Myocardial oxygen tension in isolated erythrocyte-perfused rat hearts and comparison with crystalloid media

Contribution of Harold M. Swartz to In Vivo EPR and EPR Dosimetry

In 2015, we are celebrating half a century of research in the application of Electron Paramagnetic Resonance (EPR) as a biodosimetry tool to evaluate the dose received by irradiated people. During the EPR Biodose 2015 meeting, a special session was organized to acknowledge the pioneering contribution of Harold M. (Hal) Swartz in the field. The article summarizes his main contribution in physiology and medicine. Four emerging themes have been pursued continuously along his career since its beginning: (1) radiation biology; (2) oxygen and oxidation; (3) measuring physiology in vivo; and (4) application of these measurements in clinical medicine. The common feature among all these different subjects has been the use of magnetic resonance techniques, especially EPR. In this article, you will find an impressionist portrait of Hal Swartz with the description of the 'making of' this pioneer, a time-line perspective on his career with the creation of three National Institutes of Health-funded EPR centers, a topic-oriented perspective on his career with a description of his major contributions to Science, his role as a mentor and his influence on his academic children, his active role as founder of scientific societies and organizer of scientific meetings, and the well-deserved international recognition received so far.

Maternal hypoxia decreases capillary supply and increases metabolic inefficiency leading to divergence in myocardial oxygen supply and demand

Maternal hypoxia is associated with a decrease in left ventricular capillary density while cardiac performance is preserved, implying a mismatch between metabolism and diffusive exchange. We hypothesised this requires a switch in substrate metabolism to maximise efficiency of ATP production from limited oxygen availability. Rat pups from pregnant females exposed to hypoxia (FIO₂=0.12) at days 10-20 of pregnancy were grown to adulthood and working hearts perfused ex vivo. ¹⁴C-labelled glucose and ³H-palmitate were provided as substrates and metabolism quantified from recovery of ¹⁴CO₂ and ³H₂O, respectively. Hearts of male offspring subjected to Maternal Hypoxia showed a 20% decrease in cardiac output (P<0.05), despite recording a 2-fold increase in glucose oxidation (P<0.01) and 2.5-fold increase (P<0.01) in palmitate oxidation. Addition of insulin to Maternal Hypoxic hearts, further increased glucose oxidation (P<0.01) and suppressed palmitate oxidation (P<0.05), suggesting preservation in insulin signalling in the heart. In vitro enzyme activity measurements showed that Maternal Hypoxia increased both total and the active component of cardiac pyruvate dehydrogenase (both P<0.01), although pyruvate dehydrogenase sensitivity to insulin was lost (NS), while citrate synthase activity declined by 30% (P<0.001) and acetyl-CoA carboxylase activity was unchanged by Maternal Hypoxia, indicating realignment of the metabolic machinery to optimise oxygen utilisation. Capillary density was quantified and oxygen diffusion characteristics examined, with calculated capillary domain area increased by 30% (P<0.001). Calculated metabolic efficiency decreased 4-fold (P<0.01) for Maternal Hypoxia hearts. Paradoxically, the decline in citrate synthase activity and increased metabolism suggest that the scope of individual mitochondria had declined, rendering the myocardium potentially more sensitive to metabolic stress. However, decreasing citrate synthase may be essential to preserve local PO₂, minimising regions of hypoxia and hence maximising the area of myocardium able to preserve cardiac output following maternal hypoxia.

Changes to both cardiac metabolism and performance accompany acute reductions in functional capillary supply

BACKGROUND

The relative importance of arteriole supply or ability to switch between substrates to preserve cardiac performance is currently unclear, but may be critically important in conditions such as diabetes.

METHODS

Metabolism of substrates was measured before and after infusion of polystyrene microspheres in the perfused working heart to mimic random capillary loss due to microvascular disease. The effect of acute loss of functional capillary supply on palmitate and glucose metabolism together with function was quantified, and theoretical tissue oxygen distribution calculated from histological samples and ventricular VO(2) estimated.

