Endocrine peptides
Endocrine peptides are the proteinaceous subset of the hormones produced by the glands of the endocrine system. The major endocrine glands include the pineal gland, pituitary gland, pancreas, ovaries, testes, thyroid gland, parathyroid gland, hypothalamus, gastrointestinal tract and adrenal glands. Endocrine hormones regulate a vast array of bodily functions including metabolism, growth and development, tissue function, sexual function, reproduction, sleep, and mood.
The table below provides details of the endocrine glands, the hormones they secrete and some of the principal effects of the hormones. Non-peptide hormones are shown in italics for clarity.
| Endocrine organ | Secreted hormone(s) | Effect(s) |
|---|---|---|
| Hypothalamus | thyroid-releasing hormone (TRH) | stimulates release of TSH from anterior pituitary |
| dopamine | inhibits prolactin release from anterior pituitary | |
| growth hormone-releasing hormone (GHRH) | stimulates GH release from anterior pituitary | |
| somatostatin | inhibits release of GH and TSH from anterior pituitary | |
| gonadotropin-releasing hormone (GnRH) | stimulates release of FSH and LH from anterior pituitary | |
| corticotrophin-releasing hormone (CRH) | stimulates ACTH release from anterior pituitary | |
| vasopressin | increases water permeability in distal convoluted tubule: increases blood volume | |
| Pineal gland | melatonin | regulates circadian rhythm |
| Pituitary gland- anterior | growth hormone (GH)-somatotrophs | stimulates growth and cell proliferation; stimulates hepatic insulin-like growth factor 1 release |
| thyroid-stimulating hormone (TSH)- thyrotrophs | stimulates thyroxine (T4) and triiodothyronine (T3) synthesis and release from thyroid gland; stimulates iodine uptake by thyroid gland | |
| adrenocorticotropic hormone (ACTH)- corticotrophs | stimulates corticosteroid (glucocorticoid and mineralocorticoid) and androgen synthesis and release from adrenocortical cells | |
| beta endorphin- corticotrophs | inhibits pain sensation | |
| follicle-stimulating hormone (FSH)- gonadotrophs | stimulates ovarian follicle maturation in females; stimulates maturation of seminiferous tubules, spermatogenesis and production of androgen-binding protein in males | |
| luteinizing hormone (LH)- gonadotrophs | stimulates ovulation and formation of corpus luteum in females; stimulates testosterone synthesis from Leydig cells (interstitial cells) in males | |
| prolactin (PRL)- lactotrophs | stimulates milk synthesis and release from mammary glands; mediates sexual gratification | |
| melanocyte-stimulating hormone (MSH)- melanotrophs | stimulates melanin synthesis and release from melanocytes in hair and skin | |
| Pituitary gland- posterior | oxytocin- magnocellular neurosecretory cells | stimulates uterine contraction during labour; stimulates the letdown reflex in nursing mothers |
| vasopressin (ADH or AVP)- parvocellular neurosecretory neurons | increases water permeability in distal convoluted tubule: increases blood volume | |
| Thyroid gland | T3-thyroid epithelial cells | more potent form of thyroid hormone- increases basal metabolic rate; stimulates protein synthesis |
| T4- thyroid epithelial cells | pro-hormone for T3- same effects | |
| calcitonin- parafollicular cells | reduces blood calcium; stimulates bone formation | |
| GI tract-stomach | gastrin- G cells | stimulates secretion of gastric acid by parietal cells |
| ghrelin- P/D1 cells | increases appetite | |
| neuropeptide Y (NPY) | increases food intake | |
| somatostatin- D cells | suppresses release of gastrin, cholecystokinin (CCK), secretin, motilin, vasoactive intestinal peptide (VIP), gastric inhibitory polypeptide (GIP), enteroglucagon; slows gastric emptying; reduces smooth muscle contraction and blood flow in intestine | |
| histamine- ECL cells | stimulates gastric acid secretion | |
| endothelin- X cells | regulates smooth muscle contraction in stomach | |
| GI tract-duodenum | secretin- S cells | regulates secretion of bicarbonate from liver, pancreas and Brunner's gland (duodenum); enhances effects of CCK, stops gastric juice production |
| cholecystokinin (CCK)- I cells | promotes release of digestive enzymes from pancreas and release of bile from the gall bladder; hunger suppressant | |
| GI tract-liver | insulin-like growth factor 1 (somatomedin, IGF)- hepatocytes | insulin-like effects; regulates growth and development |
| angiotensinogen- hepatocytes | vasoconstriction; stimulates release of aldosterone from adrenal cortex | |
| angiotensin- hepatocytes | vasoconstriction; stimulates release of aldosterone from adrenal cortex | |
