Circulating GH isoforms and GH bioactivity in preterm neonates

Among the molecular variants of human GH, the monomeric 22-kD is the predominant isoform, whereas the 20-kD is the second most abundant isoform. Because little is known on the pattern of human GH isoforms in the early postnatal period, we evaluated serum levels of 22-kD GH by an immunofluorometric assay and of 20-kD GH by an ELISA using an anti-20-kD antibody, and measured GH bioactivity with the Nb2 cell bioassay in 19 preterm neonates (gestational age, 32 +/- 0.5 wk; mean +/- SEM) on the fourth and 15th days of life. As control subjects, we studied 19 full-term neonates (gestational age, 39 +/- 0.3 wk) on the fourth day of life and 20 healthy adults, aged 20 +/- 0.3 y. Four-day-old preterm neonates showed significantly higher serum values of 20-kD GH (0.99 - 0.14 ng/mL) than full-term neonates (0.33 +/- 0.07 ng/mL; p < 0.001) and adults (0.09 +/- 0.02; p < 0.0001). Likewise, 22-kD GH and GH levels by Nb2 cell bioassay were also significantly higher (p < 0.001) in preterm than in full-term neonates and young adults. A significant decrease (p < 0.01) in 20-kD, 22-kD, and Nb2-determined GH was observed in preterm neonates on the 15th day of life The percentage of the 20-kD isoform was similar in the preterm infants at the fourth and 15th day, in full-term-infants, and in adults (2.7%, 2.7%, 2.8%, and 3.16%, respectively). Our results indicate that 20-kD GH serum levels change throughout life as regards total amount, but not as regards percentage.