Question

What does the Cori cycle accomplish during moderate exercise (jogging) that is critical to sustained activity?

Answer 1

At rest, skeletal muscle has a low metabolic rate. In response to contractile activity, the energy consumption of skeletal muscle can easily rise >100-fold. The body meets this increased energy demand by mobilizing energy stores both locally from muscle glycogen and triacylglycerols, and systemically from liver glycogen and adipose tissue triacylglycerols. The integrated physiological response to exercise involves the delivery of sufficient O2 and fuel to ensure that the rate of ATP synthesis rises in parallel with the rate of ATP breakdown. Indeed, skeletal muscle precisely regulates the ratio of ATP to ADP even with these large increases in ATP turnover.

Physical performance can be defined in terms of power (work/time), speed, or endurance. Skeletal muscle has three energy systems,

• Oxidative
• Non Oxidative
• Immediate

each designed to support a particular type of performance . For power events, which typically last a few seconds or less (e.g., hitting a ball with a bat), the immediate energy sources include ATP and phosphocreatine (PCr). For spurts of activity that last several seconds to a minute (e.g., sprinting 100 m), muscles rely primarily on the rapid nonoxidative breakdown of carbohydrate stored as muscle glycogen to form ATP. For activities that last 2 minutes or longer but have low power requirements (e.g., jogging several kilometers), the generation of ATP through the oxidation of fat and glucose derived from the circulation becomes increasingly important. We now consider the key metabolic pathways for producing the energy that enables skeletal muscle to have such a tremendous dynamic range of activity.