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 protein levels (i.e., 31.6%, 23.1%, and 14.6% on a dry matter basis) on body weight, growth in length, and age-related plasma GH and IGF levels. Significant differences occurred in weekly body weight gain of the 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 micrograms/l in the second week to 2.3 +/- 1, 0.7 +/- 0.4, and 1.8 +/- 0.7 microgram/l in the last week of the study for the high-, normal- and low-protein groups, respectively. Differences among groups were not significant. There was a positive correlation between the decrease in weekly body weight gain and plasma GH concentration in all 3 groups. IGF-I concentrations in plasma did not change consistently with age and were not correlated with plasma GH values. Significant differences in plasma IGF-I concentrations were found between groups at 15 weeks of age. Preliminary results of measurements of IGF receptors in the membranes of growth plate cartilage of long bones of 5 dogs revealed that homologous displacement of 125I-labeled IGF-I binding resulted in similar curves. A single binding site for IGF-I was assumed. The large number of type II IGF receptors in 7 dogs suggests a key role for IGF-II in postnatal skeletal growth. The results of this study demonstrate for the first time that in Great Dane pups basal GH concentrations in plasma decrease between 7 and 27 weeks of age, parallel to the decrease in growth velocity, whereas the plasma IGF-I concentrations remain relatively high and unchanged. The slight but statistically significant effects of low-protein feeding on growth velocity and circulating concentrations of IGF-I indicate that a diet containing 14.6% protein on a dry matter basis (13% protein as metabolizable energy) is marginal for growing Great Dane pups between 7 and 17 weeks of age.