Normal Canine Myocardium Research Articles

Comparative effects of the opioids fentanyl and buprenorphine on ventricular vulnerability during acute coronary artery occlusion

Fentanyl, a mu selective opioid agonist in wide clinical use, raises the ventricular fibrillation threshold in the normal canine myocardium. We have previously shown that this effect is amplified by haemorrhagic stress. In order to determine if mu receptor activation is antifibrillatory during acute myocardial ischaemia, we compared the effects of two mu selective agents, fentanyl and buprenorphine, in open chest chloralose anaesthetised dogs. Each drug was administered intravenously in two doses 1 h apart (fentanyl 30 micrograms.kg-1.dose; buprenorphine 0.3 mg.kg-1.dose). Ventricular fibrillation threshold was measured during right ventricular pacing using the single stimulus technique. The threshold was determined before and during a 10 min left anterior descending coronary artery occlusion. Prior to fentanyl administration, ventricular fibrillation threshold decreased from a control value of 19(SEM 2) mA to 12(1) mA during coronary artery occlusion. After the first dose of this drug an attenuation in the ischaemia induced fall in fibrillation threshold from 23(4) mA to 15(2) mA was observed. After the second dose of fentanyl the decline in fibrillation threshold was significantly blunted at 22(4) mA during control and 18(3) mA during occlusion, p less than 0.05 compared to no drug. In an additional series of experiments atropine sulphate abolished the antifibrillatory action of fentanyl, indicating that vagal efferent activation is responsible for the protective effect of the drug during acute myocardial ischaemia. This is in contrast with its mode of action during haemorrhage, when it enhances vagal afferent inhibition of sympathetic tone, and atropine pretreatment is without effect.(ABSTRACT TRUNCATED AT 250 WORDS)

Transmural metabolite distribution in regional myocardial ischemia as studied with 31P NMR.

Phosphorus-31 nuclear magnetic resonance (31P NMR) has been applied to study the canine heart prior to and during regional myocardial ischemia induced by partial flow reduction in the left anterior descending coronary artery (LAD). NMR data were acquired in a transmural fashion by restricting the signal to a column perpendicular to the heart wall using B0 gradients and obtaining spectroscopic spatial resolution along the third dimension using the B1 gradient and adiabatic excitation. With this approach, transmural spectra were accumulated in five separate voxels spanning the wall of the left ventricle from the epicardium to the endocardium. In the normal canine myocardium the levels of high-energy phosphates CP and ATP were relatively constant throughout the left ventricular wall, with only minor evidence of free inorganic phosphate in any of the transmural voxels. However, during sustained partial occlusion of the LAD, significant regional differences between the epi- and the endocardium were noted. The data demonstrate the importance of studying cardiac bioenergetics with transmural differentiation.

Stimulus-induced critical point. Mechanism for electrical initiation of reentry in normal canine myocardium.

The hypothesis was tested that the field of a premature (S2) stimulus, interacting with relatively refractory tissue, can create unidirectional block and reentry in the absence of nonuniform dispersion of recovery. Simultaneous recordings from a small region of normal right ventricular (RV) myocardium were made from 117 to 120 transmural or epicardial electrodes in 14 dogs. S1 pacing from a row of electrodes on one side of the mapped area generated parallel activation isochrones followed by uniform parallel isorecovery lines. Cathodal S2 shocks of 25 to 250 V lasting 3 ms were delivered from a mesh electrode along one side of the mapped area to scan the recovery period, creating isogradient electric field lines perpendicular to the isorecovery lines. Circus reentry was created following S2 stimulation; initial conduction was distant from the S2 site and spread towards more refractory tissue. Reentry was clockwise for right S1 (near the septum) with top S2 (near the pulmonary valve) and for left S1 with bottom S2; and counterclockwise for right S1 with bottom S2 and left S1 with top S2. The center of the reentrant circuit for all S2 voltages and coupling intervals occurred at potential gradients of 5.1 +/- 0.6 V/cm (mean +/- standard deviation) and at preshock intervals 1 +/- 3 ms longer than refractory periods determined locally for a 2 mA stimulus. Thus, when S2 field strengths and tissue refractoriness are uniformally dispersed at an angle to each other, circus reentry occurs around a "critical point" where an S2 field of approximately 5 V/cm intersects tissue approximately at the end of its refractory period.

