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Somatomedin activity was determined by porcine cartilage bioassay and somatomedin-C RIA in matched samples of tracheal fluid (TF), amniotic fluid (AF), and cord (CP) and maternal plasma collected during the delivery of normal infants of 36-40 weeks gestation. Somatomedin levels determined by both assay systems were significantly greater in TF than in AF [TF, 0.40 +/- 0.04 (+/- SE) U/ml; AF, 0.20 +/- 0.02 U/ml by bioassay in 11 infants; TF, 0.25 +/- 0.04 U/ml; AF, 0.18 +/- 0.03 U/ml, by RIA in 6 infants; P less than 0.005]. When chromatographed on Bio-Gel P10 at pH 3.5, the major part of the somatomedin immunoreactivity from both TF and CP eluted in the same position as iodinated somatomedin-C (mol wt, 7600). Somatomedin activity recovered after chromatography of TF and CP increased the incorporation of radiothymidine by growth-restricted human fetal fibroblasts in vitro. Maternal plasma contained 2.49 U/ml radioimmunoassayable somatomedin-C, a value higher than that in the nonpregnant adult. The results suggest that the human fetal lung contributes somatomedin to TF and raises the possibility that these peptides are involved in lung growth or maturation.

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