No significant increase in O2 consumption of KCl-arrested dog heart with filling and dobutamine

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.

Muscle heat: a window into the thermodynamics of a molecular machine

The contraction of muscle is characterized by the development of force and movement (mechanics) together with the generation of heat (metabolism). Heat represents that component of the enthalpy of ATP hydrolysis that is not captured by the microscopic machinery of the cell for the performance of work. It arises from two conceptually and temporally distinct sources: initial metabolism and recovery metabolism. Initial metabolism comprises the hydrolysis of ATP and its rapid regeneration by hydrolysis of phosphocreatine (PCr) in the processes underlying excitation-contraction coupling and subsequent cross-bridge cycling and sliding of the contractile filaments. Recovery metabolism describes those process, both aerobic (mitochondrial) and anaerobic (cytoplasmic), that produce ATP, thereby allowing the regeneration of PCr from its hydrolysis products. An equivalent partitioning of muscle heat production is often invoked by muscle physiologists. In this formulation, total enthalpy expenditure is separated into external mechanical work (W) and heat (Q). Heat is again partitioned into three conceptually distinct components: basal, activation, and force dependent. In the following mini-review, we trace the development of these ideas in parallel with the development of measurement techniques for separating the various thermal components.

Rescue of Ca2+ overload-induced left ventriclur dysfunction by targeted ablation of phospholamban

In failing hearts, a deficiency in sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA)2a results in abnormal Ca2+ handling and diminished contraction. In addition, a decrease in the phosphorylation of phospholamban (PLB) has been reported. Gene transfer of antisense PLB (asPLB) can improve contractile function in the failing human myocardium. Gene transfer of SERCA2a improves survival and the energy potential in failing hearts. The aim of present study was to evaluate whether enhancement of SERCA2a function prevents acute Ca2+ overload-induced left ventricular (LV) dysfunction in rat hearts. We ablated PLB using adenoviral gene transfer of asPLB by a new and less invasive gene delivery method, which involved a percutaneous technique. Experiments were performed on 13 excised cross-circulated rat hearts: 5 rats underwent sham operations, 4 rats underwent gene transfer of the reporter gene β-galactosidase (Ad.β-gal), and 4 rats underwent gene transfer of asPLB (Ad.asPLB). After clearance of high Ca2+ infused into the coronary, there was LV contractile dysfunction associated with the decreased myocardial O2 consumption per beat (Vo2) intercept (equal to decreased Vo2 for Ca2+ handling in excitation-contraction coupling) of the Vo2-systolic pressure-volume area (PVA; total mechanical energy per beat) linear relation in the hearts that underwent sham operation and had been infected with Ad.β-gal. Hearts that had been infected with Ad.asPLB were rescued from LV contractile dysfunction associated with an unchanged Vo2 intercept of the Vo2-PVA linear relation. We conclude that SERCA2a function enhanced by adenoviral gene transfer of asPLB prevents Ca2+ overload-induced LV contractile dysfunction in terms of mechanical work and especially energetics.

Left ventricular mechanoenergetics after hyperpolarized cardioplegic arrest by nicorandil and after depolarized cardioplegic arrest by KCl

We hypothesized that there are no differences in left ventricular (LV) mechanoenergetics between after hyperpolarized cardioplegic arrest by nicorandil (nicorandil arrest) and after depolarized one by high potassium chloride (KCl arrest). The aim of the present study was to test this hypothesis using LV curved end-systolic pressure-volume relation (ESPVR) and linear pressure-volume area (PVA)-myocardial oxygen consumption per beat (Vo2) relation. All hearts underwent 30 min global ischemia (30°C) after infusion of 5 ml of cardioplegia. Cardioplegia consisted of either 30 mmol/l KCl (7 hearts) or nicorandil (100 μmol/l) in Tyrode solution (6 hearts). After a 30-min blood reperfusion, ESPVR and Vo2-PVA relation were assessed again. Mean end-systolic pressure (ESPmLVV) and mean PVA at midrange LV volume (PVAmLVV) significantly ( P < 0.05) decreased to 79.1 ± 13.4% and 85.4 ± 17.1% of control after KCl arrest and to 85.3 ± 14.8% and 86.4 ± 16.9% of control after nicorandil arrest. There were no significant differences in both decreases of mean ESPmLVV and PVAmLVV between each arrest. The slopes of Vo2-PVA relations were also unchanged after each arrest. There was a significant ( P < 0.005) difference in the decreases of mean Vo2 intercepts of Vo2-PVA relations between post-KCl arrest (73.9 ± 8.2% of control) and post-nicorandil arrest (99.2 ± 10.1% of control), however. Proteolysis of α-fodrin due to Ca2+ overload was significantly marked after KCl arrest. The present results indicate that the total calcium handling in excitation-contraction coupling is transiently impaired after KCl arrest, whereas it is unchanged after nicorandil arrest. This suggests the possibility that nicorandil is a better cardioplegia than KCl.

