HEART AS ENDOCRINE GLAND
· The atrial myocytes secrete a hormone called Atrial Natriuretic Peptide (ANP).
· Structure
· Atrial natriuretic peptide is a 28-aminoacid residue polypeptide.
2. Acylation-Stimulation Protein (ASP)
· ASP helps in postprandial clearance of lipids by stimulating triglyceride synthesis in adipocytes.
3. Adiponectin
· Adiponectin increases fatty acid oxidation in muscle and liver and may regulate fat deposition without significantly affecting food intake.
· Adiponectin increases insulin sensitivity, probably by increasing fatty acid oxidation resulting in lowering of circulating fatty acid level and decreased triglyceride synthesis in muscle and liver.
· It can integrate and respond to signals from chemoreceptors and mechanoreceptors, in regulating the production of vasoactive substances involved in the regulation of blood pressure and vascular growth.
· Vascular endothelium secretes several substances some of which have diverse effects.
1. Endothelins
· Endothelins are a group of peptides that are the most potent long-acting vasopressors produced by vascular endothelium.
1. Renin
2. Erythropoietin (EPO)
3. 1,25-Dihydroxycholecalciferol (Calcitriol)
2. Erythropoietin
· Erythropoietin is a glycoprotein containing 165 aminoacids residues.
3. 1,25-Dihydroxycholecalciferol (Calcitriol)
B. Osteomalacia: It is due to deficiency of Vitamin–D in adult and show rickets like syndrome.
C. Osteoporosis: It is a common bone disease of old age.
· Prostaglandins were discovered from human semen in 1930.
· The name prostaglandin was introduced by Von Euler in 1937.
· The seminal vesicles are the chief source of prostaglandins in semen.
· The commonest prostaglandins are PGA1, PASCAL, PGE1 PGE2, PGF, etc.
· The immediate precursors of PG in the body are essential unsaturated fatty acids, e.g., Linoleic and arachidonic acids.
3. LOCAL HORMONES
· Local hormones are secreted at one place and work upon adjacent tissue. e.g., acetylcholine, Histamine, 5‑hydroxytryptamine, Bradykinin, etc.
· Atrial natriuretic peptide is a 28-aminoacid residue polypeptide.
· It is formed from a larger precursor molecule containing 151 amino acid residues.
Biological action
· Atrial natriuretic peptide, in general, antagonizes the actions of the angiotensin- aldosterone system.
a. ANP inhibits the secretion of renin and aldosterone,
b. ANP inhibits NaCl reabsorption in the collecting ducts of the kidney,
c. ANP inhibits the secretion of ADH and its action on the collecting ducts of the kidney,
d. ANP is a vasodilator.
· Note: The net effect of ANP will be natriuresis, diuresis and hypotension.
· Atrial natriuretic peptide, in general, antagonizes the actions of the angiotensin- aldosterone system.
a. ANP inhibits the secretion of renin and aldosterone,
b. ANP inhibits NaCl reabsorption in the collecting ducts of the kidney,
c. ANP inhibits the secretion of ADH and its action on the collecting ducts of the kidney,
d. ANP is a vasodilator.
· Note: The net effect of ANP will be natriuresis, diuresis and hypotension.
ADIPOSE TISSUE AS ENDOCRINE GLAND
· Adipose tissue, especially visceral fat, is an active endocrine organ.· Hormones produced by adipose tissue play an important role in the regulation of energy intake, energy expenditure, and lipid and carbohydrate metabolism.
· Adipocytes secrete three hormones:
1. Leptin,
2. Acylation-stimulation protein (ASP) &
3. Adiponectin.
2. Acylation-stimulation protein (ASP) &
3. Adiponectin.
· The secretion of these hormones is influenced by nutritional status.
1. Leptin
· Leptin is a peptide hormone secreted by the adipocytes in proportion to the body fat content.
1. Leptin
· Leptin is a peptide hormone secreted by the adipocytes in proportion to the body fat content.
· Leptin along with insulin are important regulators of energy homeostasis.
· Leptin crosses the blood-brain barrier and binds to receptors in the hypothalamus, which contains the neural mechanisms governing food intake and energy metabolism.
· Leptin inhibits food intake and simultaneously increases energy expenditure.
· A reduction in fat cell mass produces a fall in leptin production. This results in stimulation of food intake and conservation of energy.
· In contrast, an increase in fat cell mass (obesity) will increase leptin secretion with consequent inhibition of food intake and increase in energy expenditure.
· Instead, obese individuals have high levels of leptin. This has been attributed to leptin resistance in obese persons.
· Studies on individuals with absolute leptin deficiency (or receptor defects) and partial leptin deficiency have revealed leptin's critical role in the normal regulation of appetite and body adiposity in humans.
