Somatomedin-A bioactivity in rabbit serum after hypophysectomy

Hall's bioassay was used to determine the somatomedin (Sm) activity pattern after hypophysectomy in young (40 days) and adult (180 days) rabbits. In the young animals, Sm activity decreased a little, but the serum remained slightly active. The serum of the older rabbits progressively lost its sulphation activity and inhibited SO4 uptake after 2 weeks following hypophysectomy. The inhibitory activity could not be destroyed by heating. Concomitantly, the same serum continued to stimulate thymidine uptake. From the present results on bioactivity, it appeared that Sm generation was only partially pituitary-dependent, that the pituitary might control inhibitor synthesis, and that the SO4 and thymidine factors were separate entities, each having its own inhibitor(s).