Use of the perfused rat heart to study cardiac metabolism: retrospective and prospective views

Use of the Isolated Perfused Heart for Evaluation of Cardiac Toxicity

The isolated perfused rat heart model can be used to evaluate cardiotoxicity, and is especially useful in distinguishing direct vs indirect cardiac injury. Various perfusion systems can be used to characterize the pathophysiologic as well as morphologic changes induced by drugs or chemicals of interest. The isolated perfused heart was used in the studies described herein to characterize the mechanism of allylamine cardiotoxicity. Rat hearts were perfused with Krebs-Henseleit buffer containing 10 mm allylamine and a latex balloon was inserted into the left ventricle to monitor pressure. Coronary flow in hearts perfused with 10 mm allylamine was similar to control hearts at 5, 10, and 30 min, but was reduced by 1 hr (11.5 ± 0.6 ml/min/g wet heart weight vs 16.0 ± 0.7, p < 0.01). Peak left ventricular systolic pressure increased in hearts perfused with allylamine for 5 min (156 ± 8 mm Hg vs 103 ± 9, p < 0.01), but by 2 hr was decreased compared to controls (89 ± 6 vs 105 ± 5, p < 0.05). End diastolic pressure was markedly increased at 2 hr (58 ± 3 vs 4 ± 0.8, p < 0.01). Morphologically, allylamine perfused hearts exhibited significant contraction band changes as well as numerous cells with marked swelling of the sarcoplasmic reticulum. The findings in this study suggest that allylamine produces direct myocardial damage that appears to be independent of coronary flow. These studies demonstrate that the isolated perfused rat heart model can be used to evaluate mechanisms of acute cardiotoxicity.

Oxygen demand of perfused heart preparations: how electromechanical function and inadequate oxygenation affect physiology and optical measurements

NEW FINDINGS

What is the topic of this review? This review discusses how the function and electrophysiology of isolated perfused hearts are affected by oxygenation and energy utilization. The impact of oxygenation on fluorescence measurements in perfused hearts is also discussed. What advances does it highlight? Recent studies have illuminated the inherent differences in electromechanical function, energy utilization rate and oxygen requirements between the primary types of excised heart preparations. A summary and analysis of how these variables affect experimental results are necessary to elevate the physiological relevance of these approaches in order to advance the field of whole-heart research. The ex vivo perfused heart recreates important aspects of in vivo conditions to provide insight into whole-organ function. In this review we discuss multiple types of ex vivo heart preparations, explain how closely each mimic in vivo function, and discuss how changes in electromechanical function and inadequate oxygenation of ex vivo perfused hearts may affect measurements of physiology. Hearts that perform physiological work have high oxygen demand and are likely to experience hypoxia when perfused with a crystalloid perfusate. Adequate myocardial oxygenation is critically important for obtaining physiologically relevant measurements, so when designing experiments the type of ex vivo preparation and the capacity of perfusate to deliver oxygen must be carefully considered. When workload is low, such as during interventions that inhibit contraction, oxygen demand is also low, which could dramatically alter a physiological response to experimental variables. Changes in oxygenation also alter the optical properties of cardiac tissue, an effect that may influence optical signals measured from both endogenous and exogenous fluorophores. Careful consideration of oxygen supply, working condition, and wavelengths used to acquire optical signals is critical for obtaining physiologically relevant measurements during ex vivo perfused heart studies.

PET imaging of cardiac hypoxia: opportunities and challenges

Myocardial hypoxia is a major factor in the pathology of cardiac ischemia and myocardial infarction. Hypoxia also occurs in microvascular disease and cardiac hypertrophy, and is thought to be a prime determinant of the progression to heart failure, as well as the driving force for compensatory angiogenesis. The non-invasive delineation and quantification of hypoxia in cardiac tissue therefore has the potential to be an invaluable experimental, diagnostic and prognostic biomarker for applications in cardiology. However, at this time there are no validated methodologies sufficiently sensitive or reliable for clinical use. PET imaging provides real-time spatial information on the biodistribution of injected radiolabeled tracer molecules. Its inherent high sensitivity allows quantitative imaging of these tracers, even when injected at sub-pharmacological (≥pM) concentrations, allowing the non-invasive investigation of biological systems without perturbing them. PET is therefore an attractive approach for the delineation and quantification of cardiac hypoxia and ischemia. In this review we discuss the key concepts which must be considered when imaging hypoxia in the heart. We summarize the PET tracers which are currently available, and we look forward to the next generation of hypoxia-specific PET imaging agents currently being developed. We describe their potential advantages and shortcomings compared to existing imaging approaches, and what is needed in terms of validation and characterization before these agents can be exploited clinically.