N-Acylation of ethanolamine phospholipids by acyl transfer does not involve hydrolysis

N-Acylethanolamine phospholipids occur in infarcted but not in normal canine myocardium. Their synthesis is catalyzed by a membrane-bound, Ca 2+-requiring N-acyltransferase (transacylase) which transfers acyl groups from the sn-1 position of various phospholipids including phosphatidylethanolamine to the amino group of ethanolamine phospholipids. When dog heart mitochondria are incubated in media containing Ca 2+ and H 2 18O, the resulting N-acylethanolamine phospholipids do not accumulate 18O in either the amide or 1- O-acyl groups. The results indicate that acyl transfer occurs without hydrolysis, most likely through an acyl-enzyme complex which may be covalently linked. Ca 2+

Immunohistochemical localization of procainamide in normal, ischemic, and necrotic canine myocardium during acute experimental myocardial infarction

This report represents the first application of immunohistochemical methods for localizing an exogenously administered drug. Intravenously administered procainamide was localized in normal, ischemic, and necrotic myocardium in 23 dogs. Rabbit antiprocainamide antibodies were used in an avidin-blotin-peroxidase complex staining method. Normal myocardium demonstrated diffusely positive immunostaining for procainamide, as did the cardiac conduction system and vascular endothelial cells. Necrotic myocardium demonstrated markedly reduced to absent immunostaining. By contrast, in regions of myocardial ischemia without necrosis, immunostaining for procainamide was similar to that in the normal myocardium. Procainamide myocardial tissue levels were reduced in necrotic and ischemic zones compared to normal (p < 0.05) only in those animals in which procainamide was administered after rather than before the onset of coronary occlusion. The demonstration of the absence of drug binding in the necrotic cells suggests that myocardial tissue levels or radiolabelled assessment of drug distribution can be misleading when nonhomogenous tissue is sampled. The immunohistochemical technique provides additional information about the regional and cellular distribution of procainamide that is complementary to the information obtainable by radiolabelling microspheres and from biochemical assays.

Theoretical Analysis of a Technique for the Characterization of Myocardium Contraction Based Upon Temporal Correlation of Ultrasonic Echoes

The temporal correlation properties of ultrasonic echoes from contracting myocardium are investigated theoretically. Myocar- dium is modeled as a dense suspension of moving particles, with time- varying scattering amplitudes. Single scattering is assumed. Echoes from different particles are added coherently. The variance of particle velocity is proposed as an indicator of contractile performance. It is shown how this parameter may be extracted from the average corre- lation of pairs of echoes arising from separate ultrasound pulses. An experiment is performed in which a manufactured target with known motion characteristics is interrogated using a commercial medical ul- trasound scanner. The experimental results are found to be in good agreement with theory. Finally, practical considerations for perform- ing and interpreting these measurements in human myocardium are discussed. EVERAL INVESTIGATIONS have demonstrated that it may be possible to evaluate cardiac contractile per- formance accurately with ultrasound. One technique in- volves the measurement of the frequency average of the intensity of ultrasonic echoes emanating from myocar- dium. Echo intensity has been found to vary throughout the cardiac cycle in normal canine myocardium, with the maximum level occurring at end-diastole and the mini- mum at end-systole ( l), (2). In addition, the magnitude of this effect has a regional dependence throughout the heart, with areas of greater contractile activity exhibiting greater amplitudes of variation (3). The extent of the cyclic variation of echo intensity has also been observed to drop significantly in canine myocardium making a tran- sition from normal to ischemic 141. Thus the magnitude of cyclic variation may be interpreted as a measure of contractile performance. A second technique entails quantitative analysis of video images of the heart produced by ultrasound scan-

Effects of adrenergic and muscarinic receptor stimulation on serum potassium concentrations and myocardial electrical stability.

