ESTIMATES OF THE KINETIC PARAMETERS OF GROWTH HORMONE METABOLISM IN FED AND FASTED CALVES AND SHEEP 1
Iowa State University, A m e s 50011
different physiological states may be the result of changes in either or both of these factors. It also follows that a lack of change in the concentration of a hormone in plasma may not be indicative of no change in secretion if there is a corresponding change in rate of removal from circulation. It has been reported that fasting has no effect on plasma growth hormone concentrations in ruminant species (Machlin et al., 1968b; McAtee and Trenkle, 1971; Wallace and Bassett, 1970; Trenkle, 1971a), but increased growth hormone secretion in several nonruminant species has been observed with fasting (Roth et al., 1963; Kipnis et al., 1968; Machlin et al., 1968a; Garcia and Geschwind, 1968). Before concluding that plasma concentrations of growth hormone are indicative of secretion in ruminants, it seemed necessary to measure the kinetic parameters of growth hormone metabolism. The purpose of the present experiments was to measure the rate of disappearance of growth hormone from plasma of young calves and sheep after feeding and a period of fasting. Secretion rates were .calculated from the turnover of the g r o w t h hormone pool, v o l u m e o f distribution of the injected hormone and basal endogenous growth hormone concentrations.
SUMMARY
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 o f plasma glucose and insulin were significantly (P
Iowa State University, A m e s 50011
different physiological states may be the result of changes in either or both of these factors. It also follows that a lack of change in the concentration of a hormone in plasma may not be indicative of no change in secretion if there is a corresponding change in rate of removal from circulation. It has been reported that fasting has no effect on plasma growth hormone concentrations in ruminant species (Machlin et al., 1968b; McAtee and Trenkle, 1971; Wallace and Bassett, 1970; Trenkle, 1971a), but increased growth hormone secretion in several nonruminant species has been observed with fasting (Roth et al., 1963; Kipnis et al., 1968; Machlin et al., 1968a; Garcia and Geschwind, 1968). Before concluding that plasma concentrations of growth hormone are indicative of secretion in ruminants, it seemed necessary to measure the kinetic parameters of growth hormone metabolism. The purpose of the present experiments was to measure the rate of disappearance of growth hormone from plasma of young calves and sheep after feeding and a period of fasting. Secretion rates were .calculated from the turnover of the g r o w t h hormone pool, v o l u m e o f distribution of the injected hormone and basal endogenous growth hormone concentrations.
SUMMARY
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 o f plasma glucose and insulin were significantly (P
