A 64 kDa protein is a candidate for a TRH receptor in prolactin-producing rat pituitary tumour cells

Binding of the hypothalamic tripeptide TRH to specific membrane receptors stimulates Prl secretion and synthesis. The action of TRH (Fig.1) has been extensively studied in clonal strains of pituitary tumour cells (Table 1) which have retained their ability to secrete hormone and are sufficiently differentiated to generate and express hormone receptor signal systems analogous to normal anterior pituitar cells (for review see Tashjian and Hoyt 1972, Gautvik et al.1984). TRH induces the activation of adenylate cyclase (AC) and phospholipase C (PLC) through Gs- and Gplc-proteins, respectively (Gordeladze et al.1987). [Ca2+]i peaks rapidly within seconds due to the enhanced production of inositolpolyphosphates while cAMP elevation occurs within minutes (Fig.2, K.M.Gautvik et al.1988). The TRH stimulated adenylate cyclase is rapidly and totally desensitized by TRH, while phospholipase C is only partially down regulated (Table 2). The two signal pathways are crosslinked by the inactivation of Gi by protein kinase C, thereby enhancing cAMP synthesis further (Gordeladze et al.1988).