Relation between work and labile phosphate content in the isolated dog heart

Energy metabolism design of the striated muscle cell

The design of the energy metabolism system in striated muscle remains a major area of investigation. Here, we review our current understanding and emerging hypotheses regarding the metabolic support of muscle contraction. Maintenance of ATP free energy, so called energy homeostasis, via mitochondrial oxidative phosphorylation is critical to sustained contractile activity, and this major design criterion is the focus of this review. Cell volume invested in mitochondria reduces the space available for generating contractile force, and this spatial balance between mitochondria acontractile elements to meet the varying sustained power demands across muscle types is another important design criterion. This is accomplished with remarkably similar mass-specific mitochondrial protein composition across muscle types, implying that it is the organization of mitochondria within the muscle cell that is critical to supporting sustained muscle function. Beyond the production of ATP, ubiquitous distribution of ATPases throughout the muscle requires rapid distribution of potential energy across these large cells. Distribution of potential energy has long been thought to occur primarily through facilitated metabolite diffusion, but recent analysis has questioned the importance of this process under normal physiological conditions. Recent structural and functional studies have supported the hypothesis that the mitochondrial reticulum provides a rapid energy distribution system via the conduction of the mitochondrial membrane potential to maintain metabolic homeostasis during contractile activity. We extensively review this aspect of the energy metabolism design contrasting it with metabolite diffusion models and how mitochondrial structure can play a role in the delivery of energy in the striated muscle.

Role of mitochondrial Ca2+ in the regulation of cellular energetics

Calcium is an important signaling molecule involved in the regulation of many cellular functions. The large free energy in the Ca(2+) ion membrane gradients makes Ca(2+) signaling inherently sensitive to the available cellular free energy, primarily in the form of ATP. In addition, Ca(2+) regulates many cellular ATP-consuming reactions such as muscle contraction, exocytosis, biosynthesis, and neuronal signaling. Thus, Ca(2+) becomes a logical candidate as a signaling molecule for modulating ATP hydrolysis and synthesis during changes in numerous forms of cellular work. Mitochondria are the primary source of aerobic energy production in mammalian cells and also maintain a large Ca(2+) gradient across their inner membrane, providing a signaling potential for this molecule. The demonstrated link between cytosolic and mitochondrial Ca(2+) concentrations, identification of transport mechanisms, and the proximity of mitochondria to Ca(2+) release sites further supports the notion that Ca(2+) can be an important signaling molecule in the energy metabolism interplay of the cytosol with the mitochondria. Here we review sites within the mitochondria where Ca(2+) plays a role in the regulation of ATP generation and potentially contributes to the orchestration of cellular metabolic homeostasis. Early work on isolated enzymes pointed to several matrix dehydrogenases that are stimulated by Ca(2+), which were confirmed in the intact mitochondrion as well as cellular and in vivo systems. However, studies in these intact systems suggested a more expansive influence of Ca(2+) on mitochondrial energy conversion. Numerous noninvasive approaches monitoring NADH, mitochondrial membrane potential, oxygen consumption, and workloads suggest significant effects of Ca(2+) on other elements of NADH generation as well as downstream elements of oxidative phosphorylation, including the F(1)F(O)-ATPase and the cytochrome chain. These other potential elements of Ca(2+) modification of mitochondrial energy conversion will be the focus of this review. Though most specific molecular mechanisms have yet to be elucidated, it is clear that Ca(2+) provides a balanced activation of mitochondrial energy metabolism that exceeds the alteration of dehydrogenases alone.

Domestication of the cardiac mitochondrion for energy conversion

The control of mitochondria energy conversion by cytosolic processes is reviewed. The nature of the cytosolic and mitochondrial potential energy homeostasis over wide ranges of energy utilization is reviewed and the consequences of this homeostasis in the control network are discussed. An analysis of the major candidate cytosolic signaling molecules ADP, Pi and Ca(2+) are reviewed based on the magnitude and source of the cytosolic concentration changes as well as the potential targets of action within the mitochondrial energy conversion system. Based on this analysis, Ca(2+) is the best candidate as a cytosolic signaling molecule for this process based on its ability to act as both a feedforward and feedback indicator of ATP hydrolysis and numerous targets within the matrix to provide a balanced activation of ATP production. These targets include numerous dehydrogenases and the F1-F0-ATPase. Pi is also a good candidate since it is an early signal of a mismatch between cytosolic ATP production and ATP synthesis in the presence of creatine kinase and has multiple targets within oxidative phosphorylation including NADH generation, electron flux in the cytochrome chain and a substrate for the F1-F0-ATPase. The mechanism of the coordinated activation of oxidative phosphorylation by these signaling molecules is discussed in light of the recent discoveries of extensive protein phosphorylation sites and other post-translational modifications. From this review it is clear that the control network associated with the maintenance of the cytosolic potential energy homeostasis is extremely complex with multiple pathways orchestrated to balance the sinks and sources in this system. New tools are needed to image and monitor metabolites within sub-cellular compartments to resolve many of these issues as well as the functional characterization of the numerous matrix post-translational events being discovered along with the enzymatic processes generating and removing these protein modifications.