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 microgram X kg-1 X 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 beta1 adrenoceptor blocking agent metoprolol (0.5 mg X kg-1), adrenaline did not alter vulnerability to ventricular fibrillation but still elicited hypokalaemia. In contrast, selective beta2 adrenoceptor blockade (ICI 118551, 100 micrograms X kg-1) prevented the adrenaline induced lowering of serum potassium concentration but not of ventricular vulnerability. Muscarinic receptor activation by methacholine (3.0 micrograms X kg-1 X 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 beta 1 adrenoceptors and that the beta 2 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.

Electrophysiologic Effects of Bepridil in Normal and Infarcted Canine Myocardium

The electrophysiologic effects of bepridil, 10 mg/kg i.v., were determined in normal noninfarcted and in infarcted ventricular myocardium in 8 urethane-anesthetized dogs 4-6 days after anterior myocardial infarction. At drive cycle lengths of 400 and 333 ms, bepridil significantly increased relative (RRP) and effective (ERP) refractory periods in both normal ventricular tissue (mean increases, RRP 7-14%, ERP 5-6%, p less than 0.05-0.01) and in infarcted ventricular tissue (mean increases, RRP 12-15%, ERP 13-14%, p less than 0.01). Bepridil also selectively prolonged the local activation delay in infarcted ventricular myocardium (mean increases 37.5-45.1%, p less than 0.01), while ventricular excitation thresholds were not altered by bepridil in either normal or infarcted myocardium. Before bepridil administration, programmed ventricular stimulation initiated sustained ventricular tachycardias in 6 of the 8 postinfarction dogs tested. After bepridil, 2 of the 6 previously responsive animals were rendered noninducible, 3 animals responded to programmed stimulation with nonsustained tachyarrhythmias of relatively slower rates, and the one remaining dog responded with sustained ventricular tachycardia (VT). These data suggest that increases in refractoriness in both normal noninjured and in ischemically injured ventricular tissue, with a selective delay in conduction in ischemically injured tissue, contribute to the antiarrhythmic actions of bepridil in the setting of myocardial infarction.

Sodium nuclear magnetic resonance imaging of myocardial tissue of dogs after coronary artery occlusion and reperfusion

Nuclear magnetic resonance (NMR) imaging techniques have been applied to the observation of tissue sodium-23 in normal and ischemic canine myocardium. To produce a region of ischemia and infarction in the myocardium, in six dogs a coronary artery was subjected to 1 hour of surgical occlusion followed by 1 hour of reperfusion. The dogs were then killed and sodium-23 NMR images of the excised hearts were obtained using a high field NMR imaging system. These images were compared with tissue sodium contents measured by flame photometry. The regions of ischemic damage were clearly visible as areas of increased sodium NMR signal on the three-dimensional images. A good correspondence was found between the relative intensity of the sodium signals and the sodium contents of normal myocardium and myocardium subjected to coronary artery occlusion and reperfusion. The data suggest the feasibility of NMR sodium imaging to detect the location and extent of myocardial damage in patients with coronary artery disease.

Myocardial kinetics of thallium-201 after stress in normal and perfusion-reduced canine myocardium

