Both liver and muscle can produce and release glutamine into the
bloodstream, but muscle may be quantitatively the most important tissue. Since
the cells of the intestine have a large capacity to utilize glutamine, most of
the glutamine that enters the body via the diet is utilized by the intestine.
Hence glutamine production and release by muscle become of considerable
physiological and immunological importance.
The systematic study of glutamine flux in skeletal muscle, using
the principles of metabolic-control logic, has highlighted two areas of
potential misunderstanding in the literature. These relate to the processes of
glutamine synthesis in muscle and the processes of glutamine uptake and release
across the muscle membrane. It should be emphasized that glutamine synthesis,
uptake and release by muscle are three distinct processes. Evidence has been
obtained that the process of glutamine release from skeletal muscle may be the
limiting step - indeed may be the flux-generating step - for maintenance of the
plasma glutamine level and hence its uptake by other cells (e.g., immune