Local Hormones

Introduction

Local hormones are chemical messengers that usually act near their site of synthesis, though some also produce systemic effects. They regulate hemostasis, gastrointestinal activity, cardiovascular function, renal filtration, respiration, reproduction, and neural signaling, making them important in health and disease.

Definition And Types

• Local hormones are chemical mediators produced within tissues that mainly act near their site of release. They regulate neighboring cells through paracrine or autocrine mechanisms.
• Some may also produce wider systemic effects.
• Examples include renin, erythropoietin, atrial natriuretic peptide, melatonin, thymosin, histamine, serotonin, bradykinin, and prostaglandins.

Histamine

Source, Synthesis and Metabolism

Histamine acts through several receptor subtypes, mainly H1, H2, and H3 receptors.

H1 Receptors

• H1 receptors are linked to phospholipase C signaling.
• Activation increases inositol trisphosphate and diacylglycerol. They mediate allergy, inflammation, itching, bronchoconstriction, and vasodilation.

H2 Receptors

• H2 receptors increase intracellular cyclic adenosine monophosphate. They are prominent in gastric parietal cells.
• Activation stimulates gastric acid secretion and may increase heart rate.

H3 Receptors

• H3 receptors are mainly present in the central nervous system, especially presynaptic nerve terminals.
• They inhibit release of histamine and several other neurotransmitters. These receptors help regulate wakefulness, appetite, and neural signaling.

Histamine Receptors

• Histamine acts through three major receptor types: H1, H2, and H3. These receptors are present in both peripheral tissues and the nervous system.
• H1 receptors activate phospholipase C signaling. They mediate allergy, inflammation, vasodilation, itching, and bronchoconstriction.
• H2 receptors increase intracellular cyclic adenosine monophosphate. They are prominent in gastric mucosa and stimulate gastric acid secretion.
• H3 receptors are mainly located in the central nervous system on presynaptic terminals. They inhibit release of histamine and other neurotransmitters.

Physiological Actions

• Histamine acts mainly on the cardiovascular system, smooth muscle, secretory glands, immune responses, and the central nervous system. It is an important mediator of inflammation and allergy.

On CVS

• Histamine is a potent vasodilator, especially in small blood vessels. It increases local blood flow and may lower systemic blood pressure.
• Increased vascular permeability allows plasma leakage, producing edema.
• In severe reactions such as anaphylaxis, marked vasodilation may cause shock.

On Smooth Muscles

• Histamine increases tone of several smooth muscles. It can increase intestinal motility and cause abdominal cramping. It produces bronchoconstriction, especially in susceptible individuals such as patients with asthma.

On Exocrine Secretions

• Histamine strongly stimulates hydrochloric acid secretion from gastric parietal cells through H2 receptors. It may also enhance salivary, pancreatic, and intestinal secretions..

Role in Inflammation

• Histamine is released mainly from mast cells and basophils during tissue injury. It causes vasodilation, redness, warmth, and swelling at the affected site.
• Increased capillary permeability contributes to local edema. It participates in the classic wheal-and-flare skin response.

Role in Allergy

• Many immediate hypersensitivity reactions are mediated by histamine.
• Antigen binding to immunoglobulin E on mast cells triggers degranulation and histamine release. This contributes to urticaria, allergic rhinitis, hay fever, conjunctival irritation, and bronchospasm.
• In systemic anaphylaxis, widespread histamine release may cause hypotension and airway compromise.
• Antihistamines are commonly used for symptomatic relief of allergic disorders.

Relation with Itch

• Histamine activates sensory nerve endings and is an important mediator of itching.
• Intradermal release commonly causes pruritus in urticaria and insect bites.

On CNS

• Histamine functions as a neurotransmitter in the brain.
• Histaminergic neurons arise mainly from the tuberomammillary nucleus.
• They help regulate arousal, alertness, appetite, autonomic activity, and neuroendocrine functions.

Clinical Correlation

Antihistaminic Drugs

• Antihistamines are widely used for allergic and inflammatory conditions.
• H1 receptor antagonists reduce itching, sneezing, urticaria, and histamine-induced bronchial or intestinal smooth muscle contraction.
• Examples include promethazine and similar agents.
• H2 receptor antagonists reduce gastric acid secretion.
• Examples include famotidine and related drugs. They are used in acid-peptic disorders and reflux disease.

Serotonin

Source

• Serotonin is 5-hydroxytryptamine, an important neurotransmitter and local signaling molecule. It is widely distributed in the body.
• High concentrations are present in the gastrointestinal tract, central nervous system, skin, platelets, and basophils.
• Because of its abundance in the intestine, it is also called enteramine.

Synthesis and Metabolism

Synthesis

• Serotonin is synthesized from the amino acid tryptophan.
• Tryptophan is converted to 5-hydroxytryptophan by tryptophan hydroxylase. It is then decarboxylated to form serotonin.

