ATP & Energy Sources for Muscles

Why do skeletal muscle fibers need ATP?

ATP is required for Na+/K+ pumps that maintain ion gradients, for the crossbridge cycle during contraction, and for processes involved in muscle relaxation.

How much ATP is stored in muscle fibers?

Muscle fibers store only a few seconds' worth of ATP, so ATP must continually be regenerated during activity.

Three ways muscle fibers regenerate ATP

1) Immediate reactions in the cytosol that add a phosphate to ADP, 2) glycolytic catabolism in the cytosol, and 3) oxidative catabolism in the mitochondria.

Creatine phosphate (CP)

Compound stored in muscle fibers that can rapidly donate a phosphate group to ADP to regenerate ATP.

Creatine kinase (CK)

Enzyme that transfers a phosphate group from creatine phosphate to ADP to rapidly regenerate ATP.

Creatine phosphate reaction

Creatine phosphate + ADP → creatine + ATP; provides ATP almost immediately.

How long can the creatine phosphate system support maximal muscle activity?

About an additional 10 seconds of maximal muscle activity.

Glycolytic (anaerobic) catabolism

Series of reactions in the cytosol that splits one 6-carbon glucose molecule into two 3-carbon pyruvate molecules and produces 2 net ATP.

Glycogen

Storage form of glucose found in muscle fibers and the liver; muscle fibers can use stored glycogen as a source of glucose for glycolysis.

How long can glycolysis provide adequate ATP for sustained muscle contraction?

About 30–40 seconds.

Does glycolysis directly require oxygen?

No. Glycolysis itself does not directly require oxygen, but the fate of the resulting pyruvate depends on oxygen availability.

What happens to pyruvate when oxygen is abundant?

Pyruvate enters the mitochondria and undergoes oxidative catabolism.

What happens to pyruvate when oxygen is not abundant?

Pyruvate is converted into lactic acid; about 20% diffuses into the bloodstream, where much of it is converted into glucose by the liver.

Oxidative (aerobic) catabolism

Series of reactions in the mitochondria that removes electrons from carbon-based compounds and uses the released energy to produce ATP.

What fuels can oxidative catabolism use?

Products of glycolysis, fatty acids, and amino acids; glucose is used first, with fatty acids and amino acids used more as glucose becomes less available.

Why does oxidative catabolism require oxygen?

The final step of oxidative catabolism requires oxygen, so muscle fibers need a plentiful oxygen supply for sustained ATP production.

Myoglobin

Oxygen-binding protein in muscle fibers that stores oxygen and releases it as available oxygen is depleted during oxidative catabolism.

How long can oxidative catabolism provide ATP?

It can provide ATP for hours as long as oxygen and fuel are available.

When does oxidative catabolism become the predominant ATP source?

After about 1 minute of muscle activity, oxidative catabolism becomes the predominant source of ATP; after several minutes it provides essentially 100% of the ATP.

Immediate vs. glycolytic vs. oxidative energy sources

Immediate creatine phosphate = about 10 additional seconds of maximal activity; glycolysis = about 30–40 seconds of sustained contraction; oxidative catabolism = prolonged activity lasting minutes to hours.

Creatine supplementation

Can mildly improve performance during activities requiring short bursts of muscle activity, but has minimal to no effect on endurance activities.

Why can creatine supplementation cause weight gain?

It causes water retention.

Potential problem with massive doses of creatine

The PowerPoint states that massive doses may have negative health effects such as kidney damage, and excess creatine is excreted because skeletal muscle has a maximum storage capacity.

How is creatine regulated?

Creatine is a dietary supplement and is regulated as a food rather than a drug; the PowerPoint notes that it does not require FDA approval.