Serum somatomedin stimulation in thyroxine-treated hypophysectomized rats

Somatomedin activity in rat serum was measured by sulfate incorporation into embryonic chick cartilage. Hypophysectomized male rats were treated daily for 21 days with 20 microgram/kg L-thyroxine (T4) and/or 25 microgram bovine (b) GH. Serum from rats given T4 alone or with bGH had somatomedin potencies of 0.88 and 0.83, respectively, comparable to normal serum (1.00) and significantly greater than serum from bGH-treated hypophysectomized rats (0.62). Hypophysectomized rats had no measurable somatomedin activity. T4 treatment resulted in a return to normal of the basal metabolic rate and serum T4 and T3 concentrations. Addition of T4 or T3 to serum from normal rats to produce concentrations comparable to those found in serum of hypophysectomized rats after T4 and/or bGH treatment did not alter the somatomedin activity. Higher concentrations of T4 (10(-7) M) or T3 (10(-8) or 10(-7) M) did, however, enhance the sulfation potency of normal serum. T4 (10(-9) M to 10(-6) M) or T3 (10(-10) M to 10(-7) M) added to serum-free medium provided minimal sulfation activity that represented only a fraction of the activity of serum. T4 may stimulate production of somatomedin in the absence of adequate bGH or may be metabolized to an active sulfation factor.