Hormone

Hierarchical nature of hormonal control

Hormonal regulation of some physiological activities involves a hierarchy of cell types acting on each other either to stimulate or to modulate the release and action of a particular hormone. The secretion of hormones from successive levels of endocrine cells is stimulated by chemical signals originating from cells higher up the hierarchical system. The master coordinator of hormonal activity in mammals is the hypothalamus, which acts on input that it receives from the central nervous system.^[3]

Other hormone secretion occurs in response to local conditions, such as the rate of secretion of calcium levels in extracellular fluid.

Hormone signalling

Hormonal signalling across this hierarchy involves the following:

1. Biosynthesis of a particular hormone in a particular tissue
2. Storage and secretion of the hormone
3. Transport of the hormone to the target cell(s)
4. Recognition of the hormone by an receptor protein.
5. Relay and amplification of the received hormonal signal via a signal transduction process: This then leads to a cellular response. The reaction of the target cells may then be recognized by the original hormone-producing cells, leading to a down-regulation in hormone production. This is an example of a homeostatic negative feedback loop.
6. Degradation of the hormone.

As can be inferred from the hierarchical diagram, hormone biosynthetic cells are typically of a specialized cell type, residing within a particular insulin, which triggers a diverse range of systemic physiological effects. Different tissue types may also respond differently to the same hormonal signal. Because of this, hormonal signaling is elaborate and hard to dissect.

Interactions with receptors

Most hormones initiate a cellular response by initially combining with either a specific intracellular or signal transduction pathways, or alternatively different hormones and their receptors may invoke the same biochemical pathway.

For many hormones, including most protein hormones, the receptor is membrane associated and embedded in the plasma membrane at the surface of the cell. The interaction of hormone and receptor typically triggers a cascade of secondary effects within the cytoplasm of the cell, often involving cyclic AMP). Some protein hormones also interact with intracellular receptors located in the cytoplasm or nucleus by an intracrine mechanism.

For hormones such as DNA sequences, effectively amplifying or suppressing the action of certain genes, and affecting protein synthesis.^[4] However, it has been shown that not all steroid receptors are located intracellularly, some are plasma membrane associated.^[5]

An important consideration, dictating the level at which cellular concentration of hormone-receptor complexes that are formed. Hormone-receptor complex concentrations are effectively determined by three factors:

1. The number of hormone molecules available for complex formation
2. The number of receptor molecules available for complex formation and
3. The binding affinity between hormone and receptor.

The number of hormone molecules available for complex formation is usually the key factor in determining the level at which affinity between the hormone and its receptor.

Physiology of hormones

Most cells are capable of producing one or more molecules, which act as signalling molecules to other cells, altering their growth, function, or metabolism. The classical hormones produced by endocrine glands mentioned so far in this article are cellular products, specialized to serve as regulators at the overall organism level. However they may also exert their effects solely within the tissue in which they are produced and originally released.

The rate of hormone biosynthesis and secretion is often regulated by a homeostatic negative feedback control mechanism. Such a mechanism depends on factors which influence the excretion of hormones. Thus, higher hormome concentration alone can not trigger the negative feedback mechanism. Negative feedback must be triggered by overproduction of an "effect" of the hormone.

Hormone secretion can be stimulated and inhibited by:

• Other hormones (stimulating- or releasing-hormones)
• Plasma concentrations of ions or nutrients, as well as binding globulins
• Neurons and mental activity
• Environmental changes, e.g., of light or temperature

One special group of hormones is the thyroid hormones.

A recently-identified class of hormones is that of the "hunger hormones" - obestatin, nesfatin-1.

In order to release active hormones quickly into the circulation, hormone biosynthetic cells may produce and store biologically inactive hormones in the form of pre- or prohormones. These can then be quickly converted into their active hormone form in response to a particular stimulus.

Hormone effects

Hormone effects vary widely, but can include:

• stimulation or inhibition of growth,
• In puberty hormones can effect mood and mind
• induction or suppression of apoptosis (programmed cell death)
• activation or inhibition of the immune system
• regulating metabolism
• preparation for a new activity (e.g., fighting, fleeing, mating)
• preparation for a new phase of life (e.g., puberty, caring for offspring, menopause)
• controlling the reproductive cycle

In many cases, one hormone may regulate the production and release of other hormones

Many of the responses to hormone signals can be described as serving to regulate metabolic activity of an organ or tissue.

Chemical classes of hormones

Vertebrate hormones fall into three chemical classes:

• thyroxine.
• thyroid-stimulating hormone are glycoprotein hormones.
• prostaglandins.

Pharmacology

Many hormones and their vitamin D creams are used extensively in dermatological practice.

