Some Physiological Causes of Genetically Different Rates of Growth in Swine

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Abstract
The growth hormone content of anterior pituitaries from 56 pigs belonging to two genetically different lines selected for rapid and slow rates of gain was assayed using hypophysectomized female rats. These indices of growth hormone content were used: ability to retain nitrogen, width of the epiphyseal cartilaginous disk and increase in body weight of test animals. Powder from individual anterior pituitaries was injected for four days at the rate of 50 gamma per day. The nitrogen retention test was found to be least reliable and most variable as an index of growth potency, highly repeatable results being obtained by the other two methods. In relation to body weight, pituitary and thyroid weights of pigs from the two lines were not found to be significantly different. Anterior pituitaries from rapid-line pigs at all ages contained significantly higher amounts of growth hormone per unit tissue than did the same amount of tissue from the slow line. There were no significant differences between sexes. Each line contained the same amounts of growth hormone per unit tissue at all ages. In relation to anterior pituitary size and body weight the amounts of growth hormones contained per unit of living animal, increased in the rapid line from 56 to 115 days of age and then decreased until mature size was reached. In the slow line this amount reached a plateau where it remained from 75 to 154 days of age and dropped in the mature animal. These relations probably account for the growth stasis observed in all animals at maturity. The larger amounts of growth hormone consistently secreted by the rapid line at all ages of comparison accounts satisfactorily for the more rapid rate of gain observed in that line. Copyright © . .