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The main determinants of body size are GH and IGFs. The aim of this study was to investigate whether differences in adult body size of medium-sized and giant dog breeds can be explained by differences in GH release and/or in plasma IGF-I and IGF-II concentrations at a young age. The basal plasma concentrations of GH, IGF-I and IGF-II were determined once weekly in six Great Danes and six beagles from the age of 6 weeks until the age of 24 weeks. In addition, the 6 h secretory profile of GH was determined every 2 weeks. Basal plasma GH concentrations as well as the total area under the curve (AUC) and the AUC above the baseline for GH were significantly higher in Great Danes than in beagles of the same age. In contrast, plasma IGF-I and IGF-II concentrations did not differ significantly between the two breeds. Compared with values in adults, the basal plasma GH concentrations were high until the age of 7 weeks in the beagles, whereas in the Great Danes the basal plasma GH levels remained high during the entire observation period, albeit with a gradual decline. The mean frequency and the mean amplitude of GH pulses tended to be higher in Great Danes than in beagles, although a significant difference was only reached at the age of 19 and 23 weeks for the frequency and at the ages of 9, 11 and 13 weeks for the amplitude. An age-dependent decrease in pulse frequency occurred in the Great Danes. The results of this study demonstrate that differences in adult body size of medium-sized and giant dog breeds are preceded by differences in GH release and not by differences in circulating IGF-I or IGF-II concentrations. Both young Great Danes and young beagles experience a period of high GH release, but this period persists much longer in Great Danes. It is discussed that this difference may be due to delayed maturation of the inhibitory influences of somatostatin on pituitary GH release in the latter dogs.
