BIOCHEMICAL ASPECTS OF HORMONES

Ashikh Seethy

Department of Biochemistry

Maulana Azad Medical College

New Delhi

Body activity is co-ordinated by various chemical messenger systems

1. Neurotransmitters

Released by axon terminals of neurons into the synaptic junctions

Act locally to control nerve cell functions

Eg: ACh, GABA.

2. Endocrine hormones

Released by glands or specialized cells into the circulating blood

Influence the function of cells at another location in the body

3. Neuroendocrine hormones

Secreted by neurons into the circulating blood

Influence the function of cells at another location in the body

Eg: Oxytocin, ADH

4. Paracrines

5. Autokines

Hormones are classified according to the distance over which they act

Endocrine hormones

Eg: Insulin, Epinephrine

Paracrine hormones (local mediators)

Eg: IL-1 from macrophages that stimulates the bound T cell to proliferate anddifferentiate

Autocrine hormones

Eg: Autostimulatory release of IL-2 enhances the response of a T cell to IL-1

Endocrine glands and hormones:

•Hypothalamus:TRH, CRH, GHRH, PIF, GHIH, GnRH.

•Anterior Pituitary:FSH, LH, Prolactin, TSH, ACTH, Growth hormone.

•Posterior Pituitary:Oxytocin and ADH

•Thyroid: T3, T4, Calcitonin

•Parathyroid: PTH

•Stomach: Gastrin

•Kidneys:Erythropoetin, Vit D3, Angiotensin

•Adrenal Medulla:Epi- and Nor-epinephrine

•Adrenal Cortex:Cortisol, Aldosterone

•Pancreas: Insulin, Glucagon

•Intestine:Secretin, CCK

•Ovary: Estrogen, Progesterone

•Testes: Testosterone

•Placenta:HCG, Estrogen,

Progesterone

•Adipocytes: Leptin

Hormones are of Three Chemical Classes:

1. Proteins and polypeptides

Hormones secreted by anterior and posterior pituitary gland

Insulin and glucagon

Parathyroid hormone

2. Steroids

Adrenal cortex (cortisol and aldosterone)

Ovaries (estrogen and progesterone)

Testes (testosterone)

Placenta (estrogen and progesterone)

3. Derivatives of the amino acid tyrosine [Amines]

Thyroid (thyroxine and triiodothyronine)

Adrenal medullae (epinephrine and norepinephrine)

Dopamine [Prolactin Inhibiting Factor]

• There are no known polysaccharides or nucleic acid hormones.

Growth hormone and Insulin are Peptide Hormones

Synthesis, Storage and Release of Peptide Hormones:

• Preprohormones

• Prohormones

• Golgi apparatus

• Secretory vesicles

Prohormones Hormones

Stored

Exocytosis

Endoplasmic Reticulum

Stimulus

Steroid hormones are derived from cholesterol; they are not stored

Amine Hormones are Derived from Tyrosine:

CV

CV

CV

Transport of Hormones:• Water-soluble hormones (peptides

and catecholamines)

Dissolved in the plasma andtransported to target tissues, wherethey diffuse out of the capillaries, intothe interstitial fluid and to target cells.

• Steroid and thyroid hormones

Mainly bound to plasma proteins

Usually less than 10 per cent of steroidor thyroid hormones in the plasmaexist free in solution.

Protein-bound hormones cannot easilydiffuse across the capillaries

Hence, biologically inactive until theydissociate from plasma proteins

Bound forms act as a reservoir.

The Two Major Types of Control of Endocrine Gland Function

• Hypothalamic-pituitary-target gland systems

• Free-standing endocrine glands

Eg. Parathyroid, Islet cells

Release hormones that stimulate a target tissue to produce an effect which in turn modifies the function of the gland

Eg. A rise in serum calcium or a fall in blood sugar

Hypothalamic-Pituitary-Target Gland Systems

Feedback Control of Hormone Secretion:

• Negative feedback

• Positive feedback- not very common

Estrogen from ovary

Luteinizing hormone [LH] from Ant. Pituitary

Estrogen from ovary

• Cyclical variations in hormone release:

Growth hormone surges during early stages of sleep

Cortisol level peaks during early morning

Functions of the Endocrine System

• Maintain homeostasis

Insulin and glucagon maintain the blood glucose level within rigidlimits during feast or famine.

