Body size parallels insulin-like growth factor I levels but not growth hormone secretory capacity.

The relationships between body size, growth hormone (GH) secretory capacity and circulating insulin-like growth factor I (IGF I) levels were studied in genetically-determined subgroups of disparate size within one breed of dogs, the Poodle. Standard (large) Poodles exhibited six times the mean plasma IGF I concentration found in Toy Poodles. The mean IGF I level found in Standard Poodles significantly differed from the one found in Miniature and Toy Poodles (P less than 0.001). The correlation between circulating IGF I levels and body size was found to be highly significant (P less than 0.001; r = 0.88). All dogs secreted similar, normal amounts of GH in response to clonidine administration. The results show that body weight is correlated with IGF I levels rather than with the GH secretory capacity, thus providing indirect evidence for IGF I as an important in vivo growth-promoting principle.