Prolactin
Prolactin Structure and Synthesis
Prolactin. This peptide hormone, synthesized in the pituitary gland, consists of 199 amino acid residues (M – 23 kDa). It has one peptide chain and three disulfide bonds, and it is known as luteotropic hormone. As the monomeric form of prolactin, “little prolactin” circulates in the blood alongside “big prolactin” (M – 48-56 kDa) and its polymer (M > 200 kDa). The number of lactotrophs increases during pregnancy due to estrogens. About 45% of prolactin’s amino acid sequence matches that of growth hormone. Both hormones share common antigenic determinants, similar C receptor structures, and similar signal transduction pathways in cells.
Regulation of Prolactin Secretion
Prolactoliberin and dopamine control prolactin secretion, with dopamine reducing cAMP concentration by interacting with receptors in pituitary gland cells. This interaction decreases prolactin secretion and suppresses prolactin gene transcription. Progesterone, estrogens, endorphins, serotonin, oxytocin, acetylcholine, stress, sexual intercourse, and nipple stimulation during breastfeeding stimulate prolactin secretion. Like many hormones, prolactin is released into the blood episodically—every 30-90 minutes, with the secretion peak occurring 6-8 hours after the start of sleep. Its concentration in women’s blood plasma is 8-20 ng/ml and 5-8 ng/ml in men. The half-life of secretion is 15-20 minutes. Increased prolactin concentration occurs at the end of pregnancy and during lactation.
Functions and Effects of Prolactin
The hormone’s main functions include maturing the mammary glands and preparing them for lactation. Prolactin promotes the synthesis of milk proteins (casein, lactalbumin), activates the synthesis of phospholipids and triglycerides, retains water and electrolytes, helps establish and maintain lactation, and promotes uterine smooth muscle contractions during childbirth. In men, prolactin increases the sensitivity of Leydig cells to luteinizing hormone, thus maintaining necessary testosterone synthesis and sperm production and promoting erection. Dopamine controls prolactin secretion by binding to lactotrophs in the pituitary gland. The D2 receptor reduces prolactin secretion, while thyrotropin hormone and vasoactive intestinal peptide decrease it. Estrogens activate pituitary gland lactotroph cells. Increased prolactin concentration blocks the pulsatile secretion of gonadotropin hormones and competes for gonadotropin receptors, disrupting follicle maturation and sex hormone synthesis. During breastfeeding, increased prolactin concentration blocks the synthesis of gonadotropin hormones in the pituitary gland, preventing ovulation and ceasing menstruation. Prolactin receptors are also present in sexual glands, liver, kidneys, and other tissues.
Source | Glossary of Most Commonly Used Biomedical Terms and Concepts | Lithuanian University of Health Sciences | Academician Professor Antanas Praškevičius, Professor Laima Ivanovienė