Vit D, PTH and Calcitonin regulate calcium homeostasis

• Respond to a wide variety of external stimuli

The preparation for “fight or flight” engendered by epinephrine andnorepinephrine).

• Follow various cyclic and developmental programs

Sex hormones regulate sexual differentiation, maturation, themenstrual cycle, and pregnancy.

• Maintenance of diurnal rhythm

• Growth and differentiation

• Digestion and absorption of nutrients

HORMONE

SIGNALING

3 Types of Signalling

Hormones are of 2 classes based on the location of their receptors:

Class 1:

• Lipophilic hormones

• Intracellular receptors

Class 2:

• Peptide and catecholamines

• Membrane receptors

• Generate second messengers

Signaling in Class 2 Hormones Involves 5 Steps:

1. Release of the primary messenger [hormone]

2. Reception of the primary messenger

3. Delivery of the message inside the cell by the second messenger [transduction]

4. Activation of effectors that directly affect physiological responses

5. Termination of the signal

Receptors for Class 2 Hormones:

• Located in the plasma membrane

• They can be:

G protein coupled receptors [GPCRs]

Ion channel linked receptors

Enzyme linked receptors

G-protein Coupled Receptors:

• Has 7 hydrophobic plasma membrane spanning domains [Serpentine Receptors]

• Hormone binding site is extracellular

• Coupled with heterotrimeric GTP binding proteins in the cytoplasm [G proteins]

• G protein:

α subunit and βγ subunit

GDP binds to the α subunit

Binding of hormone to the receptor activates the G-protein

Binding of hormone to the receptor activates the G-protein

• There are different classes of G-proteins

Gs stimulates adenyl cyclase converting ATP to cAMP

Gi inhibits adenyl cyclase decreasing cAMP levels

Gq activates Phospholipase C: PIP2 >>> DAG + IP3

The target protein and subsequent actions vary depending on the class of G-protein activated

Classes and functions of G proteins:

Second Messengers:

• Molecules that relay signals from receptors on the cell surface to target molecules inside the cell, in the cytoplasm or nucleus

• They can be:

Hydrophobic molecules: DAG, IP3

Hydrophilic molecules: cAMP, cGMP, IP3, and Ca2+

Gases: NO, CO, H2S

• Consequences:

The hormone signal may get amplified significantly

They can diffuse throughout the cellular compartments and can influence various processes

Multiple signaling pathways can use same second messenger leading onto ‘cross talk’.

Actions of cAMP

Several Hormones Act Through Calcium or Phosphatidyl Inositols

OxytocinVasopressinAngiotensin IITRH

NO and ANP Has cGMP as the Second Messenger

Insulin and IGF have Receptors With Tyrosine KinaseActivity [Enzyme linked receptors]

Ion Channel Linked Receptors:

Class I Hormones Have Nuclear/ Cytoplasmic Receptors and Can Activate or Repress Transcription:

The Number and Sensitivity of Hormone Receptors are Regulated

• The number of receptors in a target cell usually does not remain constant

• Down-regulation of the receptors:

Increased hormone concentration and increased binding with its target cellreceptors sometimes cause the number of active receptors to decrease by:

Inactivation of some of the receptor molecules

Inactivation of some of the intracellular protein signaling molecules

Temporary sequestration of the receptor to the inside of the cell

Destruction of the receptors by lysosomes after they are internalized

Decreased production of the receptors

In each case, receptor down-regulation decreases the target tissue’sresponsiveness to the hormone.

• Up-regulation of receptors :

The stimulating hormone induces greater than normal formation ofreceptor or intracellular signaling molecules by the protein-manufacturingmachinery of the target cell, or greater availability of the receptor forinteraction with the hormone.

When this occurs, the target tissue becomes progressively more sensitiveto the stimulating effects of the hormone.