Despite the emerging use of quantitative computer programs for assessing myocardial thallium uptake and clearance after exercise, little is known about the kinetics of thallium after exercise stress. Accordingly, 11 mongrel dogs with experimental left anterior descending coronary stenoses were given thallium during norepinephrine infusion to simulate exercise. The infusion was discontinued and thallium activity was monitored regionally using miniature radiation detectors for 3 hours. Heart rate, arterial pressure and double product all increased significantly during norepinephrine infusion. The mean fractional myocardial thallium clearance was lower (0.47 ± 0.03 [± standard error of the mean]) for the stenosis zone than for the no-stenosis zone (0.57 ± 0.03) (p < 0.0001). The stress blood flow ratio (stenosis/no-stenosis zone = 0.27 ± 0.06) was significantly lower than the final thallium activity ratio (0.68 ± 0.07) (p < 0.001), consistent with thallium redistribution occurring over the 3-hour period. Myocardial thallium activity in the stenosis zone peaked in a mean of 2.2 minutes, then washed out biexponentially with a final decay constant of 0.0035 ± 0.0005 min −1. Myocardial thallium activity in the no-stenosis zone peaked within 1 minute in all dogs, then washed out biexponentially, with a final decay constant of 0.0043 ± 0.0003 (p < 0.001 compared with stenosis zone). In conclusion, fractional clearance of thallium can differentiate myocardium distal to a coronary artery stenosis from that supplied by a normal coronary vessel.

Effect of hyperaemia on thallium-201 redistribution in normal canine myocardium.

The specific effect of hyperaemia on thallium redistribution is not known. Therefore we evaluated the effect of initial (group A) and delayed (group B) hyperaemia on thallium kinetics in normal canine myocardium. In group A dogs (n = 14) adenosine was infused into the left circumflex artery (LCx) prior to thallium injection and was continued for either 5, 30 or 120 min after thallium administration. In group B dogs (n = 10) LCx adenosine was begun 10 min post thallium injection and continued for 2 h. Radioactive microspheres were injected simultaneously with thallium and just prior to death in all but the 5 min experiments. The region of the left anterior descending artery (LAD) served as a normal control for each heart. The mean LCx/LAD flow ratio in all group A hearts was 3.81 +/- 1.24 (SD) and the final thallium LCx/LAD ratios averaged 2.29 +/- 0.26, 1.52 +/- 0.27 and 0.93 +/- 0.16 at 5, 30 and 120 min respectively, which represents a significant redistribution of thallium (p less than 0.05). In group B hearts, the mean LCx/LAD flow ratio was 1.06 +/- 0.07 during thallium injection and after LCx adenosine infusion the flow ratio rose to 2.91 +/- 0.75 (p less than 0.001). This resulted in a final thallium LCx/LAD ratio of 0.96 +/- 0.07 which was significantly less than the initial microsphere determined flow ratio (p less than 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

Metabolic fate of radioiodinated heptadecanoic acid in the normal canine heart.

To clarify the metabolic fate of radioiodinated heptadecanoic acid in myocardium, the time course and distribution of the radioactivity over 131I-heptadecanoic acid, free radioiodide, and various lipids (with incorporated iodoheptadecanoic acid) were determined in normal canine myocardium. In 10 dogs seven biopsy specimens were taken over 30 min after injection of 131I-heptadecanoic acid. The radioactivity in the specimens increased until the fifth minute and decreased thereafter, with a half-time of 36 min. In the fifth minute, 61% of the radioactivity was free iodide, and its curve paralleled the curve of the total radioactivity. As early as the first minute 131I-heptadecanoic acid activity was reduced to 14% and decreased further. Activity of radioiodinated phospholipids, (mono, di, tri)-glycerides, and cholesterol-esters remained constant after an initial increase. These results indicate that immediately after uptake, 131I-heptadecanoic acid is either metabolized, liberating the radioiodide, or stored in lipids. Because the activity of radiolabeled lipids remained constant during the study period and because iodide activity paralleled the total activity in biopsy specimens, it is concluded that in normal myocardium, washout of free radioiodide determines the elimination rate as observed during a scintigraphic study. Thus the elimination rate cannot be related to the beta-oxidation rate as previously supposed.

