The Effect of Age and Plane of Nutrition on Growth Hormone and Thyrotropic Hormone Content of Pituitary Glands of Holstein Heifers

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Abstract
Bioassays were carried out on anterior pituitaries of Holstein heifers raised on 3 levels of nutrition and slaughtered at 6 different ages to determine their growth hormone and thyrotropic hormone contents. Growth hormone per gram of anterior pituitary, growth hormone per 100 lb. of body weight and total thyrotropic hormone were greater in the younger heifers and gradually decreased with increasing age. No significant differences in any of these measurements could be detected due to plane of nutrition, although differences in total thyrotropin came very close to significance (P>0.05), the levels being highest in the heifers on the high plane of nutrition and lowest in heifers on the low plane. Rates of body growth during the 16-week periods immediately prior to slaughter were found to be positively correlated with growth hormone per gram of anterior pituitary, growth hormone per 100 lb. body weight, and total thyrotropin content of pituitaries. Thyrotropin contents were also positively correlated with relative thyroid weights of the heifers from which the pituitaries were taken. These results have been interpreted as indicating that the quantities of growth hormone and thyrotropin detected in pituitary assays are fairly close approximations of the actual rates of secretion of these hormones by the pituitary. They suggest that differences in rates of growth of heifers at different ages are at least partly due to differences in amounts of pituitary growth hormone and thyrotropin being produced, but that differences in growth rates of heifers on different feeding levels are due to differences in amounts of nutrients available for growth, rather than to any differences in hormone production by the pituitary. Copyright © . .