Summary:

Class I

1. Cytoplasmic receptor Glucocorticoids

Estrogen

Progesterone

Testosterone

2. Nuclear receptor Vitamin A

Thyroid hormones

Class II

1. Ion channel linked receptor

2. Enzyme linked receptor Insulin Receptor- Tyrosine kinase

3. GPCR Gs stimulates adenyl cyclase

Gi inhibits adenyl cyclase

Gq activates Phospholipase C

INSULIN

AND

DIABETES

MELLITUS

Pancreas has Exocrine and Endocrine Functions:

Exocrine pancreas [Pancreatic acini]

• Trypsin, chymotrypsin, carboxypolypeptidase and proelastase

• α-amylase

• Pancreatic lipase

Endocrine pancreas [Islets of Langerhans]

Cell Percentage Hormones(s)

β cell 60 Insulin, Amylin

α cell 25 Glucagon

δ cell 10 Somatostatin

F cell - Pancreatic polypeptide

Insulin:

•Banting [NP 1923] and Best in 1921•2 Chains- A and B•Joined by disulfide bonds between cysteine residues•A- chain: 21 A.A•B- chain: 30 A.A•t1/2 of 6 minutes in the plasma•Catabolized by insulinases in liver, kidney and placenta

Insulin Synthesis

Pre-pro-insulin Proinsulin by microsomes

Pro-insulin [86 A.A]

Insulin [51 A.A] + C-peptide [31 A.A] + 2 Dipeptides

Golgi

Insulin Secretion:

Insulin Signaling:

Tyrosine Phosphatases

Ser/Thr kinases

Serine Phosphatases

PTEN

TERMINATION OF SIGNALING

Insulin is an Anabolic Hormone

• It increases transport of glucose into the muscle and adipose tissue

• Increases permeability of some amino acids, potassium, phosphate ions

• Alters activity of many cellular enzymes

• Alters gene expression

Insulin Action- Summary

Effects of Insulin on Carbohydrate Metabolism

• Stimulates:

Uptake of glucose into muscles and adipose tissue

Activity of the enzyme glucokinase trapping glucose inside the hepatocytes as G-6-P

Glycogen synthase [by inhibiting Glycogen synthase kinase]

Expression of glycolytic enzymes [PFK, PK]

• Inhibits:

Glycogen phosphorylase

Gluconeogenesis

Effect of Insulin on Tandem Enzyme [PFK-FBPase2]

Effects of Insulin on Lipid Metabolism

• Insulin promotes fatty acid synthesis in liver by stimulating Acetyl CoA carboxylase.

•These fatty acids enter the circulation as lipoproteins.

• Insulin stimulates lipoprotein lipase releasing free fatty acids from these lipoproteins, facilitating their storage within the adipose tissue and muscle.

• Insulin inhibits hormone sensitive lipase, inhibiting mobilization stored of fatty acids

Effects of Insulin on Protein Metabolism

• Insulin stimulates transport of amino acids like valine, leucine, isoleucine, phenyl alanine, tyrosine etc. into the cell

• Insulin inhibits catabolism of proteins

• Insulin inhibits gluconeogenesis, thus conserving protein stores of the body

• Insulin increases transcription and translation, thus affecting protein synthesis

DIABETES

MELLITUS

•Diabetes mellitus is a clinical syndrome characterized by hyperglycemia due to absolute or relative deficiency in insulin and/or its action.

•The classical symptoms include:

1. Polydipsia

2. Polyuria and

3. Polyphagia

Classification:

I. Type 1 diabetesa. Immune-mediated

b. Idiopathic

II. Type 2 diabetes

III. Other specific types of diabetesa. Genetic defects of beta cell function

b. Genetic defects in insulin action

c. Diseases of the exocrine pancreas

d. Endocrinopathies

e. Drug- or chemical-induced

f. Infections—congenital rubella, cytomegalovirus, coxsackie virus

g. Uncommon forms of immune-mediated diabetes

h. Other genetic syndromes sometimes associated with diabetes

IV.Gestational diabetes mellitus (GDM)

