Growth hormone isoforms in newborns and postpartum women

The neonatal and postpartum periods are characterized by alterations in pit uitary GH secretion. We have investigated the proportion of circulating non -22 kDa GH isoforms in newborns, in women within the early postpartum phase (just after the disappearance of placental GH from the maternal circulatio n) and in women during late postpartum (during the somatotroph recovery pha se). We studied 10 newborns (7 males: 3 females: median postnatal age, 45h) , who had been admitted because of polycythaemia, 10 women in the early pos tpartum phase (median, 48h after delivery; range, 42-54 h) 18 women in the late postpartum phase (median, 10 weeks after delivery; range, 325 weeks) a nd;3 healthy non-pregnant women. The proportion of non-22 kDa GH isoforms w as determined by the 22kDa GH exclusion assay, which is based on immunomagn etic extraction of 22 kDa GH from serum, and quantitation of non-221 kDa GH isoforms using a polyclonal GH assay. In newborns, non-22 kDa GH isoforms were measured in two arterial blood sam ples obtained with a 5-6 h interval. In the other groups, serum samples wer e obtained 40 min after an i.v. bolus administration of the GH secretagogue , GH releasing peptide-1 (GHRP-1). In newborns, the median proportion of no n-22 kDa 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-22 kDa GH isofo rms 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 w ith a decrease in the fraction of non-22 kDa GH isoforms, In conclusion, we found that (i) the proportion of non-22 kDa GH isoforms i n the newborn is comparable to that in the adult (non-pregnant women), (ii) in early postpartum, the non-22 kDa 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-22 kDa GH isoforms.