Calcium as a Regulatory and Signaling ion

Chiranjeevi Kumar

Tutor/Demonstrator

Dept of Physiology

AIIMS Bhopal

Calcium facts

Calcium history

Plasma calcium levels

Cellular Distribution

Ca2+ channels

Ca2+ binding proteins

Role of calcium as a regulatory and signaling ion

Calcium homeostatsis

Research on Calcium

Applied aspects

Summary

Calcium facts 2% of body weight

99% in bones

1% in body fluids

Soft grey alkaline earth metal

Occurs naturally in limestone, gypsum, and fluorite

Symbol Ca & Single oxidation state +2

Atomic Number 20 & Atomic weight 40 g/mol

Group II element in Periodic table

Divalent cation

Fifth most abundant element in Earth´s crust & Essential for life.

Calcium history Latin calx or calcis meaning ”lime”

Known as early as in first century when ancient Romans prepared lime as calcium oxide

Isolated in 1808 by Englishman Sir Humphrey Davy

In 1883 Sydney Ringer demonstrated the biological significance of calcium

Frog hearts needed the presence of calcium in the bathing solution in order to continue beating

Sir Humphry Davy, 1st Baronet (17  December  1778 –  29  May  1829)  was a Cornish chemist and  inventor. He  is  best  remembered  today  for  his  discoveries  of several alkali and alkaline earth metals as well as contributions to the discoveries of the elemental nature of chlorine and iodine.

He also electrolyzed a mixture of lime & mercuric acid

 Plasma calcium levels In humans the concentration of calcium in the blood is – 9 - 11

mg/dL. Calcium in plasma or serum exists in three forms or fractions: 1) Protein-bound calcium 2) Ionized or free calcium 3) Complexed or chelated calcium

Ionized or free calcium is the physiologically active form 

Complexed or chelated calcium is bound to phosphate, bicarbonate, sulfate, citrate, and lactate 

Protein-bound calcium cannot diffuse through membranes and thus is not usable by tissues

Cellular DistributionIonic cytosol Ca is maintained at about 10-7 molar.

The differential electrical charges across the cell plasma membrane creates an electrical gradient that also favors Ca entry.

Therefore, the major threat to cell viability is excessive Ca influx from the extracellular space along the electrochemical gradients.

ECF Ca is 10-3 molar, the 1000-fold chemical gradient favors Ca entry into the cell

The defense against excess Ca influx into cells includes

Ca2+ channels Ligand gated.

Calcium channels opened after ligand binding to the receptor (e.g.

glutamate/NMDA receptor;ATP receptor; nicotinic ACh receptors ; prostaglandin

receptors

Voltage gated.

Action potental depolarizes plasma membrane, which results in the opening of

“voltage”dependent calcium channels

Each channel protein has four homologous domains, each containing six

membrane spanning α-helices (the fourth one functions as the “voltage” sensor Transient.

Long-lasting.

Store operated calcium channels.

Activated by emptying of intracellular stores, exact mechanism unknown

Type Properties Location/Function Blockers

L High activation threshold;slow inactivation

Plasma membrane of many cells;main Ca++ source for contraction insmooth and cardiac muscle

Dihydropyridine;verapamil; diltiazem

N Low activation threshold;slow inactivation

Main Ca++ source for neurotransmitterrelease by nerve terminals

ω-Conotoxin(snail venom)

T Low activation threshold;fast inactivation

Widely distributed; important in cardiac pacemaker and Purkinjecells

Mibefradil; (verapamil;diltiazem)

Ca2+ binding proteins

• Troponin.

• Calmodulin.

• Calbindin.

• Calexcitin

• Prothrombin.

• Phospholiphase A2.

• Ca2+ ATPase.

• Calsequestrin.

• Synexin

Role of calcium as a regulatory and signaling ion

Formation of bones and teeth

As a cofactor for many enzymes and proteins

As component in the blood clotting cascade

In the relaxation and constriction of blood vessels

In muscle protein degradation

In secretion of hormones as insulin

In nerve impulse transmission

As a Second messenger.

In Learning and memory.

In Muscle contraction.

In fertilization

In immune response

In Gene regulation

In Cell injury

In Modulation of ion channel activity

In visual adaptation

Role of calcium as a regulatory and signaling ion

Formation of bones and teeth

Calcium is a major structural element in bones and teeth.

Bone also acts as a store of calcium for other body functions.

