Oxidative metabolism of Polytron versus Nagarse mitochondria in hearts of genetically diabetic mice

Targeting mitochondrial quality control for diabetic cardiomyopathy: Therapeutic potential of hypoglycemic drugs

Diabetic cardiomyopathy is a chronic cardiovascular complication caused by diabetes that is characterized by changes in myocardial structure and function, ultimately leading to heart failure and even death. Mitochondria serve as the provider of energy to cardiomyocytes, and mitochondrial dysfunction plays a central role in the development of diabetic cardiomyopathy. In response to a series of pathological changes caused by mitochondrial dysfunction, the mitochondrial quality control system is activated. The mitochondrial quality control system (including mitochondrial biogenesis, fusion and fission, and mitophagy) is core to maintaining the normal structure of mitochondria and performing their normal physiological functions. However, mitochondrial quality control is abnormal in diabetic cardiomyopathy, resulting in insufficient mitochondrial fusion and excessive fission within the cardiomyocyte, and fragmented mitochondria are not phagocytosed in a timely manner, accumulating within the cardiomyocyte resulting in cardiomyocyte injury. Currently, there is no specific therapy or prevention for diabetic cardiomyopathy, and glycemic control remains the mainstay. In this review, we first elucidate the pathogenesis of diabetic cardiomyopathy and explore the link between pathological mitochondrial quality control and the development of diabetic cardiomyopathy. Then, we summarize how clinically used hypoglycemic agents (including sodium-glucose cotransport protein 2 inhibitions, glucagon-like peptide-1 receptor agonists, dipeptidyl peptidase-4 inhibitors, thiazolidinediones, metformin, and α-glucosidase inhibitors) exert cardioprotective effects to treat and prevent diabetic cardiomyopathy by targeting the mitochondrial quality control system. In addition, the mechanisms of complementary alternative therapies, such as active ingredients of traditional Chinese medicine, exercise, and lifestyle, targeting mitochondrial quality control for the treatment of diabetic cardiomyopathy are also added, which lays the foundation for the excavation of new diabetic cardioprotective drugs.

Application of Animal Models in Diabetic Cardiomyopathy

Diabetic heart disease is a growing and important public health risk. Apart from the risk of coronary artery disease or hypertension, diabetes mellitus (DM) is a well-known risk factor for heart failure in the form of diabetic cardiomyopathy (DiaCM). Currently, DiaCM is defined as myocardial dysfunction in patients with DM in the absence of coronary artery disease and hypertension. The underlying pathomechanism of DiaCM is partially understood, but accumulating evidence suggests that metabolic derangements, oxidative stress, increased myocardial fibrosis and hypertrophy, inflammation, enhanced apoptosis, impaired intracellular calcium handling, activation of the renin-angiotensin-aldosterone system, mitochondrial dysfunction, and dysregulation of microRNAs, among other factors, are involved. Numerous animal models have been used to investigate the pathomechanisms of DiaCM. Despite some limitations, animal models for DiaCM have greatly advanced our understanding of pathomechanisms and have helped in the development of successful disease management strategies. In this review, we summarize the current pathomechanisms of DiaCM and provide animal models for DiaCM according to its pathomechanisms, which may contribute to broadening our understanding of the underlying mechanisms and facilitating the identification of possible new therapeutic targets.

Age and sex as confounding factors in the relationship between cardiac mitochondrial function and type 2 diabetes in the Nile Grass rat

Our study revisits the role of cardiac mitochondrial adjustments during the progression of type 2 diabetes mellitus (T2DM), while considering age and sex as potential confounding factors. We used the Nile Grass rats (NRs) as the animal model. After weaning, animals were fed either a Standard Rodent Chow Diet (SRCD group) or a Mazuri Chinchilla Diet (MCD group) consisting of high-fiber and low-fat content. Both males and females in the SRCD group, exhibited increased body mass, body mass index, and plasma insulin compared to the MCD group animals. However, the females were able to preserve their fasting blood glucose throughout the age range on both diets, while the males showed significant hyperglycemia starting at 6 months in the SRCD group. In the males, a higher citrate synthase activity-a marker of mitochondrial content-was measured at 2 months in the SRCD compared to the MCD group, and this was followed by a decline with age in the SRCD group only. In contrast, females preserved their mitochondrial content throughout the age range. In the males exclusively, the complex IV capacity expressed independently of mitochondrial content varied with age in a diet-specific pattern; the capacity was elevated at 2 months in the SRCD group, and at 6 months in the MCD group. In addition, females, but not males, were able to adjust their capacity to oxidize long-chain fatty acid in accordance with the fat content of the diet. Our results show clear sexual dimorphism in the variation of mitochondrial content and oxidative phosphorylation capacity with diet and age. The SRCD not only leads to T2DM but also exacerbates age-related cardiac mitochondrial defects. These observations, specific to male NRs, might reflect deleterious dietary-induced changes on their metabolism making them more prone to the cardiovascular consequences of aging and T2DM.

