Ontogenesis of plasma growth hormone in large and midget white strains of turkeys.

Abstract

The ontogeny of growth hormone (GH) secretion was determined in large and midget white strains of turkeys from day 14 of embryonic development through 32 weeks of age. The strains of turkeys used differed markedly in early growth rate, and exhibited a twofold difference in body size. Growth hormone was not detected in the plasma of either strain prior to day 25 of incubation, and was not found in most birds until the day before hatching. A highly significant (fivefold) increase in plasma GH concentration occurred between hatching and 2 weeks of age in both strains followed by a decline to 1.4% of peak values by 16 and 24 weeks in the large and midget white strains, respectively. Peak levels of circulating GH did not differ between strains, but the rate of decline to adult levels was significantly faster in the large white birds. Sex differences in plasma GH levels were observed only in the large white strain and only between the ages of 9 and 18 weeks. A highly significant positive correlation was observed between weekly percentage rate of growth and plasma GH concentration in both strains, while age and GH concentration were negatively correlated. These results suggest that growth hormone may nor be important in the developing embryo, but may be associated with growth during the rapid phase of postembryonic growth. The slow growth strain was characterized by elevated GH levels, indicating that physiological changes related to depressed growth may occur at some point subsequent to pituitary GH secretion.