Increased uncoupling proteins and decreased efficiency in palmitate-perfused hyperthyroid rat heart

The physiological role of mitochondrial uncoupling proteins (UCPs) in heart and skeletal muscle is unknown, as is whether mitochondrial uncoupling of oxidative phosphorylation by fatty acids occurs in vivo. In this study, we found that UCP2 and UCP3 protein content, determined using Western blotting, was increased by 32 and 48%, respectively, in hyperthyroid rat heart mitochondria. Oligomycin-insensitive respiration rate, a measure of mitochondrial uncoupling, was increased in all mitochondria in the presence of palmitate: 36% in controls and 71 and 100% with 0.8 and 0.9 mM palmitate, respectively, in hyperthyroid rat heart mitochondria. In the isolated working heart, 0.4 mM palmitate significantly lowered cardiac output by 36% and cardiac efficiency by 38% in the hyperthyroid rat heart. Thus increased mitochondrial UCPs in the hyperthyroid rat heart were associated with increased uncoupling and decreased myocardial efficiency in the presence of palmitate. In conclusion, a physiological effect of UCPs on fatty acid oxidation has been found in heart at the mitochondrial and whole organ level.

Acute cardiotoxicity of gadolinium-based contrast media: findings in the isolated rat heart

RATIONALE AND OBJECTIVE

The study was designed to compare acute and direct cardiotoxicity of gadopentetate dimeglumine, gadoteridol injection, and gadodiamide injection.

METHODS

Two consecutive injections of contrast material (0.3 to 1.5 mmol/kg body weight) were given to spontaneously beating, isolated rat hearts. Cardiac function was assessed by measuring left ventricular developed pressure (LVDP), perfusion pressure, and electrocardiographic (ECG) parameters.

RESULTS

Gadopentetate dimeglumine decreased LVDP, and its first derivatives (+/- dP/dt max) substantially more than gadodiamide injection. Gadoteridol increased these variables. PR and RT intervals lengthened after 1.5 mmol/kg gadopentetate dimeglumine and gadoteridol injection. Bradycardia and ventricular tachyarrhythmias were seen after injection of 1.5 mmol/kg gadopentetate dimeglumine. Except for isolated ventricular premature beats, gadodiamide injection and gadoteridol injection did not provoke any serious arrhythmia.

CONCLUSION

Gadopentatate dimeglumine induced negative inotropy and more pronounced ECG disturbances. Gadoteridol injection and gadodiamide injection induced only small changes in left ventricular inotropy and minor electrophysiologic effects.

Inhibition of cardiolipin biosynthesis in the hypoxic rat heart

Cardiolipin is the principal polyglycerophospholipid in the heart. The effect of hypoxia on cardiolipin biosynthesis was investigated in isolated rat hearts perfused in the Langendorff mode. Hearts were pulsed-labeled for 60 min with 0.1 mM [1,(3)-3H]glycerol in Krebs Henseleit buffer saturated with either 95% O2/5% CO2 (control) or 95% N2/5% CO2 (hypoxic). Radioactivity incorporated into phosphatidylglycerol and cardiolipin were reduced 88% (P < .05) and 79% (P < .05), respectively, in hypoxic hearts compared to controls. In other experiments, hearts were pulse-labeled for 15 min with 1.4 mM [32P]Pi in Krebs Henseleit buffer saturated with 95% O2/5% CO2 and subsequently perfused for 60 min under control or hypoxic conditions. The radioactivity incorporated into CDP-1,2-diacyl-sn-glycerol, phosphatidylglycerol, and cardiolipin were reduced 61% (P < .05), 71% (P < .05), and 70% (P < .05), respectively, in the hypoxic hearts compared to controls, indicating a decreased formation of CDP-1,2-diacyl-sn-glycerol in the hypoxic heart. The activities of the enzymes involved in cardiolipin biosynthesis and the cardiac pool sizes of cardiolipin, phosphatidylglycerol, and CDP-1,2-diacyl-1,2-diacyl-sn-glycerol were unaltered between hypoxic and control hearts. In contrast, cardiac adenosine-5'-triphosphate and CPT levels were decreased 94% (P < .05) and 92% (P < .05), respectively, in hypoxic hearts compared to controls. We postulate that the biosynthesis of the cardiac polyglycerophospholipid cardiolipin may be inhibited by a decreased adenosine-5'-triphosphate and cytidine-5'-triphosphate level in the heart.

