Calcium channels – basic aspects of their structure, function &

gene encoding; anesthetic action on the channels – a review

Tariq AlzahraniDemonstrator

College of Medicine King Saud University

Introduction

• Intracellular ca+2 conce. 100 nM.• Extracellular ca+2 conce. 1 mM.• Intracellular to extracellular of free ca+2 conce. is

1/10,000.• This conce. gradient is maintained by 3 main

mechanisms :• 1. Extrusion ( ca+2 – ATPase )• 2. Sequestration ( releasable & non releasable )• 3. Binding

• When the cells are stimulated by some receptor agonists, ca+2 conce. increases from resting level to approximately 1M.

• Intracellular ca+2 conc. may be elevated by opening membrane ca+2 channels or releasing ca+2 from intracellular (releasable) storage sites.

• There are 2 classes of ca+2 channels:• 1. Voltage- Sensitive (VDCCs)• 2. Receptor- Operated (Ligand- Gated ion

channels)

VDCCsClassification

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

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

T-type N-type L-type P/Q-type

R-type

Voltage dependence

LVA HVA HVA HVA IVA

Threshold activation(mV)

-70 -20 -30_-10 -60 -40

Inactivation range(mV)

-100_-60 -120_-30 -60_-10

Rate of inactivation(msec)

20_50 50_80 >500

Structure & Function

• L-type ca+2 channel :• It is high conce. in skeletal muscle.• It is composed of 5 different polypeptide subunits,

having different molecular masses:• 1.1(175KD) , which forms the ion channel &

contains ca+2 antagonist binding sites.• 2.2(143KD), which is associated with 1 & does

not contain any high-affinity binding site.• 3.(54KD), 4.(30KD), 5.(27KD).

• L-type ca+2 channels are linked to ryanodine receptor of sarcoplasmic reticulum.

• Abnormal ryanodine receptor causes malignant hyperthermia which is a hypermetabolic crisis triggered by suxamethonium & volatile anesthetics.

• However, as yet, there has been no report on the effects of anesthetics on abnormal L-type ca+2 channel activity in malignant hyperthermia.

• The functions of the L-type ca+2 channel are related to the generation of action potentials & to signal transduction events at the cell membrane.

• Except the platelets, L-type VDCCs are expressed ubiquitously in neuronal, endocrine, cardiac, smooth, & skeletal muscle, as well as in fibroblasts & kidney cells.

• Recent report suggest a role for L-type VDCCs in the process of neurotransmitter secretion of the central nervous system.

• N-type ca channel :• It is purified from the rat brain.• It is composed of 4 subunits: 1 , 2 , , & .• It plays a role in some forms of neurotransmitter

release.

• P/Q-type ca channel :• It is composed of 1, 2, & subunits.• Immunohistological studies have shown that the

P/Q-type channel is widely expressed in the mammalian central nervous system & that the channel appears to serve both as a generator of intrinsic activity & as a modulator of neuronal integration & transmitter release.

• T-type ca channel :• Because 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 gene

The main subunit 1 can function as ca+2 channel. Other subunits (2 / & ) contribute to the regulation of a ca+2 channel function by changing drug affinity & / or voltage dependence.

Isoform Gene name Chromosomal

localization

Tissue distribution Biophysical

properties

HVA

1A

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

IVA

1E CACNA1E 1q25-q31 Brain,neuronal cells R-type

LVA

1G

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)

• It is found on the plasma membrane & is composed of 4 or 5 subunits in various combinations depending on the particular receptor.

Effects of anesthetics on channel activity

Volatile anesthetics• Ikemoto first demonstrated in 1985 that halothane

decreased inward ca+2 slow currents in ventricular myocytes in rats, & then Terrar reported the inhibitory effect of halothane & isoflurane on ca+2 channels of cardiac myocytes from the guinea pig ventricle.

• In general , volatile 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.

• Volatile anesthetics do not alter the voltage dependence of the currents.

• Based on the percent anesthetic conces. in the gas phase, the order of inhibitory potencies for the currents is halothane > isoflurane / enflurane > sevoflurane.

• Single channel analysis has shown that halothane decreased the likelihood of channel opening & enhanced the rate at which the channel closed & became inactivated.

• Recent studies have revealed that the receptors for inhibitory neurotransmitters such as GABA & glycine are sensitive to volatile anesthetics at clinically relevant concentration.

Intravenous anesthetics

• Ikemoto also demonstrated the inhibitory effect of thiamylal on ca+2 inward current in rat ventricular cell.

• Propofol also has significant inhibitory effects on T & L- type components of the ca+2 current in cultured dorsal root ganglion neurons from chick embryos, this inhibition might play a role in cardiovascular side effect observed clinically.

• Ketamine in vitro showed inhibitory effects on activation & inactivation of ca+2 currents of ventricular myocytes in guinea pig, leading to the direct myocardiac depression. However , ketamine can support vascular tone & cardiac function presumably secondary to ketamine-induced catecholamine release.

• Also ketamine have their own binding site on the N-methyl-D-aspartat ( NMDA ) receotpr.

• The I.V anesthetics thiopental, ketamine & propofol all inhibited inward ca+2 currents through L- type VDCCs of porcine tracheal smooth muscle cells, demonstrating a cellular effect of these anesthetics that can account for their airway smooth muscle relaxant effects.

• Thiopental, ketamine & propofol showed similar effects on activation & inactivation of ca+2 currents; however, the concentration required to produce these effects appear to be substantially higher than the free conces. observed clinically in serum.

• Benzodiazepines have their own binding site on the GABAa receptor & the clinical effect of these drugs ( e.g, sedation, amnesia & anticonvulsion ) may be accounted for by these interaction.

• Other investigators have also found that benzodiazepines had inhibitory effects on L- type VDCCs in canine myocardial cells, in canine tracheal smooth muscle cells & porcine intestinal mucosa cells.

Local anesthetics

• Lidocaine at clinically relevant conces. has been shown to inhibit inward ca+2 currents in Helix 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.

• The inhibition is voltage-dependent & the peak amplitude of the ca+2 current cannot be restored to the control level by washout.

• Note : the inhibition by local anesthetics of VDCCs in cardiac myocytes might contribute to local anesthetic-induced cardiodepression.

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

Anesthetic interaction

Volatile

intravenous

Sensitive

Sensitive

Sensitive

Sensitive

Controversial

Controversial

Unknown

Unknown

Sensitive

Controversial

Conclusions

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

• Increase in conce. 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.

• The ca+2 channel can be divided into subtypes according to their electrophysiological characteristics & each subtype is encoded by its own gene.

• The effects of various kinds of anesthetics in a variety of cell types have been demonstrated & a number of clinical effects of anesthetics can be explained by their effects on ca+2 channels.

• Ligand-Gated ion channel is very important from the anesthetic viewpoint in that the nicotinic acetylcholine receptor is the target for neuromascular relaxants, the NMDA receptor is the target for ketamine & the GABAa receptor is a major target for a range of inhalation & intravenous general anesthetics agents ( excluding ketamine ).