Insulin and Na-dependent alanine transport in skeletal muscle of obese Zucker (fa/fa) rats

Obese Zucker (fa/fa) rat skeletal muscle is characterized by a reduced rate of muscle protein deposition possibly due to alterations in amino acid transport. The purpose of the present study was to investigate alanine transport in plasma membrane vesicles from skeletal muscle of lean and obese Zucker rats, facilitating the study of alanine transport independent of cellular metabolism. Initial rates of alanine transport were measured in the presence and absence of Na using a rapid filtration technique, and the properties of membranes from control and maximally insulin-treated lean and obese Zucker rats were studied. For lean rats, the maximal stimulation (V(max)) for Na-dependent alanine transport was 207 pmol · mg^-1 · s^-1, and the half-maximal affinity constant (K( 1/2 )) was 2.3 mM. Insulin treatment increased the V(max) to 387 pmol · mg^-1 · s^-1 with no changes in K( 1/2 ). For the obese rats, the V(max) for Na-dependent alanine transport was 248 pmol · mg^-1 · s^-1, and the K( 1/2 ) was 2.8 mM. These values were not changed by insulin treatment. Thus Na-dependent alanine transport in obese rat skeletal muscle is resistant to stimulation by insulin; this alteration may contribute to the abnormal muscle protein metabolism observed in these animals.