6] Measurement of somatomedin by cartilage in Vitro

Abstract

Somatomedin (SM) is a growth hormone (GH)-dependent peptide, circulating in normal serum, which directly causes skeletal growth. Its existence was proposed following studies of the action of GH on hypophysectomized (hypox) rat cartilage in vivo and in vitro. Administration of GH in vivo causes increased proliferation of cartilage, measured by incorporation of [35S] sulfate into chondroitin sulfate, but the addition of GH in vitro has no effect. Serum from normal rats or GH-treated hypox rats, added in vitro, stimulates the incorporation of [35S] sulfate, but serum from untreated hypox rats has little effect. It is hypothesized that GH produces skeletal growth indirectly by causing the generation of a “sulfation factor,” now called “SM,” which acts directly to cause proliferation of cartilage. As preparation of SM is not available in sufficient quantity or purity to permit chemical or radioimmunoassays, SM activity is determined in bioassays. Somatomedin is now known to have many metabolic effects, but only [35S] sulfate or [3H] thymidine incorporation into cartilage have been widely used for bioassays.