Muscle intracellular and plasma free amino acid concentrations in Belgian blue bulls on two different growth patterns

In this experiment the growth rate patterns were studied in 12 Belgian Blue (BB) young bulls. Initial weight was 230 kg. Six bulls (group + or control) received conventional diets in order to obtain daily growth rates of 1 kg and 1.3 kg, respectively during growth (6 months) and fattening periods. In the second group (group -) the growth diet contained 60 per cent of straw to limit growth rate to about 300 g per day. After 6 months the bulls received the same fattening diet as in the (+) group. At 3 periods, half way the growth period (I), after 2 weeks fattening (II) and 1 month before slaughter (IIIA and B) longissimus dorsi muscle biopsy samples and jugular venous blood were taken for measuring free amino acids (FAA). Animals were slaughtered at 575 to 600 kg. The bulls received the growing diets for 202 days. At this moment the + animals weighed on average 462.5 kg while the - bulls had only reached 301.7 kg. Over the whole fattening period daily growth rates were similar: 1.444 and 1.458 kg although at onset of compensatory growth, growth rate was higher in the group. The + group reached slaughter wheight after 82 days while the - group needed 194 days to achieve the goal. Slaughter yields were similar in both groups but the group - carcasses were fatter. For all free amino acids, intracellular (AA)i were substantially higher than plasmatic concentrations (AA)p. During the growth period (AA)i were distinctly higher in the restricted growth group: total concentrations were 4670 and 7332 micromol per kg while total (AA)p were similar in both groups: 1867 and 1895 micromol per l, for control and restricted animals respectively. At the beginning of the fattening period, changing to more adequate rations resulted in increased total FAA concentrations in both groups, especially for (AA)i. FAA concentrations levelled somewhat off towards the end but remained substantially higher than during the growth period. Between both groups, total FAA were not different. Most AAi were present at concentrations below 1 mmol per kg. However, some AAi were much higher. Glutamic acid and glutamin were present at the highest levels, but ALA and GLY were also high, both in muscle biopsy and blood samples. This is not surprising in view of their role in cellular N-transport. The ratio of free intracellular to protein bound essential amino acids as an estimate of requirement matching, suggests that at intracellular level LEU might be limiting.