Myosin heavy chain synthesis in white trunk muscle of cod (Gadus morhua) fed different ration sizes

In this study the expression of myosin heavy chain was investigated at the messenger RNA and protein level of the muscle ribosome. Cod (Gadus morhua) weighing 80 g were fed for 70 days eitherad libitum or with rations consisting of 75%, 50%, or 25% of thead libitum food intake. Protein and RNA contents of the ribosome from the white trunk muscle decreased with the diminished ration sizes. Myosin heavy chain content relative to total ribosome protein increased with the 50% ration size but fell again to thead libitum level at 25%. Expressed relative to RNA, a decrease in the protein content occurred in parallel with the decreasing ration size. The amount of protein/total muscle homogenate fell with diminished ration sizes, and that of myosin heavy chain/mg of protein remained unchanged with a slight increase at the 25% food energy intake. The messenger RNA for myosin heavy chain (relative to poly(A)(+) messenger (RNA) was increased in response to the decreased ration size but was decreased when calculated per g wet weight of the muscle. The changes in messenger RNA for myosin heavy chain were less pronounced than those of the protein itself. The levels of gene transcription and translation for myosin heavy chain were affected to a lesser extent by the low food intake than the synthesis of total RNA and protein. Immunological methods and messenger RNA hybridization to cloned DNA for myosin heavy chain permitted the precise determinations of the events taking place during a food-energy supply shortage. Translation appeared to diminish prior to changes in messenger RNA concentrations.