AGE-RELATED PLASMA-CONCENTRATIONS OF GROWTH-HORMONE (GH) AND INSULIN-LIKE GROWTH-FACTOR I (IGF-I) IN GREAT DANE PUPS FED DIFFERENT DIETARY LEVELS OF PROTEIN

During a 20-week study, 17 Great Dane dogs, 7 weeks of age, were used to study the influence of 3 isoenergetic diets differing in dietary pr otein levels (i.e., 3 1.6%, 23. 1 %, and 14.6% on a dry matter basis) on body weight, growth in length, and age-related plasma GH and IGF le vels. Significant differences occurred in weekly body weight gain of t he high- and low- protein groups only in the fourth week of the study. There was a significant decrease in mean basal plasma GH values with time from 14.8 +/- 2.2, 13.7 +/- 2.2, and 14.3 +/- 2.2 mug/l in the se cond week to 2.3 +/- 1, 0.7 +/- 0.4, and 1.8 +/- 0.7 mug/l in the last week of the study for the high-, normal- and low-protein groups, resp ectively. Differences among groups were not significant. There was a p ositive correlation between the decrease in weekly body weight gain an d plasma GH concentration in all 3 groups. IGF-I concentrations in pla sma did not change consistently with age and were not correlated with plasma GH values. Significant differences in plasma IGF-I concentratio ns were found between groups at 15 weeks of age. Preliminary results o f measurements of IGF receptors in the membranes of growth plate carti lage of long bones of 5 dogs revealed that homologous displacement of I-125-labeled IGF-I binding resulted in similar curves. A single bindi ng site for IGF-I was assumed. The large number of type Il IGF recepto rs in 7 dogs suggests a key role for IGF-II in postnatal skeletal grow th. The results of this study demonstrate for the first time that in G reat Dane pups basal GH concentrations in plasma decrease between 7 an d 27 weeks of age, parallel to the decrease in growth velocity, wherea s the plasma IGF-I concentrations remain relatively high and unchanged . The slight but statistically significant effects of low-protein feed ing on growth velocity and circulating concentrations of IGF-I indicat e that a diet containing 14.6% protein on a dry matter basis (13% prot ein as metabolizable energy) is marginal for growing Great Dane pups b etween 7 and 17 weeks of age.