AGE, PROTEIN LEVEL AND SEX AS FACTORS INFLUENCING GASTROCNEMIUS-MUSCLE GROWTH IN BALB C MICE FROM WEANING TO 50 DAYS OF AGE/

The effects of age, dietary protein level [12% (medium protein) or 20% (high protein)], and sex on muscle growth of female (f) and male (m) BALB/c mice between weaning (21 d) and 50 d of age were studied. Anima ls were housed individually and feed intake and body weight (BW) were recorded daily. In the total experimental period (29 d), six partial e xperimental periods were established. At 25, 30, 35, 40, 45 and 50 d o f age, five mice of each group (MP(f), MP(m), HPf, HPm) were slaughter ed and gastrocnemius muscle growth was studied. Based on BW and muscle weight (MW) measures, values of growth parameters were estimated. Fee d consumption rate (FCR) increased in all groups, but it nearly stabil ized with age in HP mice. BW, estimated muscle weight (EMW), muscle we ight gain (MWG), absolute muscle protein (AMP) and muscle protein gain (MPG) increased in MP and HP mice during the experiment. Relative mus cle protein (RMP) increased in MP while it decreased in HP mice with a ge. Also, absolute and fractional muscle weight growth rates (AWGR and FWGR, respectively) and absolute and fractional muscle protein growth rates (APGR and FPGR, respectively) decreased in all mice tested (age -dependent), except in HPf mice where it remained plateaued in time (a ge-independent). At 21 d the highest AWGR, FWGR, APGR, and FPGR values were registered in MP mice. Diet and sex influenced the relative allo metric proportions of muscle to the organism as a whole. Muscle protei n growth of MP mice versus MW growth was enhanced, resulting in an inc rease of growth coefficients as compared to HP mice. Also, the contrib ution of muscle protein to carcass protein and the efficiency of prote in intake utilization were higher in MP mice than in HP mice, while ef ficiency of energy intake was similar in both groups. In conclusion, h yperphagia and increased efficiency of protein utilization induced a n ear-normal muscle growth in MP growing BALB/c mice.