DEVELOPMENTAL EXPRESSION OF THE GROWTH-HORMONE RECEPTOR GENE IN RABBIT-TISSUES

Total RNA from several adult (6-18-month-old) rabbit tissues was characterized using an oligonucleotide probe derived from the extracellular domain of the nucleotide sequence of the rabbit growth hormone receptor (GH-R) cDNA. Multiple GH-R mRNA species of approximately 4.6, approximately 3.3, 2.1 and approximately 1.4 kb were detected. The major 4.6 kb transcript was detectable in all tissues examined but with quite marked abundance differences. The highest level of expression was observed in liver followed closely by muscle. A qualitative assessment of insulin-like growth factor I (IGF-I) mRNA abundance was made in these same tissues. The data showed that the tissue abundance of GH-R mRNA was not necessarily parallel to that of IGF-I mRNA. The ontogeny of GH-R mRNA was studied in rabbit liver, muscle, heart and kidney. Low levels of GH-R mRNA were detectable in all fetal tissues studied except kidney which showed relatively high levels, suggesting that GH may play an important role in early kidney development. The overall developmental pattern of GH-R mRNA was similar in heart, muscle and liver, being low in fetal and early neonatal (day 3) periods and reaching maximal levels between 2 and 6 months. However, in kidney the pattern contrasted markedly. Relatively high levels of GH-R mRNA were observed in fetal and early neonatal (day 3) kidney with little change throughout development. The developmental pattern of IGF-I gene expression was not necessarily co-ordinately regulated with the ontogenic pattern of GH-R gene expression. This suggests that at different stages of development GH may act either directly and/or indirectly via IGF-I and that IGF-I in the absence of GH regulation may be controlled by additional mechanisms such as nutrition and/or hormonal factors.