Diagnosis of Diabetes Mellitus

• Random Plasma Glucose of ≥200 mgdL-1 in a person with classical symptoms of Diabetes Mellitus Or

• HbA1c > 6.5 % Or

If the above combinations do not make a clear diagnosis, do an oral GTT

•If any 2 values in Oral GTT ≥200 mgdL-1 DIABETES MELLITUS

Fasting Plasma Glucose

2 hour Post PrandialPlasma Glucose

Day 1 ≥126 mgdL-1 ≥200 mgdL-1 DIABETES MELLITUS

Day 2 ≥126 mgdL-1 ≥200 mgdL-1

DIABETES MELLITUS

DIABETES MELLITUS

Type I DM is an Autoimmune Disease

• Islet destruction is caused primarily by immune effector cells reacting against endogenous β-cell antigens due to failure of self-tolerance in T-cells

• Pathogenesis of type 1 diabetes represents interplay of:

1. Genetic susceptibility: HLA-DR3, HLA-DR4 or HLA-DQ8 haplotype

2. Environmental factors: ? mumps, rubella, coxsackie B or CMV

• TH1 cells injure β cells by secreted cytokines, including IFN-γ and TNF.

• CD8+ CTLs directly kill β cells.

Type 2 DM is a Multifactorial Complex Disease

• Environmental and Genetic factors lead to Type 2 DM.

• The two metabolic defects that characterize type 2 diabetes are

(1) Insulin resistance: A decreased response of peripheral tissues to insulin and

(2) β-cell dysfunction: Manifested as inadequate insulin secretion in the face of insulin resistance and hyperglycemia

• Insulin resistance leads to decreased uptake of glucose in muscle, reduced glycolysis and fatty acid oxidation in the liver, and an inability to suppress hepatic gluconeogenesis.

• Loss of insulin sensitivity in the hepatocytes is likely to be the largest contributor to the pathogenesis of insulin resistance.

Obesity Leads to Development of Insulin Resistance

Lipolysis from adipose tissue [Central >> Peripheral]

Increased NEFA in circulation

Excess NEFA in muscles and liver

Excess intracellular NEFAs overwhelm the fatty acid oxidation pathways

Accumulation of diacylglycerol (DAG) and Ceramide

Activates Protein Kinase C Inhibits AKT

Activation of Serine and Threonine kinases

which phosphorylate Insulin receptor and IRS

Attenuation of Insulin Signaling

Obesity Leads to Development of Insulin Resistance

• Adipokines:

Leptin and adiponectin improve insulin sensitivity by directlyenhancing the activity of the AMP-activated protein kinase(AMPK), an enzyme that promotes fatty acid oxidation, in liverand skeletal muscle.

Adiponectin levels are reduced in obesity, thus contributing toinsulin resistance.

• Inflammation:

Adipose tissue also secretes a variety of pro-inflammatorycytokines like tumor necrosis factor, interleukin-6

These cytokines induce insulin resistance by increasingcellular “stress,” which in turn, activates multiple signalingcascades that antagonize insulin action on peripheral tissues

Metabolic Syndrome/ Syndrome X/ Insulin Resistance Syndrome:

• A constellation of metabolic abnormalities that confer increased risk of cardiovascular disease (CVD) and diabetes mellitus

• Criteria:

Three or more of the following:

1. Central obesity: Waist circumference >102 cm (M), >88 cm (F)

2. Hypertriglyceridemia: Triglycerides 150 mg/dL or specific medication

3. Low HDL cholesterol: <40 mg/dL and <50 mg/dL, respectively, or specific medication

4. Hypertension: Blood pressure 130 mm systolic or 85 mm diastolic or specific medication

5. Fasting plasma glucose 100 mg/dL or specific medication or previously diagnosed Type 2 diabetes

NCEP:ATPIII 2001

Metabolic Syndrome is an Inflammatory Disease

Beta Cell Dysfunction

Insulin Resistance

Increased Glucose levels in blood

Increased Insulin secretion by Beta Cells

Compromised ability of ER to process Insulin and Pro-insulin

[ER Stress]

