The erythrocyte-perfused "working heart" model: hemodynamic and metabolic performance in comparison to crystalloid perfused hearts

Importance of cardiac-synchronized vagus nerve stimulation parameters on the provoked chronotropic response for different levels of cardiac innervation

Introduction

The influence of vagus nerve stimulation (VNS) parameters on provoked cardiac effects in different levels of cardiac innervation is not well understood yet. This study examines the effects of VNS on heart rate (HR) modulation across a spectrum of cardiac innervation states, providing data for the potential optimization of VNS in cardiac therapies.

Materials and Methods

Utilizing previously published data from VNS experiments on six sheep with intact innervation, and data of additional experiments in five rabbits post bilateral rostral vagotomy, and four isolated rabbit hearts with additionally removed sympathetic influences, the study explored the impact of diverse VNS parameters on HR.

Results

Significant differences in physiological threshold charges were identified across groups: 0.09 ± 0.06 μC for intact, 0.20 ± 0.04 μC for vagotomized, and 9.00 ± 0.75 μC for isolated hearts. Charge was a key determinant of HR reduction across all innervation states, with diminishing correlations from intact (r = 0.7) to isolated hearts (r = 0.44). An inverse relationship was observed for the number of pulses, with its influence growing in conditions of reduced innervation (intact r = 0.11, isolated r = 0.37). Frequency and stimulation delay showed minimal correlations (r < 0.17) in all conditions.

Conclusion

Our study highlights for the first time that VNS parameters, including stimulation intensity, pulse width, and pulse number, crucially modulate heart rate across different cardiac innervation states. Intensity and pulse width significantly influence heart rate in innervated states, while pulse number is key in denervated states. Frequency and delay have less impact impact across all innervation states. These findings suggest the importance of customizing VNS therapy based on innervation status, offering insights for optimizing cardiac neuromodulation.

A novel ex-vivo isolated rabbit heart preparation to explore the cardiac effects of cervical and cardiac vagus nerve stimulation

The cardiac responses to vagus nerve stimulation (VNS) are still not fully understood, partly due to uncontrollable confounders in the in-vivo experimental condition. Therefore, an ex-vivo Langendorff-perfused rabbit heart with intact vagal innervation is proposed to study VNS in absence of cofounding anesthetic or autonomic influences. The feasibility to evoke chronotropic responses through electrical stimulation ex-vivo was studied in innervated isolated rabbit hearts (n = 6). The general nerve excitability was assessed through the ability to evoke a heart rate (HR) reduction of at least 5 bpm (physiological threshold). The excitability was quantified as the charge needed for a 10-bpm HR reduction. The results were compared to a series of in-vivo experiments rabbits (n = 5). In the ex-vivo isolated heart, the baseline HR was about 20 bpm lower than in-vivo (158 ± 11 bpm vs 181 ± 19 bpm). Overall, the nerve remained excitable for about 5 h ex-vivo. The charges required to reduce HR by 5 bpm were 9 ± 6 µC and 549 ± 370 µC, ex-vivo and in-vivo, respectively. The charges needed for a 10-bpm HR reduction, normalized to the physiological threshold were 1.78 ± 0.8 and 1.22 ± 0.1, in-vivo and ex-vivo, respectively. Overall, the viability of this ex-vivo model to study the acute cardiac effects of VNS was demonstrated.

Closed-loop vagus nerve stimulation for heart rate control evaluated in the Langendorff-perfused rabbit heart

Persistent sinus tachycardia substantially increases the risk of cardiac death. Vagus nerve stimulation (VNS) is known to reduce the heart rate, and hence may be a non-pharmacological alternative for the management of persistent sinus tachycardia. To precisely regulate the heart rate using VNS, closed-loop control strategies are needed. Therefore, in this work, we developed two closed-loop VNS strategies using an in-silico model of the cardiovascular system. Both strategies employ a proportional-integral controller that operates on the current amplitude. While one control strategy continuously delivers stimulation pulses to the vagus nerve, the other applies bursts of stimuli in synchronization with the cardiac cycle. Both were evaluated in Langendorff-perfused rabbit hearts (n = 6) with intact vagal innervation. The controller performance was quantified by rise time (T_r), steady-state error (SSE), and percentual overshoot amplitude (%OS). In the ex-vivo setting, the cardiac-synchronized variant resulted in T_r = 10.7 ± 4.5 s, SSE = 12.7 ± 9.9 bpm and %OS = 5.1 ± 3.6% while continuous stimulation led to T_r = 10.2 ± 5.6 s, SSE = 10 ± 6.7 bpm and %OS = 3.2 ± 1.9%. Overall, both strategies produced a satisfying and reproducible performance, highlighting their potential use in persistent sinus tachycardia.