RESULTS

Microsphere infusion led to a dose-dependent decrease in rate-pressure product (RPP) and oxygen consumption (P<0.001). Microsphere infusion also increased work/unit oxygen consumption of hearts ('efficiency') by 25% (P<0.01). When corrected for cardiac work palmitate oxidation remained tightly coupled to very low workloads (RPP<2500 mmHg/min), illustrating a high degree of metabolic control. Arteriole occlusion by microspheres decreased the density of patent capillaries (P<0.001) and correspondingly increased the average capillary supply area by 40% (P<0.01). Calculated rates of oxygen consumption declined from 16.6±7.2 ml/100 ml/min to 12.4±9 ml/100 ml/min following arteriole occlusion, coupled with increases in size of regions of myocardial hypoxia (Control=22.0% vs. Microspheres=42.2%).

CONCLUSIONS

Cardiac mechanical performance is very sensitive to arteriolar blockade, but metabolite switching from fatty acid to glucose utilisation may also support cardiac function in regions of declining PO(2).

GENERAL SIGNIFICANCE

Preserving functional capillary supply may be critical for maintenance of cardiac function when metabolic flexibility is lost, as in diabetes.

Cardiac electrophysiology and the susceptibility to sustained ventricular tachycardia in intact, conscious mice

Cardiac electrophysiological dysfunction is a major cause of death in humans. Accordingly, electrophysiological testing is routinely performed in intact, conscious, humans to evaluate arrhythmias and disorders of cardiac conduction. However, to date, in vivo electrophysiological studies in mice are limited to anesthetized open-chest or closed-chest preparations. However, cardiac electrophysiology in anesthetized mice or mice with surgical trauma may not adequately represent what occurs in conscious mice. Accordingly, an intact, conscious murine model of cardiac electrophysiology has the potential to be of major importance for advancing the concepts and methods that drive cardiovascular therapies. Therefore, we describe, for the first time, the use of an intact, conscious, murine model of cardiac electrophysiology. The conscious mouse model permits measurements of atrioventricular interval, sinus cycle length, sinus node recovery time (SNRT), SNRT corrected for spontaneous sinus cycle, Wenckebach cycle length, the ventricular effective refractory period (VERP) and the electrical stimulation threshold to induce sustained ventricular tachyarrhythmias in an intact, complex model free of the confounding influences of anesthetics and surgical trauma. This is an important consideration because anesthesia and surgical trauma markedly reduced cardiac output and heart rate as well as altered cardiac electrophysiology parameters. Most importantly, anesthesia and surgical trauma significantly increased the VERP and virtually eliminated the ability to induce sustained ventricular tachyarrhythmias. Accordingly, the methodology allows for the accurate documentation of cardiac electrophysiology in complex, conscious mice and may be adopted for advancing the concepts and ideas that drive cardiovascular research.

Myocardial ischemia, reperfusion, and infarction in chronically instrumented, intact, conscious, and unrestrained mice

In the United States alone, the National Heart, Lung, and Blood Institute (NHLBI) has invested several hundred million dollars in pursuit of myocardial infarct-sparing therapies. However, due largely to methodological limitations, this investment has not produced any notable clinical application or cardioprotective therapy. Among the major methodological limitations is the reliance on animal models that do not mimic the clinical situation. In this context, the limited use of conscious animal models is of major concern. In fact, whenever possible, studies of cardiovascular physiology and pathophysiology should be conducted in conscious, complex models to avoid the complications associated with the use of anesthesia and surgical trauma. The mouse has significant advantages over other experimental models for the investigation of infarct-sparing therapies. The mouse is inexpensive, has a high throughput, and presents the ability of one to create genetically modified models. However, successful infarct-sparing therapies in anesthetized mice or isolated mouse hearts may not be successful in more complex models, including conscious mice. Accordingly, a conscious mouse model of myocardial ischemia and reperfusion has the potential to be of major importance for advancing the concepts and methods that drive the development of infarct-sparing therapies. Therefore, we describe, for the first time, the use of an intact, conscious, and unrestrained mouse model of myocardial ischemia-reperfusion and infarction. The conscious mouse model permits occlusion and reperfusion of the left anterior descending coronary artery in an intact, complex model free of the confounding influences of anesthetics and surgical trauma. This methodology may be adopted for advancing the concepts and ideas that drive cardiovascular research.

