Protein turnover in the tissues of fish : the effect of nutrition, exercise and salinity
The rate of protein synthesis, RNA and protein content in the whole body and specific tissues in 3 economically important fish species was determined under a variety of conditions known to affect the growth rate. In experiment 1, the relationships between protein synthesis in the whole body of juvenile Atlantic cod (Gadus morhua) and ration size and growth rate were identified. The ration induced increases in growth rate were accompanied by linear increases in protein synthesis and RNA/protein ratio in the whole body. In experiment 2, the rate of protein synthesis was measured in the stomach of rainbow trout (Salmo gairdneri) following food withdrawal and subsequent refeeding. Protein synthesis fell markedly within 2 days after food withdrawal but was increased rapidly (within 3 hours) after refeeding. In experiment 3, protein synthesis was measured in the liver, gill, red muscle and white muscle of rainbow trout 40 minutes after the onset of moderate exercise. Within this time, there was a significant elevation in protein synthesis in the gill but a significant reduction in protein synthesis in the liver. In experiment 4, protein synthesis was measured in Atlantic salmon (Salmo salar) at different stages in the process of smoltification - smolting fresh water, smolted sea water and post smolt fresh water. The highest rates of growth were observed in the smolted fish despite the fact that these were heavier than the smolting salmon. In contrast to the high growth rate, the rate of protein synthesis was extremely low in this group with the result that the calculated efficiency of retention of synthesised protein was high and the calculated rate of protein degradation was low. These studies have shown the extremely plastic nature of protein synthesis in the tissues of fish.