BIOREGULATORY ROLE OF THE KALLIKREIN-KININ SYSTEM IN THE NORMAL PITUITARY-GLAND AND ITS TUMORS

Tissue kallikrein, a serine protease, is present in the prolactin-secreting cells of the normal anterior pituitary gland and pituitary adenomas. It is mainly located in the Golgi apparatus, but is also present in secretory granules. There is a distinct sexual dimorphism, with amount of tissue kallikrein being greater in anterior pituitary tissue from female rats. The intracellular levels of tissue kallikrein are increased by estradiol and in pituitary tumours, and decreased by ovariectomy, dopamine and its agonists. There is preliminary in vitro evidence that tissue kallikrein may be involved in the intracellular processing of the prolactin molecule before secretion. Tissue kallikrein synthesizes kinins which are present in the anterior pituitary and are capable of stimulating prolactin and growth hormone secretion by activating the phosphoinositide second messenger system. Prolactin physiology is uniquely linked to the kallikrein-kinin system in the normal pituitary and its tumours. Tissue kallikrein may have an important role in the pathophysiology of prolactin-secreting pituitary adenomas.