The role of Ca(2+) signaling in the coordination of mitochondrial ATP production with cardiac work

The heart is capable of balancing the rate of mitochondrial ATP production with utilization continuously over a wide range of activity. This results in a constant phosphorylation potential despite a large change in metabolite turnover. The molecular mechanisms responsible for generating this energy homeostasis are poorly understood. The best candidate for a cytosolic signaling molecule reflecting ATP hydrolysis is Ca(2+). Since Ca(2+) initiates and powers muscle contraction as well as serves as the primary substrate for SERCA, Ca(2+) is an ideal feed-forward signal for priming ATP production. With the sarcoplasmic reticulum to cytosolic Ca(2+) gradient near equilibrium with the free energy of ATP, cytosolic Ca(2+) release is exquisitely sensitive to the cellular energy state providing a feedback signal. Thus, Ca(2+) can serve as a feed-forward and feedback regulator of ATP production. Consistent with this notion is the correlation of cytosolic and mitochondrial Ca(2+) with work in numerous preparations as well as the localization of mitochondria near Ca(2+) release sites. How cytosolic Ca(2+) signaling might regulate oxidative phosphorylation is a focus of this review. The relevant Ca(2+) sensitive sites include several dehydrogenases and substrate transporters together with a post-translational modification of F1-FO-ATPase and cytochrome oxidase. Thus, Ca(2+) apparently activates both the generation of the mitochondrial membrane potential as well as utilization to produce ATP. This balanced activation extends the energy homeostasis observed in the cytosol into the mitochondria matrix in the never resting heart.

Maintenance of the metabolic homeostasis of the heart: developing a systems analysis approach

The heart is almost unique in the body with a constant requirement to conduct work well beyond the normal maintenance of cellular integrity. With this constant workload, it is not surprising that cardiac energy conversion is highly specialized to maintain a constant supply of energy. This maintenance of cellular metabolites during alterations in workload has been termed metabolic homeostasis. Here we discuss our efforts to understand the cellular and mitochondrial control network that orchestrates the metabolic homeostasis of the heart. This begins with a better definition of the metabolic pathways, acute posttranslational control sites, and proper kinetic evaluation of the reaction steps in the intact mitochondrial environment. First, a quantitative model of mitochondrial energy conversion is presented and demonstrates several serious gaps in our knowledge of this process. Toward filling these gaps, screens of the entire mitochondrial proteome have been conducted to establish the metabolic pathways that need to be considered. In addition, the dynamic phosphoproteome of intact mitochondria, using 2D gel electrophoresis coupled to (32)P labeling, has revealed a remarkably extensive protein phosphorylation network throughout the mitochondrial metabolic network that has essentially been overlooked. Initial studies on evaluating the functional significance of these protein phosphorylations and the kinase-phosphatase system involved will be reviewed. One of the major deficits in the consensus quantitative model of oxidative phosphorylation to explain intact mitochondria activities is in complex I, where even the initiation of Nicotinamide Adenine Dinucleotide (reduced) (NADH) oxidation is problematical using in vitro kinetic data. Studies will be described where the NADH binding and oxidation kinetics at complex I in the intact mitochondria were determined using fluorescence lifetime and enzyme dependent-fluorescence recovery after photo-oxidation (ED-FRAP) techniques. These later studies suggest that matrix NADH binding characteristics are much different (>10(3) binding constant errors) than isolated proteins. In addition, complex I is far from equilibrium and may play an important role in regulating the rate of reducing equivalent delivery to the cytochromes.

