Growth Hormone Secretion Rates in Cattle

Two experiments were conducted to measure plasma levels and clearance rates of growth hormone in cattle. Clearance from circulation was obtained by measuring plasma levels of the hormone after injection of exogenous bovine growth hormone. A growth hormone secretion rate was calculated from volume of distribution, clearance rate and basal plasma level of the hormone. In five 3-month-old bull calves, average values were 18.0 ng/ ml for growth hormone plasma levels, 21.8 min. for half-life and 47.0 µg/kg/day for rate of secretion. In four bulls and four heifers studied during three periods when the animals were 7 to 17 months old, average plasma growth hormone levels were 7.1 and 4.7 ng/ ml for the bulls and heifers. In bulls and heifers, average values were 24.7 and 17.8 min. for the half-life and 9.8 and 7.6 µg/kg/ day for the secretion rate. A correlation coefficient of 0.97 was observed between secretion rates and plasma levels of growth hormone.