The metabolic clearance and secretion rates of growth hormone have been determined in anesthetized adult rats using a constant infusion of 131I-rat growth hormone and single injections of both 131I-growth hormone and unlabeled pituitary growth hormone. Similar values were obtained for the metabolic clearance rate using the 3 techniques, with a mean of 1.19 ml/min. The mean basal secretion rate determined by the constant infusion technique was 13.1 ng/min. In rats subjected to ventromedial hypothalamic stimulation, plasma growth hormone levels rose 5-fold within 10 min, but no depletion of pituitary growth hormone was noted as compared to rats stimulated in the cerebral cortex where plasma growth hormone did not rise. Calculations using the changes in plasma growth hormone and the metabolic clearance rate indicated that the quantity of growth hormone secreted in response to hypothalamic stimulation was less than 1 % of the pituitary growth hormone content, thereby explaining the lack of observed pituitary depletion. (Endocrinology86: 305, 1970)
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Frohman et al. (1970) studied this question.