FUNCTIONAL GROWTH-HORMONE (GH) RECEPTORS AND GH ARE EXPRESSED BY PREIMPLANTATION MOUSE EMBRYOS - A ROLE FOR GN IN EARLY EMBRYOGENESIS

The results of this study challenge the widely held view that growth h ormone (GH) acts only during the postnatal period. RNA phenotyping sho ws transcripts for the GH receptor and GH-binding protein in mouse pre implantation embryos of all stages from fertilized eggs (day 1) to bla stocysts (day 4). An antibody specific to the cytoplasmic region of th e GH receptor revealed receptor protein expression, first in two-cell embryos, the stage of activation of the embryonic genome (day 2), and in all subsequent stages, In cleavage-stage embryos this immunoreactiv ity was localized mainly to the nucleus, but clear evidence of membran e labeling was apparent in blastocysts. GH receptor immunoreactivity w as also observed in cumulus cells associated with unfertilized oocytes but not in the unfertilized oocytes. The blastocyst receptor was demo nstrated to be functional, exhibiting the classic bell-shaped dose-res ponse curves for GH stimulation of both 3-O-methyl glucose transport a nd protein synthesis. Maximal stimulation of 40-50% was seen for both responses at less than 1 ng/ml recombinant GH, suggesting a role for m aternal GK. However mRNA transcripts for GH were also detected from th e morula stage (day 3) by using reverse transcription-PCR, and GH immu noreactivity was seen in blastocysts. These observations raise the pos sibility of a paracrine/autocrine GH loop regulating embryonic develop ment in its earliest stages.