Opening of the mitoKATP channel and decoupling of mitochondrial complex II and III contribute to the suppression of myocardial reperfusion hyperoxygenation

Diazoxide, a mitochondrial ATP-sensitive potassium (mitoK(ATP)) channel opener, protects the heart from ischemia-reperfusion injury. Diazoxide also inhibits mitochondrial complex II-dependent respiration in addition to its preconditioning effect. However, there are no prior studies of the role of diazoxide on post-ischemic myocardial oxygenation. In the current study, we determined the effect of diazoxide on the suppression of post-ischemic myocardial tissue hyperoxygenation in vivo, superoxide (O(2)(-*)) generation in isolated mitochondria, and impairment of the interaction between complex II and complex III in purified mitochondrial proteins. It was observed that diazoxide totally suppressed the post-ischemic myocardial hyperoxygenation. With succinate but not glutamate/malate as the substrate, diazoxide significantly increased ubisemiquinone-dependent O(2)(-*) generation, which was not blocked by 5-HD and glibenclamide. Using a model system, the super complex of succinate-cytochrome c reductase (SCR) hosting complex II and complex III, we also observed that diazoxide impaired complex II and its interaction with complex III with no effect on complex III. UV-visible spectral analysis revealed that diazoxide decreased succinate-mediated ferricytochrome b reduction in SCR. In conclusion, our results demonstrated that diazoxide suppressed the in vivo post-ischemic myocardial hyperoxygenation through opening the mitoK(ATP) channel and ubisemiquinone-dependent O(2)(-*) generation via inhibiting mitochondrial complex II-dependent respiration.

The low oxygen-carrying capacity of Krebs buffer causes a doubling in ventricular wall thickness in the isolated heart

The buffer-perfused Langendorff heart is significantly vasodilated compared with the in vivo heart. In this study, we employed ultrasound to determine if this vasodilation translated into changes in left ventricular wall thickness (LVWT), and if this effect persisted when these hearts were switched to the "working" mode. To investigate the effects of perfusion pressure, vascular tone, and oxygen availability on cardiac dimensions, we perfused hearts (from male Wistar rats) in the Langendorff mode at 80, 60, and 40 cm H2O pressure, and infused further groups of hearts with either the vasoconstrictor endothelin-1 (ET-1) or the blood substitute FC-43. Buffer perfusion induced a doubling in diastolic LVWT compared with the same hearts in vivo (5.4 ± 0.2 mm vs. 2.6 ± 0.2 mm, p < 0.05) that was not reversed by switching hearts to "working" mode. Perfusion pressures of 60 and 40 cm H2O resulted in an increase in diastolic LVWT. ET-1 infusion caused a dose-dependent decrease in diastolic LVWT (6.6 ± 0.4 to 4.8 ± 0.4 mm at a concentration of 10–9 mol/L, p < 0.05), with a concurrent decrease in coronary flow. FC-43 decreased diastolic LVWT from 6.7 ± 0.5 to 3.8 ± 0.7 mm (p < 0.05), with coronary flow falling from 16.1 ± 0.4 to 8.1 ± 0.4 mL/min (p < 0.05). We conclude that the increased diastolic LVWT observed in buffer-perfused hearts is due to vasodilation induced by the low oxygen-carrying capacity of buffer compared with blood in vivo, and that the inotropic effect of ET-1 in the Langendorff heart may be the result of a reversal of this wall thickening. The implications of these findings are discussed.Key words: ultrasound, endothelin, ventricular wall thickness, vasodilation.