Live Intravital Imaging of Cellular Trafficking in the Cardiac Microvasculature-Beating the Odds

Although mortality rates from cardiovascular disease in the developed world are falling, the prevalence of cardiovascular disease (CVD) is not. Each year, the number of people either being diagnosed as suffering with CVD or undergoing a surgical procedure related to it, such as percutaneous coronary intervention, continues to increase. In order to ensure that we can effectively manage these diseases in the future, it is critical that we fully understand their basic physiology and their underlying causative factors. Over recent years, the important role of the cardiac microcirculation in both acute and chronic disorders of the heart has become clear. The recruitment of inflammatory cells into the cardiac microcirculation and their subsequent activation may contribute significantly to tissue damage, adverse remodeling, and poor outcomes during recovery. However, our basic understanding of the cardiac microcirculation is hampered by an historic inability to image the microvessels of the beating heart-something we have been able to achieve in other organs for over 100 years. This stems from a couple of clear and obvious difficulties related to imaging the heart-firstly, it has significant inherent contractile motion and is affected considerably by the movement of lungs. Secondly, it is located in an anatomically challenging position for microscopy. However, recent microscopic and technological developments have allowed us to overcome some of these challenges and to begin to answer some of the basic outstanding questions in cardiac microvascular physiology, particularly in relation to inflammatory cell recruitment. In this review, we will discuss some of the historic work that took place in the latter part of last century toward cardiac intravital, before moving onto the advanced work that has been performed since. This work, which has utilized technology such as spinning-disk confocal and multiphoton microscopy, has-along with some significant advancements in algorithms and software-unlocked our ability to image the "business end" of the cardiac vascular tree. This review will provide an overview of these techniques, as well as some practical pointers toward software and other tools that may be useful for other researchers who are considering utilizing this technique themselves.

Paradoxical arteriole constriction compromises cytosolic and mitochondrial oxygen delivery in the isolated saline-perfused heart

The isolated saline-perfused heart is used extensively to study cardiac physiology. Previous isolated heart studies have demonstrated lower tissue oxygenation compared with in vivo hearts based on myoglobin oxygenation and the mitochondrial redox state. These data, consistent with small anoxic regions, suggest that the homeostatic balance between work and oxygen delivery is impaired. We hypothesized that these anoxic regions are caused by inadequate local perfusion due to a paradoxical arteriole constriction generated by a disrupted vasoregulatory network. We tested this hypothesis by applying two exogenous vasodilatory agents, adenosine and cromakalim, to relax vascular tone in an isolated, saline-perfused, working rabbit heart. Oxygenation was monitored using differential optical transmission spectroscopy and full spectral fitting. Increases in coronary flow over control with adenosine (27 ± 4 ml/min) or cromakalim (44 ± 4 ml/min) were associated with proportional spectral changes indicative of myoglobin oxygenation and cytochrome oxidase (COX) oxidation, consistent with a decrease in tissue anoxia. Quantitatively, adenosine decreased deoxymyoglobin optical density (OD) across the wall by 0.053 ± 0.008 OD, whereas the reduced form of COX was decreased by 0.039 ± 0.005 OD. Cromakalim was more potent, decreasing deoxymyoglobin and reducing the level of COX by 0.070 ± 0.019 OD and 0.062 ± 0.019 OD, respectively. These effects were not species specific, as Langendorff-perfused mouse hearts treated with adenosine demonstrated similar changes. These data are consistent with paradoxical arteriole constriction as a major source of regional anoxia during saline heart perfusion. We suggest that the vasoregulatory network is disrupted by the washout of interstitial vasoactive metabolites in vitro. NEW & NOTEWORTHY Regional tissue anoxia is a common finding in the ubiquitous saline-perfused heart but is not found in vivo. Noninvasive optical techniques confirmed the presence of regional anoxia under control conditions and demonstrated that anoxia is diminished using exogenous vasodilators. These data are consistent with active arteriole constriction, occurring despite regional anoxia, generated by a disrupted vasoregulatory network. Washout of interstitial vasoactive metabolites may contribute to the disruption of normal vasoregulatory processes in vitro.

Normothermic Ex Vivo Heart Perfusion: Effects of Live Animal Blood and Plasma Cross Circulation

Prolonged normothermic ex vivo heart perfusion could transform cardiac transplantation. To help identify perfusate components that might enable long-term perfusion, we evaluated the effects of cross-circulated whole blood and cross-circulated plasma from a live paracorporeal animal on donor porcine hearts preserved via normothermic ex vivo heart perfusion. Standard perfusion (SP; n = 40) utilized red blood cell/plasma perfusate and Langendorff technique for a goal of 12 hours. Cross-circulation groups used a similar circuit with the addition of cross-circulated venous whole blood (XC-blood; n = 6) or cross-circulated filtered plasma (XC-plasma; n = 7) between a live paracorporeal pig under anesthesia and the perfusate reservoir. Data included oxygen metabolism, vascular resistance, lactate production, left ventricular function, myocardial electrical impedance, and histopathologic injury score. All cross-circulation hearts were successfully perfused for 12 hours, compared with 22 of 40 SP hearts (55%; p = 0.002). Both cross-circulation groups demonstrated higher oxygen consumption and vascular resistance than standard hearts from hours 3-12. No significant differences were seen between XC-blood and XC-plasma hearts in any variable, including left ventricular dP/dT after 12 hours (1478 ± 700 mm Hg/s vs. 872 ± 500; p = 0.17). We conclude that cross circulation of whole blood or plasma from a live animal improves preservation of function of perfused hearts, and cross-circulated plasma performs similarly to cross-circulated whole blood.

Organ preservation: from the past to the future

Organ transplantation is the most effective therapy for patients with end-stage disease. Preservation solutions and techniques are crucial for donor organ quality, which is directly related to morbidity and survival after transplantation. Currently, static cold storage (SCS) is the standard method for organ preservation. However, preservation time with SCS is limited as prolonged cold storage increases the risk of early graft dysfunction that contributes to chronic complications. Furthermore, the growing demand for the use of marginal donor organs requires methods for organ assessment and repair. Machine perfusion has resurfaced and dominates current research on organ preservation. It is credited to its dynamic nature and physiological-like environment. The development of more sophisticated machine perfusion techniques and better perfusates may lead to organ repair/reconditioning. This review describes the history of organ preservation, summarizes the progresses that has been made to date, and discusses future directions for organ preservation.