Unfolded and misfolded proteins accumulate

Unfolded Protein Response

Unfolded Protein Response

Complications of Diabetes Mellitus: Mechanism

• Advanced Glycation End Products [AGE]

Formed by nonenzymatic reactions of intracellular glucose-derived dicarbonyl precursors (glyoxal, methylglyoxal, and 3deoxyglucosone) with the amino groups of proteins

Generates pro-inflammatory cytokines and ROS

Increase procoagulant activity on endothelial cells and macrophages

Enhance proliferation of vascular smooth muscle cells and synthesis of extracellular matrix

Microangiopathy

Complications of Diabetes Mellitus: Mechanism

• Activation of Protein Kinase C

Production of proangiogenic vascular endothelial growth factor (VEGF), implicated in the neovascularization characterizing diabetic retinopathy

Production of profibrogenic factors like TGF-β, leading to increased deposition of extracellular matrix and basement membrane material

Production of PAI-1, leading to reduced fibrinolysis and possible vascular occlusive episodes

Production of pro-inflammatory cytokines by the vascular endothelium

• Disturbances in Polyol Pathways

Long Term Complications

Treatment:

Lifestyle modifications

Diet modifications

Exercise

Drugs

Insulin

Oral Hypoglycemic Agents

Exercise

Increases fatty acid oxidation Increases biogenesis of mitochondria

Increases insulin sensitivity

THYROID

HORMONES

THYROID GLAND:

• 15-20 g

• Located in front of the trachea

• Secretes

1. Thyroxine [T4]

2. Tri-iodo thyronine [T3]

3. Calcitonin [by the parafollicular cells/ C-cells]

• T3 and T4 secretion are controlled primarily by TSH [ThyroidStimulating Hormone] from the anterior pituitary.

• Calcitonin- Calcium metabolism

Functional Anatomy:

• Follicles filled with colloid

• Rich blood supply

Chemistry of Thyroid Hormones

• T4:

93% of the hormones secreted.

Converted to T3 in the peripheraltissues- Liver, kidney etc.

• T3:

4 times more potent than T4.

• rT3:

Secreted in minimal amounts

Biologically inactive

Synthesis

1. Formation & secretion of Thyroglobulin2. Iodide trapping3. Oxidation of Iodide ion4. Iodination of Tyrosine

5. Coupling6. Pinocytosis7. Proteolysis8. Secretion

Formation & Secretion of Thyroglobulin

Thyroglobulin:

335 kDa glycoprotein

Synthesized by ER and Golgi apparatus

Forms the colloid

70 tyrosine residues

Iodide Trapping

• The minimum daily iodine to maintain normal thyroid function is150 μg in adults [1mg/week]

• Dietary iodide absorbed by the intestine enters the circulation

• In the basolateral membranes of thyrocytes, Na+/I– symporter[NIS] transports 2 Na+ ions and 1 I– ion into the cell against theelectrochemical gradient for I–.[Secondary active transport]

• Produces intracellular I– concentrations 20 to 40 times as greatas plasma concentration

• In the thyrocyte apical membrane, a Cl–/I– exchanger known asPendrin transports I– into the colloid.

22

Significance:

• Defect in Pendrin gene leads to Pendred syndrome, whosepatients suffer from thyroid dysfunction and deafness.

• TSH induces both NIS expression and the retention of NIS in thebasolateral membrane where it can mediate sustained iodideuptake.

• Thiocyanate and perchlorate ions competitively inhibit I–

transport via NIS.

Oxidation of Iodide ion

• Enzyme thyroid peroxidase [TPO] located in the apicalmembrane.

• Autoantibodies against TPO is associated with Hashimoto’sthyroiditis.

• Peroxidase is inhibited by Propyl thio-uracil and Carbimazole

Iodination of Tyrosine and Coupling

• Iodination:

Also called organification of thyroglobulin

By Iodinase

Forms MIT and DIT

Coupling:•MIT + DIT = T3

•DIT + DIT = T4

Pinocytosis, Proteolysis & Secretion:

• Thyroglobulin with hormones are stored in the gland.

