Relationships between muscle growth potential, intramuscular fat content and different indicators of muscle fibre types in young Charolais bulls.

Genetic selection in favor of muscle growth at the expense of fat should affect characteristics of muscles, and therefore beef quality. This study was conducted with two extreme groups of six animals selected among 64 Charolais young bulls ranked according to their genetic potential for muscle growth. Muscle characteristics were assessed in Rectus abdominis (RA, slow oxidative) and Semitendinosus (ST, fast glycolytic) muscles. Intramuscular fat content and proportions of myosin heavy chains I (slow) and IIA (fast oxido-glycolytic) and certain indicators of oxidative metabolism (activities of citrate synthase (CS), isocitrate dehydrogenase and cytochrome-c oxidase (COX); expression of H-fatty acid binding protein (FABP)) were higher in RA than in ST muscle. Genetic selection for muscle growth reduced intramuscular fat content and the activities of some oxidative metabolism indicators (namely CS, COX only). The positive correlation between muscle triacylglycerol content and A-FABP messenger RNA level (a marker of adipocyte differentiation) (r = 0.53, P < 0.05) suggests that A-FABP may be a good marker of the ability of bovines to deposit intramuscular fat. In conclusion, the metabolic muscle characteristics which respond to the selection process in favor of muscle growth clearly differ from the muscle characteristics which allow muscle types to be differentiated.