Either leucine or isoleucine dissolved at a concentration of 8 mmoles/kg/1 was administered into the cranial pancreaticoduodenal artery at a rate of 5/6 ml/min to observe any direct effect of these essential L—amino acids on secretion of glucagon and insulin in anesthetized dogs. The infusion of leucine induced a prompt, short—lived, small but statistically significant elevation in plasma immunoreactive glucagon (IRG) concentration and a more than 2–fold augmentation in plasma immunoreactive insulin (IRI) level in the cranial pancreaticoduodenal vein. Plasma IRI concentration in the femoral artery reflected a significant enhancement, but the peripheral plasma level of IRG failed to show any definite rise, signifying a possibility of only a trivial increase in glucagon secretion. An equimolar load of isoleucine did not cause any consistent elevation in the pancreatic vein plasma level of IRG, but raised pancreatic effluent plasma concentration of IRI to about 3–fold values. A significant rise in plasma IRI occurred in the femoral artery upon infusion of isoleucine. A slight but delayed fall in arterial blood sugar concentration was shown after the first infusion of leucine. The results indicate that there is a difference between leucine and isoleucine in their capacity to stimulate release of glucagon despite a similar magnitude of insulin releasing potency, and suggest that these essential L—amino acids infused intrapancreatically are capable of stimulating insulin secretion, not through the sole mediation of secretion of glucagon, but directly. (Endocrinology91: 691, 1972)
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KANETO et al. (1972) studied this question.