Metabolism

• Serotonin is mainly metabolized by monoamine oxidase. Its principal urinary metabolite is 5-hydroxyindoleacetic acid (5-HIAA).
• Urinary 5-HIAA can reflect serotonin production.

Serotonin Receptors

• Serotonin acts through seven major receptor families: 5-HT1 to 5-HT7.
• Several families have subtypes, including 5-HT1 and 5-HT2 groups.
• Most are G protein-coupled receptors.
• 5-HT3 is unique because it is a ligand-gated ion channel.

Physiological Actions

• Serotonin acts on the cardiovascular, respiratory, renal, gastrointestinal, and nervous systems. Its effects depend on receptor subtype and tissue distribution.

Cardiovascular System

• Serotonin can produce local vasoconstriction in several vascular beds. It promotes platelet aggregation and contributes to temporary hemostasis after vascular injury.
• Platelets store serotonin and release it during activation. It may also increase heart rate in some settings.

Respiratory System

• Serotonin increases bronchial smooth muscle tone and may cause bronchoconstriction.
• In susceptible individuals, it can worsen bronchospasm. It may also stimulate respiratory drive.

Kidney

• Serotonin may reduce urine formation transiently by decreasing renal blood flow and glomerular filtration. It can increase ureteric smooth muscle tone and cause spasm.

GI Tract

• Large amounts of serotonin are present in enterochromaffin cells of the intestine. It increases gastrointestinal motility, promotes peristalsis, and enhances secretion.
• Excess serotonin may produce abdominal cramps and diarrhea. It is also involved in nausea and vomiting reflexes.

Central Nervous System

• Serotonin is an important neurotransmitter in multiple brain pathways. It participates in endogenous pain modulation within the spinal cord and brainstem. It influences mood, anxiety, emotional behavior, and cognition.
• Reduced serotonergic activity is associated with depression in some patients.
• Selective serotonin reuptake inhibitors increase synaptic serotonin and are used as antidepressants.
• Serotonin helps regulate appetite and can reduce food intake. It contributes to circadian rhythm and sleep-wake regulation, though effects vary by receptor and brain region. It can influence prolactin secretion through hypothalamic pathways.
• Activation of 5-HT3 receptors in the area postrema contributes to vomiting.

Clinical Correlation

• Increased serotonin production occurs in some neuroendocrine tumors, especially carcinoid syndrome.
• Urinary 5-hydroxyindoleacetic acid (5-HIAA) is commonly used as a biochemical marker.

Bradykinin

• Bradykinin is a peptide mediator composed of nine amino acids. It is generated in plasma and tissues from high-molecular-weight kininogen.

Synthesis

• Plasma kallikrein and tissue kallikrein catalyze its formation.
• Plasma kallikrein is activated through factor XII-dependent pathways.

Physiological Actions

• Bradykinin is a potent vasodilator and increases local blood flow. It increases capillary permeability, promoting edema at injured sites. It can contract visceral smooth muscle, including bronchial and intestinal muscle.
• It produces pain by stimulating sensory nerve endings. It may increase salivary, pancreatic, and sweat secretions.

Clinical Importance

• Excess bradykinin contributes to angioedema and inflammatory pain.
• Elevated levels may occur in some tumors and carcinoid states.

Prostaglandins

• Prostaglandins are lipid mediators derived from 20-carbon unsaturated fatty acids, mainly arachidonic acid. They contain a characteristic cyclopentane ring.
• Despite the name, they are synthesized in many tissues, not only the prostate. They act locally in the kidney, lungs, gastrointestinal tract, uterus, skin, and blood vessels.

Types

• Major prostanoid groups include PGE, PGF, PGI (prostacyclin), PGD, and thromboxanes.
• Common subtypes include PGE2, PGF2 alpha, and PGI2.

Synthesis

• Prostaglandins are synthesized from arachidonic acid released from membrane phospholipids by phospholipase A2.
• Cyclooxygenase enzymes convert arachidonic acid into prostaglandin endoperoxides. These intermediates are then transformed into prostacyclin, thromboxane A2, PGE2, PGF2 alpha, and related compounds by specific tissue enzymes.

Physiological Actions

• Prostaglandins are produced in most tissues and act locally as short-lived mediators. They regulate organ function, inflammation, vascular tone, reproduction, and hemostasis.

Cardiovascular System

• Some prostaglandins, such as certain E and A series compounds, cause peripheral vasodilation.
• Prostacyclin (PGI2) is a strong vasodilator and improves blood flow.
• Thromboxane A2 causes vasoconstriction and promotes vascular spasm when excessive.
• Local balance among these mediators influences blood pressure and tissue perfusion.