Mechanism of the effect of bretylium on the ventricular fibrillation threshold in dogs

Experiments were performed to determine the importance of sympathetic blockade vs a direct myocardial effect as a mechanism for the antifibrillatory action of bretylium. The ventricular fibrillation (VF) threshold was determined in open-chest, anesthetized dogs by scanning the vulnerable period with either a single electrical stimulus (10 ms) or a train of electrical stimuli (14 pulses, 4 ms, 100 Hz). Using the train-of-pulses technique, the VF threshold increased from 6.8 ± 0.6 mA to 29.7 ± 6.4 mA 15 minutes after a 10-mg/kg intravenous bolus of bretylium (p < 0.001, n = 8). There was no further significant change in the train-of-pulses VF threshold at 2 or 4 hours. Beta-adrenergic blockade with timolol (0.2 mg/kg) increased the train-of-pulses VF threshold from 6.7 ± 1.6 mA to 24.5 ± 5.2 mA (p >; 0.01 n = 8) and prevented any further significant change in response to bretylium. When single electrical pulses were used to scan the vulnerable period, bretylium at doses of 10 mg/kg (n = 8) and 100 mg/kg (n = 6) did not alter the VF threshold over a 4-hour observation period. The administration of timolol, alone or in combination with bretylium, did not significantly alter the single-pulse VF threshold. The failure of bretylium to alter the single-pulse VF threshold was not dependent on the site of stimulation. Stimulation of the right sympathetic cardiac nerves showed that 15 minutes of bretylium treatment was sufficient to completely inhibit adrenergic neuronal transmission to the myocardium. The data indicate that bretylium has little direct antifibrillatory action on the normal canine myocardium. The ability of bretylium to elevate the VF threshold determined with a train of stimuli appears to be primarily a result of adrenergic neuronal blockade.

Effect of Adenosine on Transmural Flow Gradients in Normal Canine Myocardium

Summary: Previous reports suggest that in comparison to epicardial flow the response of endocardial blood flow to systemic adenosine is limited. However, intravenous adenosine results in reflex tachycardia and hypotension, which we avoided in six anesthetized dogs by infusing adenosine into the left circumflex (LCx) artery to evaluate the endocardial/epicardial ratio during partial and full vasodilation. The mean (± SD) LCx endocardial/epicardial ratio was 1.37 ± 0.18 with partial vasodilation and 0.81 ± 0.06 with full vasodilation, which represents an initial increase followed by a decrease when compared with the normal left anterior descending values (p < 0.05). This biphasic response in the endocardial/epicardial ratio with increasing vasodilation was a direct effect of adenosine, as no hemodynamic alterations occurred during the protocol. These data suggest that in an experimental model with a relative tachycardia the endocardial layer is more sensitive to adenosine but there is a greater capacity to increase flow in the epicardial layer.

Effect of Adenosine on Transmural Flow Gradients in Normal Canine Myocardium

Previous reports suggest that in comparison to epicardial flow the response of endocardial blood flow to systemic adenosine is limited. However, intravenous adenosine results in reflex tachycardia and hypotension, which we avoided in six anesthetized dogs by infusing adenosine into the left circumflex (LCx) artery to evaluate the endocardial/epicardial ratio during partial and full vasodilation. The mean (+/- SD) LCx endocardial/epicardial ratio was 1.37 +/- 0.18 with partial vasodilation and 0.81 +/- 0.06 with full vasodilation, which represents an initial increase followed by a decrease when compared with the normal left anterior descending values (p less than 0.05). This biphasic response in the endocardial/epicardial ratio with increasing vasodilation was a direct effect of adenosine, as no hemodynamic alterations occurred during the protocol. These data suggest that in an experimental model with a relative tachycardia the endocardial layer is more sensitive to adenosine but there is a greater capacity to increase flow in the epicardial layer.

Afterdepolarizations as a mechanism for the long QT syndrome: Electrophysiologic studies of a case

A young woman with palpitation and syncope was found to have ventricular tachyarrhythmia and a congenital long QT interval. The QT interval was shortened and the arrhythmia suppressed by propranolol. Electrograms recorded at various sites in both ventricles revealed a distinct diastolic slow wave that followed the T wave and measured 1.1 mV. Epinephrine infusion and emotion augumented this diastolic wave and induced ventricular ectopic complexes arising from this potential. Similar endocardial recordings in eight patients without a long QT interval showed diastolic slow waves that never exceeded 0.28 mV. In normal canine myocardium, afterdepolarizations can be induced by norepinephrine and blocked by propranolol. These findings suggest that the long QT syndrome is associated with abnormally large afterdepolarizations in ventricular myocardial cells, which are enhanced by beta-adrenergic stimulation to attain threshold and produce firing.

