Studies on the Disappearance of Equine Chorionic Gonadotropin from the Circulation in the Rat: Tissue Uptake and Degradation*

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Abstract
Equine CG (eCG) both radioiodinated and unlabeled, was injected iv into male rats, and the disappearance of the hormone from the circulation was studied. The uptake and inactivation of the hormone by various tissues was also examined. The disappearance curve of eCG consisted of two exponential components with apparent t} of 0.2 and 6 h. Gel filtration analysis of a blood sample 60 min after injection of eCG indicated that residual plasma eCG is intact and is active in both RIA and bioassay. Along with the disappearance of radiolabeled eCG from plasma, its uptake and release was recorded in liver, kidneys, and testis. The label also appeared rapidly in the urine and the thyroid gland. However, gel filtration profiles of urine samples did not show the presence of any intact eCG. Furthermore, overnightin vitro incubation of eCG with plasma, liver, and kidney tissues destroyed 58%, 64%, and 96% of the biological activity of the hormone, respectively. Thus, these results suggest that eCG disappears from the circulation in a biphasic manner, and the plasma, liver, and kidneys may play an important role in the inactivation and metabolism of the hormone.