Investigation of cardiac metabolism using stable isotopes and mass spectrometry

The technique described in this communication enables detailed investigations of cardiac metabolism using 13C-labeled substrates and mass spectrometric measurements of 13CO2 in the coronary effluent. To validate this technique for further studies isolated working rat hearts were perfused with 13C-labeled substrates in a bicarbonate-free perfusion fluid. The fraction of CO2 produced by oxidation of labeled substrate was calculated by the 13CO2/CO2 ratio in the coronary perfusate. The oxidation of 13C-acetate showed a linear correlation with 13C-acetate concentrations between 0.015 and 0.16 mmol/l. An inhibitor of acylcarnitine translocase, 2-(3-methylcinnamylhydrazono)-propionate (BM42.304) decreased CO2 production from 13C-palmitate from 48% +/- 4% to 31% +/- 3% (n = 11, SEM). Taking into account considerations of tracer kinetic theory rapidly accessible intracellular palmitate stores were estimated to be less than 900 nmol/g ww. This technique allows specific investigations of the oxidation of labeled substrates in the heart and may be useful for basic research and/or clinical diagnosis, thus avoiding the hazards of radiolabeled substrates.

Isolated biventricular working rat heart preparation

The isolated perfused heart from small animals has been used extensively for hemodynamic and metabolic studies. The left working heart preparation proved superior to the Langendorff model for functional evaluations but has not allowed study of right heart function. A simple and inexpensive biventricular working heart preparation has been developed by modifying the left working rat heart model. Under general anesthesia the heart was removed surgically leaving sufficient vessels attached to it. Cannulation of the aorta, left atrium, right atrium, and pulmonary artery was completed in 10 minutes. A pressurized compliance chamber allowed rapid and reliable regulation of aortic impedance. For the 7 hearts that were subjected to 3-hour biventricular perfusion (their end points expressed as percent of their initial values), the aortic output (95% +/- 3%), pulmonary flow (88% +/- 9%), mean aortic pressure (109% +/- 5%), mean pulmonary pressure (100% +/- 2%), heart rate (106% +/- 8%), myocardial adenosine triphosphate level (85% +/- 8%), and creatine phosphate level (89% +/- 4%) were all maintained at physiologic levels. For the 11 hearts that were converted from left working heart preparation to biventricular working mode, significant improvement in stroke volume, aortic and cardiac output, and pressure development were observed. Experimental results indicate that the biventricular working model for isolated perfused rat hearts is superior to the left working preparation for studying the function of the total heart. Further study of the biventricular perfused working rat heart appears warranted.

Mechanisms for cardiac depression induced by phorbol myristate acetate in working rat hearts

1. The effects of the phorbol ester, phorbol myristate acetate (PMA) were examined on function and energy metabolism in the isolated working heart of the rat. 2. At a concentration of 10(-9) M PMA produced a rapid loss in cardiac function in terms of aortic flow rate (AFR) and coronary flow rates (CFR) whereas a similar concentration of 4 alpha-phorbol 12,13-didecanoate was ineffective. At a concentration of 10(-10) M, the PMA-induced depression was more gradual but nevertheless very pronounced with an almost total loss in AFR after 30 min perfusion. The reduction in CFR was more moderate than that observed with respect to AFR. 3. The protein kinase C (PKC) inhibitor (+/-)-1-O-hexadecyl-2-O-acylglycerol significantly attenuated the loss in AFR and CFR following addition of PMA. 4. Two inhibitors of Na+/H+ exchange, amiloride and quinacrine, totally prevented the reduction in AFR. Although the PMA-induced depression in CFR was also attenuated by both amiloride and quinacrine, these effects were not significant, probably reflecting the less pronounced effect of PMA on this parameter. 5. Nifedipine, a dihydropyridine calcium channel blocker reduced PMA toxicity to a similar degree as Na+/N+ exchange inhibition whereas the calcium channel agonist Bay K 8644 was without effect. 6. Tissue content of energy metabolites including high energy phosphates, total adenine nucleotides or lactate were not significantly affected by PMA perfusion. 7. We conclude that PKC activation is necessary for phorbol ester-induced cardiac dysfunction. The consequence of PKC stimulation includes (1) Na+/H+ exchange activation and a subsequent elevation in intracellular calcium [Ca2+]i via Na+/Ca2+ exchange and (2) PKC-dependent phosphorylation of the calcium channel, both of which would produce toxicity by elevation of [Ca21]i. Pharmacological manipulation of any of these steps prevents PMA toxicity by virtue of a reduction in the accumulation of [Ca21]i. PMA effects or their prevention are unrelated to any changes in energy metabolism.

