Amino acids are more important insulinotropins than glucose in a teleost fish, barfin flounder (Verasper moseri)

The insulinotropic effects of eighteen l-amino acids, two d-amino acids, and glucose were investigated to evaluate the priority of those as stimulators of insulin secretion in barfin flounder (Verasper moseri). This is also the first step in characterizing the insulinotropin-sensing molecule. After intramuscular injection of amino acids or glucose at doses of 3.50 and 1.75 mmol/kg body weight, plasma was collected periodically to determine plasma insulin level. Twelve amino acids and glucose showed insulinotropic effects. Four l-amino acids (Arg, Ala, Met, Ser) produced significantly higher integrated levels of plasma insulin (12.4–34.8 ng/ml) than glucose (average: 4.7 ng/ml) during 3 h after injection. d-Amino acids (Arg, Ala) showed no activity. This indicates that many amino acids have strong insulinotropic activities and supports a classic idea, which is well known but has not been confirmed, that amino acids rather than glucose are the important insulinotropins in fish. This study also indicates that the insulinotropic activity of amino acids is restricted to l-amino acids and establishes which amino acids are the strongest stimulators of the insulinotropin sensor in barfin flounder. Co-injection of insulin and l-Thr, l-Ala, or glucose produced a hypoglycemic and hypoaminoacidemic state, indicating that insulin can lower blood amino acid level as well as blood sugar level. This study suggests that insulin plays a more important role than glucose in the regulation of blood l-amino acid metabolism, at least in flounder.