Cardiac performance is limited by oxygen delivery to the mitochondria in the crystalloid-perfused working heart

The left ventricular working, crystalloid-perfused heart is used extensively to evaluate basic cardiac function, pathophysiology, and pharmacology. Crystalloid-perfused hearts may be limited by oxygen delivery, as adding oxygen carriers increases myoglobin oxygenation and improves myocardial function. However, whether decreased myoglobin oxygen saturation impacts oxidative phosphorylation (OxPhos) is unresolved, since myoglobin has a much lower affinity for oxygen than cytochrome c oxidase (COX). In the present study, a laboratory-based synthesis of an affordable perfluorocarbon (PFC) emulsion was developed to increase perfusate oxygen carrying capacity without impeding optical absorbance assessments. In left ventricular working hearts, along with conventional measurements of cardiac function and metabolic rate, myoglobin oxygenation and cytochrome redox state were monitored using a novel transmural illumination approach. Hearts were perfused with Krebs-Henseleit (KH) or KH supplemented with PFC, increasing perfusate oxygen carrying capacity by 3.6-fold. In KH-perfused hearts, myoglobin was deoxygenated, consistent with cytoplasmic hypoxia, and the mitochondrial cytochromes, including COX, exhibited a high reduction state, consistent with OxPhos hypoxia. PFC perfusate increased aortic output from 76 ± 6 to 142 ± 4 ml/min and increased oxygen consumption while also increasing myoglobin oxygenation and oxidizing the mitochondrial cytochromes. These results are consistent with limited delivery of oxygen to OxPhos resulting in an adapted lower cardiac performance with KH. Consistent with this, PFCs increased myocardial oxygenation, and cardiac work was higher over a wider range of perfusate Po₂. In summary, heart mitochondria are limited by oxygen delivery with KH; supplementation of KH with PFC reverses mitochondrial hypoxia and improves cardiac performance, creating a more physiological tissue oxygen delivery. NEW & NOTEWORTHY Optical absorbance spectroscopy of intrinsic chromophores reveals that the commonly used crystalloid-perfused working heart is oxygen limited for oxidative phosphorylation and associated cardiac work. Oxygen-carrying perfluorocarbons increase myocardial oxygen delivery and improve cardiac function, providing a more physiological mitochondrial redox state and emphasizing cardiac work is modulated by myocardial oxygen delivery.

Improving the physiological realism of experimental models

The Virtual Physiological Human (VPH) project aims to develop integrative, explanatory and predictive computational models (C-Models) as numerical investigational tools to study disease, identify and design effective therapies and provide an in silico platform for drug screening. Ultimately, these models rely on the analysis and integration of experimental data. As such, the success of VPH depends on the availability of physiologically realistic experimental models (E-Models) of human organ function that can be parametrized to test the numerical models. Here, the current state of suitable E-models, ranging from in vitro non-human cell organelles to in vivo human organ systems, is discussed. Specifically, challenges and recent progress in improving the physiological realism of E-models that may benefit the VPH project are highlighted and discussed using examples from the field of research on cardiovascular disease, musculoskeletal disorders, diabetes and Parkinson's disease.

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.

The impact of crystalloidal and colloidal infusion preparations on coronary vascular integrity, interstitial oedema and cardiac performance in isolated hearts

INTRODUCTION

Recent data suggested an interaction between plasma constituents and the endothelial glycocalyx to be relevant for vascular barrier function. This might be negatively influenced by infusion solutions, depending on ionic composition, pH and binding properties. The present study evaluated such an influence of current artificial preparations.

METHODS

Isolated guinea pig hearts were prepared in a modified Langendorff mode and perfused with Krebs-Henseleit buffer augmented with 1g% human albumin. After equilibration the perfusion was switched to replacement of one half buffer by either isotonic saline (NaCl), ringer's acetate (Ri-Ac), 6% and 10% hydroxyethyl starch (6% and 10% HES, resp.), or 4% gelatine (Gel), the artificial colloids having been prepared in balanced solution. We analysed glycocalyx shedding, functional integrity of the vascular barrier and heart performance.

RESULTS

While glycocalyx shedding was not observed, diluting albumin concentration towards 0.5g% by artificial solutions was associated with a marked functional breakdown of vascular barrier competence. This effect was biggest with isotonic saline and significantly attenuated with artificial colloids, the difference in the pressure dependent transvascular fluid filtration (basal vs. during infusion in groups NaCl, Ri-Ac, 6% HES, 10% HES and Gel, n = 6 each) being 0.31 ± 0.03 vs. 1.00 ± 0.04; 0.27 ± 0.03 vs. 0.81 ± 0.03; 0.29 ± 0.03 vs. 0.68 ± 0.02; 0.32 ± 0.03 vs. 0.59 ± 0.08 and 0.31 ± 0.04 vs. 0.61 ± 0.03 g/5min, respectively. Heart performance was directly related to pH value (7.38 ± 0.06, 7.33 ± 0.03, 7.14 ± 0.04, 7.08 ± 0.04, 7.25 ± 0.03), the change in the rate pressure product being 21,702 ± 1969 vs. 21,291 ± 2,552; 22,098 ± 2,115 vs. 14,114 ± 3,386; 20,897 ± 2,083 vs. 10,671 ± 1,948; 21,822 ± 2,470 vs. 10,047 ± 2,320 and 20,955 ± 2,296 vs. 15,951 ± 2,755 mmHg × bpm, respectively.

