1-mm thick slice across the ventricles of a canine heart was used to simulate termination of transmural reentry with AC shocks. Cardioversion defibrillation thresholds (DFTs) were determined for 200-msec (L) AC shocks at varying frequencies. At the DFT, the entire tissue is entrained by the AC shock. DFT decreases as the frequency of the long-lasting AC shock increases. We hypothesize that this decrease is due to the short period of the high-frequency AC waveform, leaving strong virtual electrode polarization (VEP) after the shock ends. To test this hypothesis, the end-shock VEP were compared for different frequencies, demonstrating stronger polarization as frequency increased. To examine whether entrainment by the long-lasting AC shock contributes to the VEP at the end of the shock, additional simulations were conducted using single-period (Z) AC waveforms. Z waveform DFTs were higher than L waveform DFTs; the Z waveform VEP was weaker than the L waveform VEP at the same frequency. This indicates that entrainment contributes to the development of stronger VEP and, thus, to lower DFT at high frequencies. Conclusion: This study offers for the first time a mechanistic insight into cardioversion with long-lasting AC shocks. [ABSTRACT FROM AUTHOR]

Introduction: Postinfarction monomorphic ventricular tachycardias induced by programmed stimulation may display initial cycle length (CL) variations before stabilizing. Methods and Results: To show that tachycardia onset dynamics depend on rate-dependent electrical properties of the reentrant substrate, we extracted activation times and maximum negative slopes of local activation complexes (-dV/dtmax) from 191 unipolar electrograms recorded In the anterior left ventricular wall of anesthetized, 3-day-old infarct canine preparations. Measurements were made of the responses to programmed stimulation, as well as in early and later beats of tachycardias, which displayed either a constant trend in CL (group A, n = 5 preparations) or one in which CL prolongation occurred according to an exponential course before stabilizing (group B, n = 9). Stimulation protocols inducing the tachycardias were more aggressive and their CL was significantly shorter (CL = 159 ± 24 msec) in group A than in group B (stabilized CL = 206 ± 34 msec). Reentrant activity occurred In subepicardial areas in which the absolute value of -dV/dtmax (I-dV/dtmax) was heterogeneously depressed (<2 mV/msec). &verbar;-dV/dtmax&verbar; was reduced and activation delay increased in the successive responses to extrastimuli. Further reductions in &verbar;-dV/dtmax&verbar; (10% to 23%) were shown to occur between early and later beats in S of the 9 tachycardias in group B (no change in the 4 others), and they were associated with localized prolongation of conduction times in reentrant pathways. In contrast, &verbar;-dV/dtmax improved in all group A tachycardias (7% to 25%). Conclusion: This study provides evidence that the onset dynamics of postinfarction ventricular tachycardias are determined by Intervaledependent electrical changes occurring in the reentrant substrate. [ABSTRACT FROM AUTHOR]

The number and intensity of stimuli that set basic cycle length in cardiac electrophysiological studies can influence the electrical properties assessed by extrastimuli. The relative contribution of drive (S1 and test (S2) stimulus intensity in defining myocardial excitability and vulnerability has not been reported. The purpose of this investigation was to assess this interaction and to determine whether atrial and ventricular findings differed. The effects of S1 and S2 intensity on atrial and ventricular stimulus-intensity-refractory-period curves were determined in open-chest dogs: comparisons were made between curves with S1 intensity varied between diastoiic threshold (DT) and 10 mA and S2 intensity maintained at DT and those with S1 intensity maintained at DT and S2 intensity varied between DT and 10 mA. S1-S1 was held constant and S1-S2 varied. The effects of different stimulation sites, cycle length, number of stimulations, and neural blockade were assessed. S1 intensity amplification shifted atrial stimulus-intensity-refractory period curves in the direction of increased excitability and vulnerability; the changes were more pronounced than those obtained by modulating S2 intensity. The changes produced by increasing S1 intensity were evident at different cycle lengths and were enhanced by an increased number of stimulations, but were not evident when S1 and S2 were delivered at different atrial sites. Although beta-blockade attenuated the effects of increasing S1 intensity somewhat, the addition of cholinergic blockade virtually abolished it. Ventricular refractoriness was also changed by modulation of S1 intensity, but the changes were less striking. In the atrium, modulation of S1 intensity has greater effects of stimulus-intensity-refractory-period relations than modulation of S2 intensity; in the ventricle, the converse is true. [ABSTRACT FROM AUTHOR]

