Characteristics of growth hormone binding to liver microsomes of pregnant women

Effect of oestrogen status on serum levels of growth hormone-binding protein and insulin-like growth factor-I in non-pregnant and pregnant women

OBJECTIVE

Since there appears to be a relationship between circulating oestrogens and growth hormone, we have investigated the effect of the oestrogen status of adult women on serum levels of GHBP and IGF-I.

DESIGN AND PATIENTS

The investigation was performed on serum samples of 14 spontaneously menstruating women, 10 women taking oral contraceptives containing 20-50 micrograms ethinyloestradiol, and 30 pregnant women at different stages of pregnancy.

MEASUREMENTS

Serum levels of GHBP were measured by HPLC gel filtration and IGF-I levels were measured by RIA after acid-ethanol extraction.

RESULTS

In the spontaneously menstruating women the mean +/- SD serum level of GHBP was 34.6 +/- 6.7% and of IGF-I 30 +/- 7 nmol/l. Serum GHBP levels were negatively (r = -0.67; P < 0.01) and IGF-I levels were positively related (r = 0.69; P < 0.01) to serum oestradiol concentrations. In the women taking oral contraceptives serum levels of GHBP were 47.0 +/- 7.4%. This was significantly (P < 0.001) higher than in spontaneously menstruating women. In contrast, IGF-I levels were not different from those obtained in spontaneously menstruating women. In the pregnant women, the mean +/- SD serum level of GHBP was not different from that observed in non-pregnant spontaneously menstruating women. Polynomial regression analysis, however, showed a significant (P = 0.01) second-order relationship between the duration of pregnancy and serum GHBP levels, with increasing levels during the first half of pregnancy and decreasing levels thereafter. Serum concentrations of IGF-I increased during the second half of pregnancy and were significantly (P < 0.005) elevated in the third trimester.

CONCLUSIONS

In non-pregnant women the endogenous oestrogen status seems to modulate negatively GHBP levels and positively IGF-I levels, whereas oral oestrogen administration, in contrast, increases serum levels of GHBP without modification of IGF-I levels. During pregnancy serum GHBP levels increase slightly during the first half of pregnancy and decrease thereafter, whereas IGF-I concentrations increase during the second part of pregnancy. The oestrogen status of women has a complex effect on serum concentrations of GHBP and IGF-I and has therefore to be taken into account when evaluating serum levels of GHBP and IGF-I.

Influence of Mg2+ on detection of somatogenic and lactogenic components of growth-hormone-binding protein in mammalian sera

We recently classified the growth-hormone (GH)-binding protein (GH-BP) in a wide range of mammalian [including human (h)] sera and reported the existence of a major lactogenic component in GH-BP of type-III sera (rabbit, horse, dog, pig and cat), based on the capacity of bovine (b) and ovine prolactin (PRL) to displace 125I-labelled human growth hormone (hGH) binding and on direct 125I-bPRL binding studies. In this study, we demonstrate the high degree of Mg2+ dependence of the binding of the classically lactogenic hGH and bPRL, but not that of the somatogenic bGH to various mammalian sera (types I-IV). Serum GH-BP was assayed using a previously described and validated charcoal-separation assay. 125I-hGH binding to rat, ovine, bovine, rabbit, horse, dog and human sera was enhanced 1.5-2.5-fold in the presence of 70 mM Mg2+. The Mg2+ effect was concentration-dependent between 3.7 mM and 70 mM, causing a significant and proportional increase in 125I-hGH binding to serum. Like 125I-hGH, 125I-bPRL binding to type-III sera was also Mg(2+)-dependent. In contrast, 125I-bGH binding to all types of serum GH-BP was not affected by Mg2+ concentrations of up to 35 mM, while 70 mM Mg2+ slightly, but significantly, reduced (by approx. 15%) bGH binding to rabbit serum. In keeping with the Mg(2+)-dependent stimulation of lactogenic hormone binding to GH-BP, 70 mM Mg2+ caused a shift to the left in the displacement curves of hGH and bPRL competing with 125I-hGH binding to rabbit, dog, horse and human sera, while the effects of the somatogens bGH and rabbit GH were shifted to the right. Scatchard analysis of hGH displacement curves with sera from various species yielded linear plots and revealed that Mg2+ significantly increased (2.3-3.0-fold) the affinity constants, but not the binding capacities. These results demonstrate the ability of changes in Mg2+ concentration to determine the degree of differential recognition of somatogens versus lactogens by serum GH-BP. It remains to be determined whether such bivalent cation effects may account, at least in part, for the growth retardation seen in Zn2+ or Mg2+ ion deficiencies.