The Actions of Insulin, Trypsin, and Electrical Stimulation on Amino Acid Transport in Muscle

The manner in which intact skeletal muscle cells regulate amino acid transport across the cell membrane was examined with the use of α-aminoisobutyrate in isolated frog sartorius muscles, and the results were compared with the behavior of sugar transport in this tissue. α-Aminoisobutyrate entered the muscle cells through two mechanisms, one involving active transport by a readily saturable system, the other resembling passive diffusion. Glucose did not compete with α-aminoisobutyrate for entry. Insulin enhanced the rate of penetration of α-aminoisobutyrate by increasing the Vmax of the saturable system without significantly changing its apparent Km for the amino acid; the rate of the diffusion-like process was not affected. A steady state of active accumulation of α-aminoisobutyrate was attained in muscles incubated for several days. The intracellular steady state concentration was increased by insulin. In contrast, 3-O-methylglucose equilibrated passively across the cell membrane, both in the presence and absence of insulin. Electrical stimulation of muscles and treatment with a low concentration, 2 µg per ml, of trypsin augmented the initial rate of entry of α-aminoisobutyrate into muscle cells; trypsin also raised the steady state level of accumulation. The response of this amino acid transport system to the stimulatory agents resembled that of the sugar transport system in the following particular ways. Insulin acted on both systems at 19° but not at 0° in a 3½-hour exposure. Electrical stimulation affected both systems at 0 and 19°. The effects of electrical stimulation on the transport of the amino acid or sugar persisted essentially undiminished for 5 to 6 hours after cessation of the treatment. Insulin increased the Vmax of entry without altering the apparent Km for either type of substrate. The amino acid transport system differed from the sugar transport system in its ability to accomplish an active accumulation of substrate, in its dependence on Na+ for optimal rates of transport, and in its notably smaller responses to insulin, electrical stimulation, and trypsin. Observations are cited which support the hypothesis that insulin, trypsin, and electrical stimulation enhance the penetration of sugar into the cells of frog sartorius muscles by altering the organization of the cell membrane so that a sugar carrier can move more rapidly within the lipid barrier. Our results suggest that a similar hypothesis is applicable to the transport of amino acids, provided that the additional assumption is made that energy coupling, accomplished through an interaction of the amino acid carrier with Na+, introduces steps that can become rate-limiting when transport is stimulated by the various agents.