Metabolic Clearance Rate of Growth Hormone in Mice during Various Physiological States*

Abstract

In conjunction with studies relating the clearance rate of PRL with mammary tumor incidence, ws measured the MCR of GH in male, female, pregnant, and lactating mice of two strains, C3H/St (high incidence of mammary tumors) and C57BL/St (low incidence of mammary tumors). Mouse GH was administered by a single injection in the jugular vein of unanesthetized mice, and sera were collected from the orbit at frequent intervals up to 60 min. Sera were assayed for GH with a homologous RIA. The MCR of GH in male mice (C3H/St, 0.12 ± 0.02; C57BL/ St, 0.13 ± 0.01 ml/min) was not significantly different from that in females (C3H/St, 0.11 ± 0.01; C57BL/St, 0.11 ± 0.01 ml/min). Advanced pregnancy did not cause appreciable changes in the MCR of GH (C3H/St, 0.13 ± 0.01; C57BL/St, 0.08 ± 0.01 ml/min). The MCR increased nearly 2-fold, however, in lactating mice (C3H/St, 0.26 ± 0.01; C57BL/St, 0.22 ± 0.02 ml/min). The clearance rates in the two strains were similar in the male and virgin female states but averaged higher in the C3H/St strain during pregnancy and lactation. The secretion rate of GH was highest in pregnant mice (C3H/ St, 18.8 ± 3.1; C57BL/St, 8.2 ± 2.0 ng/min) and lowest in virgin females (C3H/St, 1.3 ± 0.1; C57BL/St, 1.0 ± 0.1 ng/min). Male mice of both strains had secretion rates (C3H/St, 1.9 ± 0.1; C57BL/St, 1.2 ± 0.3 ng/min) similar to those in females. Lactating mice had 3–4 times higher rates of GH secretion (C3H/St, 5.8 ± 0.5; C57BL/St, 4.0 ± 1.4 ng/min) than virgin females. The secretion rates in the two strains were virtually alike during all physiological states, except during pregnancy when the values were higher in the C3H/St strain. The results show that the serum concentrations of GH in mice do not always reflect the actual level of its secretion. Furthermore, they suggest that GH may play an important part in the maintenance of pregnancy and lactation in the mouse.