Muscle Glucose Metabolism and Insulin Resistance

In contrast to most mammals, in which 30-40% of body weight consists of muscle, more than
half of a mature horse's body weight comprises skeletal muscle (Gunn 1987) and is the
primary tissue of insulin stimulated glucose uptake, disposal, and storage. This underscores
the importance of glucose utilization as an energy source in muscle. Consequently, skeletal
muscle has a significant role in insulin sensitivity and obesity. Moreover, caloric excess,
obesity and physical inactivity may lead to skeletal muscle insulin resistance (Smith and
Muscat 2005). Glucose from the blood can not pass the sarcolemma of skeletal muscle itself
because of the lipophobic nature of the molecule. In skeletal muscle, the most abundant
glucose transport proteins are GLUT-1 and GLUT-4. GLUT-4 is a protein that resides in an
intracellular compartment and translocates to the plasma membrane after distinct stimuli
among which insulin and contraction are the most potent (Ploug et al. 1990; Goodyear et al.
1991).