EPR oximetry in the beating heart: myocardial oxygen consumption rate as an index of postischemic recovery

Oxygen plays a critical role in the pathophysiology of myocardial injury during both ischemia and subsequent reperfusion (I/R). Thus, oxygen concentration is an important variable to measure during I/R. In the present work, electron paramagnetic resonance (EPR)-based oximetry was used to measure the oxygen concentration during a series of I/R episodes and oxygenation levels were correlated with the contractile and hemodynamic functions of the heart. A custom-developed electronically tunable surface coil resonator working at 1.1 GHz was used to determine tissue pO(2) in the beating heart. Microcrystalline particulate of lithium phthalocyanine was used as an EPR oximetry probe. Isolated and perfused rat hearts were subjected to 1 or 3 hr durations of preischemic perfusion, followed by 15-min I/R cycles. In hearts perfused for 3 hr prior to 15-min I/R cycles, the myocardial pO(2) decreased gradually on subsequent reperfusions of three successive I/R cycles. However, in hearts perfused for 1 hr there was almost 100% recovery of myocardial pO(2) in all three I/R cycles. The extent of oxygenation recovered in each reperfusion cycle correlated with the recovery of hemodynamic and contractile function. The results also showed that the oxygen consumption rate of the heart at the end of each I/R episode decreased in direct proportion to the functional recovery. In summary, it was observed that the amount of myocardial oxygen consumption during I/R could provide a reliable index of functional impairment in the heart.

Slaughterhouse blood as a perfusate for studying myocardial function under ischemic conditions

Metabolic studies using the in vitro non-recirculating blood-perfused isolated heart model require large volumes of blood. The present study was designed to determine whether heterologous pig blood collected from a slaughterhouse can be used as perfusate for isolated pig hearts perfused under aerobic and constant reduced flow conditions. Eight isolated working pig hearts perfused for 90 min at a constant flow of 1.5 ml g(-1) min(-1) with non-recirculated blood diluted with Krebs-Henseleit bicarbonate buffer at a hematocrit of 23% were compared to eight hearts subjected to the same protocol but perfused only with Krebs-Henseleit bicarbonate buffer solution. Hearts were paced at 100 bpm and subjected to aerobic perfusion at 38 degrees C. Hearts were weighed before perfusion and at the end of the experiment and the results are reported as percent weight gain (mean +/- SD). Comparisons between groups were performed by the Student t-test (P<0.05). After 90 min of perfusion with modified Krebs-Henseleit, perfused hearts presented a larger weight gain than blood-perfused hearts (39.34 +/- 9.27 vs 23.13 +/- 5.42%, P = 0.003). Left ventricular end-diastolic pressure was higher in the modified Krebs-Henseleit-perfused group than in the blood group (2.8 +/- 0.4 vs 2.3 +/- 0.3 mmHg, respectively, P = 0.01). We conclude that heterologous blood perfusion, by preserving a more physiological myocardial water content, is a better perfusion fluid than modified Krebs-Henseleit solution for quantitative studies of myocardial metabolism and heart function under ischemic conditions.

Acute hemodynamic and coronary circulatory effects of experimental autoimmune myocarditis

Myocarditis and progression to cardiomyopathy is associated with focal spasm and reperfusion of the coronary microcirculation. Experimental autoimmune myocarditis (EAM), induced with cardiomyosin peptide-specific T cells in Lewis rats, was hypothesized to cause acute hemodynamic and coronary vasculature changes. Fifteen experimental animals (5 each at 1, 2, and 3 weeks after T-cell injection) and eight controls were studied using the constant pressure variant of the isolated heart. Coronary resistant decreased while coronary flow increased (P < 0.05) in EAM hearts after the first week. Rate-pressure product, +dP/dt and -dP/dt, decreased while the heart/body weight ratio increased (P < 0.05) compared with controls at 1 week but not at 2 or 3 weeks. Mean local myocardial PO2, which reflects local oxygen delivery and consumption, and MVO2 were not different for EAM hearts. However, compared with controls EAM myocardial PO2 varied more widely and was often beyond the usual range, suggesting the occurrence of localized hypoxic and hyperoxic areas. In summary, after the first week there was a significant decrease in coronary resistance in the EAM animals, which required higher flow to maintain a similar perfusion pressure. These changes in coronary resistance and flow along with the heterogeneity and extremes of local myocardial PO2 levels without a significant change in MVO2 may be explained by postulating development of low-resistance, high-flow hyperoxic areas which steal flow, thus causing hypoxia in other areas.