A "pharmacologic dose" of a hormone is a medical usage referring to an amount of a hormone far greater than naturally occurs in a healthy body. The effects of pharmacologic doses of hormones may be different from responses to naturally-occurring amounts and may be therapeutically useful. An example is the ability of pharmacologic doses of glucocorticoid to suppress inflammation.

Important human hormones

Spelling is not uniform for many hormones. Current North American and international usage is estrogen, gonadotropin, while British usage retains the Greek diphthong in oestrogen and the unvoiced aspirant h in gonadotrophin.

Structure	Name	Abbrev- iation	Tissue	Cells	Mechanism	Target Tissue	Effect
amine - tryptophan	antioxidant and causes drowsiness
amine - tryptophan	Serotonin	5-HT	CNS, GI tract	enterochromaffin cell			Controls mood, appetite, and sleep
amine - tyrosine	catecholamines, affect protein synthesis
amine - tyrosine	Triiodothyronine (a catecholamines, affect protein synthesis
amine - tyrosine (Epinephrine (or adrenaline)	EPI	adrenal medulla	chromaffin cell			Fight-or-flight response: Boosts the supply of lipids in fat cells. dilate the pupils Suppress non-emergency bodily processes (e.g. digestion) Suppress immune system
amine - tyrosine (Norepinephrine (or noradrenaline)	NRE	adrenal medulla	chromaffin cell			Fight-or-flight response: Boosts the supply of lipids in fat cells. Increase skeletal muscle readiness.
amine - tyrosine (TRH from anterior pituitary
peptide	TRH from anterior pituitary
peptide	Adiponectin	Acrp30	adipose tissue
peptide	androgens) in adrenocortical cells
peptide	angiotensin	AGT	liver		IP3		vasoconstriction release of aldosterone from adrenal cortex dipsogen.
peptide	ACTH in anterior pituitary
peptide	Atrial-natriuretic peptide (or atriopeptin)	ANP	heart		cGMP
peptide	Ca2+
peptide	pancreas Release of bile from gallbladder hunger suppressant
peptide	ACTH from anterior pituitary
peptide	Erythropoietin	EPO	kidney	Extraglomerular mesangial cells			Stimulate erythrocyte production
peptide	Follicle-stimulating hormone	FSH	anterior pituitary	gonadotrope	cAMP		In female: stimulates maturation of Graafian follicles in ovary. In male: spermatogenesis, enhances production of androgen-binding protein by the Sertoli cells of the testes
peptide	gastric acid by parietal cells
peptide	Ghrelin		stomach	P/D1 cell			Stimulate appetite, secretion of growth hormone from anterior pituitary gland
peptide	gluconeogenesis in liver increases blood glucose level
peptide	LH from anterior pituitary.
peptide	Growth hormone-releasing hormone	GHRH	hypothalamus		IP3		Release GH from anterior pituitary
peptide	Human chorionic gonadotropin	hCG	placenta	syncytiotrophoblast cells	cAMP		promote maintenance of corpus luteum during beginning of pregnancy Inhibit immune response, towards the human embryo.
peptide	IGF-1 increase carbohydrate intolerance
peptide	Growth hormone	GH or hGH	anterior pituitary	somatotropes			stimulates growth and cell reproduction Release Insulin-like growth factor 1 from liver
peptide	FSH
peptide	glycolysis in liver and muscle from blood intake of triglycerides in adipocytes Other anabolic effects
peptide	tyrosine kinase		insulin-like effects regulate cell growth and development
peptide	metabolism.
peptide	Luteinizing hormone	LH	anterior pituitary	gonadotropes	cAMP		In female: ovulation In male: stimulates Leydig cell production of testosterone
peptide	Melanocyte stimulating hormone	MSH or α-MSH	anterior pituitary/pars intermedia	Melanotroph	cAMP		melanogenesis by melanocytes in skin and hair
peptide	Oxytocin	OXT	posterior pituitary	Magnocellular neurosecretory cells	IP3		release breast milk Contraction of cervix and vagina Involved in orgasm, trust between people.[6] and circadian homeostasis (body temperature, activity level, wakefulness) [7].
peptide	Ca2+: *indirectly stimulate osteoclasts Ca2+ reabsorption in kidney activate vitamin D (Slightly) decrease blood phosphate: (decreased reuptake in kidney but increased uptake from bones activate vitamin D)
peptide	Prolactin	PRL	anterior pituitary, uterus	lactotrophs of anterior pituitary Decidual cells of uterus			milk production in mammary glands sexual gratification after sexual acts
peptide	Relaxin	RLN	uterus	Decidual cells			Unclear in humans
peptide	pancreas and duodenal Brunner's glands Enhances effects of cholecystokinin Stops production of gastric juice