CONCLUSIONS

It appears important to maintain the pH value within a physiological range to maintain optimal myocardial contractility. Using colloids prepared in calcium-containing, balanced solutions for volume replacement therapy may attenuate the breakdown of vascular barrier competence in the critically ill.

Effects of autologous, cross-linked erythrocytes on isolated hypoperfused rabbit heart dynamics

The present study was aimed at assessing the effects of either red blood cells (RBC) or RBC cross-linked with the bifunctional dimethyl suberimidate reagent (C-RBC) on contractile force (CFo), heart rate (HR) and coronary flow (CF) of the isolated rabbit heart hypoperfused with RBC suspensions under 30 mm Hg constant pressure. RBC or C-RBC caused a rapid and marked reduction of CF, CFo and HR. In RBC-treated hearts, however, reperfusion with Tyrode solution partially restored the initial myocardial parameters, while in C-RBC-treated hearts a rapid impairment of diastolic relaxation with a subsequent, steady and increasing heart contracture was observed. Histological analysis showed that in C-RBC-perfused hearts either capillaries or precapillary arterioles were occluded by C-RBC in spite of extensive washings with Tyrode solution. These findings indicate that C-RBC impair coronary circulation markedly and irreversibly.

The continuing evolution of the Langendorff and ejecting murine heart: new advances in cardiac phenotyping

The isolated retrograde-perfused Langendorff heart and the isolated ejecting heart have, over many decades, resulted in fundamental discoveries that form the underpinnings of our current understanding of the biology and physiology of the heart. These two experimental methodologies have proven invaluable in studying pharmacological effects on myocardial function, metabolism, and vascular reactivity and in the investigation of clinically relevant disease states such as ischemia-reperfusion injury, diabetes, obesity, and heart failure. With the advent of the genomics era, the isolated mouse heart preparation has gained prominence as an ex vivo research tool for investigators studying the impact of gene modification in the intact heart. This review summarizes the historical development of the isolated heart and provides a practical guide for the establishment of the Langendorff and ejecting heart preparations with a particular emphasis on the murine heart. In addition, current applications and novel methods of recording cardiovascular parameters in the isolated heart preparation will be discussed. With continued advances in methodological recordings, the isolated mouse heart preparation will remain physiologically relevant for the foreseeable future, serving as an integral bridge between in vitro assays and in vivo approaches.

Improved myocardial protection in the failing heart by selective endothelin-A receptor blockade

OBJECTIVE

Ischemia/reperfusion injury caused by cardioplegic arrest is still a major challenge in patients with reduced left ventricular function. We investigated the effect of chronic versus acute administration of the selective endothelin-A receptor antagonist (ERA) TBC-3214Na during ischemia/reperfusion in failing hearts.

METHODS

Male Sprague-Dawley rats underwent coronary ligation. Three days after myocardial infarction (MI), 19 randomly assigned animals (ERA chronic) were administered TBC-3214Na continuously with their drinking water, 29 MI rats received placebo, and 3 rats died during the observation period. Six weeks after infarction, hearts were evaluated in a blood-perfused working heart model during 60 minutes of ischemia and 30 minutes of reperfusion. In 14 MI rats, TBC-3214Na (ERA acute) was added to the cardioplegic solution during ischemia. Thirteen MI rats served as control.

RESULTS

At a similar infarct size, postischemic recovery of cardiac output (ERA chronic: 91% +/- 10%, ERA acute: 86% +/- 11% vs control: 52% +/- 15%; P < .05) and external heart work (ERA chronic: 90% +/- 10%, ERA acute: 85% +/- 13% vs control: 51% +/- 17%; P < .05) was significantly enhanced in both TBC-3214Na-treated groups whereas recovery of coronary flow was only improved in ERA acute rats (ERA acute: 121% +/- 23% vs ERA chronic: 75% +/- 13%; control: 64% +/- 15%; P < .05). Blood gas measurements showed enhanced myocardial oxygen delivery and consumption with acute TBC-3214Na therapy. Additionally, high-energy phosphates (phosphocreatine) were significantly higher and transmission electron microscopy revealed less ultrastructural damage under acute TBC-3214Na administration.

CONCLUSION

Acute endothelin-A receptor blockade is superior to chronic blockade in attenuating ischemia/reperfusion injury in failing hearts. Therefore, acute endothelin-A receptor blockade might be an interesting option for patients with heart failure undergoing cardiac surgery.

S-nitroso human serum albumin attenuates ischemia/reperfusion injury after cardioplegic arrest in isolated rabbit hearts

BACKGROUND

Depletion of nitric oxide (NO) is associated with ischemia/reperfusion injury. The novel NO donor, S-nitroso human serum albumin (S-NO-HSA), could bridge NO depletion during reperfusion in cardiac transplantation and minimize ischemia/reperfusion injury.

METHODS

In an isolated erythrocyte-perfused working heart model, rabbit hearts were randomly assigned after assessment of hemodynamic baseline values to receive S-NO-HSA (0.2 micromol/100 ml, n = 8), L-arginine (10 mmol/100 ml, n = 8) or albumin (control) (0.2 micromol/100 ml, n = 8). After 20 minutes of infusion, the hearts were arrested and stored in Celsior (4 degrees C) enriched with respective drugs for 6 hours, followed by 75 minutes of reperfusion. Hemodynamic values were assessed and biopsy specimens were taken to determine calcium-ionophore stimulated release of NO and superoxide.