Cardiac energy metabolism homeostasis: role of cytosolic calcium

The heart is capable of dramatically altering its overall energy flux with minimal changes in the concentrations of metabolites that are associated with energy metabolism. This cardiac energy metabolism homeostasis is discussed with regard to the potential cytosolic control network responsible for controlling the major energy conversion pathway, oxidative phosphorylation in mitochondria. Several models for this cytosolic control network have been proposed, but a cytosolic Ca(2+) dependent parallel activation scheme for metabolism and work is consistent with most of the experimental results. That model proposes that cytosolic Ca(2+) regulates both the utilization of ATP by the work producing ATPases as well as the mitochondrial production of ATP. Recent studies have provided evidence supporting this role of cytosolic Ca(2+). These data include the demonstration that mitochondrial [Ca(2+)] can track cytosolic [Ca(2+)] and that the compartmentation of cytosolic [Ca(2+)] can facilitate this process. On the metabolic side, Ca(2+) has been shown to rapidly activate several steps in oxidative phosphorylation, including F(1)F(0)-ATPase ATP production as well as several dehydrogenases, which results in a homeostasis of mitochondrial metabolites similar to that observed in the cytosol. Numerous problems with the Ca(2+) parallel activation hypothesis remain including the lack of specific mechanisms of mitochondrial Ca(2+) transport and regulation of F(1)F(0)-ATPase, the time dependence of Ca(2+) activation of cytosolic ATPases as well as oxidative phosphorylation, and the role of cytosolic compartmentation. In addition, the lack of cytosolic or mitochondrial [Ca(2+)] measurements under in vivo conditions is problematic. Several lines of investigation to address these issues are suggested. A model of the cardiac energy metabolism control network is proposed that includes a Ca(2+) parallel activation component together with more classical elements including metabolite feedback and cytosolic compartmentation.

Hemodynamic changes in hypophysectomized rats

The following changes were observed in hypophysectomized rats, 14 days after the operation: greatly reduced cardiac output, bradycardia, slight increase in total peripheral resistance, less work done by the left ventricle, low blood pressure, reduced respiratory rate and oxygen uptake, no change in blood volume and erythrocyte count. During an increased input or resistance load, the output and work of the heart rose greatly both in normal and in hypophysectomized rats. The maximum work which the heart did under these conditions, especially if the size of the heart was taken into consideration, was not significantly less in hypophysectomized than in normal rats. It is concluded that the low cardiac output after hypophysectomy is not primarily of cardiac origin.

Oxidative Phosphorylation in the Failing Dog Heart-Lung Preparation

Utilizing dog heart-lung preparations mitochondria were obtained from control preparations and from hearts failing spontaneously, or as a result of pressure-loading or venous inflow-loading. The mitochondria were compared with respect to their oxidative phosphorylating capacity. The data on the P:O ratios indicate that there is an uncoupling of oxidative phosphorylation in the mitochondria from hearts in spontaneous failure, and in the mitochondria of hearts failing as a result of pressure-loading. In the case of failure due to venous inflow-loading of the heart the mitochondrial P:O ratios are within the control range. The data definitely support the conclusion that in certain types of failure in the dog heart-lung preparation the energy generating or liberating mechanisms may be impaired. This conclusion is contrary to that in the literature based on measurements of high-energy compounds and oxygen consumption, namely, that energy liberation, including transfer of oxidative energy in the form useful for contraction, is not impaired in the failing heart.

Effects of Experimental Congestive Heart Failure, Ouabain, and Asphyxia on the High-Energy Phosphate and Creatine Content of the Guinea Pig Heart

The left ventricles of guinea pigs have been assayed for adenosine triphosphate, adenosine diphosphate, adenylic acid, phosphorylcreatine, free creatine, and inorganic phosphate. These determinations have been made under various experimental conditions which affect the performance of the heart in vivo, such as experimental congestive heart failure, acute asphyxia, and the influence of the cardiac glycoside, ouabain. In 20 guinea pigs with experimental congestive heart failure, sacrificed 1 to 18 days after surgically producing a coarctation of the ascending aorta, a significant fall in PC (54 per cent), ATP (24 per cent), and total creatine (34 per cent) was observed. The extent of the loss of high-energy phosphate compounds and total creatine paralleled the severity of cardiac failure as determined by abnormal intraventricular pressure pulses, ventricular hypertrophy, and gross and microscopic pathology. In animals with experimental congestive heart failure, the cardiac glycoside ouabain produced significant slowing of the heart, a positive inotropic effect, and a lowering of abnormally high right ventricular systolic and left ventricular diastolic pressures. At the height of the response to ouabain, there was a further fall in PC to 25 per cent of normal without any change in ATP concentration. The positive inotropic action of ouabain is, therefore, not due to an ability of the drug to increase high-energy phosphate levels in failing heart muscle. Acute asphyxia produces a very rapid fall in the PC concentration (85 per cent lost in two minutes) and a much slower fall in ATP (33 per cent lost in eight minutes). Recovery of normal mechanical activity, after a period of asphyxia, by rebreathing occurs with little, if any, restoration of ATP and at a time when the PC concentration is only about 25 per cent of normal. These experiments indicate that steady-state levels of high-energy phosphate compounds are not as important in determining the mechanical capacity of the heart as are their rates of synthesis.