THE SERUM HALF-LIFE OF SOMATOMEDIN ACTIVITY: EVIDENCE FOR GROWTH HORMONE DEPENDENCE

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Abstract
The serum half-life of somatomedin (SM) activity was measured following the i.v. injection of SM activity into hypophysectomized rats. A [3H]thymidine incorporation assay in chick embryo fibroblasts (CEF) was utilized to measure SM activity. Cell cycle analysis data obtained with the flow microfluorometer shows that the [3H]thymidine incorporation data reflects actual DNA synthesis. When normal rat serum was injected, a half-life for SM activity of approximately 3 h was determined. In marked contrast, when serum from hypophysectomized (hypox) rats acutely treated with growth hormone (GH) was used as the source of SM actively, the half-life of the SM actively was short, approximately 8 min. When serum from hypox rats chronically treated with GH was injected, the half-life was again long, approximately 4 h. SM activity was separated from its binding proteins by boiling under acid conditions or chromatography on Sephadex G-50 in 1 N acetic acid. The half-life of this partially purified SM activity was short, in the rnage of 10-30 min. Recombination of the partially purified SM activity with the large proteins in normal serum extended the half-life of the SM from 10-30 min to about 2 h. The serum half-life of SM activity may be GH dependent and the serum binding protein, like SM itself, is under GH control.

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