Resting Skeletal Muscle

RESTING SKELETAL MUSCLE

In the fed state, muscle takes up glucose via GLUT-4 (for energy and glycogen synthesis) and amino acids (for energy and protein synthesis). [Note: The energy metabolism of skeletal muscle is unique in being able to respond to substantial changes in the demand for ATP that accompanies muscle contraction. At rest, muscle accounts for approximately 30% of the O₂ consumption of the body, whereas during vigorous exercise, it is responsible for up to 90% of the total O₂ consumption. This graphically illustrates the fact that skeletal muscle, despite its potential for transient periods of anaerobic glycolysis, is an oxidative tissue. In contrast to liver, there is no covalent regulation of PFK-2 in skeletal muscle. In the cardiac isozyme, however, the kinase domain is activated by epinephrine-mediated phosphorylation.]

Figure 24.7 Major metabolic pathways in skeletal muscle in the absorptive state. [Note: The numbers in circles, which appear both in the figure and in the text, indicate important pathways for carbohydrate or protein metabolism.] CoA = coenzyme A; P = phosphate; GLUT = glucose transporter; BCAAs = branched-chain amino acids; TCA = tricarboxylic acid.

A. Carbohydrate metabolism

1. Increased glucose transport: The transient increase in plasma glucose and insulin after a carbohydrate-rich meal leads to an increase in glucose transport into muscle cells by GLUT-4 (Figure 24.7, ), thereby reducing blood glucose. Glucose is phosphorylated to glucose 6-phosphate by hexokinase and metabolized to provide the energy needs of the cells.

2. Increased glycogen synthesis: The increased insulin-to-glucagon ratio and the availability of glucose 6-phosphate favor glycogen synthesis, particularly if glycogen stores have been depleted as a result of exercise (Figure 24.7, ).

B. Fat metabolism

FAs are released from chylomicrons and VLDL by the action of LPL. However, fatty acids are of secondary importance as a fuel for resting muscle during the fed state, in which glucose is the primary source of energy.

C. Amino acid metabolism

1. Increased protein synthesis: An increase in amino acid uptake and protein synthesis occurs in the absorptive period after ingestion of a meal containing protein (see Figure 24.7, 3 and 4 ). This synthesis replaces protein degraded since the previous meal.

2. Increased uptake of branched-chain amino acids: Muscle is the principal site for degradation of the BCAAs (leucine, isoleucine, and valine) because it contains the required transaminase. The BCAAs escape metabolism by the liver and are taken up by muscle, where they are used for protein synthesis (see Figure 24.7, 3) and as sources of energy.