Left Ventricular Mechanoenergetics in Small Animals

Studies on left ventricular mechanical work and energetics in rat and mouse hearts are reviewed. First, left ventricular linear end-systolic pressure-volume relation (ESPVR) and curved end-diastolic pressure-volume relation (EDPVR) in canine hearts and left ventricular curved ESPVR and curved EDPVR in rat hearts are reviewed. Second, as an index for total mechanical energy per beat in rat hearts as in canine hearts, a systolic pressure-volume area (PVA) is proposed. By the use of our original system for measuring continuous oxygen consumption for rat left ventricular mechanical work, the linear left ventricular myocardial oxygen consumption per beat (VO2)-PVA relation is obtained as in canine hearts. The slope of VO2-PVA relation (oxygen cost of PVA) indicates a ratio of chemomechanical energy transduction. VO2 intercept (PVA-independent VO2) indicates the summation of oxygen consumption for Ca2+ handling in excitation-contraction coupling and for basal metabolism. An equivalent maximal elastance (eEmax) is proposed as a new left ventricular contractility index based on PVA at the midrange left ventricular volume. The slope of the linear relation between PVA-independent VO2 and eEmax (oxygen cost of eEmax) indicates changes in oxygen consumption for Ca2+ handling in excitation-contraction coupling per unit changes in left ventricular contractility. The key framework of VO2-PVA-eEmax can give us a better understanding for the biology and mechanisms of physiological and various failing rat heart models in terms of mechanical work and energetics.

Energetics of the Frank-Starling effect in rabbit myocardium: economy and efficiency depend on muscle length

We tested the hypothesis that economy and efficiency are independent of length in intact cardiac muscle over its normal working range. We measured force, force-time integral, force-length area, and myocardial oxygen consumption in eight isometrically contracting rabbit right ventricular papillary muscles. 2,3-Butanedione monoxime was used to partition nonbasal oxygen consumption into tension-independent and tension-dependent components. Developed force, force-time integral, and force-length area increased by factors of 2.4, 2.7, and 4.8, respectively, as muscle length was increased from 90% to 100% maximal length, whereas tension-dependent oxygen consumption increased only 1.6-fold. Economy (the ratio of force-time integral to tension-dependent oxygen consumption) increased significantly with muscle length, as did contractile efficiency, the ratio of force-length area to tension-dependent oxygen consumption. The average force-length area-nonbasal oxygen consumption intercept was more than the twice tension-independent oxygen consumption. We conclude that economy and efficiency increase with length in rabbit myocardium. This conclusion is consistent with published data in isolated rabbit and dog hearts but at odds with studies in skinned myocardium.

Left ventricular diastolic dysfunction in type 2 diabetes mellitus model rats

To gain insight into the pathogenesis of diabetic cardiomyopathy, we investigated cardiac function in terms of the coupling of left ventricular mechanical work and the energetics in Otsuka Long-Evans Tokushima Fatty rats, which are well known as a model of type 2 diabetes mellitus (DM). Neither left ventricular systolic function and mean coronary flow nor coronary flow reserve differed even in late DM rats. The amount of oxygen required for mechanical work and contraction was unaltered, although myosin isozyme was finally transformed from V(1) to V(3). The maximum pacing rate was decreased from 300 to 240 beats/min, and the left ventricular relaxation rate was significantly (P < 0.05) slower only in late DM rats, resulting in decreased oxygen consumption per minute for total Ca(2+) handling in excitation-contraction coupling mainly consumed by sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA2) without significant changes in basal metabolism or in mitochondrial oxidative phosphorylation. The protein level of SERCA2 in membranes was significantly (P < 0.001) lower in severe DM rats. We conclude that the only lusitropic dysfunction due to the depressed expression of SERCA2 is related to generating diabetic cardiomyopathy even in the present type 2 diabetic rats.

Rat cardiac contractile dysfunction induced by Ca2+ overload: possible link to the proteolysis of alpha-fodrin

The aim of the present study was to examine the mechanisms of Ca2+ overload-induced contractile dysfunction in rat hearts independent of ischemia and acidosis. Experiments were performed on 30 excised cross-circulated rat heart preparations. After hearts were exposed to high Ca2+, there was a contractile failure associated with a parallel downward shift of the linear relation between myocardial O(2) consumption per beat and systolic pressure-volume area (index of a total mechanical energy per beat) in left ventricles from all seven hearts that underwent the protocol. This result suggested a decrease in O(2) consumption for total Ca2+ handling in excitation-contraction coupling. In the hearts that underwent the high Ca2+ protocol and had contractile failure, we found marked proteolysis of a cytoskeleton protein, alpha-fodrin, whereas other proteins were unaffected. A calpain inhibitor suppressed the contractile failure by high Ca2+, the decrease in O(2) consumption for total Ca2+ handling, and membrane alpha-fodrin degradation. We conclude that the exposure to high Ca2+ may induce contractile dysfunction possibly by suppressing total Ca2+ handling in excitation-contraction coupling and degradation of membrane alpha-fodrin via activation of calpain.

Ryanodine wastes oxygen consumption for Ca2+ handling in the dog heart. A new pathological heart model

Ryanodine (RYA) at a low concentration (several tens of nM) is known to selectively bind to Ca2+ release channels in sarcoplasmic reticulum (SR) and to fix them open. The present study was designed to investigate the effects of the selective change in Ca2+ release channel activity on cardiac mechanoenergetics as a model of Ca(2+)-leaky SR observed in pathological hearts. We analyzed the negative inotropic effect of RYA at a low concentration (up to 30 +/- 13 nM) on left ventricular (LV) mechanoenergetics using frameworks of LV Emax (a contractility index) and the myocardial oxygen consumption (LV VO2)-systolic pressure-volume area (PVA) (a measure of total mechanical energy) relation in 11 isolated, blood-perfused dog hearts. RYA significantly decreased Emax by 42%, whereas PVA-independent VO2 remained disproportionately high (93% of control). This oxygen-wasting effect of RYA was quite different from ordinary inotropic drugs, which alter Emax and PVA-independent VO2 proportionally. The present result suggests that RYA suppresses force generation of cardiac muscle for a given amount of total sequestered Ca2+ by SR in a similar way to myocardial ischemia and stunning. We speculate about the underlying mechanism that RYA makes SR leaky for Ca2+ and thereby wastes energy for Ca2+ handling by SR.