Mitochondrial Mechanisms in Diabetic Cardiomyopathy

Mitochondrial medicine is increasingly discussed as a promising therapeutic approach, given that mitochondrial defects are thought to contribute to many prevalent diseases and their complications. In individuals with diabetes mellitus (DM), defects in mitochondrial structure and function occur in many organs throughout the body, contributing both to the pathogenesis of DM and complications of DM. Diabetic cardiomyopathy (DbCM) is increasingly recognized as an underlying cause of increased heart failure in DM, and several mitochondrial mechanisms have been proposed to contribute to the development of DbCM. Well established mechanisms include myocardial energy depletion due to impaired adenosine triphosphate (ATP) synthesis and mitochondrial uncoupling, and increased mitochondrial oxidative stress. A variety of upstream mechanisms of impaired ATP regeneration and increased mitochondrial reactive oxygen species have been proposed, and recent studies now also suggest alterations in mitochondrial dynamics and autophagy, impaired mitochondrial Ca²⁺ uptake, decreased cardiac adiponectin action, increased O-GlcNAcylation, and impaired activity of sirtuins to contribute to mitochondrial defects in DbCM, among others. In the current review, we present and discuss the evidence that underlies both established and recently proposed mechanisms that are thought to contribute to mitochondrial dysfunction in DbCM.

Alterations in fatty acid metabolism and sirtuin signaling characterize early type-2 diabetic hearts of fructose-fed rats

Despite the fact that skeletal muscle insulin resistance is the hallmark of type‐2 diabetes mellitus (T2DM), inflexibility in substrate energy metabolism has been observed in other tissues such as liver, adipose tissue, and heart. In the heart, structural and functional changes ultimately lead to diabetic cardiomyopathy. However, little is known about the early biochemical changes that cause cardiac metabolic dysregulation and dysfunction. We used a dietary model of fructose‐induced T2DM (10% fructose in drinking water for 6 weeks) to study cardiac fatty acid metabolism in early T2DM and related signaling events in order to better understand mechanisms of disease. In early type‐2 diabetic hearts, flux through the fatty acid oxidation pathway was increased as a result of increased cellular uptake (CD36), mitochondrial uptake (CPT1B), as well as increased β‐hydroxyacyl‐CoA dehydrogenase and medium‐chain acyl‐CoA dehydrogenase activities, despite reduced mitochondrial mass. Long‐chain acyl‐CoA dehydrogenase activity was slightly decreased, resulting in the accumulation of long‐chain acylcarnitine species. Cardiac function and overall mitochondrial respiration were unaffected. However, evidence of oxidative stress and subtle changes in cardiolipin content and composition were found in early type‐2 diabetic mitochondria. Finally, we observed decreased activity of SIRT1, a pivotal regulator of fatty acid metabolism, despite increased protein levels. This indicates that the heart is no longer capable of further increasing its capacity for fatty acid oxidation. Along with increased oxidative stress, this may represent one of the earliest signs of dysfunction that will ultimately lead to inflammation and remodeling in the diabetic heart.