Role of ancillary properties of beta-adrenoceptor antagonists in protecting the heart from anoxia

The mechanism of the cardioprotective action of beta-blocking drugs against anoxia or ischemia is still not clear. We used beta-blockers (5 x 10(-7) M) of various pharmacodynamic profiles in a model of isolated, perfused working guinea pig heart subjected to 20 min of anoxia to study this. The cardioprotective effects were evaluated by measuring the recovery of the flow indices after 15 min of reoxygenation. There was a significant cardioprotective action (as measured by the effect on stroke volume recovery and on recovery of other flow and work indices) of the beta-blocking properties (nadolol, P less than 0.05), of the membrane-stabilizing property [+)-propranolol, P less than 0.05) and of a combination of these two properties with (+/-)-propranolol, which had a significantly greater effect than nadolol (P less than 0.05). The addition of weak (acebutolol) or strong (pindolol) intrinsic partial agonist activity had no clear unfavourable effect, as the degree of cardioprotection was comparable with that obtained with (+/-)-propranolol. The stroke volume recovery (percent recovery after anoxia) in the control hearts was 42.57 +/- 12.75 compared to 54.84 +/- 6.94 in hearts pretreated with nadolol, 62.99 +/- 11.41 with (+)-propranolol, 71.02 +/- 11.36 with (+/-)-propranolol, 72.63 +/- 13.08 with acebutolol and 68.01 +/- 15.42 with pindolol. In vitro heart protection from anoxia with beta-blockers would appear to be related to beta-blocking activity and/or membrane stabilizing property but not a function of partial agonist activity. These ancillary properties of beta-blockers should thus be taken into account in studies on cardioprotection.

Carbon flux through citric acid cycle pathways in perfused heart by 13C NMR spectroscopy

Mathematical models of the TCA cycle derived previously for 14C tracer studies have been extended to 13C NMR to measure the 13C fractional enrichment of [2-13C]acetyl-CoA entering the cycle and the relative activities of the oxidative versus anaplerotic pathways. The analysis is based upon the steady-state enrichment of 13C into the glutamate carbons. Hearts perfused with [2-13C]acetate show low but significant activity of the anaplerotic pathways. Activation of two different anaplerotic pathways is demonstrated by addition of unlabeled propionate or pyruvate to hearts perfused with [2-13C]acetate. In each case, the amount of [2-13C]acetate being oxidized and the relative carbon flux through anaplerotic versus oxidative pathways are evaluated.

Importance of endogenous substrates for cultured adult rat cardiac myocytes

In Ca-tolerant adult cardiomyocytes the contribution of endogenous substrates (glycogen, tri- and diacylglycerol) to oxidative substrate metabolism was investigated. After 4 h in culture medium (M 199 plus 4% fetal calf serum) the cellular triacylglycerol content is 3.6-fold higher than in fresh myocardium and reflects the free fatty acid composition of the medium. When triacylglycerol is degraded, all long-chain fatty acids are hydrolysed at equal rates. In these quiescent cells, the activity of pyruvate dehydrogenase is low (10% of full activity, in Tyrode solution with 5 mM glucose). Up to 30% of full pyruvate dehydrogenase activity, the contribution of non-lipid substrates (glycogen, glucose, lactate and pyruvate) to oxidative energy production is correlated to pyruvate dehydrogenase activity. At 5 mM medium concentration, glucose, lactate and pyruvate share in energy production the proportions of 15, 36 and 50%, whereas endogenous lipolysis accounts for 78, 61 and 46%. It is concluded that these quiescent cardiomyocytes represent cardiac metabolism in a basal state in which the preference for fatty acids, especially from endogenous lipids, is very pronounced. The utilization of endogenous substrates therefore has to be considered in all studies investigating the oxidative metabolism of these isolated cells.