RESULTS

During early reperfusion, recovery of cardiac output (75% +/- 6% vs 49% +/- 5%, p < 0.05) and coronary flow (99% +/- 8% vs 70% +/- 5%, p < 0.05) were higher, and myocardial oxygen consumption was reduced in the S-NO-HSA Group compared with Control (4.08 +/- 0.46 ml/min/0.1 kg vs 6.78 +/- 0.38 ml/min/0.1 kg, p < 0.01). At the end of the experiment cardiac output (53% +/- 5% vs 27% +/- 5%, p < 0.01) was higher and left atrial pressure (115% +/- 9% vs 150% +/- 8%, p < 0.05) was lower in the S-NO-HSA Group compared with Control. NO release was increased (1,040 +/- 50 nmol/liter and 1,070 +/- 60 nmol/liter vs 860 +/- 10 nmol/liter, p < 0.01) and superoxide release diminished (31 +/- 5 nmol/liter and 38 +/- 5 nmol/liter vs 64 +/- 5 nmol/liter, p < .01) in the S-NO-HSA and L-arginine Groups compared with Control.

CONCLUSION

S-NO-HSA improved hemodynamic functions after prolonged hypothermic cardiac arrest by supplementing NO and thereby decreasing ischemia/reperfusion injury.

Supplementation of nucleoside-nucleotide mixture enhances functional recovery and energy metabolism following long-time hypothermic heart preservation

BACKGROUND

OG-VI, a well-balanced mixture of nucleoside and nucleotides, has been demonstrated to have a favorable effect on energy metabolism. In this study, we tested the hypothesis that addition of OG-VI to the University of Wisconsin solution can improve the cardiac functional recovery following long-time hypothermic preservation.

MATERIALS AND METHODS

Forty-two male Wistar rats were randomized into four groups. After 30-min of isolated working heart perfusion, the rat hearts were arrested with St. Thomas cardioplegic solution and preserved at 4 degrees C in saline, OG-VI, UW, and UW+OG-VI, respectively. After 12-h of preservation, the hearts were reperfused for 60-min during which the recovery of cardiac functions were monitored continuously. Myocardial adenine nucleotides were analyzed using high-performance liquid chromomatograph.

RESULTS

In the UW+OG-VI group, the recovery of cardiac output, coronary flow, aortic flow, rate-pressure product, left ventricle stroke volume, and stroke work were significantly higher than other groups (P < 0.05). Furthermore, all phosphate high-energy compounds were significantly higher in the UW+OG-VI group than in the other groups (P < 0.05). Coronary vascular resistance and myocardial wet/dry weight ratio were obviously lower in the UW+OG-VI group, compared to the other groups (P < 0.05).

CONCLUSIONS

Heart function was better recovered when nucleoside-nucleotide mixture was added to UW solution during long-time hypothermic rat heart preservation. The mechanism is not totally clear, but enhancement of high-energy phosphate production is possible.

Effects of nutrient and hemoglobin enriched cell free perfusates upon ex vivo isolated rat heart preparation

We evaluated the effects of nutrient enriched medium and hemoglobin based oxygen carrier (HBOC) upon myocardial functional recovery after 15 minutes of warm ischemia in an isovolumic Langendorff rat heart model. Hearts (n = 8/group) were perfused at constant pressure (90 mm Hg) with Krebs-Henseleit buffer or HEPES modified cell culture medium (M199) in the absence and presence of HBOC. Hearts received 15 minutes of normothermic no flow ischemia followed by 60 minutes reperfusion. Hemodynamics, coronary flow, and tissue water content were measured, and microscopic evidence of injury including TUNEL assay was assessed. Preischemic left ventricular performance (left ventricular developed pressure and maximum rate of positive and negative change in systolic pressure) and coronary flow were similar among groups. At 60 minutes of reperfusion, M199 alone provided more stable and complete left ventricular systolic and diastolic functional recovery than any other perfusate. Coronary flow rates reflected left ventricular function observed under each perfusate condition. TUNEL assay showed arterial endothelial cell death in some hearts perfused with HBOC. Tissue water content did not reflect functional recovery. The combination of M199 and HBOC was associated with poor recovery and elevated perfusate methemoglobin. In this system, postischemic dysfunction is prevented by components in M199. Added HBOC does not improve functional recovery and negates the salutary effects of M199, possibly by augmenting methemoglobin formation.

Reduced oxygen supply explains the negative force-frequency relation and the positive inotropic effect of adenosine in buffer-perfused hearts

In isolated rat hearts perfused with HEPES and red blood cell-enriched buffers, we examined changes in left ventricular pressure induced by increases in heart rate or infusion of adenosine to investigate whether the negative force-frequency relation and the positive inotropic effect of adenosine are related to an inadequate oxygen supply provided by crystalloid perfusates. Hearts perfused with HEPES buffer at a constant flow demonstrated a negative force-frequency relation, whereas hearts perfused with red blood cell-enriched buffer exhibited a positive force-frequency relation. In contrast, HEPES buffer-perfused hearts showed a concentration-dependent increase in left ventricular systolic pressure [EC50 = 7.0 ± 1.2 nM, maximal effect (Emax) = 104 ± 2 and 84 ± 2 mmHg at 0.1 μM and baseline, respectively] in response to adenosine, whereas hearts perfused with red blood cell-enriched buffer showed no change in left ventricular pressure. The positive inotropic effect of adenosine correlated with the simultaneous reduction in heart rate ( r = 0.67, P < 0.01; EC50 = 3.8 ± 1.4 nM, baseline 228 ± 21 beats/min to a minimum of 183 ± 22 beats/min at 0.1 μM) and was abolished in isolated hearts paced to suppress the adenosine-induced bradycardia. In conclusion, these results indicate that the negative force-frequency relation and the positive inotropic effect of adenosine in the isolated rat heart are related to myocardial hypoxia, rather than functional peculiarities of the rat heart.