In vivo creatine kinase reaction kinetics at rest and stress in type II diabetic rat heart

The effects of type II diabetes on cardiac creatine kinase (CK) enzyme activity and/or flux are unknown. We therefore measured steady‐state phosphocreatine (PCr) and adenosine triphosphate (ATP) content and forward CK reaction kinetic parameters in Zucker Diabetic Fatty (ZDF) rat hearts, a type II diabetes research model. At baseline the PCr to ATP ratio (PCr/ATP) was significantly lower in diabetic heart when compared with matched controls (1.71 ± 0.21 vs. 2.26 ± 0.24, P < 0.01). Furthermore, the forward CK reaction rate constant (k_f) was higher in diabetic animals (0.52 ± 0.09 s⁻¹ vs. 0.35 ± 0.06 s⁻¹, P < 0.01) and CK flux calculated as a product of PCr concentration ([PCr]) and k_f was similar between two groups (4.32 ± 1.05 μmol/g/s vs. 4.94 ± 1.23 μmol/g/s, P = 0.20). Dobutamine administration resulted in similar increases in heart rate (~38%) and k_f (~0.12 s⁻¹) in both groups. No significant change in PCr and ATP content was observed with dobutamine. In summary, our data showed reduced PCr/ATP in diabetic myocardium as an indicator of cardiac energy deficit. The forward CK reaction rate constant is elevated at baseline which might reflect a compensatory mechanics to support energy flux through the CK shuttle and maintain constant ATP supply. When hearts were stimulated similar increase in k_f was observed in both groups thus it seems that CK shuttle does not limit ATP supply for the range of workload studied.

Noninvasive ³¹P MRS was used to measure PCr concentration ([PCr]) and creatine kinase (CK) reaction flux in type II diabetic rat hearts. [PCr] was reduced in diabetic myocardium as compared to controls, indicative of impairment in mitochondrial ATP production. The forward CK reaction rate constant was elevated, possibly reflecting a compensatory mechanism to support increased flux through the CK shuttle required to support cardiac work. CK reaction velocity increased in both diabetic and control hearts to maintain constant ATP content at higher work.

Molecular mechanisms of diabetic cardiomyopathy

In recent years, diabetes mellitus has become an epidemic and now represents one of the most prevalent disorders. Cardiovascular complications are the major cause of mortality and morbidity in diabetic patients. While ischaemic events dominate the cardiac complications of diabetes, it is widely recognised that the risk for developing heart failure is also increased in the absence of overt myocardial ischaemia and hypertension or is accelerated in the presence of these comorbidities. These diabetes-associated changes in myocardial structure and function have been called diabetic cardiomyopathy. Numerous molecular mechanisms have been proposed to contribute to the development of diabetic cardiomyopathy following analysis of various animal models of type 1 or type 2 diabetes and in genetically modified mouse models. The steady increase in reports presenting novel mechanistic data on this subject expands the list of potential underlying mechanisms. The current review provides an update on molecular alterations that may contribute to the structural and functional alterations in the diabetic heart.

Cardiac dysfunction and oxidative stress in the metabolic syndrome: an update on antioxidant therapies

The metabolic syndrome (MetS) is a cluster of risk factors including obesity, insulin resistance, dyslipidemia, elevated blood pressure and glucose intolerance. The MetS increases the risk for cardiovascular disease (CVD) and type 2 diabetes. Each component of the MetS causes cardiac dysfunction and their combination carries additional risk. The mechanisms underlying cardiac dysfunction in the MetS are complex and might include lipid accumulation, increased fibrosis and stiffness, altered calcium homeostasis, abnormal autophagy, altered substrate utilization, mitochondrial dysfunction and increased oxidative stress. Mitochondrial and extra-mitochondrial sources of reactive oxygen species (ROS) and reduced antioxidant defense mechanisms characterize the myocardium of humans and animals with the MetS. The mechanisms for increased cardiac oxidative stress in the MetS are not fully understood but include increased fatty acid oxidation, mitochondrial dysfunction and enhanced NADPH oxidase activity. Therapies aimed to reduce oxidative stress and enhance antioxidant defense have been employed to reduce cardiac dysfunction in the MetS in animals. In contrast, large scale clinical trials using antioxidants therapies for the treatment of CVD have been disappointing because of the lack of efficacy and undesired side effects. The focus of this review is to summarize the current knowledge about the mechanisms underlying cardiac dysfunction in the MetS with a special interest in the role of oxidative stress. Finally, we will update the reader on the results obtained with natural antioxidant and mitochondria-targeted antioxidant therapies for the treatment of CVD in the MetS.