Introduction: Our goals were to study the role of development in determining the cardiac effects of sympathetic neural activation, and to identify the roles of α- and β-adrenergic receptor-mediated pathways in modulating the effects of sympathetic stimulation. Methods and Results: We compared responses of young and adult canine hearts in situ to right, left, and bilateral stellate ganglion stimulation. We focused on changes in heart rate, rhythm, QT interval, rate-corrected QT interval (QTc), and T wave amplitude. Right stellate stimulation (RSS) induced more pronounced sinus tachycardia in adult than young animals. Left stellate stimulation (LSS) Induced junctional tachycardia in adult more than young animals. In adults. LSS and RSS prolonged QTc (LSS > RSS), whereas 1-week-olds manifested QTc shortening with RSS. LSS also increased T wave amplitude, most markedly in adults. In all studies, bilateral stellate stimulation induced responses intermediate between those seen with RSS and LSS. β-Adrenergic blockade (propranolol) abolished all responses to LSS in adult hearts, but α-blockade (prazosin) attenuated only the LSS-induced prolongation in QTc. Conclusion: In the postnatal modulation of cardiac rhythm, rate, and repolarization by the sympathetic nervous system, β-adrenergic receptors play a major role at all ages, whereas α-adrenergic receptors play a lesser role, which is manifested only in adults. Moreover, expression of junctional tachycardias, which are β-adrenergically modulated, is seen only in the adults. [ABSTRACT FROM AUTHOR]

Introduction: A comparison of the effects of shock timing and strength on the effective refractory periods (ERPs) of open chest canine hearts to the duration of depolarization in optically recorded action potentials from the rabbit heart was conducted in order to investigate the mechanism of ERP prolongation in the canine. Methods and Results: Studies were conducted in four open chest canines hearts and three isolated, perfused rabbit hearts. Shocks were applied during ventricular pacing and the strength and timing were varied. The shock voltage gradient was measured at the stimulating site used to assess the ERP in the canine and at the optical recording site in the rabbit. Shock voltage gradients ranged from 7.5 to 53 V/cm in the canine and 2 to 25 V/cm in the rabbit. Shock coupling intervals (CIs) were varied over a range of 15% to 105% of the control ERP (canine) and 10% to 115% of the control action potential duration (APD) (rabbit). Shocks prolonged the ERP in the canine and they caused an additional period of depolarization that prolonged the action potential in the rabbit. ERP and APD prolongation showed similar dependencies on the shock strength and CI: both increased with increasing shock strength and CI. Conclusion: ERP prolongation in the canine may be due to the same type of responses seen on the optically recorded action potentials in the rabbit. ERP prolongation by low-strength shocks at long CIs in the canine may be due to graded responses and premature action potentials. ERP prolongation by high-strength shocks over a range of CIs may be due to the additional depolarization time response that prolongs the action potential in rabbits. [ABSTRACT FROM AUTHOR]

Sarcolemmal permeability during reperfusion in early myocardial ischemia was investigated in dogs by the vital ionic lanthanum probe (La) technique and electron microscopy. The left anterior descending coronary artery was occluded for 10-120 min and released for 10 min, and then a 4 mM solution of La ions was injected, and perfusion fixation was performed. In the normal myocardium, La ions were localized in extracellular spaces. After 20 min of ischemia and subsequent reperfusion, deposits of lanthanum were found in 17% of the myocytes prior to the development of irreversible myocardial damage. The number of cells with such deposits increased in proportion to the increased duration of ischemia. These findings indicate that the disruption of sarcolemmal permeability, during the early stages of myocardial injury due to ischemia and reperfusion, contributes to the development of irreversible myocardial damage. [ABSTRACT FROM AUTHOR]

