Growth hormone isoforms in newborns and postpartum women.

The neonatal and postpartum periods are characterized by alterations in pituitary GH secretion. We have investigated the proportion of circulating non-22kDa GH isoforms in newborns, in women within the early postpartum phase (just after the disappearance of placental GH from the maternal circulation) and in women during late postpartum (during the somatotroph recovery phase). We studied 10 newborns (7 males; 3 females; median postnatal age, 45h), who had been admitted because of polycythaemia, 10 women in the early postpartum phase (median, 48h after delivery; range, 42-54h), 18 women in the late postpartum phase (median, 10 weeks after delivery; range, 3-25 weeks) and 9 healthy non-pregnant women. The proportion of non-22kDa GH isoforms was determined by the 22kDa GH exclusion assay, which is based on immunomagnetic extraction of 22kDa GH from serum, and quantitation of non-22kDa GH isoforms using a polyclonal GH assay. In newborns, non-22kDa GH isoforms were measured in two arterial blood samples obtained with a 5-6h interval. In the other groups, serum samples were obtained 40min after an i.v. bolus administration of the GH secretagogue, GH releasing peptide-1 (GHRP-1). In newborns, the median proportion of non-22kDa GH isoforms was 10% (range, 7. 2-19.4%) and the values were similar in samples collected at different times. In early postpartum women, total GH levels after GHRP-1 were lower and the proportion of non-22kDa GH isoforms was higher compared with the values in non-pregnant and late-postpartum women. In late postpartum, there was a partial recovery of GH response to GHRP-1, as shown by an increment in total GH levels, which was associated with a decrease in the fraction of non-22kDa GH isoforms. In conclusion, we found that (i) the proportion of non-22kDa GH isoforms in the newborn is comparable to that in the adult (non-pregnant women), (ii) in early postpartum, the non-22kDa fraction is high within the small pool of readily releasable GH, (iii) in late postpartum, recovery of pituitary GH responsiveness is associated with a relative decrease in the release of non-22kDa GH isoforms.