Glutamine transport and its metabolic effects.

This paper surveys current information concerning the characteristics of glutamine transport in skeletal muscle and heart and presents evidence for a regulatory role of amino acid transporters under certain physiological and pathophysiological circumstances. In particular, attention is focused upon the properties and regulatory modulation of System Nm, the transporter with the highest capacity for transmembrane glutamine exchange in rat and human skeletal muscle. Much of the observed behaviour of the glutamine-free pool in skeletal muscle in health and disease can be explained by alterations in the activity of this transporter. Like the System A transporters in liver and muscle System Nm shows adaptive regulation in response to altered availability of extracellular glutamine. The effects of glutamine in skeletal muscle include the stimulation of protein synthesis which occurs in the absence or presence of insulin, the response being greater with insulin. Integrative features of the transport and metabolic effects of glutamine and other amino acids are discussed.