Pattern differences between distributions of microregional myocardial flows in crystalloid- and blood-perfused rat hearts

Regional myocardial flow distributions in Langendorff rat hearts under Tyrode and blood perfusion were assessed by tracer digital radiography (100-μm resolution). Flow distributions during baseline and maximal hyperemia following a 60-s flow cessation were evaluated by the coefficient of variation of regional flows (CV; related to global flow heterogeneity) and the correlation between adjacent regional flows (CA; inversely related to local flow randomness). These values were obtained for the original images (642 pixels) and for coarse-grained images (322, 162, and 82 blocks of nearby pixels). At a given point in time during baseline, both CV and CA were higher in blood ( n = 7) than in Tyrode perfusion ( n = 7) over all pixel aggregates ( P < 0.05, two-way ANOVA). During the maximal hyperemia, CV and CA were still significantly higher in blood ( n = 7) than in Tyrode perfusion ( n = 7); however, these values decreased substantially in blood perfusion and the CV and CA differences became smaller than those at baseline accordingly. During basal blood perfusion, the 60-s average flow distribution ( n = 7) showed a smaller CV and CA than those at a given point in time ( P < 0.05, two-way ANOVA). Coronary flow reserve was significantly higher in blood than in Tyrode perfusion. In conclusion, the flow heterogeneity and the local flow similarity are both higher in blood than in Tyrode perfusion, probably due to the different degree of coronary tone preservation and the presence or absence of blood corpuscles. Under blood perfusion, temporal flow fluctuations over 60-s order are largely involved in shaping microregional flow distributions.

Comparison of buffer and red blood cell perfusion of guinea pig heart oxygenation

Myocardial mean myoglobin oxygen saturation was determined spectroscopically from isolated guinea pig hearts perfused with red blood cells during increasing hypoxia. These experiments were undertaken to compare intracellular myoglobin oxygen saturation in isolated hearts perfused with a modest concentration of red blood cells (5% hematocrit) with intracellular myoglobin saturation previously reported from traditional buffer-perfused hearts. Studies were performed at 37 degrees C with hearts paced at 240 beats/min and a constant perfusion pressure of 80 cmH2O. It was found that during perfusion with a hematocrit of 5%, baseline mean myoglobin saturation was 93% compared with 72% during buffer perfusion. Mean myoglobin saturation, ventricular function, and oxygen consumption remained fairly constant for arterial perfusate oxygen tensions above 100 mmHg and then decreased precipitously below 100 mmHg. In contrast, mean myoglobin saturation, ventricular function, and oxygen consumption began to decrease even at high oxygen tension with buffer perfusion. The present results demonstrate that perfusion with 5% red blood cells in the perfusate increases the baseline mean myoglobin saturation and better preserves cardiac function at low oxygen tension relative to buffer perfusion. These results suggest that caution should be used in extrapolating intracellular oxygen dynamics from buffer-perfused to blood-perfused hearts.

Erythrocyte-containing versus crystalloid cardioplegia in the rat: effects on myocardial capillaries

BACKGROUND

The purpose of this study was to investigate the effects of crystalloid and erythrocyte-containing cardioplegia on capillary morphology of the isolated erythrocyte-perfused rat heart.

METHODS

Hearts from adult Sprague-Dawley rats were perfused throughout with resuspended sheep erythrocytes and subjected to the following protocols (n = 6, all groups): (1) 15 minutes nonworking and 30 minutes working heart mode (control; group 1); (2) as for group 1, with 30 minutes erythrocyte-containing (BL) or crystalloid (CR) cardioplegic arrest without reperfusion (groups 2BL and 2CR); (3) as for group 2, with 30 minutes nonworking reperfusion (groups 3BL and 3CR); and (4) as for group 3, with 30 minutes working heart mode (groups 4BL and 4CR). After each protocol troponin I from coronary effluent was measured. Corrosion casts were then made of the coronary microvasculature. Cast density was calculated as cast volume per left ventricular dry weight. Casts also underwent scanning electron microscopy. Analysis was by analysis of variance. Values are mean +/- standard deviation.

RESULTS

Prearrest working heart coronary flow averaged 15.1 +/- 4.7 mL/min without any differences among groups. Coronary flow in group 4 working hearts was the same before and after either cardioplegia. Cardiac outputs were similarly consistent in all groups. Cast density in group 1 (control) was 9.60 +/- 1.17 x 10(-2) mm3/mg. It was unaltered by erythrocyte-containing cardioplegia, but after crystalloid cardioplegia (group 2CR), it was 6.52 +/- 0.93 x 10(-2) mm3/mg (p = 0.0001 versus group 1 and p = 0.0007 versus group 2BL). With 30 minutes of nonworking reperfusion (group 3CR, there was slight improvement in cast density at 7.60 +/- 0.90 x 10(-2) mm3/mg (p = 0.0072 versus group 1; p = 0.0242 versus group 3BL). No further improvement was seen in group 4CR. Electron micrographs showed circumferential angularities or narrowings in crystalloid-perfused, arrested hearts, consistent with ischemic damage. Troponin I rose significantly after reperfusion in all groups, but it was higher in crystalloid-perfused, arrested hearts: 0.054 +/- 0.013 microg/L versus 0.024 +/- 0.017 microg/L (p = 0.0273).

