Localization of endothelin A receptors in the rat pituitary TSH cells: light- and electron-microscopic immunohistochemical studies

Abstract.

Endothelins modulate hormonal secretion in the pituitary gland. Intense signaling of endothelin A receptors (ET_AR) has been detected by in situ hybridization, binding assay and receptor autoradiography. We used light- and electron-microscopic immunohistochemistry of ET_AR with polyclonal antibody against a synthetic peptide corresponding to the carboxyl terminus (403–427) of human ET_AR. Immunoreactivity was observed in 6–8% of anterior pituitary cells, which were rather large polygonal or stellate cells. These cells were often clustered. Double-staining immunofluorescence showed that the ET_AR-positive cells immunoreacted with antibody against the β-subunit of thyroid-stimulating hormone (TSH), but not adrenocorticotropic hormone (ACTH) or lutenizing hormone β (LHβ). Pre- and postembedding electron-microscopic immunohistochemistry showed that ET_AR-positive cells had vacuolated or parallel-lined rough endoplasmic reticulum (rER) and numerous round granules in their periphery and the elongated processes. By pre-embedding immunohistochemistry, diaminobenzidine tetrahydrochloride (DAB) products were shown to be mostly located around the granules and occasionally underneath the plasma membrane. By postembedding immunohistochemistry, granules in the ET_AR-positive cells were 90–150 nm in diameter, and colloidal gold particles due to ET_AR were associated with about 10% of these granules. These results indicate that ET_A receptors are associated mostly with the secretory granules of TSH cells.