IGF-I AND INSULIN-RECEPTORS IN BOVINE SKELETAL-MUSCLE - COMPARISONS OF DIFFERENT DEVELOPMENTAL AGES, 2 DIFFERENT GENOTYPES AND VARIOUS INDIVIDUAL MUSCLES

To investigate the regulation of the IGF-I and the insulin receptor in bovine skeletal muscle, we determined their concentrations and their affinity constants in animals of different age, muscle type and breed. Receptors were solubilized and measured by an enzyme-immunoassay. During ontogenesis the concentration of IGF-I receptors decreased from 1650 +/- 600 fmol/mg protein in 3 - 5-month old fetuses to 105 +/- 20 fmol/mg protein in 16-month old heifers (p less than or equal to 0.01). The insulin receptor concentrations also decreased with age from 136 +/- 28 fmol/mg protein in 3 - 5-month old fetuses to 50 +/- 11 fmol/mg protein (p less than or equal to 0.05) in 16-month old heifers. In order to compare different muscle types, seven muscles, which represent large differences in fibre type composition and growth impetus, were selected from 6 month old female Jersey calves and were assayed for IGF-I and insulin receptors. We observed significant differences of the IGF-I as well as the insulin receptor concentrations between distinct muscles. However, no relationship could be established between receptor concentration and fibre type composition or growth impetus. In muscle of two cattle breeds, differing markedly with regard to muscle growth intensity, the Jersey and the German Fleckvieh breed, we observed no divergence in IGF-I nor insulin receptor concentrations. We found no differences in IGF-I and in insulin receptor affinities in any of the adult animals. In conclusion, we suggest that the regulation of IGF-I and insulin activity at the receptor level is most prominent during the transition from prenatal to postnatal life, whereas for postnatal muscle growth IGF-I and Insulin receptor regulation appears to be less important.