Effect of Hypophysectomy on Growth Hormone Receptor Gene Expression in Rat Tissues*

Abstract

Two cDNA probes derived from the nucleic acid sequence for the rabbit GH receptor were used to study RNA samples from normal and hypophysectomized (hypox) rat tissues by Northern analysis. Results obtained with a probe that contained a nucleotide sequence corresponding to part of the extracellular domain of the GH receptor indicated that rat liver, gastrocnemius muscle, and epididymal fat each contain a 4.4-kilobase (kb) message and one or more shorter messages that appear to be homologous to the rabbit GH receptor message. The other probe, which contained a nucleotide sequence that corresponds to the intracellular domain of the GH receptor, detected only one 4.4-kb message in these rat tissues. These results suggest that rat tissues may synthesize several forms of the GH receptor, but only one form that contains a region homologous to the intracellular domain of the rabbit liver GH receptor. Hypophysectomy increased the abundance of the 4.4-kb message 5-fold in muscle and reduced it by a factor of 2 in adipose tissue. No significant difference was seen between GH receptor message levels of normal and hypox rat liver when the results were expressed as a fraction of the total RNA. The level of the beta-actin message was also measured in liver, muscle, and fat from normal and hypox rats. No significant differences were found when the message levels in normal rats were compared to those for the corresponding tissue in hypox rats. When normalized to the beta-actin message levels, a significant increase was seen in the relative amount of the GH receptor mRNA in muscle and liver of hypox rats. The increased levels of the GH receptor message in muscle and liver and the simultaneous decreased level in fat suggest that GH receptor synthesis may be regulated selectively in these tissues by hormonal factors that are altered by hypophysectomy.