Kinetics of somatomedin C/insulin-like growth factor-I(SMC) response to hGH in GH deficient children

Abstract

Studies in animals indicate that the half-life (t½) of SMC is longer (2-4 hrs) than that of other polypeptide hormones. This is believed to be due to the fact that SMC circulates as part of a binding protein complex. Studies of the t½ of SMC in humans have not been possible due to the scarcity of purified SMC. We have estimated the t½ of SMC in 11 children (8M, 3F) with GH deficiency. After 2 baseline serum samples were obtained, each child received 5 doses of NPA hGH (0.1 u/kg/dose) at 12 hr intervals. Serum samples were obtained at 24 hrs and 48 hrs of treatment and at 6 hr intervals after the last dose of hGH. Total SMC was determined on acid-ethanol extracted samples using an RIA specific for SMC. The t½ for SMC disappearance was 18.7 ± 2.6 hrs (x ± SEM). T½ was not correlated with age, bone age or peak SMC. However, the time elapsed from the last hGH injection to SMC = ½ peak SMC was correlated with bone age (r = 0.67, p 0.05). In 8 children, growth velocities on hGH are available (8.2 ± 2.5 cm/yr). Growth velocity was not significantly correlated with bone age, peak SMC or t½. Conclusion: The t½ of SMC in GH deficient children is considerably longer than the t½ observed in small animals. The prolonged t½ of SMC may partially explain the success of present GH treatment regimens in promoting skeletal growth.

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