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Specific binding of hGH to liver microsomes of nonpregnant women and men is low, usually 10% of less in RRA. In pregnant women, however, we found that this binding is 10 times higher. The binding reaction shared the properties common to receptor systems: time, temperature and cation dependence, saturability and reversibility. hGH specific binding without cations was 10 times lower. The cross-reactions of hPRL and human placental lactogen with hGH were 0.49 +/- 0.16% and 0.10 +/- 0.05%, respectively. I-125-hPRL and I-125-human placental lactogen binding to microsomes of two controls and two pregnant women were very low and poorly reproducible. The Scatchard analysis revealed two hGH binding sites, one with an association constant (K(A)) of 2.7 +/- 0.1 x 10(10) M-1, and the other with a K(A) of 1.5 +/- 0.1 x 10(9) M-1. In one nonpregnant woman, we found a single hGH binding site with a K(A) of 1.5 x 10(9) M-1, confirming results previously reported in the literature. A hGH RRA was set up with microsomes of pregnant women. Acromegalic sera produced curves parallel to the hGH standard and pituitary dwarf serum had no I-125-hGH displacing activity. Sera of pregnant women produced curves divergent to the hGH standard and showed a I-125-hGH displacing activity 20 to 40 times higher than could be predicted by hGH levels determined by RIA. Cord sera and sera from puerperal women had similar hGH levels as determined by either RRA or RIA (r = 0.93, P < 0.001, slope = 0.85, n = 25). Our results show the existence of specific GH receptors and serum factor(s) with high I-125-hGH displacing activity from these receptors in pregnancy. These findings must be related to several metabolic changes of pregnancy, such as glucose intolerance, hyperinsulinemia, increased lipolysis, and ketogenesis