• Hormones attached to Thyroglobulin are endocytosed and fusedwith lysosomes

• Lysosomal proteases will release T3 & T4 from Thyroglobulin

• They diffuse to the basal surface of the cell from where they aresecreted in the blood.

• Iodides inhibit endocytosis of colloid.

Synthesis

1. Formation & secretion of Thyroglobulin2. Iodide trapping3. Oxidation of Iodide ion4. Iodination of Tyrosine

5. Coupling6. Pinocytosis7. Proteolysis8. Secretion

Transport of Thyroid Hormones in Plasma:

• 99.98% of T4 and 99.8% of T3 in circulation is protein bound.

• Because of the high affinity to the binding proteins, T3 and T4

are released slowly in the tissues.

• Numerous factors like illness, medications and genetic factorscan influence protein binding.

• Hence assay of free hormones [f T3 and fT4] are betterindicators of thyroid function

ProteinPl. Conc.

[mg/dL]

% of bound T4 % of bound T3

Thyroxine-binding globulin (TBG)

2 67 46

Transthyretin (thyroxine-binding prealbumin, TBPA)

15 20 2

Albumin 3500 13 53

T3 is potent than T4

• 5’ Deiodinase:

Mainly in liver, kidneys, thyroid, brain and brown adipose tissue

Types 1, 2 and 3

Contains Selenocysteine

1 and 2 convert T4 to T3

Inhibited by Propranolol and Propyl thio-uracil.

• T4 to T3 enter the cell by passive diffusion and through special carriers.

Mechanism of Action: Nuclear Receptors

•TRα or TR β•Forms heterodimerwith Retinoic acid X Receptor [RXR]•Binds to Thyroid Response Elements [TRE]

(1) T4 or T3 enters the nucleus

(2) T3 binding dissociates Co-repressors (CoR) from TR

(3) Coactivators (CoA) are recruited to the T3-bound receptor

(4) Gene expression is altered.

(5) Synthesis of various proteins are altered in different rates.

Thyroid Hormones Increase Cellular Metabolic Activity

• Thyroid hormones increase the number and activity of mitochondria

• Increase the rate of ATP formation

• Thyroid hormones increase active transport of ions through cell membranes

Na+-K+ ATPase activity is increased

More energy utilization and heat production

Effects of Thyroid Hormone on Growth and Development

• Development of brain

– Hypothyroidism during fetal life, infancy or childhood >>> Cretinism

• Rate of growth is affected by the thyroid hormone status

• Effect on skeletal growth

Accelerated in hyperthyroidism

Inhibited in hypothyroidism

Other Effects:

• Carbohydrate metabolism:

– Stimulate

Uptake of glucose by the cells

Enhanced glycolysis and gluconeogenesis

Increased rate of absorption of glucose from the gastrointestinal tract

• Lipid metabolism:Lipolysis and thus free fatty acid concentration

β-oxidation

LDL Receptor expression in the liver

Decrease the cholesterol, phospholipids, and triglycerides in the plasma

Hypothyroidism >>> atherosclerosis

• Increase BMR

• Decrease body weight

• Increase expression of catecholamine receptors

Regulation of Thyroid Hormone Secretion:

TRH: from hypothalamus•Tripeptide amide—pyroglutamylhistidyl proline amide•Binds to TRH receptors in pituitary and enhances TSH release

TSH:from anterior pituitary•Glycoprotein•Two subunits α and β•α subunit is identical to the α subunitof LH, FSH, and hCG-α•β subunit is specific for TSH•TSH receptor: G protein-coupled, 7-transmembrane segment receptorthat activates adenylyl cyclasethrough Gs•Enhances iodide trapping, iodinationof tyrosine, size, activity and numberof thyroid cells

Thyroid Function Tests:

HORMONES

MAINTAINING

CALCIUM AND

PHOSPHATE

HOMEOSTASIS

Calcium:• Most prevalent cation in the body

• About 1100 g in an adult

• Distribution:

99% in bones

0.1% in Plasma

Remaining intracellular

• Plasma Calcium is present in 3 forms:

Non-DiffusibleDiffusible

Biological Reference Interval:

• The plasma and interstitial fluids have a calcium ion concentration of only one half the total plasma calcium concentration.

