Carbohydrates affect protein-turnover rates, growth, and nucleic acid content in the white muscle of rainbow trout (Oncorhynchus mykiss)

We have investigated the effect of dietary carbohydrate on different parame ters of protein-turnover rate, nature of growth, and nucleic acid content i n the muscle of rainbow trout in order to better understand the molecular n ature of these growth parameters in the absence of this dietary component. For this, we used a methodology based on the incorporation rate of tritium labelled phenylalanine in muscle protein. Juvenile rainbow trout of an init ial body weight of 110 g were fed near to satiety with a control or a non-c arbohydrate diet during 7 weeks. The absence of dietary carbohydrate signif icantly depressed fish growth, as well as daily body weight gain, as a cons equence of muscular hypotrophy (the cell size diminished by almost 50%) and not by a reduction of number of cells (hypoplasia). This nutritional situa tion also significantly slowed (by almost 11%) muscle-protein accumulation rate (K-G) as a result of a significant increase (eight-fold) in muscle-pro tein degradation rate (K-D), without changing the other protein-turnover ra tes, protein synthesis rate (K-S), protein synthesis capacity (C-S), protei n synthesis efficiency (K-RNA), protein synthesis rate per cell unit (K-DNA ), or protein retention efficiency (PRE). These results, together with the nucleic acid content, clearly indicate that the absence of carbohydrate sig nificantly exacerbates the muscular-protein degradation without affecting p rotein synthesis. In conclusion. carbohydrates are needed to prevent amino acids released during protein degradation from being used to synthesize car bohydrates and/or to be used for energy and not for growth. (C) 1999 Elsevi er Science B.V. All rights reserved.