Mitochondria in the diabetic heart

Diabetes mellitus increases the risk of developing cardiovascular diseases such as coronary artery disease and heart failure. Studies have shown that the heart failure risk is increased in diabetic patients even after adjusting for coronary artery disease and hypertension. Although the cause of this increased heart failure risk is multifactorial, increasing evidence suggests that derangements in cardiac energy metabolism play an important role. In particular, abnormalities in cardiomyocyte mitochondrial energetics appear to contribute substantially to the development of cardiac dysfunction in diabetes. This review will summarize these abnormalities in mitochondrial function and discuss potential underlying mechanisms.

Molecular mechanisms for myocardial mitochondrial dysfunction in the metabolic syndrome

The metabolic syndrome represents a cluster of abnormalities, including obesity, insulin resistance, dyslipidaemia and Type 2 diabetes, that increases the risk of developing cardiovascular diseases, such as coronary artery disease and heart failure. The heart failure risk is increased even after adjusting for coronary artery disease and hypertension, and evidence is emerging that changes in cardiac energy metabolism might contribute to the development of contractile dysfunction. Recent findings suggest that myocardial mitochondrial dysfunction may play an important role in the pathogenesis of cardiac contractile dysfunction in obesity, insulin resistance and Type 2 diabetes. This review will discuss potential molecular mechanisms for these mitochondrial abnormalities.

Effects of Propionyl-Carnitine on Mitochondrial Respiration and Post-Ischaemic Cardiac Function in???the Ischaemic Underperfused Diabetic???Rat Heart

BACKGROUND AND OBJECTIVE

Carnitine and its derivatives, namely propionyl-carnitine (PC), have been shown to protect cardiac metabolism and function in diabetes mellitus and ischaemic heart disease. Since diabetes is associated with abnormalities in mitochondrial metabolism of fuels, we examined the effects of PC on mitochondrial respiration in ischaemic hearts from streptozotocin-diabetic rats.

METHODS

Diabetes was induced in Sprague-Dawley rats by an intravenous injection of streptozotocin. Following the diagnosis of diabetes, oral PC treatment was initiated for a period of 6 weeks. After treatment, cardiac function was determined from working hearts perfused under aerobic conditions and in a separate group of hearts subjected to ischaemia and reperfusion. Mitochondrial respiration was determined under aerobic conditions and following low-flow ischaemia.

RESULTS

Rates of state 3 mitochondria respiration with pyruvate were significantly lower in diabetic (n = 4) hearts compared with control (n = 6) hearts (80 +/- 5 vs 112 +/- 5 nanoatoms O2/mg protein/min, respectively), but those with palmitoylcarnitine were similar (101 +/- 11 vs 106 +/- 6 nanoatoms O2/mg protein/min). Diabetic rat heart (n = 8) function, expressed as rate pressure product, was also significantly decreased compared with control (n = 8) hearts (21.5 +/- 1.0 vs 29.5 +/- 0.9 beats x mm Hg x 10(-3)/min, respectively). In PC-treated diabetic (n = 6) hearts, state 3 respiration with pyruvate was increased, and a marked improvement in left ventricular function from 21.5 +/- 1.0 to 26.0 +/- 0.6 beats x mm Hg x 10(-3)/min was observed. During low-flow ischaemia, state 3 respiration with pyruvate remained lower in diabetic (n = 5) hearts compared with control (n = 5) hearts (64 +/- 3 vs 46 +/- 5 nanoatoms O2/mg protein/min, respectively). Following treatment with PC (n = 4), however, respiration with this substrate was significantly increased to 57 +/- 4 nanoatoms O2/mg protein/min. PC was also associated with a significant improvement in cardiac function in reperfused diabetic rat (n = 4) hearts (18.4 +/- 0.2 beats x mm Hg x 10(-3)/min).

CONCLUSION

Our results showed that PC has a beneficial effect on cardiac function and increases ischaemic tolerance of the diabetic rat heart. This beneficial effect of PC can be explained, in part, as an improvement in mitochondrial metabolism of pyruvate during the actual ischaemic period.