• This ionic calcium is the form that is important for most functions of calcium in the body, including the effect of calcium on the heart, the nervous system, and bone formation.

mmol/L mg/dL

Serum Calcium 2.2–2.6 8.7–10.2

Calcium, ionized 1.12–1.32 4.5–5.3

Functions of Calcium:1. Calcium activates enzymes

• Indirectly

• Or Directly Pancreatic lipase Enzymes of coagulation system

Functions of Calcium:

2. Binding of Calcium to Troponin I initiates muscle contraction

Functions of Calcium:

3. Nerve and Junctional Conduction

Calcium also decreases neuromuscular excitability

In hypocalcemia >>>>> Tetany

Functions of Calcium:

4. Secretion of Hormones: Insulin, PTH, Calcitonin, ADH

Functions of Calcium:

5. Second Messenger in Signal Tranduction

Functions of Calcium:

6. Coagulation:

– EDTA chelates Calcium-anticoagulant

7. In myocardium, Calcium prolongs systole:

– Intravenous Calcium is administered very slowly

8. Bone and Teeth formation

9. Vesicle degranulation, Endocytosis, Cell motility

Absorption and Excretion of Calcium

• Absorbed from 1st and 2nd part of duodenum by Calbindin and Ca2+

dependant ATPase.

• Absorption is enhanced by:

Calcitriol

PTH

Acidity

• Inhibited by:

Oxalates

Phytates

Phosphates

Fatty acids

• Unabsorbed Calcium is excreted by the intestines

• Plasma Calcium: Filtered in the kidneys, but 98–99% is reabsorbed.

• About 60% of the reabsorption occurs in PCT and the remainder in the ascending limb of the loop of Henle and the distal tubule.

Phosphate:

• 600 g in adults

85% in skeleton

15% in soft tissues

<1% is extracellular

• Phosphates can be

Organic: DNA, RNA, ATP, Creatine Phosphate, Phospholipids…

Inorganic [Pi]: H2PO42- and HPO4

- [1:4 at pH 7.4]

• Laboratory methods estimate inorganic phosphate only

• Biological Reference Interval: 2.5–4.3 mg/dL

• In Serum:

10 % is protein bound

35 % is complexed with Sodium, Calcium, Magnesium

55 % is free.

Functions

1. Major structural component of bone in the formof hydroxyapatite- Ca10 (PO4)6(OH)2

2. Phospholipids are major structural components of cellmembranes.

3. All energy production and storage are dependenton phosphorylated compounds, such as ATP and creatinephosphate.

4. Nucleic acids

5. Co-enzymes- NADP, NADPH

6. A number of enzymes, hormones, and cell-signaling moleculesdepend on phosphorylation for their activation.

7. Phosphorus also helps to maintain normal acid-base balance(pH) by acting as one of the body's most important buffers.

8. 2,3-DPG binds to hemoglobin in red blood cells and affectsoxygen delivery to the tissues of the body

Absorption and Excretion

• Pi is absorbed in the duodenum and small intestine.

• Uptake occurs by a sodium-dependent Pi

cotransporter, NaPi-IIb.

• Many stimuli that increase Ca2+ absorption, including 1,25-dihydroxycholecalciferol, increase Pi absorption via increased NaPi-IIb expression.

• Pi in the plasma is filtered and 85–90% of the filtered Pi is reabsorbed.

