The rat anterior pituitary expresses ß₂-adrenoceptors (ARs) on somatotrophs, lactotrophs and corticotrophs. The present study investigates whether ß₁-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 ß₁-AR mRNA and protein. Unlike the ß₂-AR, expression fell to very low levels after 5 days of 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 ß₁-AR mRNA levels. As examined by immunofluorescence confocal microscopy, ß₁-AR-immunoreactivity (-ir) was detected in a subpopulation of gonadotrophs but not in somatotrophs, lactotrophs, corticotrophs, thyrotrophs or folliculo-stellate cells. ß₁-AR-ir cells were often surrounded by cup-shaped lactotrophs. Consistent with these findings, ß₁-AR mRNA was considerably more abundant in the gonadotrophic T3–1 and LßT2 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 ß₁-AR mRNA levels in aggregates from a subpopulation enriched in large gonadotrophs obtained by gradient sedimentation. In contrast, excessive DEX-dependent up-regulation of ß₁-AR mRNA was found in a subpopulation enriched in small non-hormonal cells.

The present data indicate that ß₁-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.