Objective: The aim was to determine whether a 120 min exposure of ventricular myocytes to β agonist challenge alters ventricular cell surface p adrenergic receptors concomitant with desensitisation of ventricular function. Methods: Supramaximal doses of isoprenaline (1 (μg·kg−1·min−1 intravenously) were given continuously to anaesthetised dogs for 15 (n = 6), 60 (n = 2), and 120 (n = 6) min. Changes induced during β adrenoceptor challenge in right and left ventricular systolic pressures were correlated with right and left ventricular myocyte β adrenoceptor number and affinity. Results: Isoprenaline initiated early augmentation in right [23(SEM 3)-78(10) mm Hg] and left [82(6)-186(11) mm Hg] venticular intramyocardial systolic pressures. After 5 min of continuous exogenous β agonist challenge these pressures were reduced to 42(4) mm Hg and 104(16) mm Hg, respectively, even though the agonist challenge persisted. Throughout the subsequent 115 min exposure period these pressures remained relatively similar. Despite these inotropic effects the number (Bmax) and affinity (Kd) of ventricular myocyte β adrenergic cell surface receptors, as determined by the tissue slice technique, were similar before and after 15, 60, and 120 min exposure to isoprenaline. Conclusions: (1) After the desensitisation of the initial enhancement of ventricular inotropism that occurs during the first 5 min of a β agonist challenge, inotropism remains relatively constant for the next 115 min exposure. (2) Desensitisation of ventricular inotropism elicited during 2 h exposure of the in situ heart to a β agonist challenge is not primarily due to altered myocyte cell surface β adrenergic receptor number or affinity.Cardiovascular Research 1994;28:680-683 [ABSTRACT FROM PUBLISHER]

Objective: The aim was to test whether a brief period of non-ischaemic hypoxia can precondition myocardium. Methods: 60 anaesthetised adult mongrel dogs of either sex underwent 60 min occlusion of the left anterior descending coronary artery, followed by 5 h reperfusion. In treated groups, hearts were either preconditioned with 5 min coronary perfusion with hypoxic blood [O2 content 9.2(SEM 0.6) ml·litre−1] or 5 min occlusion followed by a 10 min reperfusion period prior to 60 min occlusion. The effect of these treatments on myocardial infarct size and regional contractile function was assessed. Results: Infarct size, determined by tetrazolium staining, as a percentage of anatomical area at risk was markedly decreased in hypoxia preconditioned hearts, at 7.2(1.8)% v 22.4(4.6)% in controls (p<0.01), but did not differ from ischaemia preconditioned hearts [4.6(1.7)%; p<0.01 v control]. Anatomical area at risk, expressed as a percentage of left ventricular mass, and collateral blood flow to the inner two thirds of the ischaemic wall did not differ among the groups. Regional contractile function was depressed following ischaemic preconditioning but not following hypoxic preconditioning. During reperfusion following 60 min occlusion, marked paradoxical systolic lengthening was evident in ischaemia preconditioned and control hearts but not in hypoxia preconditioned myocardium. Conclusions: Five minutes of hypoxic and ischaemic preconditioning were equipotent in preventing infarction, whereas ischaemic preconditioning caused a greater decrement in postischaemic contractile function. [ABSTRACT FROM PUBLISHER]

The vagus nerve and adenosine exert a negative chronotropic effect on the mammalian sinus node. In addition, adenosine is released from myocardial cells under both physiological and pathophysiological conditions, which are characterised by variable vagal tone. To determine the interaction between adenosine and the vagus nerve, 21 barbiturate anaesthetised mongrel dogs with bilateral cervical vagotomy and stellectomy were studied. In group 1 (n = 16) adenosine (3 μmol·kg−1) was rapidly (≤ 1 s) administered before (control) and during 60 s of repetitive vagal stimulation. Each stimulus (consisting of a burst of 5 square wave pulses, 0.3 mA in amplitude and 1 ms in duration at 0.1 kHz) was given at a fixed point in time in each sinus cycle (that is, one burst per cycle). This protocol was repeated after a 50 ms incremental change in the phase at which vagal stimulation was given until the entire cycle length was scanned. Recovery time of at least 2 min separated any two consecutive adenosine administrations. In four animals the above protocol was performed twice using vagal stimulation amplitude of 0.3 mA and 0.6 mA in the first and second runs respectively. In this group (group 1), the effect of adenosine was altered by the vagus in a phase dependent manner. In each animal, however, there was at least one phase of the sinus cycle during which vagal stimulation significantly enhanced the effect of adenosine. Adenosine prolonged the baseline mean (SEM) sinus cycle length of 578(24) ms by a maximum of 262(26) ms; during increased background vagal tone, simulated by repetitive vagal stimulation; it prolonged sinus cycle length of 741 (47) ms by 446(45) ms (p<0.02). In addition, an inverse relation was found between background vagal tone and the superimposed action of adenosine. As vagal tone was further increased, the negative chronotropic action of adenosine was decreased (p<0.01).In group 2 (n=5) single bursts (with characteristics as in group 1) were given one at a time, at different phases of the sinus cycle length. The latter were varied in 50 ms increments until the entire sinus cycle was scanned by a single vagal stimulation. This protocol was performed before, during, and after infusions of 2 chloro-adenosine into the sinus node artery. In this group (group 2), adenosine consistently caused an upward and rightward shift of the phasic vagal stimulation-sinus cycle length response curve. Thus, although vagal stimulation prolonged the baseline sinus cycle length of 644(46) ms by a maximum of 383(78) ms in the absence of 2 chloro-adenosine, it prolonged the sinus cycle length of 673(58) ms by a maximum of 617(57) ms in the presence of 2 chloro-adenosine (p<0.05). These results indicate the complex interactive negative chronotropic effects of adenosine and the vagus nerve on the sinus node. Background vagal tone has a modulating effect on the action of adenosine, whereas increased extracellular adenosine concentrations potentiate the depressant action of the vagus nerve on the sinus node. [ABSTRACT FROM PUBLISHER]

