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Abstract
An oral phenylalanine load provokes a significant drop in serum tyrosine levels in children with phenylketonuria [8]. The aim of the present investigation was to examine the response of insulin and glucagon to oral phenylalanine loading as these hormones are known to have a hypoaminoacidaemic effect. Six adult normal weight and healthy men were loaded orally with 0.6 mmol L-phenylala-nine per kg body weight after an overnight fast. Serum phenylalanine increased within 10 min after the load and reached a maximum concentration at 30 min. Serum tyrosine increased within 10 min after the load and reached a maximum concentration at 2 h. Plasma glucagon and insulin increased during the first 10 min after the load and reached a peak twice the fasting levels at 30 min after the load. The molar insulin/glucagon ratio remained unchanged during the first 20 min after the load but then declined by 50% at 2 h. Associated with this decline plasma amino acid concentration (except phenylalanine and tyrosine) declined by approximately 15%. The decline was most marked for isoleucine, leucine, methionine and valine. As the hypoaminoacidaemic effect of insulin and glucagon is known to be most marked for these four amino acids plus phenylalanine and tyrosine, the response of insulin and glucagon to a phenylalanine load may influence not only the fate of phenylalanine given but also the blood tyrosine level.