· The primary biological role of leptin appears to be an adaptation to low energy intake rather than inhibition of overeating and obesity. Leptin production is mainly regulated by insulin-induced changes and adipocyte metabolism.
· Consumption of fat and fructose, which do not initiate insulin secretion, resulting in lower circulating leptin levels with resultant overeating and weight gain in people consuming diets high in energy derived from these nutrients.
· Leptin crosses the blood-brain barrier and binds to receptors in the hypothalamus, which contains the neural mechanisms governing food intake and energy metabolism.
· Leptin inhibits food intake and simultaneously increases energy expenditure.
· A reduction in fat cell mass produces a fall in leptin production. This results in stimulation of food intake and conservation of energy.
· In contrast, an increase in fat cell mass (obesity) will increase leptin secretion with consequent inhibition of food intake and increase in energy expenditure.
· Instead, obese individuals have high levels of leptin. This has been attributed to leptin resistance in obese persons.
· Studies on individuals with absolute leptin deficiency (or receptor defects) and partial leptin deficiency have revealed leptin's critical role in the normal regulation of appetite and body adiposity in humans.
· The primary biological role of leptin appears to be an adaptation to low energy intake rather than inhibition of overeating and obesity. Leptin production is mainly regulated by insulin-induced changes and adipocyte metabolism.
· Consumption of fat and fructose, which do not initiate insulin secretion, resulting in lower circulating leptin levels with resultant overeating and weight gain in people consuming diets high in energy derived from these nutrients.
2. Acylation-Stimulation Protein (ASP)
· ASP helps in postprandial clearance of lipids by stimulating triglyceride synthesis in adipocytes.
· The secretion of ASP is regulated by the circulating diet-derived chylomicrons.
· Mice that lack the gene for ASP exhibit reduced body fat, obesity resistance and improved insulin sensitivity.
3. Adiponectin
· Adiponectin increases fatty acid oxidation in muscle and liver and may regulate fat deposition without significantly affecting food intake.
· Adiponectin increases insulin sensitivity, probably by increasing fatty acid oxidation resulting in lowering of circulating fatty acid level and decreased triglyceride synthesis in muscle and liver.
VASCULAR ENDOTHELIUM AS ENDOCRINE GLAND
Vascular endothelium, present between blood and vascular tunica media.· It can integrate and respond to signals from chemoreceptors and mechanoreceptors, in regulating the production of vasoactive substances involved in the regulation of blood pressure and vascular growth.
· Vascular endothelium secretes several substances some of which have diverse effects.
· The substances secreted by vascular endothelium are:
(1) Endothelins &
(2) Nitric oxide.
1. Endothelins
· Endothelins are a group of peptides that are the most potent long-acting vasopressors produced by vascular endothelium.
· Three 21-aminoacid vasopressor peptides have been identified and they are termed endothelin-1 (ET-1), endothelin-2 (ET-2) and endothelin-3 (ET-3).
· They are formed from a large precursor by the action of endothelin converting enzyme (ECE) found on the endothelial cell membrane.
· ET-1 combines with a receptor, called ETA, present on vascular smooth muscle and produces contraction (vasoconstriction).
· ET-1 combines with a receptor, called ETA, present on vascular smooth muscle and produces contraction (vasoconstriction).
· ET-1 release is stimulated by angiotensin II, ADH, thrombin, cytokines, reactive oxygen species and shearing forces acting on the vascular endothelium.
· ET-1 release is inhibited by nitric oxide (NO) and by prostacyclin and ANP.
· In addition to its vasoconstrictor effect, ET-1 stimulates aldosterone secretion, positive inotropy and chronotropic in the heart, decreasing renal blood flow and GFR, and release of ANP.
· In addition to its vasoconstrictor effect, ET-1 stimulates aldosterone secretion, positive inotropy and chronotropic in the heart, decreasing renal blood flow and GFR, and release of ANP.
2. Nitric oxide (NO)
· Synthesis and release: Nitric oxide (NO) is produced by vascular endothelium and many other tissues.
· Synthesis and release: Nitric oxide (NO) is produced by vascular endothelium and many other tissues.
· NO is formed from L-arginine that is transported into the cell. The enzyme, Nitric oxide synthase (NOS) acts on L- arginine to form NO and citrulline.
· Actions: Nitric oxide has many important actions on the cardiovascular system—
· Vasodilation
· Inhibition of vasoconstrictor influences - e.g., inhibits angiotensin II and sympathetic vasoconstriction
· Inhibition of platelet adhesion to vascular endothelium
· Inhibition of leukocyte adhesion to vascular endothelium
· Inhibition of smooth muscle growth after vascular injury
· Scavenging free radicals.
Applied physiology: Damage to vascular endothelium or dysfunction cause impairment of NO production and the following effects are seen:
· Vasoconstriction
· Platelet aggregation
· Upregulation of leucocyte and endothelial adhesion molecules
· Vascular stenosis.