To evaluate the effect of beta blockade on energy metabolism in ischaemic myocardium metoprolol tartrate in doses of 0.2 or 0.5 mg·kg−1 or saline was injected into 24 anaesthetised dogs 20 min after coronary ligation. Regional myocardial blood flow was measured by the hydrogen gas clearance method, and the heart rate and aortic pressure were recorded. After 60 min coronary occlusion myocardial biopsy specimens were removed from five areas where myocardial blood flow had been recorded to determine their content of adenosine-5′-triphosphate (ATP). Respiratory function and acyl-coenzyme A contents of mitochondria isolated from ischaemic and non-ischaemic myocardium were also measured. In the control group the ATP content in severely ischaemic myocardium where myocardial blood flow was <20 ml·min−1·100 g−1 was decreased to 27.5% of that in the non-ischaemic area, whereas the administration of metoprolol 0.5 mg·kg−1 maintained ATP content at 46.3%. In ischaemic myocardium metoprolol inhibited the accumulation of long chain acyl-coenzyme A in mitochondria and significantly preserved their respiratory function. In contrast, metoprolol did not affect myocardial blood flow in myocardium where flow was <60 ml·min−1·100 g−1. Heart rate was decreased by metoprolol in proportion to the dose, whereas mean aortic pressure was not changed. These results suggest that beta blockade had beneficial effects on the energy metabolism of ischaemic myocardium and that such effects might be due not only to a reduction in cardiac work but also to a reduction in the vicious circle of fatty acid metabolism in ischaemic myocardium. [ABSTRACT FROM PUBLISHER]

The influence of aminophylline, a competitive antagonist of adenosine, on the chronotropic and dromotropic effects of adenosine and adenosine triphosphate was studied in pentobarbital anaesthetised dogs under various modifications of the autonomic nervous tone. Adenosine and adenosine triphosphate (3 μmol·kg−1 each) were rapidly (≤1 s) injected into the right atrium during both sinus rhythm and right atrial pacing (cycle length 300 ms) before and after infusion of aminophylline (5 mg·kg−1) (n=21) as well as after increasing doses of aminophylline (n=10). Some dogs underwent either muscarinic blockade with atropine (0.2 mg·kg−1) (n=10), or beta adrenergic blockade with propranolol (1 mg·kg−1) (n=10), or complete autonomic blockade with atropine and propranolol (n=10). Aminophylline (5 mg·kg−1) antagonised the negative chronotropic and dromotropic effects of adenosine triphosphate and adenosine in dogs pretreated with atropine or atropine plus propranolol but did not affect them in autonomically intact dogs. In addition, the electrophysiological effects of adenosine were antagonised by only the highest doses of aminophylline in autonomically intact dogs and by aminophylline (5 mg·kg−1) in dogs pretreated with propranolol.It was concluded that (a) alteration of the electrophysiological effects of adenosine triphosphate and adenosine by aminophylline is appreciably influenced by the autonomic nervous tone and (b) autonomic blockade is required for the manifestation of the antagonism by aminophylline of the electrophysiological action of adenosine and adenosine triphosphate. [ABSTRACT FROM PUBLISHER]

