Protein synthesis in fish in relation to oxygen consumption and free amino acids
Protein synthesis was investigated in fish with respect to energetic costs and the effects of feeding. Experiments with cycloheximide were conducted on preparations of isolated, perfused flounder gills. Cycloheximide treatment reduced both oxygen consumption and protein synthesis rates in the gills by 80%. The cost of protein synthesis was calculated to be equivalent to 85 mmol ATP g protein-1 synthesised. Oxygen consumption and protein synthesis were measured in fasted cod and in animals refed a single meal. In cod fasted for up to 14 days, oxygen consumption and protein synthesis had both fallen to stable values by six days post-feeding. Refeeding after a six day fast caused a rapid increase in both variables, with peak values occurring between 12 and 18 hours after the meal. Maximum oxygen consumption rates were about twice the pre-feeding levels, while protein synthesis increased four-fold. Protein synthesis rates in the liver and stomach responded faster to the meal than the remainder of the body, maximum values occurring at six hours post-feeding. Calculations of the contribution of increased protein synthesis to post-prandial metabolism indicate that it accounts for between 23% and 51% of the increment in oxygen consumption. Changes in the free amino acid pools of tissues and plasma were examined in cod refed a single meal. The only significant changes were in plasma from the heart, with a decrease at six hours post-feeding and an increase at 12 hours. Nevertheless, the mean concentrations of amino acids in the hepatic portal plasma, stomach and white muscle increased at 12 hours or 18 hours post-feeding. This coincided with the timing of maximum whole animal protein synthesis previously observed. It was concluded that protein synthesis is an important contributor to post-prandial metabolism in fish, and that free amino acids might play a role in stimulating protein synthesis after a meal.