Chapter 5 Mechanism of action of pituitary hormone releasing and inhibiting factors

Abstract

Publisher Summary This chapter focuses on intracellular messengers underlying the stimulus-secretion coupling of thyrotropin releasing hormone (TRH) and dopamine (DA) on prolactin (PRL) and thyrotropin secretion, of luteinizing hormone (LH) releasing hormone (LHRH) on LH secretion, of vasoactive intestinal peptide (VIP) on PRL secretion, of corticotrophin-releasing factor (CRF) and vasopressin on ACTH and β -endorphin secretion and of growth hormone (GH) releasing factor (GRF) and somatostatin (SRIF) on GH secretion. Releasing and inhibiting factors control pituitary-hormone secretion through binding to cell-surface receptors and require intracellular second messengers to couple the extracellular signal for secretion to the intracellular secretory machinery. In addition to the classical adenylate cyclase-cAMP system, various other intracellular messengers have been identified, such as the Ca 2+ -calmodulin system and the inositol polyphosphate-diacylglycerol–protein kinase C system. The relative importance of each of them appears to differ according to the particular hormone-secreting cell type and releasing factor. In contrast to TRH, VIP is considered a potent and efficient agonist of adenylate-cyclase activity in the pituitary. VIP stimulates PRL release in tumor as well as in normal lactotrophs.