Large body size in the dog is associated with transient GH excess at a young age

The main determinants of body Size are GH and IGFs. The aim of this study w as to investigate whether differences in adult body size of medium-sized an d 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 con centrations of GH, IGF-I and IGF-II were determined once weekly in six Grea t Danes and six beagles from the age of 6 weeds 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 (A UC) and the AUC above the baseline for GH were significantly higher in Grea t 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. Comp ared with values in adults. the basal plasma GH concentrations were high un til 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, albei t with a gradual decline. The mean frequency and the mean amplitude of GH p ulses tended to be higher in Great Danes than in beagles, although a signif icant difference was only reached at the age of 19 and 23 weeks for the fre quency and at the ages of 9. 11 and 13 weeks for the amplitude. An age-depe ndent decrease in pulse frequency occurred in the Great Danes. The results of this study demonstrate that differences in adult body size o f medium-sized and giant dog breeds are preceded by differences in GH relea se and not by differences in circulating IGF-I or IGF-II concentrations. Bo th young Great Danes and young beagles experience a period of high GH relea se, but this period persists much longer in Great Danes. It is discussed th at this difference may be due to delayed maturation of the inhibitory influ ences of somatostatin on pituitary GH release in the latter dogs.