Calcineurin transgenic mice have mitochondrial dysfunction and elevated superoxide production

Introduction of the constitutively active calcineurin gene into neonatal rat cardiomyocytes by adenovirus resulted in decreased mitochondrial membrane potential (P < 0.05). Infection of H9c2 cells with calcineurin adenovirus resulted in increased superoxide production (P < 0.001). Transgenic mice with cardiac-specific expression of a constitutively active calcineurin cDNA (CalTG mice) exhibit a two- to threefold increase in heart size that progresses to heart failure. We prepared mitochondria enriched for the subsarcolemmal population from the hearts of CalTG mice and transgene negative littermates (control). Intact, well-coupled mitochondria prepared from one to two mouse hearts at a time yielded sufficient material for functional studies. Mitochondrial oxygen consumption was measured with a Clark-type oxygen electrode with substrates for mitochondrial complex II (succinate) and complex IV [tetramethylpentadecane (TMPD)/ascorbate]. CalTG mice exhibited a maximal rate of electron transfer in heart mitochondria that was reduced by approximately 50% (P < 0.002) without a loss of respiratory control. Mitochondrial respiration was unaffected in tropomodulin-overexpressing transgenic mice, another model of cardiomyopathy. Western blotting for mitochondrial electron transfer subunits from mitochondria of CalTG mice revealed a 20-30% reduction in subunit 3 of complex I (ND3) and subunits I and IV of cytochrome oxidase (CO-I, CO-IV) when normalized to total mitochondrial protein or to the adenine nucleotide transporter (ANT) and compared with littermate controls (P < 0.002). Impaired mitochondrial electron transport was associated with high levels of superoxide production in the CalTG mice. Taken together, these data indicate that calcineurin signaling affects mitochondrial energetics and superoxide production. The excessive production of superoxide may contribute to the development of cardiac failure.

Altered Ca2+ homeostasis and impaired mitochondrial function in cardiomyopathy

Altered Ca2+ homeostasis and myocyte death is a major characteristic of the hereditary cardiomyopathy in the dystrophic hamster. Despite numerous studies, the mechanisms that link calcium dysregulation and cell death in this animal model remain unclear. We have shown previously that the maintenance of mitochondrial Ca2+ homeostasis is essential for cell survival, and that loss of mitochondrial Ca2+ is closely correlated with cell death in cultured cells. Here, we have further investigated the role of mitochondrial Na+-Ca2+ exchange (NCE) in the development of cardiomyopathy in the dystrophic hamster. We found that the myocyte death was associated with elevated NCE activity together with a reduced level of matrix Ca2+, and impaired mitochondrial energetics. The upregulation of NCE activity in myopathic heart was also accompanied by enhanced expression of the sarcolemmal Na+-Ca2+ exchange (NCX) without alterations in the L-type Ca2+ channel expression. Treatment of dystrophic hamsters with diltiazem (a potent inhibitor of NCE and Ca2+ channels) prevented the occurrence of cell death and restored the normal expression of NCX. Our findings implicate the dysregulation of both sarcolemmal and mitochondrial Na+-Ca2+ exchange in cell death in the myopathic heart.

Regulation of sodium-calcium exchange and mitochondrial energetics by Bcl-2 in the heart of transgenic mice

Our previous work in cultured cells has shown that the maintenance of mitochondrial Ca(2+) homeostasis is essential for cell survival, and that the anti-apoptotic protein Bcl-2 is able to maintain a threshold level of mitochondrial Ca(2+) by the inhibition of permeability transition. To test whether Bcl-2 also affects the mitochondrial Na(+)-Ca(2+) exchange (NCE), a major efflux pathway for mitochondrial Ca(2+), studies using transgenic mice that overexpress Bcl-2 in the heart have been performed. NCE activity was determined as the Na(+)-dependent Ca(2+) efflux in the isolated mitochondria. Overexpression of Bcl-2 led to a significant reduction of NCE activity as well as increased resistance to permeability transition in the mitochondria of transgenic heart. This was accompanied by increased matrix Ca(2+) level, enhanced formation of NADH and enhanced oxidation of pyruvate, an NAD(+)-linked substrate. Furthermore, there was induction of cellular Ca(2+) transport proteins including the Na(+)-Ca(2+) exchanger of the sarcolemma (NCX). Bcl-2 not only stimulates NCX expression in the sarcolemma but also attenuates the Na(+)-Ca(2+) exchange in the mitochondria. These results are consistent with the protection by Bcl-2 against apoptosis in heart following ischemia/reperfusion.