Reproductive System

• PGF2 alpha stimulates uterine smooth muscle contraction, especially during pregnancy and labor.
• Rising prostaglandin levels contribute to cervical ripening and onset of labor.
• Prostaglandins can promote luteolysis, leading to regression of the corpus luteum in some species. They may reduce progesterone support and influence reproductive cycling.
• Increased endometrial prostaglandins participate in menstrual cramps and menstrual bleeding. They may also modulate hypothalamic release of gonadotropin-releasing hormone.

Hemostasis

• Prostacyclin inhibits platelet aggregation and dilates blood vessels.
• Thromboxane A2 promotes platelet aggregation and constricts vessels.
• The balance between prostacyclin and thromboxane helps determine platelet plug formation.
• Disturbance of this balance can favor bleeding or thrombosis.

Respiratory System

• Prostaglandin E generally causes bronchodilation.
• PGF2 alpha can produce bronchoconstriction and may worsen bronchial asthma in susceptible individuals.

GI System

• Certain prostaglandins reduce sodium and water absorption in the intestine.
• Excess production may contribute to watery diarrhea. They also stimulate intestinal motility and may cause cramping.
• In the stomach, prostaglandins help protect mucosa by increasing mucus and bicarbonate secretion.

Central Nervous System

• Certain prostaglandins reduce sodium and water absorption in the intestine.
• Excess production may contribute to watery diarrhea. They also stimulate intestinal motility and may cause cramping.
• In the stomach, prostaglandins help protect mucosa by increasing mucus and bicarbonate secretion.

Inflammation

• Several prostaglandins increase vascular permeability during inflammation. They sensitize sensory nerve endings to pain mediators such as bradykinin. This contributes to pain, redness, and swelling.

Metabolism

• Some prostaglandins can inhibit lipolysis induced by hormones such as catecholamines and glucagon.

Clinical Correlation

• Glucocorticoids reduce prostaglandin synthesis by inhibiting phospholipase A2.
• Nonsteroidal anti-inflammatory drugs such as ibuprofen inhibit cyclooxygenase enzymes and reduce prostaglandin production. These drugs are widely used for pain, fever, and inflammatory disorders.

Carcinoid Syndrome

• Carcinoid tumors arise from neuroendocrine cells, commonly in the gastrointestinal tract or bronchus.
• Patients may develop episodic flushing, diarrhea, abdominal pain, wheezing, or bronchoconstriction.
• Tumors often secrete excess serotonin and other vasoactive substances.
• Increased urinary 5-hydroxyindoleacetic acid (5-HIAA) supports diagnosis.

Other Local Hormones

Renin

• Renin is an enzyme-hormone released from juxtaglomerular cells of the kidney.
• Secretion increases during hypovolemia, reduced renal perfusion, low sodium delivery, or sympathetic stimulation.
• Renin converts angiotensinogen into angiotensin I.
• Angiotensin-converting enzyme then forms angiotensin II.
• Angiotensin II causes vasoconstriction, stimulates aldosterone release, and helps restore blood pressure and extracellular fluid volume.

Erythropoietin

• Erythropoietin is a glycoprotein hormone produced mainly by peritubular interstitial cells of the kidney. Its synthesis rises when tissue oxygen delivery falls. It is the principal regulator of erythropoiesis in bone marrow.
• The hormone stimulates survival, proliferation, and maturation of erythroid precursors.
• Reduced production contributes to anemia in chronic kidney disease.

ANP

• Atrial natriuretic peptide (ANP) is synthesized and stored in atrial myocytes. It is released when atrial walls are stretched by increased blood volume or pressure.
• Related peptides include brain natriuretic peptide (BNP) and C-type natriuretic peptide.
• ANP increases sodium excretion by reducing tubular sodium reabsorption. It increases glomerular filtration rate through dilation of the afferent arteriole and relaxation of mesangial cells.
• Increased sodium loss leads to greater water excretion and mild diuresis. It causes vasodilation, thereby lowering systemic blood pressure.
• ANP reduces secretion of renin, aldosterone, and vasopressin. It decreases vascular responsiveness to vasoconstrictors such as catecholamines and angiotensin II. It may reduce heart rate and cardiac workload indirectly by lowering preload and afterload.
• BNP levels are commonly used as biomarkers in heart failure.
• Natriuretic peptides act as protective counter-regulatory hormones against volume overload and hypertension.

Melatonin

• Melatonin is secreted by the pineal gland.
• Secretion is generally higher in children and lower in adults. It helps regulate circadian rhythm and sleep timing.
• Melatonin may suppress gonadal activity and influence timing of puberty.

Thymosin

• Thymosin is produced by epithelial cells of the thymus.
• The thymus lies in the anterior superior mediastinum behind the sternum. It enlarges during childhood, reaches maximum size near adolescence, and then undergoes involution with age.
• Thymosin supports maturation and differentiation of T lymphocytes. It is essential for development of cellular immunity and normal immune competence.