Formula omitted] inhibition of thromboxane synthetase in infarcted canine myocardium

The in vivo effectiveness of the thromboxane synthetase inhibitor OKY-1581 was tested in normal and infarcted canine myocardium. A rapid in vitro assay was developed which permits an accurate assessment of the status of the tissue thromboxane synthetase at the time of sacrifice. Reperfused infarcts were created by two hours of coronary artery occlusion followed by release of occlusion and three days of recovery. OKY-1581 was infused at 100 μg/kg/min for 15 minutes, a dose previously found to cause an 85% inhibition of canine platelet thromboxane synthetase in vivo . The heart was rapidly excised and transmural tissue plugs of infarcted and normal areas were obtained. These were incubated for 5 minutes with prostaglandin endoperoxide (PGH 2) in phosphate buffer. Thromboxane production was inhibited from 16 ± 1 ng TxB 2 per tissue plug to 5 ± 1 in normal myocardium and from 27 ± 5 to 6 ± 1 in infarcted areas of myocardium. Control incubations showed no further inhibition with the in vitro addition of 20 μg/ml OKY-1581, confirming the completeness of in vivo inhibition. Thus significant inhibition of thromboxane synthetase by intravenous OKY-1581 occurs even in a reperfused zone of infarction.

Microbiopsy metabolite and paired flow analysis: a new rapid procedure for homogenisation, extraction and analysis of high energy phosphates and other intermediates without any errors from tissue loss

A new procedure is described which allows for the rapid homogenisation, extraction and analysis of the metabolite content of microbiopsy samples (milligram quantities) while completely overcoming the major errors arising as the consequence of the substantial and variable tissue loss associated with conventional procedures. In addition to allowing more accurate and faster analysis of much smaller quantities of tissue the procedure also allows for the coincident paired measurement of flow (radioactive microspheres) in each biopsy. An example of the application of the method to the measurement of flow and high energy phosphate content in multiple microbiopsy samples from normal and ischaemic canine myocardium is provided.

Distribution of LDH-1 in normal, ischemic, and necrotic myocardium. An immunoperoxidase study.

To study the distribution of lactate dehydrogenase (LDH-1) (H4) in normal, ischemic, and necrotic myocardium using the peroxidase-antiperoxidase technic, the authors studied formalin-fixed paraffin-embedded sections of human (n = 11) and canine (n = 28) myocardium. All normal control myocardium showed positive immunostaining for LDH-1 (H4). In infarcts 10 hours or more old, the histologically necrotic myocardium (by triphenyl tetrazolium chloride staining) (TTC) showed markedly diminished immunostaining. In 24-dogs ischemia was induced in a closed-chest model using a balloon-tipped catheter inflated in the left anterior descending coronary artery. In dogs with 3 hours or more of occlusion, myocardium that was necrotic by TTC staining, light and/or electron microscopy, showed diminished staining for LDH-1, while normal, control myocardium stained intensely. In four dogs, ischemia was induced by a controlled perfusion apparatus by which left main coronary flow was reduced by 50%. Ischemia without necrosis was documented by demonstration of glycogen loss with no light or electron microscopic evidence of necrosis. These ischemic fibers stained intensely for LDH-1, as did controls. Thus, by immunoperoxidase staining, LDH-1 can be demonstrated in normal human and canine myocardium. In experimental models of ischemia in dogs, tissue that was ischemic but not necrotic showed no diminished staining. LDH-1 loss can be detected in necrotic myocardium as early as 3 hours after coronary artery occlusion.

Uptake of iodinated fatty acids and thallium-201 in normal canine myocardium

Uptake of iodinated fatty acids and thallium-201 in normal canine myocardium