• Active transport in the PCT via two related sodium-dependent Pi cotransporters- NaPi-IIa and NaPi-IIc, accounts for most of the reabsorption

• NaPi-IIa is powerfully inhibited by parathyroid hormone.

v

Calcium and Phosphate homeostasis is maintained by:

1. Vitamin D

2. Parathyroid hormone [PTH] and

3. Calcitonin

Vitamin D:• Steroid hormone

• Acts via Nuclear receptor

• Provitamins:

Vitamin D3- cholecalciferol

Vitamin D2- ergocalciferol

• Vitamin D3 is formed in the epidermis from 7-dehydrocholesterol by the action of uv radiation

• Calcitriol [1,25- Dihydroxycholecalciferol] is the active form

Vitamin D:

• 25-hydroxylase is a microsomal enzyme

• 25-hydroxycholecalciferolis the major transport form

• It is bound to Vitamin D Binding Protein.

• 1α Hydroxylase is a mitochondrial enzyme

• Present in Proximal Convoluted Tubule.

Vitamin D Metabolism is Well Regulated:

• Feedback inhibition of 25-hydroxylase by 25-OH Cholecalciferol

• Role of kidneys

• PTH exerts a potent influence in determining the functional effects of vitamin D in the body

• At higher calcium concentrations, PTH is suppressed

• 25-hydroxycholecalciferol is converted to 24,25 -dihydroxycholecalciferol—that has almost no vitamin D effect

• ↓ plasma phosphate increases the activity of 1α–hydroxylase.

Actions of Vitamin D:

Intestine:

Vitamin D also increases absorption of Phosphate

Actions of Vitamin D:Kidneys:

Calcitriol facilitates Ca2+ reabsorption in the kidneys via increased TRPV5 expression in the proximal tubules

It also facilitates Phosphate reabsorption

Bone:

• Extreme quantities of vitamin D causes absorption of bone.

It is believed to result from the effect of 1,25-dihydroxycholecalciferol to increase calcium transport through cellular membranes.

• Vitamin D in smaller quantities promotes bone calcification by:

Increasing calcium and phosphate absorption from the intestines.

Transport of calcium ions through cell membranes—but in this instance, perhaps in the opposite direction through the osteoblastic or osteocytic cell membranes

Stimulates osteoblasts to synthesize the calcium-binding protein osteocalcin

Cholecalciferol is Considered as a Prohormone and Calcitriol as a Hormone

• Cyclo-pentano-perhydro-phenanthrene nucleus like a steroid hormone.

• Calcitriol has definite target organs - intestine, bone and kidney, where it specifically acts.

• Calcitriol action is similar to steroid hormones. It binds to a receptor in the cytosol and the complex acts on DNA to stimulate the synthesis of calcium binding protein.

• Like hormones, formation of both calcidiol and calcitirol are subject to feedback inhibition. Calcitriol synthesis is self-regulated by a feedback mechanism i.e., calcitriol decreases its own synthesis.

• Halflife of calcitriol is short (10 hours).

Parathyroid Hormone [PTH]

High Levels of Calcium Inhibits PTH Release and Vice Versa

Actions of PTH:

BONE Stimulates bone resorptionReleases calcium & phosphate

into blood

KIDNEY Increases reabsorption of

calcium & reduces reabsorption of phosphateNet effect is increased calcium

& reduced phosphate in plasma

INTESTINE Increases calcium

reabsorption via vitamin D

Calcitonin:

Synthesis and secretion of calcitonin occur in the parafollicular cells, or C cells

Actions

Calcitonin acts on bone osteoclasts to reduce bone resorption

Calcitonin also has minor effects on calcium handling in the kidney tubules and the intestines

Net result of its action is a decline in plasma calcium

Abnormal Ca and P Metabolism Can Lead to:

• Osteoporosis:

A condition of multiple causes in which bone volume is reduced and increased fractures are evident

A normal mineralization but a reduction in the volume of bone

• Rickets:

Nutritional rickets: Vitamin D Deficiency

Vitamin D-dependent rickets

• Type I : Due to deficiency of the renal 1 alpha-hydroxylase

• Type II: Due to intracellular vitamin D receptor (VDR) defects

Vitamin D-resistant rickets:

• X-linked dominant hypophosphatemic rickets

• Defect in renal phosphate reabsorption

Osteomalacia:

Adult counterpart of Rickets

Undermineralization and normal bone volume.

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