The ontogeny of somatotropic binding sites in ovine hepatic membranes

The binding of human [125I]GH, ovine [125I]GH, bovine [125I]GH, and ovine [125I]PRL, to hepatic microsomal membranes (100,000 g) prepared from fetal, neonatal, and infant lambs and adult ewes has been examined. The specific binding of hGH increases (P less than 0.01) from 3.4 +/- 0.8% in the fetus (n = 7) to 20.0 +/- 2.1% (n = 6) in lambs at least 6 days postpartum. The binding of oGH is low in the fetus (0.7 +/- 0.2%; n = 13) and neonatal lamb (1.3 +/- 0.5%; n = 5) and increases (P less than 0.01) in older lambs (14.6 +/- 4.7%; n = 6) and adult sheep (14.9 +/- 5.3%; n = 4). Similarly, the binding of bGH is less (P less than 0.05) to fetal tissues. In contrast, the binding of oPRL is similar in fetal and postnatal preparations. Cross-reaction studies suggest that the binding of GHs is to a site with lactogenic characteristics in the fetus. In contrast, in lambs at least 4 days postpartum and in adult sheep, binding is to a site with somatogenic characteristics. The inability to detect somatogenic sites in the fetal liver is not due to saturation by endogenous GH or chorionic somatomammotropin, as the binding characteristics do not change after MgCl2 pretreatment of the membrane fractions. No change in binding is observed 25 days after fetal decapitation at 69 days (n = 3), suggesting that circulating GH does not down-regulate the fetal GH receptor. These observations suggest an immaturity of the somatogenic receptor in the ovine fetal liver and its appearance in the perinatal period. Immaturity of this receptor is likely to be the basis for the lack of a major effect of fetal GH on fetal somatic growth.