Glucose Modulation of Amino Acid-Induced Glucagon and Insulin Release in the Isolated Perfused Rat Pancreas

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Interactions between glucose and arginine and a mixture of 20 amino acids found in normal rat serum were studied in the isolated perfused rat pancreas of normal rats, with release of immunoreactive glucagon and insulin as parameters. Secretion of both pancreatic hormones was low during the steady state, whether glucose (5 mM) was included in the perfusion medium or not. This glucose concentration significantly stimulated insulin release twofold and resulted in an 80% inhibition of basal glucagon release. Arginine and the amino acid mixture were potent stimulants of both hormones. Secretion of both hormones followed identical biphasic response patterns after addition of arginine or the amino acid mixture. However, stimulation of insulin release occurred only when glucose was included, whereas both phases of glucagon release were elicited in the absence of glucose and markedly reduced in its presence. The dose-dependency curves of hormone release due to arginine on one hand and the amino acid mixture on the other differed substantially: with arginine, release of insulin and glucagon was linear between a concentration of 0.3 and 20 mM. In contrast, the amino acid mixture resulted in half-maximal release for both hormones between a concentration of 3 and 4.5 mM, and maximal release between 6 and 8 mM. The dose-dependencies of glucose modulation of α- and β-cell activity were also different: when the amino acid mixture was maintained at 15 mM and glucose varied (0-6.25 nM), no insulin release occurred until glucose was above 2.5 mM, whereas incremental inhibition of glucagon occurred through the complete dose range. It was also observed that glucose inhibition of amino acid-stimulated glucagon release was dissociated from glucose-dependent increase of insulin release.

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