Changes in free amino acid levels in various tissues of common carp in response to insulin injection followed by force-feeding an amino acid diet.

The effects of a single injection of porcine insulin on free amino acid levels in plasma, erythrocytes, hepatopancreas and skeletal muscle were simultaneously monitored in common carp; the fish were force-fed a complete diet composed of crystalline amino acids as the sole protein precursors (amino acid diet) to study the dynamics of amino acid metabolism. The force-feeding of the amino acid diet caused surges in the concentrations of almost all amino acids in fish injected with saline, and amino acid levels reached peaks within 1 h in plasma as well as in hepatopancreas. It took more than 2.5 h for most amino acids to reach maximum levels in erythrocytes and skeletal muscle of the same fish. The injection of insulin stimulated drastic reductions of free amino acid levels in the plasma. At the same time, it facilitated reduction of free amino acid levels without elevating glutamine and ammonia levels in erythrocytes, hepatopancreas or skeletal muscle. These results suggest that exogenous insulin accelerated assimilation of dietary free amino acids and their deposition in these tissues.