Rat Growth Hormone (GH) but Not Prolactin (PRL) Induces both GH and PRL Receptors in Female Rat Liver*

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Abstract
It was elucidated which hormone is responsible for the induction of GH [growth hormone] and PRL [prolactin] receptors in rat liver. Intact female rats were implanted with osmotic minipumps delivering rat GH (rGH) or ovine GH (oGH) or PRL at various rates from 75 to 800 .mu.g/day for 7 days, and binding of radioiodinated bovine GH or ovine PRL (oPRL) tracer was measured on liver microsomal membranes. MgCl2 treatment was used to remove bound hormones from receptors before tracer binding. Infusion of rGH resulted in a significant increase (P < 0.001) in both GH and PRL binding, the effect being maximal (2.5- to 3-fold for both ligands) at rGH infusion rates from 150 to 400 .mu.g/day. Serum rGH levels were elevated 3- to 5-fold in these animals, but somatomedin-C concentrations were not higher than in controls. MgCl2 treatment showed that GH, but not PRL, binding sites in rGH-treated animals were significantly occupied by administered hormone. Analysis of competitive binding curves indicated that receptors for both GH and PRL increased in concentration without changes in binding affinity. In contrast to the rGH effect, oGH infusion from 75 to 400 .mu.g/day failed in 2 experiments to consistently alter either bovine GH or oPRL binding sites. This was not explained by the potency of the preparation at the somatogenic receptor; oGH was in fact more potent than rGH. The effects of rat PRL and oPRL infusion on receptor levels were also assessed. In contrast to previous reports, neither preparation caused induction of either PRL or GH binding sites. oPRL decreased PRL binding by 30-40% when infused between 200 and 400 .mu.g/day; rat PRL had a less consistent effect. MgCl2 stripping of membranes suggested that administered PRL preparations did not significantly occupy PRL receptors. GH receptors were unaffected in any PRL-treated group. In intact female rats, rGH regulates the concentration of both GH and PRL receptors. The slight down-regulation of PRL receptors resulting from PRL infusion casts further doubt on the concept that PRL induces its own hepatic receptors.