β1-Adrenoceptor Expression in Rat Anterior Pituitary Gonadotrophs and in Mouse αT3-1 and LβT2 Gonadotrophic Cell Lines

Abstract

The rat anterior pituitary expresses β2-adrenoceptors (ARs) on somatotrophs, lactotrophs, and corticotrophs. The present study investigates whether β1-ARs exist in the anterior pituitary, in which cell type(s) they are found, and whether they are regulated by glucocorticoids. As determined by quantitative RT-PCR and Western immunoblotting, the rat anterior pituitary expressed β1-AR mRNA and protein. Unlike the β2-AR, expression decreased to very low levels after 5-d aggregate cell culture but was strongly up-regulated in a dose- and time-dependent manner by dexamethasone (DEX). Glucocorticoids attenuated isoproterenol-induced down-regulation of β1-AR mRNA levels. As examined by immunofluorescence confocal microscopy, β1-AR immunoreactivity was detected in a subpopulation of gonadotrophs, but not in somatotrophs, lactotrophs, corticotrophs, thyrotrophs, or folliculo-stellate cells. β1-AR-immunoreactivity cells were often surrounded by cup-shaped lactotrophs. Consistent with these findings, β1-AR mRNA was considerably more abundant in the gonadotrophic αT3-1 and LβT2 cell lines than in the GHFT, GH3, and TtT/GF cell lines. DEX did not affect expression level in the cell lines. DEX also failed to up-regulate β1-AR mRNA levels in aggregates from a subpopulation enriched in large gonadotrophs obtained by gradient sedimentation. In contrast, excessive DEX-dependent up-regulation of β1-AR mRNA was found in a subpopulation enriched in small nonhormonal cells. The present data indicate that β1-AR is expressed in a subpopulation of gonadotrophs with a topographical relationship to lactotrophs. However, the glucocorticoid-induced up-regulation does not seem to occur directly in the gonadotrophs but within (an)other unidentified cell type(s), or is transduced by that cell type on gonadotrophs.