Effects of propionyl-L-carnitine on isolated mitochondrial function in the reperfused diabetic rat heart

The effects of propionyl-L-carnitine (PLC) on isolated mitochondrial respiration in the ischemic reperfused diabetic heart were studied. Oral PLC treatment of STZ-diabetic rats was initiated for a period of 6 weeks. After treatment, isolated working hearts from diabetic rats were perfused under aerobic conditions then subjected to 25 min of no-flow ischemia followed by 15 min of aerobic reperfusion. At the end of reperfusion, heart mitochondria was isolated using differential centrifugation and respiration measured in the presence of pyruvate, glutamate, and palmitoylcarnitine. Our results indicate that diabetes was characterized by a pronounced decrease in heart function under aerobic conditions as well as during reperfusion following ischemia. Treatment with PLC resulted in a significant improvement in heart function under these conditions. The depressions in state 3 mitochondrial respiration with both pyruvate and glutamate seen in reperfused hearts from diabetic rats were prevented by PLC. State 3 respiration in the presence of palmitoylcarnitine was also improved in the ischemic reperfused diabetic rat heart. Our results show that PLC improves recovery of mechanical function following ischemia in the diabetic rat heart. The beneficial effects of PLC are associated with enhanced mitochondrial oxidation of fuels.

Myocardial uptake of iodine-125-labeled 15-(p-iodophenyl)-3-(R,S)-methyl pentadecanoic acid is decreased in chronic diabetic rats with changes in subcellular distribution

Iodine-123-labeled 15-(p-iodophenyl)-3-(R,S)-methyl pentadecanoic acid (123I-BMIPP) is widely used to detect myocardial metabolic changes, but the preferred energy substrates in the myocardium would be expected to be altered in the presence of metabolic disorders such as diabetes mellitus (DM). We investigated the metabolism of branched-chain fatty acids in the myocardium of rats with DM. Streptozotocin-induced DM rats were examined 48 h (acute; AD) and 6 weeks (chronic; CD) after injection of streptozotocin. Hearts were excised 15 min or 60 min after injection of 0.185 MBq of 125I-BMIPP, followed by homogenization in an EDTA-Tris buffer. The homogenates were subjected to differential centrifugation to obtain the mitochondrial (MF) and cytoplasmic (CF) fractions. Myocardial 125I uptake tended to increase in the AD group, but the change was not significant. Myocardial 125I uptake at 15 min was significantly lower in the CD group than in the control group, even in the insulin-treated rats [control (CC), 4.4+/-0.4; not treated (CDN), 3.3+/-0.5; insulin-treated (CDI), 3.4+/-0.4 x 10(4) cpm/g, p<0.05 in each case]. The 125I count value corrected for the blood count (counts/min (cpm) per g of protein divided by blood cpm) in the MF decreased by 40% at 60 min in the CC group, but increased by 60% in the CDN group. The results of the present study suggest that the myocardial uptake of branched-chain fatty acids is decreased in rats with chronic diabetes, probably as a result of mitochondrial dysfunction.

In situ study of myofibrils, mitochondria and bound creatine kinases in experimental cardiomyopathies

Human cardiomyopathy has been extensively studied in the last decade, and knowledge of the functional and structural alterations of the heart has grown. However, understanding of the pathogenesis has come mostly from experimental studies. A number of work have been designed to elucidate if alterations of the contractile apparatus of cardiac cells contribute to the impairment of heart mechanics in cardiomyopathies. As well, an important question is to be solved: whether energy supply of the contraction-relaxation cycle is sufficient in the myopathic heart. Use of cardiac fibers skinned by different techniques allows to evaluate functional ability of myofibrils, mitochondria and bound creatine kinase which plays an important role in cardiomyocyte energy metabolism. The data presented in this chapter show that experimental cardiomyopathies of various types have some common features. These are an increase in calcium sensitivity of myofibrils and a depression of functional activity of mitochondrial creatine kinase. Possible mechanisms and physiological significance of these changes are discussed.

