Question

List the compounds used as energy sources by each of the following: a) Skeletal muscle b) Cardiac muscle c) Brain

Answer 1

Answer-

For Skeletal Muscle-Adenosine triphosphate (ATP) is the sole fuel for muscle contraction. During near maximal intense exercise, the muscle store of ATP will be depleted in < 1s, therefore, to maintain normal contractile function ATP must be continually resynthesized. During intense exercise (from approximately 75% VO2 max to near maximal workloads) this is achieved principally by the oxidation of carbohydrate and the anaerobic utilization of phosphocreatine (PCr) and carbohydrate. The relative contribution of carbohydrate oxidation to total energy provision decreases, while that from anaerobic utilization increases. During prolonged intense exercise (approximately 75% VO2 max), the oxidation of glucose derived from skeletal muscle and liver glycogen stores is the primary pathway for ATP resynthesis. It is widely accepted that the availability of carbohydrate limits performance during this type of exercise as the point of exhaustion has been shown to be closely related to the depletion of muscle and liver glycogen stores. It is probable that carbohydrate depletion results in the inability of skeletal muscle to maintain the required rate of ATP resynthesis and therefore, the work intensity must be reduced for exercise to continue. During short lasting near maximal exercise (0-30 s), the anaerobic utilization of muscle PCr and glycogen will fuel muscle contraction. The evidence is available to indicate that fatigue during this type of exercise is related to the inability of type II fibers to maintain the required very high rate of ATP resynthesis. This has been suggested to result from a rapid depletion of type II fiber PCR stores and an insufficiency of the glycogenolytic rate to compensate for the fall in ATP production when the PCR store is depleted. In this situation, the force generation has to decrease due to insufficient energy supplies.

For Cardiac Muscle-Myocardium is able to produce energy from several substrates: fatty acids, glucose, lactate, pyruvate, ketone bodies and even amino acids. Preference of individual substrates representing the particular sources of energy depends on their current concentration in both blood and cardiac muscle cells.If the oxygen supply is sufficient, the dominant fuel is represented by fatty acids which are predominantly utilized and they cover 50-70% of the total energy demands myocardium and glucose which covers the remnant 30 %. Lactate is utilized as an energy substrate under the condition of increased muscular activity, during which the lactate concentration in blood augments rapidly. Ketone bodies and amino acids are utilized exclusively under special pathological conditions (e.g. in diabetic ketoacidosis). They participate in ATP production by more than 10 %

For Brain- The mammalian brain depends on glucose as its main source of energy. In the adult brain, neurons have the highest energy demand, requiring continuous delivery of glucose from the blood. In humans, the brain accounts for ~2% of the body weight, but it consumes ~20% of glucose-derived energy making it the main consumer of glucose (~5.6 mg glucose per 100 g human brain tissue per minute). Glucose metabolism provides the fuel for physiological brain function through the generation of ATP, the foundation for neuronal and non-neuronal cellular maintenance, as well as the generation of neurotransmitters. Therefore, tight regulation of glucose metabolism is critical for brain physiology and disturbed glucose metabolism in the brain underlies several diseases affecting both the brain itself as well as the entire organism.