CONCLUSIONS

Erythrocyte-containing cardioplegia maintained capillary density and morphology. Crystalloid cardioplegia produced capillary loss, visible abnormalities, and higher troponin I release. These hearts may be more vulnerable to myocardial damage during reperfusion than hearts perfused with erythrocyte-containing cardioplegic solution.

The role of myocardial KATP-channel blockade in the protective effects of glibenclamide against ischaemia in the rat heart

Glibenclamide preserves postischaemic myocardial function in the isolated, erythrocyte perfused, working rat heart model. This study addresses the possible involvement of KATP channels in this beneficial action of glibenclamide. We hypothesized that if glibenclamide improved postischaemic cardiac function by blocking of KATP channels, opening of these KATP channels should result in the opposite, namely detrimental effects on postischaemic heart function. Postischaemic functional loss and coronary blood flow were recorded during treatment with glibenclamide (4 micromol x l(-1); n = 5), the KATP channel openers pinacidil (1 micromol x (l-1); n = 5) and diazoxide (30 micromol x l(-1); n = 5), the combination of glibenclamide with pinacidil (n = 5) and glibenclamide with diazoxide (n = 5), and vehicle (n = 8). Both pinacidil and diazoxide significantly increased coronary blood flow 2-3 times, which was abolished by glibenclamide pre- and postischaemically. This confirms that under both flow conditions glibenclamide significantly blocks KATP channels in the coronary vasculature. The 12 min. global ischaemic incident resulted in a cardiac functional loss of 22.2 +/- 2.9% during vehicle. Glibenclamide reduced the cardiac functional loss to 4.3 +/- 1.2% (P < 0.01). Interestingly, both pinacidil and diazoxide reduced the cardiac functional loss to 4.0 +/- 1.5% (P < 0.01) and 2.9 +/- 1.4% (P < 0.001), respectively. The combination pinacidil+glibenclamide resulted in additional protection compared with the individual components (0.6 +/- 0.1 versus 4.0 +/- 1.5%, P < 0.05). Thus, in contrast to its effect on coronary vascular tone, the glibenclamide-induced improvement of postischaemic cardiac function may not be mediated through blockade of the KATP channel. Alternative mechanisms may be operative, such as uncoupling of the mitochondrial respiratory chain, thereby preconditioning the hearts against stunning.

Quinaprilat during cardioplegic arrest in the rabbit to prevent ischemia-reperfusion injury

OBJECTIVES

This study evaluated intracardiac angiotensin-converting enzyme inhibition as an adjuvant to cardioplegia and examined its effects on hemodynamic, metabolic, and ultrastructural postischemic outcomes.

METHODS

The experiments were performed with an isolated, erythrocyte-perfused, rabbit working-heart model. The hearts excised from 29 adult New Zealand White rabbits (2950 +/- 200 g) were randomly assigned to four groups. Two groups received quinaprilat (1 microg/mL), initiated either with cardioplegia (n = 7) or during reperfusion (n = 7). The third group received l-arginine (2 mmol/L) initiated with cardioplegia (n = 7). Eight hearts served as a control group. Forty minutes of preischemic perfusion were followed by 60 minutes of hypothermic arrest and 40 minutes of reperfusion.

RESULTS

All treatments substantially improved postischemic recovery of external heart work (62% +/- 6%, 69% +/- 3%, and 64% +/- 5% in quinaprilat during cardioplegia, quinaprilat during reperfusion, and l-arginine groups, respectively, vs 35% +/- 5% in control group, P <.001) with similarly increased external stroke work and cardiac output. When administered during ischemia, quinaprilat significantly improved recovery of coronary flow (70% +/- 8%, P =.028 vs quinaprilat during reperfusion [49% +/- 5%] and P =.023 vs control [48% +/- 6%]). l-Arginine (55% +/- 7%) showed no significant effect. Postischemic myocardial oxygen consumption remained low in treatment groups (4.6 +/- 1.2 mL. min(-1). 100 g(-1), 6.0 +/- 2.2 mL. min(-1). 100 g(-1), and 4.7 +/- 1.6 mL. min(-1). 100 g(-1) in quinaprilat during cardioplegia, quinaprilat during reperfusion, and l-arginine groups, respectively, vs 4.2 +/- 0.8 mL. min(-1). 100 g(-1) in control group), even though cardiac work was markedly increased. High-energy phosphates, which were consistently elevated in all treatment groups, showed a significant increase in adenosine triphosphate with quinaprilat during ischemia (2.24 +/- 0.14 micromol/g vs 1.81 +/- 0.12 micromol/g in control group, P =.040). Ultrastructural grading of mitochondrial damage revealed best preservation with quinaprilat during ischemia (100% [no damage], P =.001 vs control).

CONCLUSION

These experimental findings have clinical relevance regarding prevention of postoperative myocardial stunning and low coronary reflow in patients undergoing heart surgery.

