Amino Acid Transport in Peripheral Nerve: Energy and Sodium Dependence

The energy and sodium dependence of the several carrier‐mediated mechanisms for amino acid uptake have been studied in frog sciatic nerve. The different transport mechanisms are found to be variable in their dependence on sodium and metabolic energy. Saturable uptakes of lysine, phenylalanine and valine are relatively independent of the presence or absence of sodium in the incubation medium, indicating that uptakes by those mechanisms subserving basic, large neutral amino acids, and those amino acids containing aromatic or heterocyclic ring structures are largely sodium independent. Saturable uptakes of glutamic acid, proline, glycine and β‐alanine are considerably reduced in the absence of sodium; thus carrier mechanisms for uptake of acidic, small neutral amino acids, β‐alanine and proline are highly sodium dependent. The efficacies of several cations in substituting for sodium is variable; greatest inhibitions are found when potassium is used to replace sodium. With the exception of proline, those mechanisms found to be sodium dependent are also found to be energy dependent, since they are inhibited by both DNP and lowered temperature. Although proline uptake is sodium dependent, proline uptake is stimulated by DNP and relatively insensitive to lowered temperature.