Veterinary Physiology-III

PROLACTIN (LACTOGENIC HORMONE)

• PRL , a lactogenic hormone, is a single‑chain peptide consisting of 199 amino acids and three disulfide bridges.
• PRL is structurally similar to GH and has many overlapping biological functions with GH.
• The most important influence of PRL secretion is the combination of pregnancy, estrogen and nursing of newborn. PRL secretion increases steadi­ly during pregnancy.
• PRL secretion rises at night similar to GH.
• Suckling inhibits dopamine (PIF) secretion, thus stimulates PRL release.
• TRH and VIP stimulate PRL synthesis and release,
• Dopamine, catecholamines, LH ‑ RH inhibits PRL release.
• PRL is the principle hormone responsible for lactogenesis (milk production).
• PRL together with estrogen, proges­trone, cortisol and GH cause proliferation of duct system of the mammary gland. During pregnancy prolactin, estrogen and progesterone cause development of secretory alveoli in the mammary gland thus causes initiation and maintenance of lactation in domestic animals.
• PRL level increases just prior to parturition and after parturition, milk synthesis and secretion require PRL along with cortisol and insu­lin. PRL induces enzymes necessary for lactose synthesis. It provokes synthesis of milk constituents including lactalbumin, casein, and lipids. Prolactin increases in the peripheral blood of the cow during milking.
• In pigeon it causes the crop gland to hypertrophy and stimulates the secretion of crop milk (this is used in bioassay of prolactin).
• In ewes, prolactin stimulates corpus luteum referred to as the luteotropic effect; hence it is also referred as luteotropin .
• It is necessary for maintenance of CL in rat, dog.
• An excess of PRL blocks the synthesis and release of LH‑RH which inhibits gonadotropin (FSH/LH) secretions from the pitui­tary and prevents ovulation and spermatogenesis.
• High PRL concen­tration also inhibits the synthesis of gonadal steroids both in male and female.
• In animals PRL provides productive maternal behaviour toward the newborn.
• Broodiness in birds is controlled by PRL.
• Prolactin may have a growth mediating role similar to GH and reduces lipid synthesis
• Helps in partitioning of nutrients.