Some Aspects of Endocrine Control of Growth and Protein Metabolism in Rainbow Trout, Salmo Gardneri

Abstract

The influence of insulin, growth hormone and some synthetic anabolic steroids was studied on protein synthesis, indexed by incorporation of L-(14C-U)-leucine into skeletal muscle protein and growth in rainbow trout. In rainbow trout, insulin tended to increase protein synthesis in skeletal muscle, decrease the excretion of ammonia in ambient water and induce an overall increase in muscle protein content. The incorporation of leucine-14C increased in response to high protein feeding, while short term fasting and protein restriction decreased the incorporation. These changes in protein synthesis are probably related to altered plasma insulin levels during different nutritional states. An attempt was made in vain to measure plasma immunoreactive insulin levels in rainbow trout by using anti-mammalian insulin antibody. Although a single injection of a hypoglycaemic sulphonylurea HB 419 (glibenclamide) stimulated protein synthesis, long term administration of glibenclamide and tolbutamide reduced growth rate of rainbow trout. Porcine and selachian growth hormone and rainbow trout pituitary extracts stimulated the rate of protein synthesis in skeletal muscle. Three synthetic androgens, dimethazine, norethandrolone and methenolone, and an oestrogenic compound, diethylstilboestrol were administered orally at different concentrations in the diet to juvenile rainbow trout and their effect on protein metabolism, growth and food conversion was examined. Both dimethazine and norethandrolone, when given at low concentrations (2.5 and 5.0 mg/kg food) for 60 days significantly increased weight gain. These increases in weight gain were due to both the increased rate of protein synthesis and improved food conversion efficiency. High concentrations of dimethazine (10 and 20 mg/kg food) produced an initial increase in weight gain but it was not significantly different from that of untreated fish after 48 days. There was no change in the hepatosomatic indices of the fish treated with dimethazine or norethandrolone. However, the renosomatic indices of rainbow trout fed dimethazine were significantly greater than those of the controls, apparently due to an increase in the renal haemopoietic tissue. Methenolone (10 mg/kg food) and diethylstilboestrol (1.2 mg/kg food) tended to depress growth rate and impair food conversion efficiency. It is concluded that insulin and growth hormone can stimulate protein synthesis in rainbow trout. It appears that insulin plays a dominant role as a protein anabolic hormone in rainbow trout and it may, probably, be true for other carnivorous teleostei. Synthetic androgens, dimethazine and norethandrolone can manifest pronounced proteo-anabolic activity and have potential value as growth promoters in trout culture.