Calcium channels physiology and Therapeutics uses..

Dr . Kapil Dev Doddamani.

• Function.

• Types of Calcium channels.

• Channelopathies.

• Therapeutics Uses of Calcium .

Function• Signal transduction pathways, second messenger

• Neurotransmitter release from neurons

• Contraction of all muscle cell types

• Many enzymes require calcium ions as a cofactor (blood-clotting

cascade)

• Extracellular calcium is also important for maintaining the potential

difference across excitable cell membranes, as well as proper bone

formation.

•

Function

Ventricular AP Function• Phase 4: resting membrane

potential near the K+ equilibrium potential.

• Phase 0: depolarizing impulse activates fast Na+ channels and inactivates K+ channels.

• Phase 1: Transient opening of K+ channels and Na+ channels begin to close.

• Phase 2: Ca2+ channels are open, key difference between nerve AP.

• Phase 3: repolarization, Ca2+ inactivate and K+ channels open.

• Refractory period: Na+ channels are inactive until membrane is repolarized.

FUNCTION

The synthesis and release of insulin is modulated by:

1. Glucose (most important), AAs, FAs and ketone bodies stimulate release.

2. Glucagon and somatostation inhibit relases

3. α-Adrenergic stimulation inhibits release (most important).

4. β-Adrenergic stimulation promotes release.

5. Elevated intracellular Ca2+promotes release.

Example of how an endocrine cell(pancreatic β-cell) depolarizes itsmembrane with Ca2+ to release insulin.

Classes of Ca+2 channels

– Voltage- Sensitive (VDCCs)

– Receptor- Operated (Ligand- Gated ion channels)

– Leakey channels

VDCCs

• The possible existence of VDCCs was first reported by

Hagiwara in 1975 using egg cell membrane of a starfish.

• They were initially divided into 2 classes HVA & LVA ca+2

channels.

• HVA ca+2 channels are further divided into L,N,P/Q & R-types

channels,

• LVA ca+2 channels consists of only T-type channels.

• R-type is occasionally classified as ( IV A ) channels.

Structure & Function

L-TYPE Ca+2 CHANNEL

• It is composed of 5 different polypeptide subunits, different mol

masses

i. 1(175KD) , which forms the ion channel & contains ca+2

antagonist binding sites.

ii. 2(143KD), which is associated with 1 & does not contain any

high-affinity binding site.

iii.(54KD),

iv.(30KD),

v. (27KD).

L-TYPE Ca+2 CHANNEL

N-TYPE Ca+2 CHANNEL

• It is purified from the rat brain.

• It is composed of 4 subunits:

1 , 2 , , & .

• role -- neurotransmitter release.

P/Q-TYPE Ca+2 CHANNEL• It is composed of 1, 2, & subunits.

• Play similar role - N-type calcium channel (NT release at e presynaptic

terminal & neuronal integration in many neuronal types.

• They are also found in Purkinje fibers in the electrical conduction system of

the heart.

• P channels were discovered in cerebellar Purkinje cells by Llinas and

Sugimo

T-TYPE Ca+2 CHANNEL

• T-type VDCCs are activated at negative membrane potentials

close to the resting potential.

• the T-type channel is thought to be responsible for neuronal

oscillatory activity, which is proposed to be involved in process

such as sleep / wakefulness regulation & motor coordination.

• In addition ,T-type ca+2 channels are involved in pacemaker

activity.

CHANNEL GENEIsoform Gene name Chromosomal

localization

Tissue distribution

Biophysical

properties

HVA1A

1B

1C

1D

1F

1S

CACNA1A

CACNA1B

CACNA1C

CACNA1D

CACNA1F

CACNA1S

19p13.1-2

9q34

12p13.3

3p14.3

Xp11.23

1q31-q32

Brain,neuronal cells,heart

Brain,neuronal cells

Ubiquitous

Brain,neuronal,cells,endocrine cells

Skeletal muscle

P / Q –type

N-type

L-type

L-type

L-type

L-type

IVA1E CACNA1E 1q25-q31 Brain,neuronal

cellsR-type

LVA1G

1H

1I

CACNA1G

CACNA1H

CACNA1I

17q22

16p13.3

22q13

Brain

Kidney, liver , heart

Brain

T-type

T-type

T-type

Receptor – Operated Channels( Ligand – Gated Ion Channels)

• Independent of membrane depolarization

• It is found on the plasma membrane

• composed of 4 or 5 subunits in various combinations depending

on the particular receptor.

LIGAND – GATED ION CHANNELS

Type Gated by Genes Location Function

IP3 receptor IP3

ITPR1, ITPR2, ITPR3 ER/SR

Releases calcium from ER/SR in response to IP3 by

Ryanodine receptor

Dihydropyridine receptors in T-tubules and increased intracellular calcium (CICR)

RYR1, RYR2, RYR3

ER/SR Calcium-induced calcium release in myocytes

Cation channels of sperm

store-operated channels

indirectly by ER/SR depletion of calcium

ORAI1, ORAI2, ORAI3

plasma membrane

LEAKEY Ca+2 CHANNELS

• small amount of Ca+2 leak into resting cell and pump out by

Ca+2 ATPase

• Mechanical stretch promotes inward movement in Ca+2

occurring through activation of leaky channels or separate

stretch sensitive channels.

