Circulating growth factor studies in growth plate versus resting cartilage in vitro. II. Recognition of growth factors in rat serum.

The growth-promoting effects of GH can be explained in part by the mediation of somatomedins/insulin-like growth factors (IGFs). However, large quantities of the IGFs are required to stimulate growth in vivo, and in some conditions, IGF levels may correlate poorly with GH levels and growth status. These observations suggest that other circulating factors may also be important for growth. We have examined the growth-promoting activity in rat serum, as assessed by stimulation of sulfate and/or thymidine uptake by resting and growth plate cartilage (osteochondral junction) from hypophysectomized rats in vitro. Although stimulation by a low molecular weight somatomedin fraction (approximately 5,000-12,000) accounted for about 90% of serum stimulation of sulfate uptake by resting cartilage, it explained only about 60% of stimulation of the growth plate. Growth plate and resting cartilage appeared equally insensitive to insulin, but the growth plate exhibited reduced sensitivity to inhibitor(s) in diabetic rat serum. Fractionation of normal rat serum by gel filtration at neutral pH revealed comparable stimulation of growth plate and resting cartilage by high molecular weight factors, presumably somatomedins bound to carrier proteins. After gel filtration at acid pH, both growth plate and resting cartilage responded to somatomedins with molecular weights from 5,000-12,000. However, the growth plate also responded to a 12,000-22,000 mol wt factor [Sephadex G-75; 5 X 120 cm; sulfate uptake, 68 +/- 16% above buffer (mean +/- SEM); P less than 0.01] which did not affect resting cartilage (sulfate uptake, 27 +/- 21% above buffer; P = NS). Levels of both the low and higher molecular weight factors were reduced in the serum of hypophysectomized rats. We conclude that circulating growth-promoting activity includes both the low molecular weight somatomedins and a higher molecular weight growth plate growth factor which is not recognized by resting cartilage. Use of the osteochondral junction assay system may permit elucidation of the regulation and nature of this growth factor.