A 24-year-old, healthy woman volunteers for an exercise research study. In the study, various radiolabeled compounds are used to determine the source of skeletal muscle ATP during exercise. The participants are given carbohydrate-rich food and are asked to run on a treadmill at high speed. Which of the following is the most likely source of ATP represented by the red curve on the graph below?

Question

A 24-year-old, healthy woman volunteers for an exercise research study.  In the study, various radiolabeled compounds are used to determine the source of skeletal muscle ATP during exercise.  The participants are given carbohydrate-rich food and are asked to run on a treadmill at high speed.  Which of the following is the most likely source of ATP represented by the red curve on the graph below?

Quizplus · Accepted Answer

11ec160b_3cb8_fc21_9249_3d0538324be1_MD0001_00 Skeletal muscle contraction requires the hydrolysis of ATP, which is only present in small amounts within the sarcoplasma. There are 3 main methods of ATP production during intense exercise, and the primary method varies with exercise duration. The phosphocreatine shuttle is the primary source of ATP at the beginning (eg, first 10 sec) of exercise. As ATP is consumed by the muscle cell, creatine kinase removes inorganic phosphate molecules from phosphocreatine and combines them with ADP to regenerate ATP. Phosphocreatine supplies a brief burst of energy before being quickly depleted. Increased dietary intake of creatine can enhance this phase of ATP production by increasing sarcoplasmic phosphocreatine stores. Once phosphocreatine is depleted, anaerobic glycolysis becomes the primary source of ATP for the next stage of exercise (eg, 10 to 60 sec). Muscle glycogen is rapidly broken down into glucose, which is converted to pyruvate (generating ATP in the process) and then lactic acid (Choice C). However, the process quickly becomes limited by reduced sarcoplasmic pH caused by the accumulation of lactic acid. Oxidative phosphorylation is the slowest of the 3 sources to reach peak production, but it is the most efficient generator of ATP and serves as the primary energy source for the remainder of exercise (eg, >60 sec). Pyruvate generated during glycolysis along with free fatty acids taken up from the circulation are converted to acetyl-coenzyme A, which is catabolized within the citric acid cycle (Krebs cycle) (Choice B). This process produces NADH and FADH₂, which then enter the electron transport chain, where electrons are transferred to oxygen and large quantities of ATP are generated. (Choices A and D) Amino acids and ketone bodies are sometimes used as an energy source for oxidative phosphorylation but only after glycogen and free fatty acid stores are depleted. Because amino acids are derived from muscle protein, their use as an energy source leads to muscle breakdown. Educational objective: Intense exercise creates a high demand for ATP, and the primary method of ATP production varies with the duration of exercise. The phosphocreatine shuttle is the primary source of ATP at the beginning of exercise, followed sequentially by anaerobic glycolysis and then oxidative phosphorylation.

A 24-Year-Old, Healthy Woman Volunteers for an Exercise Research Study