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Estimates of the Kinetic Parameters of Growth Hormone Metabolism in Fed and Fasted Calves and Sheep¹

Iowa State University, Ames 50011

Abstract

The metabolic clearance rate of growth hormone was determined in castrated male sheep by using a constant infusion and a single injection of pituitary growth hormone. The mean values obtained were 8.0 ± .9 and 11.0 ± 1.2 liters/hr with the constant-infusion and single-injection techniques, respectively. The rate of disappearance of injected growth hormone from plasma was measured in eight sheep at 2.5 hr after feeding and in seven sheep 48 hr after feeding. Similar experiments were conducted with six bull calves at 4 hr after feeding and with three bull calves at 23 hr after feeding. Concentrations of plasma glucose and insulin were significantly (P<.01) lower in the fasted animals. Plasma growth hormone concentrations were not significantly elevated during fasting. Fasting increased (P<.01) the half-life of the injected growth hormone from 9.2 to 13.4 min in sheep and from 20.3 to 31.9 min in calves. The volume of distribution of the immunologically reactive growth hormone was about 6 and 8% of the body weight in calves and wethers, respectively, and was not influenced by fasting. Calculated secretion rates of growth hormone were .8 µg/kg/hr in both the fed and fasted sheep and 2.6 and 1.8 µg/kg/hr in fed and fasted calves, respectively.

¹ Journal Paper No. J-7986 of the Iowa Agriculture and Home Economics Experiment Station, Ames. Project No. 2027. This study was supported in part by the Iowa State University Research Foundation.

² Department of Animal Science.

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