Maximus Gage
There are three types of muscle fibres: slow oxidative (SO), fast oxidative (FO), and fast glycolytic (FG). Slow oxidative fibres contract slowly and use aerobic respiration to produce ATP. They can function for long periods without fatiguing, making them useful for maintaining posture and stabilizing bones and joints. Cardiac muscle metabolism is predominantly aerobic, with the majority of energy supplied by oxidative phosphorylation. Fatty acids are the predominant substrate utilized in the adult myocardium, and the oxidation of pyruvate is regulated by pyruvate dehydrogenase (PDH). The oxidation of myofilament proteins has been linked to oxidative myocardial contractile depression and cardiac dysfunction, but the role of oxidation in cardiac dysfunction is not yet fully understood.
| Characteristics | Values |
|---|---|
| Muscle fiber type | Slow oxidative (SO) |
| Contraction speed | Slow |
| ATP production | Uses aerobic respiration (oxygen and glucose) to produce ATP |
| Fatigue | Low |
| Tension | Low |
| Mitochondria | High volume |
| Capillary supply | Rich |
| Aerobic respiratory enzymes | High volume |
| Myoglobin | High concentration |
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What You'll Learn
Cardiac muscle is predominantly aerobic
The heart is a muscle that exhibits a highly regulated and efficient system for adenosine triphosphate (ATP) regeneration, generating up to 6 kg of ATP every day, which is 15- to 20-fold its own weight. Cardiac muscle is predominantly aerobic, with the majority of energy supplied by oxidative phosphorylation. ATP produced by means of this pathway is used for contraction.
The heart is capable of utilizing all classes of energy substrates, including carbohydrates, lipids, amino acids, and ketone bodies, for ATP production in the mitochondrion. Fatty acids are the predominant substrate utilized in the adult myocardium, although the cardiac metabolic network is highly flexible in utilizing other substrates when they become abundantly available. For example, cardiac extraction and oxidation of lactate become predominant during exercise as skeletal muscle lactate production increases.
Mitochondria occupy one-third of the cell volume in cardiac myocytes, making them the cell type with the highest mitochondria content. The oxidation of pyruvate is regulated at the pyruvate dehydrogenase (PDH) enzyme level. The oxidation of acetyl CoA in the TCA cycle generates NADH and FADH, which donate electrons to the electron transport chain (ETC), which pumps protons into the mitochondrial intermembrane space to generate the proton-motive force that is dissipated via ATP synthase to ultimately regenerate ATP from adenosine diphosphate (ADP) by the process of oxidative phosphorylation (OXPHOS).
Oxidative stress has been linked to heart failure and muscular dystrophy, which are characterized by abnormal redox balance and nitrosative stress. However, the role of oxidation of myofilament proteins in cardiac dysfunction in the heart is still not fully understood, as studies have reported equivocal effects in different muscle types and even within a single muscle type.
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Cardiac muscle uses fatty acids as the main energy source
The heart is the most metabolically demanding organ in the body, and alterations in cardiac intermediary energy metabolism are major contributors to several cardiovascular pathologies. Fatty acids are recognised as a key source of energy for the heart, along with carbohydrates such as glucose and lactate. The oxidation of fatty acids and carbohydrates accounts for about 90-95% of ATP production in the heart.
Myocardial fatty acid metabolism is an important area of study, with increasing fatty acid availability to the heart resulting in a marked inhibition of glucose oxidation. The oxidation of other energy substrates, such as ketones and branched-chain amino acids (BCAAs), also contributes to energy production. Alterations in cardiac ketone and BCAA metabolism may impact the severity of heart failure through changes in cellular signalling, despite these fuels providing a lower contribution to overall energy production.
The heart's energy for contraction, ion movements, and intracellular protein turnover is provided by the metabolism of oxidisable substrates. The relative substrate concentration is the prime factor defining preference and utilisation rate. The healthy myocardium uses mainly fatty acids as its major energy source, with a minor contribution from glucose. However, lactate, ketone bodies, amino acids, or even acetate can be oxidised under certain circumstances.
The continuous, cyclic work performed by the heart requires a very efficient supply of energy. Myocytes, almost exclusively, rely on the energy derived from the oxidation of fatty acids.
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Cardiac muscle is flexible in using other energy sources
The heart is capable of utilizing all classes of energy substrates, including carbohydrates, lipids, amino acids, and ketone bodies, for ATP production in the mitochondrion. The cardiac metabolic network is highly flexible in utilizing other substrates when they become abundantly available. For example, cardiac extraction and oxidation of lactate become predominant during exercise as skeletal muscle lactate production increases.
The heart's metabolism is predominantly aerobic, with the majority of energy supplied by oxidative phosphorylation. ATP produced by this pathway is used for contraction. The heart exhibits a highly regulated and efficient system for adenosine triphosphate (ATP) regeneration, generating up to 6 kg of ATP every day, which is 15- to 20-fold its own weight.