· Increased inflammation and tissue damage.
· Hence, endothelial dysfunction and decreased NO production have been implicated in -hypertension, obesity, dyslipidaemias, diabetes mellitus, heart failure and atherosclerosis.
· Actions: Nitric oxide has many important actions on the cardiovascular system—
· Vasodilation
· Inhibition of vasoconstrictor influences - e.g., inhibits angiotensin II and sympathetic vasoconstriction
· Inhibition of platelet adhesion to vascular endothelium
· Inhibition of leukocyte adhesion to vascular endothelium
· Inhibition of smooth muscle growth after vascular injury
· Scavenging free radicals.
Applied physiology: Damage to vascular endothelium or dysfunction cause impairment of NO production and the following effects are seen:
· Vasoconstriction
· Platelet aggregation
· Upregulation of leucocyte and endothelial adhesion molecules
· Vascular stenosis.
· Increased inflammation and tissue damage.
· Hence, endothelial dysfunction and decreased NO production have been implicated in -hypertension, obesity, dyslipidaemias, diabetes mellitus, heart failure and atherosclerosis.
Kidney as Endocrine Gland
· Kidney synthesizes and secretes the following substances:1. Renin
2. Erythropoietin (EPO)
3. 1,25-Dihydroxycholecalciferol (Calcitriol)
1. Renin
· Renin is an enzyme with protease activity.
· Renin is an enzyme with protease activity.
· It is synthesized by juxtaglomerular cells.
· Renin is made up of 340 aminoacids.
· It is an integral part of the renin-angiotensin system.
2. Erythropoietin
· Erythropoietin is a glycoprotein containing 165 aminoacids residues.
· In adults, it is mostly produced by interstitial cells in the peritubular capillaries of the kidney(85%) and hepatocytes in the liver.
· In the foetus and neonates, the liver is the chief source of EPO.
· Function:
· EPO is necessary for normal erythropoiesis.
· Function:
· EPO is necessary for normal erythropoiesis.
3. 1,25-Dihydroxycholecalciferol (Calcitriol)
· In the liver, vitamin –D is hydroxylated to 25 – hydroxycholecalciferol and converted to 1,25- dihydroxycholecalciferol in the kidney by the action of enzyme 1- hydroxylase.
· Calcitriol increases the serum calcium and phosphate levels and favours bone mineralization.
· Calcitriol increases the serum calcium and phosphate levels and favours bone mineralization.
· The intestine is the principal action of it.
· It inhibits the proliferation of lymphocytes and decreases the formation of interleukin-2, interferon and lymphokines.
· Disorders:
A. Rickets: It is a disease of children due to lack of Vitamin–D.
A. Rickets: It is a disease of children due to lack of Vitamin–D.
· Symptoms: soft bone, calcification and ossification of epiphyseal plates are abnormal. Bones are deformed and weaker.
B. Osteomalacia: It is due to deficiency of Vitamin–D in adult and show rickets like syndrome.
C. Osteoporosis: It is a common bone disease of old age.
· There is gradual loss of minerals from bone to blood.
· Bone becomes susceptible to fracture.
OTHER CHEMICAL SUBSTANCES (HORMONES)
1. PROSTAGLANDINS· Prostaglandins were discovered from human semen in 1930.
· The name prostaglandin was introduced by Von Euler in 1937.
· The seminal vesicles are the chief source of prostaglandins in semen.
· The commonest prostaglandins are PGA1, PASCAL, PGE1 PGE2, PGF, etc.
· The immediate precursors of PG in the body are essential unsaturated fatty acids, e.g., Linoleic and arachidonic acids.
2. PHEROMONES
· Pheromones are chemical messengers produced by animals and released outside the body, hence called 'ectohormones'.
· The term 'pheromone' was proposed by Keelson.
· Pheromones are also called 'semiochemicals'.
· Bombycol, a pheromone produced by silkmoth, was the pheromone studied first.
· Pheromones are chemical messengers produced by animals and released outside the body, hence called 'ectohormones'.
· The term 'pheromone' was proposed by Keelson.
· Pheromones are also called 'semiochemicals'.
· Bombycol, a pheromone produced by silkmoth, was the pheromone studied first.
Types of Pheromones
| Type | Example |
|---|---|
| Sex pheromones | Bombycol- silkmoth Queen substance-Honey bee Civetone- Cat Muskone- Musk deer |
| Aggregation pheromones | Geradiol- Honey bee |
| Alarm pheromones | Danger signals |
| Marking pheromones | Mark the territory in wild animals |
3. LOCAL HORMONES
· Local hormones are secreted at one place and work upon adjacent tissue. e.g., acetylcholine, Histamine, 5‑hydroxytryptamine, Bradykinin, etc.
Also, Read our Other Notes related to Endocrinology of Mammals: