November 6, 2017

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

J.E. Eigenmann, D.F. Patterson and E.R. Froesch studied the relationship between body size, insulin-like growth factor I (IGF I) levels, and growth hormone (GH) secretory capacity in the Standard, Miniature, and Toy Poodle. The purpose of the study was to determine the relationship between body size and IGF I levels and GH secretory capacity in the Standard, Miniature, and Toy Poodle.

The scientists specifically used poodles because poodles have a fairly similar genetic makeup, but three distinct size variations (Standard, Miniature, and Toy). Although all of the poodle breed, these three variations tend to be bred separately in order to maintain the accepted standard sizes. This means that the breeders specifically select for the genes that affect body size, while keeping phenotypic traits and proportions the same.

To test if IGF I or GH secretory capacity is related to body size, the scientist measured the IGF I levels as well as recording the GH secretory capacity after the administration of clonidine over time (0, 15, 30, 45, 60, and 90 minutes) in 10 poodles of each variation. Clonidine was used in measuring the GH secretory capacity because clonidine is a stimulator of growth hormone.

The results were that IGF I levels are significantly correlated and parallel the body size of the different variations of poodle. The Standard Poodle has six times the amount of IGF I in circulation compared to the Toy Poodle (standard: 96 ng/ml, toy: 16 ng/ml).  For GH secretory capacity, there was no significant correlation between body size and GH. Miniature and Toy Poodles secreted at least as much of GH compared to the Standard Poodle, showing that GH secretory capacity is not significantly related to body size. Also, Miniature and Toy Poodles had similar GH secretion patterns over time, whereas Standard Poodles had differing secretion patterns over time when compared with Miniature and Toy Poodles.

In conclusion, in the poodle breed, body size is correlated with IGF I levels rather than with GH secretory capacity, indirectly supporting that IGF I is an important factor for in vivo growth.

J.E. Eigenmann, D.F. Patterson and E.R. Froesch, 1984. Body size parallels insulin-like growth factor I levels but not growth hormone secretory capacity. Acta Endocrinologica, 106: 448-453

Original article: http://www.eje-online.org/content/106/4/448.full.pdf

Reviewed by Hannah Cohen