Differential expression of growth hormone messenger ribonucleic acid by somatotropes and gonadotropes in male and cycling female rats

Past studies have reported the appearance of cells sharing phenotypic chara cteristics of gonadotropes and GH cells. During diestrus and early proestru s, a subset of somatotropes (40-608) expressed both GH antigens and gonadot ropin (LH-beta, LH beta, or FSH-beta) messenger RNAs (mRNAs) or GnRH recept ors. More recently, we reported that subsets of gonadotropes identified by LH beta or FSH beta antigens expressed GH- releasing hormone (GHRH) binding sites. The present studies were designed to learn if these putative multip otential cells also expressed GH mRNA. Biotinylated sense and antisense oli gonucleotide probes were developed and cytochemical in situ hybridization t ests were optimized for the detection of GH mRNA with GH, LH beta, and FSH beta antigens. RNase protection assays were developed with a complementary RNA probe that detected a 380-bp region at the 5' end of the GH mRNA. Both the in situ hybridization and RNase protection assays detected changes in e xpression of GH mRNA during the estrous cycle with the lowest expression oc curring during metestrus and peak expression occurring on the morning of pr oestrus. Cell counts confirmed the results of the RNase protection assays s howing that increases in mRNA levels seen from metestrus to proestrus refle cted increased percentages of GH mRNA-bearing cells. In addition, densitome tric analyses demonstrated that the higher GH mRNA levels assayed from dies trus to proestrus reflected increased area and density of label per cell. B oth types of assays showed sex differences in expression of GR mRNA; male r at cell populations had higher values than female rats in metestrus, diestr us, or estrus. However, percentages of GH cells in male rats were equal to those from proestrous female rats and levels of GB mRNA were lower in male rats than proestrous females. Dual labeling experiments showed that, in mal e rats and diestrous, proestrous, or estrous females, GH mRNA was expressed in over 70% of GH cells. Expression of GR mRNA was also found in 50-57% of cells with LR beta or FSH beta antigens in the same groups. The lowest exp ression was seen in the metestrous groups (30-40% of GH cells or gonadotrop es expressed GH mRNA). Expression of GN mRNA was first increased from metes trus to diestrous largely in GH cells, and slightly in cells with LH beta a ntigens. Further increases were seen in GII and LN cells by the morning of proestrus. In contrast, FSH gonadotropes did not show an increased expressi on of GH mRNA until the morning of proestrus (reaching the same peak reache d by LPI cells). These data confirm the working hypothesis that a multihorm onal cell type develops during diestrus to support both the somatotrope and gonadotrope populations. Collectively, our studies suggest that this multi hormonal cell may function to help support the regulatory functions of the gonadotrope during the periovulatory period. Tn addition, the appearance of significant levels of expression of GH mRNA by male rat gonadotropes sugge sts that this multihormonal cell may play a role in regulation of the male reproductive system as well.