This complex provides the hard and rigid structure of bone which is essential to its function.

In bone calcium and phosphate combined together in the crystalline complex; hydroxyapatite [Ca10(PO4)6 (OH)2].

Bone is a dynamic tissue that is 'remodeled' throughout life due to osteoclasts and osteoblasts

Calcium as a co-factor needed for the full activity of many enzymes, such as nitric oxide synthase, protein phosphatases, and adenylate kinase, but calcium activates these enzymes in allosteric regulation in a complex with calmodulin

catalyzing the production of nitric oxide (NO) from L-arginine. NO is an important cellular signaling molecule. It helps modulate vascular tone, insulin secretion, airway tone, and peristalsis

 Protein kinases (PKs) are the effectors of phosphorylation

is a phosphotransferase enzyme that catalyzes the interconversion of adenine nucleotides, and plays an important role in cellular energy homeostasis.

Calcium As A Cofactor For Many Enzymes

Intrinsic Pathway and Protein Complex

Role of Calcium in Blood Coagulation

Extrinsic Pathway and Protein Complex

Role of Calcium in Blood Coagulation

Role of Calcium in Blood CoagulationClotting factors (thrombin, VII, IX and X) contain a unique modified glutamate residue, called carboxyglutamate (Gla).

This amino acid is a natural high affinity binder (or chelator) of calcium ions, hence the designation of calcium as a co-factor in the blood clotting cascade.

Synthesis of these Gla residues results from post-translational modifications of the newly synthesized factors in the liver endoplasmic reticulum by a vitamin K

Calcium - Gla-factors complex allow specific interactions with acidic membrane lipids that ultimately lead to correct tertiary and quaternary protein structures recognized by other proteins in the pathway.

Role of calcium In the constriction and relaxation of blood vessels

• Contraction in VSM can be initiated by

Passive stretching of VSM can cause contraction that originates from the smooth muscle itself and is termed as a myogenic response

The mechanism of contraction involves different signal transduction pathways, all of which converge to increase intracellular calcium.

chemical stimuli such as norepinephrin, vasopressin, endothelin-1, angiotenisin 2 and thromboxane A2 can cause contraction

Electrical depolarization of the VSM cell membrane also elicits contraction, most likely by opening voltage dependent calcium channels (L-type calcium channels)

VSM relaxation occurs when there is reduced phosphorylation of MLC. 1) reduced release of calcium by the SR or reduced calcium entry into the cell, 2) inhibition of MLCK by increased intracellular concentration of cAMP, and 3) phosphatase-activated MLC dephosphorylation.

A calpain is a protein belonging to the family of calcium-dependent nonlysosomal cysteiene proteases expressed ubiquitously in mammals and many other organisms.

Although the physiological role of calpains is still poorly understood but a transient and localized influx of calcium into the cell activates a small local population of calpains close to Ca2+ channels

Calpains have been implicated in apoptotic cell death, and appear to be an essential component of necrosis.

Calpain

Role of calcium In muscle protein degradation

These calpains activates signal transduction pathway and catalyzing the controlled proteolysis of its target proteins

Role of Calcium In secretion of hormones as insulin

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Role Of Ca In Release Of Neurotransmitter

Role Of Ca In Release Of Neurotransmitter

Calcium influx is necessary for neurotransmitter release and Post synaptic potentiation

Voltage-gated calcium channels

Sutherland Second Messenger Hypothesis

First Messenger:Neurotransmitters

(Receptor)

AGT, GnRH, GHRH,Oxytocin, TRH

Epinephrine (α1)Acetylcholine (M1, M3)

Signal Transducer

First Messenger:Hormones

Primary effector Phospholipase C

GPCR/Gq

Catalyses PIP2Secondary effector

Second messengerIP3; DAG; Ca2+PKC; CaM

Cellular Response

Calcium – A Versatile Second Messenger

Protein kinase C:

 Regulatory domain & catalytic domain tethered together by a hinge region

C1 domain, present in all of the isoforms of PKC has a binding site for DAG

C2 domain acts as a Ca2+ sensor

Catalytic Region brings about phosphorylation Ser/Thr a.a. of proteins

Upon activation, translocated to the plasma membrane

 

Cell type EffectsSmooth muscle (vascular)

Vasoconstriction

Smooth muscle (GIT)

Contraction

Smooth muscle (bronchi)