Fatty acids are the predominant substrate utilized in the adult myocardium. However, ketone bodies can be easily metabolized by the heart. If circulating ketone body levels are elevated, they can become the heart's primary fuel. The oxidation of ketone bodies is conducive to ameliorating heart failure and cardiac hypertrophy.
Hormone level alterations also play a significant role in energy metabolism regulation. Insulin modulates myocardial glucose uptake, fatty acid oxidation, and mitochondrial metabolism. Elevated thyroxine can inhibit myocardial apoptosis and reduce energy loss, while high epinephrine can accelerate the oxidation of various cardiac energy substrates.
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Cardiac muscle exhibits high oxygen consumption
Myocardial oxygen consumption (MVo2) represents the oxygen consumption of the heart muscle and is a critical measure of the total energy utilization of the heart. It is determined by the amount of work performed by the heart per unit of time, known as myocardial power output. MVo2 is primarily utilized for contraction, with basal metabolism accounting for only a small fraction of total oxygen consumption.
The heart operates solely through aerobic metabolism, relying on myocardial mitochondria to maintain an abundance of oxygen for oxidative phosphorylation. This reliance on oxygen distinguishes cardiac muscle as a slow oxidative muscle fiber type. Slow oxidative fibers, also known as Type I or slow-twitch fibers, contract relatively slowly and utilize aerobic respiration to produce ATP. They are characterized by a rich capillary supply, numerous mitochondria, and high concentrations of myoglobin, which enhances oxygen delivery to the fibers.
The high oxygen consumption of cardiac muscle has important clinical implications. For example, in cardiopulmonary patients, achieving a sufficient cardiac output of 4 to 6 L per minute for adequate tissue perfusion becomes challenging due to their impaired heart function. Additionally, abnormal heart rates, whether too high or too low, can be indicative of cardiac or pulmonary symptoms and warrant clinical attention.
Furthermore, understanding the determinants of MVo2 is crucial, especially in clinical conditions that restrict coronary blood flow and oxygen availability to the myocardium. By recognizing the factors influencing myocardial oxygen demand and supply, clinicians can better manage conditions such as ischemia, which arises from a mismatch between oxygen supply and demand due to structural damage from plaques.
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Cardiac muscle is oxidative, but the role of oxidation in cardiac dysfunction is unclear
Cardiac or heart muscle is predominantly oxidative, with the majority of energy supplied by oxidative phosphorylation. The heart requires a lot of energy to beat 100,000 times a day for a lifetime. This energy is generated through a highly regulated and efficient system of adenosine triphosphate (ATP) regeneration, producing up to 6 kg of ATP every day.
The three types of muscle fibres are slow oxidative (SO), fast oxidative (FO), and fast glycolytic (FG). Slow oxidative fibres contract relatively slowly and use aerobic respiration (oxygen and glucose) to produce ATP. They have a rich capillary supply, numerous mitochondria, and aerobic respiratory enzymes. Their high myoglobin content gives them a red colour and allows them to function for long periods without fatiguing, making them useful in maintaining posture and stabilizing bones and joints.
Fast oxidative fibres have fast contractions and primarily use aerobic respiration. They produce ATP relatively quickly and can generate relatively high amounts of tension. They are used for movements that require more energy than postural control but less energy than explosive movements, such as walking.
Fast glycolytic fibres have fast contractions and primarily use anaerobic glycolysis as their ATP source. They have a large diameter and high glycogen volume, which allows them to generate ATP quickly. However, they fatigue more quickly than the other types of fibres due to their reliance on anaerobic metabolism and lower number of mitochondria.
While cardiac muscle is oxidative, the role of oxidation in cardiac dysfunction is not yet fully understood. Studies have reported equivocal effects, with some suggesting a link between oxidation and cardiac dysfunction, while others have found no clear relationship. For example, while decreased Ca2+ sensitivity has been reported in some studies, others have found no changes or even increases in Ca2+ sensitivity. Additionally, high oxidative stress associated with myocytes and diastolic abnormalities has been observed in mice, but no changes in cellular Ca2+-fluxes were seen. Further research is needed to clarify the role of oxidation in cardiac dysfunction.
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Frequently asked questions
Slow oxidative fibres are one of the three types of muscle fibres, the other two being fast oxidative and fast glycolytic. Slow oxidative fibres contract relatively slowly and use aerobic respiration (oxygen and glucose) to produce ATP. They have a rich capillary supply, numerous mitochondria and aerobic respiratory enzymes, and a high concentration of myoglobin.
Fast oxidative fibres produce ATP more quickly than slow oxidative fibres, and thus can produce relatively high amounts of tension. They are used for movements that require more energy than postural control but less energy than explosive movements.
Fast glycolytic fibres have fast contractions and primarily use anaerobic glycolysis as their ATP source. They fatigue more quickly than slow oxidative fibres.
Slow oxidative fibres have a rich capillary supply, numerous mitochondria and aerobic respiratory enzymes, and a high concentration of myoglobin. They can function for long periods without fatiguing, making them useful in maintaining posture, producing isometric contractions, and stabilizing bones and joints.