CHANNELOPATHIES

Hypokalemic periodic paralysis Voltage-gated Na+2 or Ca+2 channel

Malignant hyperthermia Ligand-gated Ca+2 channel

Timothy syndrome Voltage-dependent Ca+2 channel

CHANNELOPATHIES

HYPOKALEMIC PERIODIC PARALYSIS

MUTATED GENE CALCL1A3 SCN4A

CHROMOSOME 1q31 17q

DEFECTIVE CHANNEL

CALCIUM SODIUM

MODE OF INHERITENCE

AUTOSOMAL DOMINANT

TYPE 1 TYPE 2

CHANNELOPATHIES HYPOKALEMIC PERIODIC PARALYSIS

Prevelance 1:100,000

Symptoms during attacks Acute onselt flaccid paralysisProximal >>> distal

Triggers High carbohydrate,High salt, Drugs- beta agonists,InsulinRest following prolonged exercise

CHANNELOPATHIES

Malignant hyperthermia

• Mutation of the ryanodine receptor (type 1), located on the

sarcoplasmic reticulum , that stores calcium.

• RYR1 opens in response to increases in intracellular Ca2+ level

mediated by L-type

• RYR1 has two sites for reacting to changing Ca2+ concentrations; A-

site and the I-site.

Malignant hyperthermiaSkeletal muscle Rigidity and weakness

RhabdomyolysisMuscle spasms especially affecting Masseter, but canbe generalisedmyalgia

Autonomic Sympathetic overactivityHyperventilationTachycardiaHaemodynamic instabilityCardiac arrhythmia

Laboratory Increased oxygen consumptionHypercapniaLactic acidosisRaised creatine kinaseHyperkalaemia

Malignant hyperthermiaTriggers Full episodes: general anaesthesia (inhalational

agents— isoflurane, desflurane,) suxamethoniumMilder malignant hyperthermia: exercise in hotconditions, neuroleptic drugs, alcohol, infections

Treatment Dantrolene 2 mg/kg intravenously every 5 minutes toa total of 10 mg/kgAvoid calcium, calcium antagonists, b-blockers

Timothy syndrome

• AD.

• classical (type-1) and atypical (type-2).

• Physical malformations, as well as neurological and developmental

defects.

• They are both caused by mutations in CACNA1C, the gene

encoding the Ca2+ α subunit.

• Mutations in CACNA1C cause delayed channel closing & thus

increased cellular excitability.

THERAPEUTICS USES OF Ca+2 CHANNELS

• Calcium channel blockers (CCBS).

• Calcium Channels role in Anesthetics.

• Antiepileptic

• Prophylaxis of Migraine.

• Rx of infestation.

• Other roles

USES OF Ca+2 CHANNELS

CALIUM CHANNEL BLOCKERS

CCBS MECHANISM OF ACTION

• block calcium entry into cardiac and vascular smooth muscle at

the alph1 subunit of the L-type voltage-gated calcium ion

channels (slow channels)

• Increase the time that Ca 2+ channels are closed

USES OF Ca+2 CHANNELS

Ca+2 CHANNELS ROLE IN ANESTHETICS

MECHANISM OF ACTION

• volatile inhalational anesthetics at clinically relevant conces. inhibit

inward currents through VDCCs in a dose-dependent manner

without an apparent change in the time course of activation or

inactivation.

• The I.V anesthetics thiopental, ketamine & propofol all inhibited

inward ca+2 currents through L- type VDCCs of porcine tracheal

smooth muscle cells

USES OF Ca+2 CHANNELS

Local anesthetics

Mechanism

• Lidocaine at clinically relevant conces. has been shown to inhibit

inward ca+2 currents in ganglionic neurons & in frog dorsal root

ganglionic cells.

• Lidocaine, tetracaine & bupivacaine also inhibit the VDCC activity of

cardiac myocytes in the chick, guinea pig & hamster, respectively.

USES OF Ca+2 CHANNELS

As Antiepileptic ..Valproic acid (Na valproate) Ethosuximide

Absence seizures, GTCS, CPSJuvenile myoclonic epilepsy,Lennox-Gastaut syndrome,second-line treatment of status epilepticus,post-traumatic epilepsy.(neurodegenerative diseases such as Alzheimer's disease and Huntington's disease)

Absence seizures

Anorexia, vomiting drowsiness, ataxia Hypersensitivity rashes, blood dyscrasias.

•Blocks voltage-gated sodium channels & T-type calcium channels.•Affect the function of the neurotransmitter GABA•Inhibitor of the enzyme histone deacetylase 1

Reduced low-threshold Ca2+ currents in T-type Ca2+ channels in thalamic neuron

USES OF Ca+2 CHANNELS

Prophylaxis of Migraine.

Flunarizine.• non-selective calcium entry blocker + histamine H1 blocking

activity.

• Also Na channel blocker

SE;

Sedation, constipation, dry mouth, wt gain, extrapyramidal

effects, drowsiness.

USES OF Ca+2 CHANNELS

Infestation treatment• Praziquantel

– Rx Tape worms, flukes worms.

Mechanism --increases the permeability of the membranes of cells

towards calcium.

SE-• dizziness, headache, and malaise, drowsiness, somnolence,

fatigue, and vertigo.• Urticaria, rash, pruritus

Summary

• Intracellular free ca+2 is important for regulation of cell function.

• Increase in concen. of intracellular free ca+2 can be obtained by

rapid but transient ca+2 release from intracellular ca+2 stores &

by slow ca+2 influx from the extracellular space.

• VDCCS serve as one of the important mechanisms for ca+2

influx into the cells, enabling the regulation of intracellular free

ca+2 concentration.

Summary L N P/Q R T

VA HVA HVA HVA IVA LVA

location heart Neuronal Neuronal Neuronal Heart

function Contraction Release Release Release Pacemaker

The end..

Thanks.