Metabolic interactions between glucose and amino acids were studied with isolated rat islets using glucose utilization and lactate formation as indicators. Certain amino acids (8–10 mM) are capable of greatly stimulating lactate formation from 5 mM glucose. On a molar basis L-isoleucine is the most potent stimulator in a group of twenty-six amino acids. A physiological amino acid mixture (7.5–14 mM) or L-isoleucine (8 mM) profoundly altered the basic sigmoidal relation between glucose concentration in the medium and the rate of glucose utilization and lactate formation: with basal glucose (5 mM) both glucose utilization and lactate production were stimulated by the amino acid mixture and by L-isoleucine; at high glucose levels utilization was decreased by the amino acid mixture, but was unaffected by L-isoleucine, whereas lactate formation was decreased by both additions. The data indicate that amino acids may play a significant role in regulating the extent to which glucose serves as a fuel of pancreatic islet cells and in determining the pathways of glucose metabolism.
In order to elucidate the mechanisms of the amino acid effect, studies with phloridzin, ouabain, iodoacetate, cytochalasin B, and Na±deficiency were performed with the most effective amino acid, L-isoleucine. Each of these agents and Na±deficiency substantially reduced or completely blocked the extra lactate formation induced by L-isoleucine (8–10 mM). The intracellular uptake of 14C L-isoleucine by isolated islets was found to be Na±independent, and uphill transport of this amino acid was not detectable, whether basal glucose was present in the medium or not.
The action of iodoacetate in blocking glycolysis was reinvestigated. After forty-five minutes of exposure, 0.2 mM iodoacetate completely blocks lactate formation as well as glucose utilization. This confirms and extends earlier data from this laboratory and suggests that this SH-reagent indeed allows dissociation of the fuel and releasing functions of glucose.