Cerebral mitochondrial respiration in diabetic and chronically hypoglycemic rats

The respiratory function of cerebral mitochondria harvested from genetically diabetic (BB/W) and streptozotocin-diabetic rats deprived of insulin for 3-4 weeks was found to be unchanged from control values. Furthermore, insulin-deprived BB/W rats subjected to 30 min of insulin-induced hypoglycemic coma demonstrated a normal mitochondrial respiration following a 60 min period of glucose restitution, a finding consistent with earlier results in non-diabetic rats. However, in rats exposed to 1 week of moderate hypoglycemia (plasma glucose = 3.0 mumol.ml-1), both state 3 respiration and the respiratory control ratio (RCR) were reduced from control. In fact, when the chronic hypoglycemia was imposed following a 3-4 week period of diabetic hyperglycemia, the state 3 rate and RCR were found to be reduced to a greater degree than in chronically hypoglycemic, non-diabetic, previously normoglycemic rats. Finally, when 1 week of moderate hypoglycemia preceded a 30 min period of insulin-induced hypoglycemic coma, a disturbed pattern of mitochondrial respiration (i.e. increased state 4, decreased RCR) was found at 60 min of recovery following coma. These results indicate that chronic increases in glucose (and insulin deprivation) have no effect on cerebral mitochondrial respiratory function, whereas prolonged, albeit moderate, reductions in cerebral glucose supply result in perturbations in mitochondrial respiration. These results demonstrate the importance of an adequate glucose supply for normal mitochondrial activity.

Electron conduction between b cytochromes of the mitochondrial respiratory chain in the presence of antimycin plus myxothiazol

The b haems of the bc1 complex of bovine heart mitochondria were poised with succinate and fumarate so that only the high-potential haem (b-562) was reduced, and then isolated from further redox exchange with the ubiquinone pool by adding antimycin and myxothiazol. A transmembrane electric potential difference was then developed, either by electron flow from [Ru(NH3)6]Cl2 to oxygen or by ATP hydrolysis. The small difference spectrum, caused by the electric field, indicated 32-55% oxidation of b-562 with concomitant reduction of b-566. No lag greater than 0.1 s was detectable between the initiation of respiration and the development of the difference spectrum, thus providing a direct demonstration of (fairly) rapid electron transfer between the b haems.

Hormonal effects on mitochondrial respiration: potential role of endogenous lipolytic activities

Hormonal effects on heart mitochondrial metabolism are investigated by comparing respiratory rates, Ca2+ uptake capacity, and lipolytic activities of mitochondria isolated from control rats to those of mitochondria isolated from thyroparathyroidectomized animals. Two biochemically and morphologically distinct populations of heart mitochondria are prepared--one derived from the region of the cell directly beneath the sarcolemma (subsarcolemmal mitochondria), the other originally between the myofibrils (interfibrillar mitochondria). Subsarcolemmal mitochondria isolated from normal rat cardiac tissue have both lower respiratory rates and Ca2+ uptake capacity than do interfibrillar mitochondria. However, when these mitochondrial populations are isolated from hearts from thyroparathyroidectomized rats, there is a selective increase in the maximal ability of the subsarcolemmal mitochondria to accumulate Ca2+, which is accompanied by a proportionate increase in their maximal respiratory rates. Neither Ca2+ uptake capacity nor respiratory rates are similarly increased in the interfibrillar mitochondria. Cytochrome contents and mitochondrial protein recoveries are not significantly changed in either of these mitochondrial preparations. The relationship between these selective increases in respiratory properties of the subsarcolemmal mitochondria to endogenous lipolytic activities is also investigated. It was previously demonstrated that, in the absence of Ca2+, both the rate and extent of formation of free fatty acids from endogenous phospholipids is greater in subsarcolemmal than interfibrillar mitochondria (J. W. Palmer et al. (1981) Arch. Biochem. Biophys. 211, 674-682). In this study it is shown that lipolysis is also more sustained in the subsarcolemmal mitochondria when Ca2+ is added. In the subsarcolemmal mitochondria isolated from thyroparathyroidectomized rats, however, the rates of release of stearic acid and oleic acid are reduced in both the presence and absence of Ca2+. In the presence of added Ca2+, the rate of release of arachidonic acid is also decreased compared to control subsarcolemmal mitochondria, suggesting that the expressed activity of Ca2+-activated phospholipase A2 is lower in those mitochondria isolated from the thyroparathyroidectomized animals, in which respiratory rates and Ca2+ uptake capacity are increased.