The influence of adrenergic and muscarinic receptor activation on cardiac electrical stability and on serum potassium concentrations was studied in 23 anaesthetised dogs. The ventricular fibrillation threshold was assessed using the single stimulus technique. Adrenaline (1.0 μg·kg−1·min−1) caused a brief rise and a subsequent prolonged fall in serum potassium concentration, which was accompanied by a decline in ventricular fibrillation threshold when baroreceptor activation was prevented. After pretreatment with the betai adrenoceptor blocking agent metoprolol (0.5 mg·kg−1), adrenaline did not alter vulnerability to ventricular fibrillation but still elicited hypokalaemia. In contrast, selective beta2 adrenoceptor blockade (ICI 118551, 100 μg·kg−1) prevented the adrenaline induced lowering of serum potassium concentration but not of ventricular vulnerability. Muscarinic receptor activation by methacholine (3.0 μg·kg−1·min−1) had no effect on serum potassium concentration but increased the ventricular fibrillation threshold by 30%. When methacholine was administered concomitantly with adrenaline the decline in serum potassium concentration persisted, but the increase in ventricular vulnerability was completely prevented. It is concluded that in the normal canine myocardium adrenaline produces an increase in vulnerability that is mediated through beta1 adrenoceptors and that the beta2 adrenoceptor mediated hypokalaemia is dissociated from electrophysiological effects of adrenaline. Parasympathetic nervous system activation does not influence serum potassium concentrations but opposes the effects of adrenaline on susceptibility to ventricular fibrillation. [ABSTRACT FROM PUBLISHER]

Myocellular injury induced by acute ischemia and subsequent reperfusion was studied in 38 dogs, with special reference to sarcolemmal permeability as determined by the vital ionic lanthanum (La) probe technique and electron microscopy. The left anterior descending coronary artery (LAD) was occluded in 14 dogs for 10 to 60 min, and the ischemic zone was perfused slowly for 7 min with a La-containing solution. In 21 dogs, the LAD was released for 10 min after occlusion and was then reperfused for 7 min with arterial blood plus the La-containing solution. Subsequently, in both groups of animals, the ischemic myocardium was subjected to perfusion fixation in preparation for electron microscopy. In normal cardiac myocytes, La was localized exclusively in the extracellular space. After 10 to 20 min of ischemia, more than 80% of myocytes appeared normal or were damaged only slightly, and the majority continued to exclude La. After 10 min of ischemia, deposits of lanthanum were detected in 1 and 6 % of myocytes in the absence or presence of reperfusion, respectively. The number of cells with such deposits was markedly increased after 30 min of ischemia (19%), as well as after 20 min of ischemia followed by reperfusion (17%), prior to the development of irreversible myocardial damage. After 60 min of ischemia with or without reperfusion, about 30% of myocytes showed severe injury with particulate deposits of lanthanum throughout the entire cell. These results indicate that sarcolemmal permeability increases during the early stage of myocardial injury due to ischemia or ischemiareperfusion and contributes to the development of myocardial damage. [ABSTRACT FROM AUTHOR]

The role of phospholipase (PLase) in the development of heart mitochondrial dysfunction following reperfusion was studied together with the effects of dilazep on the action of PLases and reperfusion injury. In vivo experiment: Seventy six adult mongrel dogs were divided into 3 groups; the control group (n=36), the dilazep 0.5 mg group (n=17) and the dilazep 1 mg group (n=23). Fifteen min after premedication with physiological saline or dilazep (0.5 mg/kg or 1 mg/kg), the left anterior descending coronary artery was occluded for 15 min and then reperfused for 5 min. Each group was further divided into two subgroups depending on the presence or absence of reperfusion arrhythmia. Immediately after 5 min of reperfusion, myocardial mitochondria were prepared from the normal and the reperfused areas. Pretreatment with dilazep induced a dose-dependent decrease in the incidence of reperfusion arrhythmia from 31% of the control to 24% (0.5 mg/kg) and 9% (1 mg/kg). In the arrhythmia cases in each group, functional deterioration of mitochondria from the reperfused area was observed with the increase in free fatty acids and the decrease in phospholipids in the reperfused mitochondria. In vitro experiment: Using L-α-dimyristoyl phosphatidylcholine as a substrate, myristic acid released by PLase A or by PLase C with or without pretreatment by dilazep was quantitatively determined. Dilazep inhibited the release of myristic acid by PLase A or by PLase C in a concentration-dependent manner. These results suggest that decomposition of mitochondrial membrane phospholipids caused by PLase activation following reperfusion was primarily responsible for the development of mitochondrial dysfunction, and that dilazep showed beneficial effects against reperfusion injury by inhibiting the action of PLases. [ABSTRACT FROM AUTHOR]