Physiological insulin concentrations protect against ischemia-induced loss of cardiac function in rats

This study determined whether insulin at pre- (fasting) and post-prandial concentrations increases coronary blood flow and improves cardiac function after acute ischemia during a situation of myocardial stunning. The experiments were performed using an isolated, erythrocyte perfused, working rat heart model. To the perfusate we added erythrocytes and 1.5% bovine serum albumin to improve clinical relevance. The following protocol was used: 8 min baseline performance assessment, 10 min pre-ischemic treatment, 12 min global ischemia, 20 min post-ischemic treatment and 8 min recovery assessment. Vehicle, 10 mIU l(-1) and 100 mIU l(-1) human insulin were tested (all n=6). No significant vasodilator response to insulin was observed either pre- or post-ischemically. After the 12-min ischemic insult, cardiac function returned dose-dependently to pre-ischemic values (function loss with 100 mIU l(-1) insulin: -0.2+/-0.4% vs. vehicle: 10.7+/-0.8%). This study clearly shows that in our clinically relevant model of moderate ischemia (stunning), insulin is highly cardioprotective at physiological concentrations. This may be explained primarily by higher glucose uptake, improving the myocardial energetic state during ischemia. Therefore, insulin should be considered for use when the myocardium is at acute risk for ischemic incidents.

Glibenclamide attenuates ischemia-induced acidosis and loss of cardiac function in rats

Previous research has shown that the sulfonylurea derivative glibenclamide may improve post-ischemic cardiac functional recovery. Although K(ATP) channel blockade is a possible explanation for this observation, alternative mechanisms exist. Therefore, we simultaneously recorded cardiac function and the intracellular concentration of ATP, phosphocreatine, Pi and pH before and after ischemia in the presence of glibenclamide or vehicle. (31)Phosphorus magnetic resonance (MS) spectroscopy on erythrocyte-perfused, isolated working rat hearts was performed. Glibenclamide 4 micromol l(-1) or vehicle alone was tested (both n=5). The following protocol was used: 8 min performance assessment, 10 min drug treatment, 12 min global ischemia, 20 min reperfusion with drug treatment and 8 min functional recovery assessment. Compared with vehicle, glibenclamide significantly decreased coronary blood flow (59.5+/-7.0% vs. 94.3+/-1.3%, P=0.008), ischemia-induced cardiac functional loss (7.4+/-1.3% vs. 18.8+/-3.3%; P=0.019) as well as the ischemia-induced intracellular acidosis (6.75+/-0.01 vs. 6.43+/-0.03 for vehicle, P=0.03). In conclusion, glibenclamide is able to reduce the myocardial functional loss after ischemia while preserving pH but not ATP levels during ischemia. This suggests that the beneficial response to glibenclamide is probably not the result of myocardial K(ATP) channel blockade, but may be explained by inhibition of glycolysis.

Effects of sulfonylurea derivatives on ischemia-induced loss of function in the isolated rat heart

This study determined whether sulfonylurea derivatives affect cardiac function prior to and after a mild ischemic incident (stunning). This was investigated using an isolated, erythrocyte-perfused, working rat heart model. In total, 11 groups were studied: five increasing (clinically relevant) concentrations of the classical glibenclamide (range 0.005-4 micromol l(-1)), five increasing concentrations of the newly developed glimepiride (range 0.005-0.8 micromol l(-1)), and one control group. Pre-ischemically, glibenclamide and glimepiride reduced coronary blood flow concentration dependently to 55.2+/-4.5% and 58.5+/-5.5%, respectively (P<0.001). Twenty minutes after a 12-min ischemic incident, these reductions of flow were even more pronounced (to 38.3+/-6.7% and 45.8+/-5.8%, P<0.001). This shows that both sulfonylureas reduce coronary blood flow at concentrations slightly higher than therapeutic ones. In the control group, the ischemic incident significantly lowered cardiac function by 22.2+/-2.9%. In the therapeutic range, glimepiride, but not glibenclamide, significantly reduced this ischemia-induced cardiac functional loss to 4.9+/-1.2% (P<0.01). Therefore, we suggest that both sulfonylureas and in particular glimepiride can be used safely in patients with type 2 diabetes mellitus, as long as the coronary vascular system is not compromised. Because of the obvious vasocontrictor response to sulfonylurea derivatives, these drugs must be used with caution in patients with a reduced coronary reserve.

New double-tracer digital radiography for analysis of spatial and temporal myocardial flow heterogeneity

A new high-resolution digital radiographic technique based on the deposition of (125)I- and (3)H-labeled desmethylimipramine (IDMI and HDMI, respectively) was developed for the assessment of spatial and temporal myocardial flow heterogeneity at a microvascular level. The density distributions of two tracers, or relative flow distributions, were determined by subtraction digital radiography using two imaging plates of different sensitivity. The regions resolved are comparable in size to vascular regulatory units (400 x 400 microm(2)). This method was applied to the measurement of within-layer myocardial flow distributions in Langendorff-perfused rabbit hearts. The validity of this method was confirmed by the strong correlation between regional densities of two tracers injected simultaneously (r = 0.89 +/- 0.03, n = 8). The temporal flow stability was evaluated by a 90-s continuous IDMI injection and subsequent bolus HDMI injection (n = 8). Regional densities of the two tracers were fairly correlated (r = 0.86 +/- 0.03), indicating that the spatial pattern of flow distribution was stable even at a microvascular level over a 90-s period. The effect of microsphere embolization on the flow distribution was also investigated by the sequential injections of IDMI, 15-microm microspheres, and HDMI at 20-s intervals (n = 8). Microembolization increased the coefficient of variation of tracer density from 19 to 25% (P < 0.05), whereas the regional densities of two tracers were still correlated substantially, as in the case of no embolization (r = 0.84 +/- 0.06). Thus the microsphere embolization enhanced flow heterogeneity with increasing flow differences between control high-flow and control low-flow regions but rather maintained the pattern of flow distribution. In conclusion, double-tracer digital radiography will be a promising method for the spatial and temporal myocardial flow analysis at microvascular levels.