421 ACUTE SOMATOMEDIN RESPONSE TO GROWTH HORMONE THERAPY

Abstract

We have examined the acute somatomedin (SM) response to growth hormone (GH) therapy in 21 GH deficient children using a placental membrane radioreceptor assay (RRA), which measures a variety of SMs, and a radioimmunoassay (RIA) specific for SM-C and insulin-like growth factor I (IGF-I). RRA and RIA SM determinations were performed on the same acid-chromatographed samples, obtained before initiation of therapy and 13 hr after each of 4 daily injections of GH (0.1 U/kg). The 4 days of GH therapy resulted in an increase in SM levels from 0.39±0.24(SD)U/ml to 1.18±0.62(SD) U/ml determined by RRA and 0.19±0.14(SD)U/ml to 0.82±0.51(SD)U/ml determined by RIA. A single injection of GH resulted in a significant rise in plasma SM levels measured by either RRA or RIA (p<0.001). Subjects who responded poorly to 2 injections of GH also had low SM levels after 4 days and even after 6 weeks of GH therapy. The RRA resulted in consistently higher values than the RIA, particularly when compared to a pure IGF-I/SM-C standard. SM peptide content determined by RRA and RIA were strongly correlated for individual subjects as well as for the whole group. However, the RRA/RIA ratio among individual subjects varied from 0.85 to 2.50. The marked difference in SM peptide content measured by RRA and RIA and the variability in the RRA/RIA ratio among individual subjects suggests that the IGF-I/SM-C RIA measures only one of a number of GH dependent SM peptides.