| thrombopoietin (THPO)- hepatocytes | stimulates megakaryocytes to produce platelets in the bone marrow | |
| hepcidin- hepatocytes | inhibits intestinal iron absorption and iron release by macrophages | |
| GI tract-pancreas | insulin- β islet cells | stimulates glucose absorption from blood to skeletal muscles and fat tissue; promotes fat storage; inhibits hepatic glucose production |
| glucagon- α islet cells | increases blood glucose level; stimulates glycogenolysis and gluconeogenesis in liver | |
| somatostatin- δ islet cells | inhibits insulin and glucagon release; inhibits exocrine secretion from pancreas | |
| pancreatic polypeptide- PP cells | modulates hepatic glycogen levels and gastrointestinal secretions; auto-regulates pancreatic secretion | |
| Kidney | renin- juxtaglomerular cells | stimulates the renin-angiotensin system by producing angiotensin I from angiotensinogen |
| erythropoietin- extraglomerular mesangial cells | stimulates erythrocyte production | |
| calcitrol (active form of vitamin D3) | stimulates absorption of calcium and phosphate from GI tract and kidneys; inhibits release of parathyroid hormone | |
| thrombopoietin | stimulates megakaryocytes to produce platelets in the bone marrow | |
| Adrenal glands- cortex
|
glucocorticoids- zona fasciculata and zona reticularis cells | stimulate gluconeogenesis; stimulates fat breakdown in adipose tissue; inhibit protein synthesis and glucose uptake in muscle and adipose tissue; immunosuppressive; anti-inflammatory |
| mineralocorticoids- zona glomerulosa cells | stimulate active sodium reabsorption and passive water reabsorption in kidneys (increasing blood volume and pressure); stimulate renal potassium and H+ excretion | |
| androgens: DHEA and testosterone- Zona fasciculata and Zona reticularis cells | masculising effects in females; in males, effects are insignificant compared to those elicited by testicular androgens | |
| Adrenal glands- medulla | adrenaline- chromaffin cells | promotes mechanisms underlying the fight-or-flight response e.g. boosting oxygen and glucose supplies to the brain and muscles, increasing heart rate and stroke volume, increasing hepatic glycogen catalysis, and suppressing non-emergency bodily-processes |
| noradrenaline- chromaffin cells | promotes mechanisms underlying the fight-or-flight response e.g. boosting oxygen and glucose supplies to the brain and muscles, increasing heart rate and stroke volume, increasing hepatic glycogen catalysis | |
| dopamine- chromaffin cells | increases heart rate and blood pressure | |
| enkephalin- chromaffin cells | regulates pain | |
| Reproductive organs- testes | androgens- Leydig cells | anabolic effects; virulizing |
| estradiol- sertoli cells | prevents apoptosis of germ cells | |
| inhibin- sertoli cells | inhibits FSH production | |
| Reproductive organs- ovarian follicle and corpus luteum | progesterone- granulosa cells, theca cells | supports pregnancy |
| androstendione- theca cells | metabolic precursor of both testosterone and estrone | |
| estrogens (mainly estradiol)- granulosa cells | promotes development of secondary female sex characteristics; stimulates endometrial growth | |
| inhibin- granulosa cells | inhibits FSH production | |
| Reproductive organs- pregnant placenta | progesterone | supports pregnancy; inhibits onset of labour and lactation; supports fetal mineralo- and glucocorticosteroid production |
| estrogens | effects on mother similar to ovarian follicle-derived estrogen | |
| human chorionic gonadotropin (hCG)- syncytiotrophoblast | promotes maintenance of the corpus luteum at the beginning of pregnancy; suppresses the immune response to the developing embryo | |
| human placental lactogen-syncytiotrophoblast | increases insulin and IGF-1 production; increases insulin resistance and carbohydrate intolerance | |
| inhibin- fetal trophoblasts | inhibits FSH production | |
| Reproductive organs- pregnant uterus | prolactin (PRL)- decidual cells | stimulates milk production in mammary glands |
| relaxin- decidual cells | relaxes the ligaments in the pelvis and softens and widens the cervix | |
| Parathyroid gland | parathyroid hormone (PTH)- parathyroid chief cells | increases the concentration of ionic calcium (Ca2+) in the blood; decreases serum phosphate; promotes renal activation of vitamin D |
| Skin | calcidiol (25-hydroxyvitamin D, inactive form of vitamin D3) | |
| Heart | atrial-natriuretic peptide (ANP)- cardiac myocytes | reduces blood pressure |
| brain natriuretic peptide (BNP)- cardiac myocytes | reduces blood pressure | |
| Skeletal muscle | myokines | |
| Adipose tissue | leptin- adipocytes | appetite suppressant; increases metabolic rate |
| estrogens (mainly estrone)- adipocytes |