peptide	motilin, vasoactive intestinal peptide (VIP), gastric inhibitory polypeptide (GIP), enteroglucagon in gastrointestinal system Lowers rate of gastric emptying Reduces smooth muscle contractions and blood flow within the intestine [8] Inhibit release of pancreas.
peptide	platelets[10]
peptide	thyroxine (T4) and triiodothyronine (T3)
peptide	prolactin release
steroid - gluconeogenesis Inhibition of glucose uptake in muscle and adipose tissue Mobilization of amino acids from extrahepatic tissues Stimulation of fat breakdown in adipose tissue anti-inflammatory and immunosuppressive
steroid - sodium in kidneys (primarily) H+ secretion in kidney.
steroid - sex (Anabolic: growth of muscle mass and strength, increased bone density, growth and strength, Virilizing: maturation of sex organs, formation of scrotum, deepening of voice, growth of beard and axillary hair.
steroid - sex (anabolic
steroid - sex (estrogen
steroid - sex (Dihydrotestosterone	DHT	multiple		direct
steroid - sex (Estradiol	E2	females: ovary, males testes	females: granulosa cells, males: Sertoli cell	direct		Females: Structural: promote formation of female secondary sex characteristics accelerate height growth accelerate metabolism (burn fat) reduce muscle mass stimulate endometrial growth increase uterine growth maintenance of blood vessels and skin reduce bone resorption, increase bone formation Protein synthesis: increase hepatic production of binding proteins Coagulation: increase circulating level of plasminogen increase platelet adhesiveness Increase HDL, triglyceride, height growth Decrease LDL, fat depositition Fluid balance: salt (sodium) and water retention increase growth hormone increase cortisol, SHBG Gastrointestinal tract: reduce bowel motility increase cholesterol in bile Melanin: increase eumelanin Cancer: support hormone-sensitive breast cancers [11] Suppression of production in the body of estrogen is a treatment for these cancers. Lung function: promote lung function by supporting alveoli[12]. Males: Prevent apoptosis of germ cells[13]
steroid - sex (est)	Estrone		ovary	granulosa cells, Adipocytes	direct
steroid - sex (Estriol		placenta	syncytiotrophoblast	direct
steroid - sex (Progesterone		ovary, adrenal glands, placenta (when pregnant)	Granulosa cells theca cells of ovary	direct		Support pregnancy[14]: Convert endometrium to secretory stage Make cervical mucus permeable to sperm. Inhibit immune response, e.g. towards the human embryo. Decrease uterine smooth muscle contractility[14] Inhibit lactation Inhibit onset of labor. Support fetal production of adrenal mineralo- and glucosteroids. Other: Raise epidermal growth factor-1 levels Increase core temperature during ovulation[15] Reduce spasm and relax smooth muscle (widen bronchi and regulate mucus) Antiinflammatory Reduce gall-bladder activity[16] Normalize blood clotting and vascular tone, oxygen levels, and use of fat stores for energy. Assist in thyroid function and bone growth by osteoblasts Relsilience in bone, teeth, gums, joint, tendon, ligament and skin Healing by regulating collagen Nerve function and healing by regulating myelin Prevent endometrial cancer by regulating effects of estrogen.
sterol	Calcitriol (1,25-dihydroxyvitamin D3)		skin/proximal tubule of kidneys		direct		Active form of vitamin D3 Increase absorption of phosphate from gastrointestinal tract and kidneys inhibit release of PTH
sterol	Calcidiol (25-hydroxyvitamin D3)		skin/proximal tubule of kidneys		direct		Inactive form of Vitamin D3
eicosanoid	Prostaglandins	PG	seminal vesicle
eicosanoid	Leukotrienes	LT		white blood cells
eicosanoid	Prostacyclin	PGI2	endothelium
eicosanoid	Thromboxane	TXA2		platelets
	Prolactin releasing hormone	PRH	hypothalamus				Release prolactin from anterior pituitary
	Lipotropin	PRH	anterior pituitary	Corticotropes			melanin
	Brain natriuretic peptide	BNP	heart	Cardiac myocytes			(To a minor degree than ANP) reduce blood pressure by: reducing systemic vascular resistance, reducing blood water, sodium and fats
	Neuropeptide Y	NPY	Stomach				increased food intake and decreased physical activity
	gastric acid secretion
	Endothelin		Stomach	X cells			Smooth muscle contraction of stomach [17]
	Pancreatic polypeptide		Pancreas	PP cells			Unknown
	angiotensinogen
	Enkephalin		Kidney	Chromaffin cells			Regulate pain

References

See also

v • d • e NT-3