The growth hormone receptor messenger ribonucleic acid present in ovine fetal liver is a variant form

Specific binding of ovine GH (oGH) to microsomal membranes isolated from fetal sheep liver is slight to nonexistent. The complementary DNA sequence encoding the oGH receptor (oGHR) has been reported, and Northern blot analysis has indicated that oGHR messenger RNA (mRNA) is present in fetal liver and skeletal muscle from mid- to late gestation. In human tissues, the GHR mRNA exists in multiple forms, including the deletion of exon 3 and variable 5'-untranslated regions. In rodents, the GHR mRNA exists in two forms, one encoding the membrane-bound receptor and the other encoding the soluble GH-binding protein. To further characterize the oGHR mRNA transcript present in ovine fetal liver during gestation, we designed a series of primers to be used in reverse transcriptase-polymerase chain reactions (RT-PCR), which generate products that span from the 5'-untranslated region through the coding region of the oGHR mRNA. Nucleotide sequences of the resulting complementary DNAs revealed that an oGHR mRNA is present from mid- to late gestation (days 60-135) which contains the region analogous to exon 3 of the human GHR gene. However, the 5'-untranslated region previously reported in adult tissues was not present until day 135 of gestation in fetal liver, nor was it present in day 100 fetal skeletal muscle. Northern hybridization analysis indicates that the major oGHR transcript in day 105 fetal liver is 5.8 kilobases (kb) in size, with minor transcripts observed at 4.7 kb and three transcripts greater than 6.5 kb. By day 135 of gestation, the transcript size is the same as that observed in day 100 pregnant ewe liver (5.5 kb). We conclude that the oGHR mRNA present in midgestation fetal liver differs structurally from the transcript present in late gestation fetal liver and adult liver, and this difference may explain the lack of specific GH binding to ovine fetal liver membranes. Furthermore, our results suggest that there is a developmental switch in the structure of oGHR mRNA that occurs shortly before term, potentially preparing the fetus to respond to GH postnatally.