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 S0 4 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 S0 4 and thymidine factors were separate entities, each having its own inhibitor(s).Introduction.The initial work of Salmon and Daughaday (1957) on the sulphation factor (SF) demonstrated that hypophysectomy in rats caused a marked drop in the ability of the serum to stimulate cartilage sulphate uptake in vitro.Hypophysectomy in humans decreases somatomedin (Sm) activity to 30 to 40 p. 100 of its normal value (Almqvist, Ikkos and Luft, 1961 ; Hall, 1970 ; Phillips, Herington and Daughaday, 1974 ; Herington, Phillips and Daughaday, 1976 ; Takano et at., 1977).In a previous study, Vezinhet (1968) showed that hypophysectomy, carried out before the age of 100 days, did not stop ponderal growth in rabbits ; after that age, it induced arrest of ponderal gain almost immediately.At 40 days rabbit growth rate is maximum (Cantier et al., 1969 ; Beaton, 1976), while at 180 days (the post-puberal period), it has slackened.The aim of the present work is to show that the changes in rabbit SM bioactivity, induced by hypophysectomy, depend on age at the time of the operation.SM-A was originally characterized by its action on sulphate uptake in embryonic chick cartilage (Uthne, 1973).Later, both Sm-A, and A 2 were chemically identified and their amino acid composition defined (Fryklund, Uthne and Sievertsson, 1974).Moreover, not only Sm-A, but also the related peptides, Sm-C, IGF-1, IGF-2 and MSA, were shown to be active in the bioassay (Froesch et al., 1976 ; Gibson et al., 1977 ; Zapf and Froesch, 1977).Up to now, no assay method has been strictly specific for