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

Endothelins modulate hormonal secretion in the pituitary gland. Intense sig naling of endothelin A receptors (ETAR) has been detected by in situ hybrid ization, binding assay and receptor autoradiography. We used light- and ele ctron-microscopic immunohistochemistry of ETAR with polyclonal antibody aga inst a synthetic peptide corresponding to the carboxyl terminus (403-427) o f human ETAR. Immunoreactivity was observed in 6-8% of anterior pituitary c ells, which were rather large polygonal or stellate cells. These cells were often clustered. Double-staining immunofluorescence showed that. the ETAR- positive cells immunoreacted with antibody against the beta-subunit of thyr oid-stimulating hormone (TSH), but not adrenocorticotropic hormone (ACTH) o r lutenizing hormone beta (LH beta). Pre- and postembedding electron-micros copic immunohistochemistry showed that ETAR-positive cells had vacuolated o r parallel-lined rough endoplasmic reticulum (rER) and numerous round granu les in their periphery and the elongated processes. By pre-embedding immuno histochemistry, diaminobenzidine tetrahydrochloride (DAB) products were sho wn 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 ETAR were associated with about 10% of these granules. These results indicate that ETA receptors are associated mostly with the secretory granu les of TSH cells.