Abstract

Plasma triacylglycerols (TG) and free fatty acids (FFA), as well as intramuscular TG, are oxidizable lipid fuel sources for skeletal muscle metabolism during endurance exercise. Plasma FFA are a major fuel oxidized by skeletal muscle, and the mobilization of FFA from adipose tissue is the first committed step in their metabolism. The rate of FFA mobilization is dependent on the rate of adipose tissue lipolysis, the plasma transport capacity for FFA, and the rate of FFA reesterification. The rate of adipose tissue lipolysis increases during prolonged submaximal exercise. The essential hormonal changes promoting increased lipolysis during whole-body exercise are an increase in catecholamine levels and a decrease in insulin concentration, both of which facilitate the activation of the hormone-sensitive lipase system through changes in its phosphorylation state. Independently of the exercise-induced hormonal changes, glucose concentration also regulates FFA mobilization by suppressing lipolysis. The plasma transport capacity for FFA is dependent on blood flow and on the FFA/albumin molar ratio. During prolonged submaximal exercise the increase in adipose tissue blood flow compensates for the increase in the FFA/albumin molar ratio to favor an increase in FFA mobilization. As part of the triglyceride-fatty acid cycle, the exercise-induced decrease in the rate of FFA reesterification acts in concert with the exercise-induced increase in lipolysis to amplify the response and favor a net increase in FFA mobilization. Following the transport of FFA in plasma, FFA permeation across the plasma membranes is the next step in the metabolism of FFA. In all cell types studied to date, evidence shows that at least part of the permeation of FFA across the plasma membranes is carrier-mediated and that a plasma membrane fatty acid binding protein may be the functional transporter. Possible regulation of FFA metabolism through changes in the rate of permeation remain to be elucidated. Cytoplasmic transport of FFA is facilitated by another family of fatty acid binding proteins whose level in skeletal muscle is correlated with the oxidative capacity of the muscle fiber types. Intracellular metabolism of FFA is regulated by a number of factors. In skeletal muscle, the rate of FFA oxidation increases with an increase in the plasma FFA concentration. At a given FFA concentration, it increases with an increase in the metabolic rate. At high FFA concentration, the rate of FFA oxidation tends to plateau. The rate of FFA oxidation is also regulated in part by the oxidative capacity of the recruited fibers, the intramuscular concentration of malonyl-CoA, and the availability of carbohydrate sources. During prolonged submaximal exercise, the contribution of plasma TG and ketone bodies to skeletal muscle oxidative metabolism is small. The rate of utilization of plasrna TG is dependent on lipoprotein lipase (LPL) activity which is correlated with the fiber's capacity for oxidative metabolism. The rate of utilization of ketone bodies is dependent in part on their blood concentration. During prolonged exercise, blood ketone bodies' concentration increases slightly but their total contribution to skeletal muscle oxidative metabolism remains minimal. Verf.-Referat (gekuerzt)