Bronchoconstriction

Smooth muscle (ureter/ urinary bladder/ urethral sphincter)

Contraction

Platelets Aggregation

Smooth muscle cells in

Ciliary muscle Contraction

Iris constrictor Constriction

Cellular responses of Protein kinase C

Cell type Effects

Cardiomyocytes Positive ionotropic effect

Hepatocyte Glycogenolysis,Gluconeogenesis

Adipocyte Glycogenolysis,Gluconeogenesis

Proximal Convoluted tubule Stimulate H+ secretion & Na+ reabsorptionStimulate basolateral Na+-K+ ATPase →Na+ reabsorption

neurons in CNSneurons in autonomic ganglia

neuronal excitationEPSP

sweat gland cells ↑secretion

ependymal cells (choroid plexus) ↑cerebrospinal fluid secretion

parietal cells ↑ gastric acid secretion

When calmodulin binds Ca2+, it is capable of activating five different calmodulin-dependent kinases

One of the kinases is myosin light-chain kinase, which phosphorylates myosin. This brings about contraction in smooth muscle

CaMKI is concerned with synaptic function

CaMKIII is concerned with protein synthesis

Another calmodulin-activated protein is calcineurin, a phosphatase that dephosphorylates NFATC. It also plays a prominent role in activating T cells.

Calmodulin-dependent Kinases

CaMKII is concerned with neurotransmitter secretion, transcription factor regulation & glycogen metabolism

• LTP relies on calcium influx at NMDA glutamate receptors

• Calcium channels controlled by the NMDA receptor are blocked by a magnesium ion– Magnesium ion is ejected by:

1. simultaneous glutamate binding AND

2. depolarization of the post-synaptic cell (by activity at AMPA receptors on the membrane)

Role of Ca2+ in long term potentiation

Role of Ca2+ in long term potentiation

Role of Ca in Skeletal muscle contraction

Role of Ca in Skeletal muscle contraction Role of Ca in Skeletal muscle contraction

Role of Ca in Skeletal muscle contraction

Relaxation occurs when Ca2+ is reaccumulated in the sarcoplasmic reticulum by the Ca2+ ATPase of the sarcoplasmic reticulum membrane ) SERCA(

Role of Ca in Cardiac muscle contraction

Role of Ca in Smooth muscle contraction Role of Ca in smooth muscle contraction

Smooth muscle does not contain the protein troponin;instead calmodulin, caldesmon and calponin are significant proteins expressed within smooth muscle.

Caldesmon has been suggested to be involved in tethering actin, myosin and tropomyosin, and enhance the ability of smooth muscle to maintain tension.

Calponin molecules may exist in equal number as actin, and has been proposed to be a load-bearing protein.

Role Of Calcium In Fertilization

During ovulation mammalian eggs are arrested at metaphase of their second meiotic division and remain arrested until fertilized.

These ca2+ spikes can be termed as Ca2+ oscillations which switches on calmodulin-dependent protein kinase II (CamKII), which phosphorylates the egg-specific protein Emi2.

At the time of fertilisation sperm delivers phospholipase C into the egg which triggers a series of Ca2+ spikes lasting several hours

These Ca2+ spikes are necessary for all the events of fertilization, including exit from metaphase II arrest and extrusion of cortical granules that block the entry of other sperm.

Role Of Calcium In Fertilization To remain in metaphase II, arrested eggs must maintain high levels of

Maturation-Promoting Factor (MPF) activity, a heterodimer of CDK1 and cyclin B1.

Emi2 causes blocking of cyclin B1 results in degradation of MPF

CamKII also acts as the primary initiator in the extrusion of cortical granules.

Role of calcium in Immune response

TCR stimulation

[Ca++] increases

NFATc translocates to nucleus where it combines with NFATn and induces transcription of IL-2 gene

Calcineurin: target of immunesuppressive drugs FK506 and cyclosporine, which form a complex with immunophillins and

compete with Ca++/CaM for binding to calcineurin

dephosphorylates NFATc

Binds to Calmodulin and activates Calcineurin

T cell proliferation

no NFATc activation

Role of calcium in Immune response

Role Of Calcium In Gene Expression

Hogan P G et al. Genes Dev. 2003;17:2205-2232

Cold Spring Harbor Laboratory Press

Role Of Calcium In Cell Injury

Causes of Cell InjuryOxygen Deprivation (Anoxia)

Physical Agents

Chemical Agents

Infections Agents

Immunologic Reactions

Genetic Defects

Nutritional Imbalances

Injury mechanismsDecreased Atp

Mitochondrial Damage

Increased Intracellular Calcium

Increased Free Radicals

Increased Cell Membrane Permeability

Role of calcium In Modulation of ion channel activity

• Calcium-activated potassium channels are divided into BK channels, IK

channels, and SK channels based on their conductance (big, intermediate,

and small conductance).

