Effects of insulin and of metabolite loading on blood metabolites in the European silver eel, ( Anguilla anguilla L.)

Abstract

The effects of codfish and bovine insulin, and of glucose and amino acid loading on blood glucose, plasma cholesterol and amino acid nitrogen were studied in the European silver eel, Anguilla anguilla. Serial blood samples were withdrawn with minimal stress from individual animals after cannulation of the bulbus arteriosus. The operation initially caused marked elevations of the metabolites, necessitating a 48 hr post-operative recovery period. Codfish insulin at 2 IU/kg produced a significant yet recoverable lowering of blood glucose, plasma cholesterol and amino acid nitrogen, while bovine insulin at the same does level effected a reduction in amino acid nitrogen only. A single intra-arterial glucose load of 0.5 g/kg resulted in an increase in blood glucose levels followed by a slow return to normal after 24 hr. Plasma cholesterol levels were significantly lowered after such a treatment while amino acid nitrogen remained unaltered. A repeated intra-arterial glucose load 24 hr later resulted in an enhanced rate of glucose removal from the blood. A glucose load administered via the intestine resulted in a smaller increase in blood glucose, with more rapid return to normal levels, while plasma cholesterol and amino acid nitrogen remained unaltered. An intra-arterially injected mixture of amino acids (0.25 g/kg) was rapidly removed from the blood within 3 hr. In addition, plasma cholesterol levels were reduced and a rapid hyperglycaemia developed, followed by hypoglycaemia 9 hr later. An intestinal load of amino acids resulted in a slower rate of removal from the blood together with a sustained hyperglycaemia, but of lesser degree than after intra-arterial administration. Plasma cholesterol levels remained unaltered by an intestinal amino acid load. The results indicate a greater sensitivity of the European silver eel to codfish rather than bovine insulin, and point to the possibility of an important role of insulin in the metabolic control of the initial phases of its spawning migration. The metabolite-loading data indicate that mechanisms for the removal of excess glucose and amino acids are still functional in pre-migratory silver eels and may be partly insulin-dependent.