Prolactin

This is a 198 amino acid single chain polypeptide, encoded by a gene on chromosome 6. It has a molecular weight of 23 000. Prolactin is synthesised in ribosomes and rough endoplasmic reticulum of lactotrophs of the adenohypo-physis. It is concentrated in the Golgi apparatus and stored in cytoplasmic granules. Secretion occurs in a sleep-related circadian rhythm in both males and females. Hypothalamic control of secretion is primarily inhibitory, mediated by PRL-release-inhibiting factor (dopamine). Thyrotrophin releasing hormone stimulates PRL secretion. The plasma half-life is 15-20 minutes. The actions are mediated by a dimeric tyrosine kinase-linked receptor.

Actions

Increased sensitivity of the testis to LH stimulation, sustaining testosterone levels. Normal corpus luteum function.

Initiation and maintenance of lactation in the post-partum period: it induces lobulo-alveolar growth of the breasts and stimulates lactogenesis after parturition. Osmoregulation. Immune regulation.

Effects of hyperprolactinaemia

Women

Galactorrhoea

Amenorrhoea or oligomenorrhoea Infertility

Galactorrhoea and gynaecomastia Hypogonadism with reduced sex drive Impotence

Both sexes

Pituitary mass effects

Secretion is stimulated by breast-feeding, mediated by stimulation of the d g nipple during suckling, stress, sexual intercourse, hormones (oestrogens), n si sleep, dopamine antagonists and TRH. g

Secretion is inhibited by somatostatin and by dopamine agonists. n

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