• This family of ion channels are activated by intracellular Ca2+.

• Calcium-activated chloride channels (CaCCs) play important roles in

cellular physiology, including

– epithelial secretion of electrolytes and water,

– sensory transduction,

– regulation of neuronal and cardiac excitability, and

– regulation of vascular tone.

Calcium-activated potassium channels

SK channels are activated by an increase in the concentration of intracellular calcium

through N-type calcium channels. Their activation limits the firing frequency of action

potentials and is important for regulating afterhyperpolarization in the neurons of

the central nervous system.

BK channels are essential for the regulation of contraction of smooth muscle and are

involved with the electrical tuning of hair cells in the cochlea.

IK channel  is expressed mainly in peripheral tissues such as those of

the haematopoietic system, colon, placenta, lung andpancreas. The IK channel in red

blood cells was the first Ca2+–sensitive K+ channel to be identified 

• Among all, large-conductance (BK) channels is much more sensitive to Ca2+

Each BK channel alpha subunit consists of (from N- to C-terminal):

A voltage sensing domain (S1-S4).

A K+ channel pore domain (S5, selectivity filter, and S6).

A cytoplasmic C-terminal domain (CTD) consisting of a pair of RCK (Regulator of Conductance of K+) domains

Second RCK domain. contains four primary binding sites for Ca2+, called "calcium bowls"

Role of calcium In Modulation of ion channel activity

Phototransduction

•Starts with photon absorption by rhodopsin

•Transducin  binds to activated rhodopsin , exchanges GTP for GDP

•Activated transducin dissociates into  and  subunits 

•The  subunit binds to, and activates, phosphodiesterase

•Intracellular cGMP concentration decreases

•Reduction in cGMP closes cGMP-gated cation channels in the plasma membrane 

•Membrane potential hyperpolarizes

•Closing of cGMP-gated channel reduces intracellular calcium

•Reduced calcium counteracts the effects of light absorption

Role of Ca2+ in visual adaptation

Role of Ca2+ in visual adaptation

Calcium homeostatsis

Exercise and Calcium

• Normal bone function requires weight-bearing exercise

• Regular physical activity has been associated with many positive

health benefits including strong bones.

• Proper calcium consumption & adequate weight-bearing

physical activity early in life is important in reaching peak bone

mass.

• Weight-bearing physical activities cause muscles and bones to

work against gravity

• Lack of weight-bearing exercise decreases bone formation

• Total bed-rest causes bone loss and negative calcium balance

Research on CalciumSee in particular:

• Calvo MS et al., “overall trend in food consumption in the US is to drink

less milk and more carbonated soft drinks.” Nutrition 2000 Vol 16 (7/8).

• Calvo MS et al., “ High sodium associated with fast-food consumption

competes for renal reabsorption of calcium and PTH secretion “

• Harland BF et al., “ Caffeine induced calcium loss “. Nutrition 2000 Vol 16

(7/8)

• Intake of carbonated beverages (soda pop) has been associated with

increased excretion and loss of calcium

• Excessive intake of Na may cause renal hypercalciuria by impairing Ca

reabsorption

Applied aspects

Hypocalcemia

Hypercalcemia

Lambert – Eaton syndrome

COFFIN – LOWRY syndrome (RPS6kA3)

Timothy's syndrome (Long QT syndrome) (LTQ1 – 13)

WOLFRAM syndrome (DIDMOAD)

Toxicology

• The UL for calcium is 1200 - 1500 mg/day

• MAS (Milk alkali syndrome)

- Rare and potentially life threatening condition in individuals

consuming large quantities of calcium and alkali

- Characterized by renal impairment, alkalosis and hypercalcemia:

cause progressive depression of the nervous system

Final word

•Calcium

A “vital life element”

not to be ignored

A very exciting area

for research

Thank you for your attention

• Rolein surfactant• Role in peripheral chemoreceptors in chemical 

regulation of respiration