Voltage-Gated Calcium Channels

Dr. Ren-Zhi ZhanInstitute of PhysiologyShandong University School of MedicineE-mail: zhan0001@sdu.edu.cn

Contents

Classification of voltage-gated calcium channelsStructures of voltage-gated calcium channelsFunctions of voltage-gated calcium channels

What Are Ion Channels?

• Ion channels are ion-permeable pores in the lipid membranes of all cells

• Ion channels open and close in response to stimuli, gating the flow of specific ions

• The ions flow downhill thermodynamically

• http://www.scholarpedia.org/article/Ion_channels

ion-permeable pores

Functions of Ion Channels

• Setting up resting membrane potential in all cells

• Excitability• Cell signaling • Excitation-contraction coupling

• Excitation-secretion coupling

• Fluid transport, volume regulation

Basic Properties of Ion Channels

– Ion selectivity (离子选择性 )

–Gating (门控性 )

GlossaryConductance（电导） : The ability of something

(e.g. a wire or an ion channel) to pass current. Conductance (G) is measured in Siemens (S).

Gating（门控性） : Conformational changes in an ion channel induced by an extrinsic source (i.e., voltage, ligands, stretch, etc.). Such conformational changes are responsible for functional properties like activation and inactivation.

Activation (激活 ): Opening of a channel due to the presence of a gating signalDeactivation (去活 ): Closing of a channel due to removal of the gating signal (i.e. the opposite of activation)Inactivation (失活 ): Closing of a channel in the continued presence of the gating signal. The term "inactivation“ is usually only applied to voltage-gated channels, whereas "desensitization" describes the analogous process for ligand-gated channels. Once in the inactivated state, the channels can only be reopened by re-polarization to hyperpolarized membrane potentials, allowing the voltage sensor to return to its original closed conformation and the inactivation machinery to return to its de-inactivated position.Desensitization (脱敏感化 ): Closing of a ligand-gated channel despite the presence of a bound activating ligand. For example, glutamate receptors desensitize in the continued presence of glutamate.

Tail current (尾电流 ): A current that flows during the repolarizing phase of an action potential or voltage command. K+ tail currents can be used to determine the reversal potential of voltage-gated K+ currents. In a physiological context, tail currents are often carried by Ca2+ ions and result from the increased driving force as the action potential repolarizes.

Access resistance (接触电阻 ): The electrical resistance between the inside of the patch pipette and the inside of the cell during a whole-cell recording. Compromises recordings by introducing a voltage-divider error and slowing the response time of the voltage clamp. Access resistance can be reduced by using larger patch pipettes and can be compensated electronically with the patch clamp amplifier.

Rectification（整流） : current passes in one direction better than it does in another

Types of Ion Channels

• Voltage-gated ion channels– Voltage-gated sodium channels– Voltage-gated potassium channels– Voltage-gated calcium channels

• Ligand-gated ion channels– Nocotinic acetylcholine receptors– GABAA receptors– NMDA/AMPA/kainate types of glutamate receptors– 5-HT3 receptors– Glycine receptors

• miscellaneous– Mechanosensitive ion channels– Thermosensitive ion channels

Basic Structure of Voltage-Gated Calcium Channels

Cain & Snutch,2010

The alpha subunit is responsible for basic electrophysiological and pharmacological properties

The auxiliary subunits have regulatory roles

Principal one alpha subunit and several auxiliary subunits

Classification of Voltage-Gated Calcium Channels according to Physiological &

Pharmacological Properties

TypesL-Type: “L”for long-lastingN-Type: “N” for neuronalP/Q-Type: “P/Q” for Purkinje cells with questionR-Type: “R” for resistant or residualT-Type: “T” for transient (only the T-type belongs to

LVA)

Cain & Snutch, 2010

Excitation-Contraction Coupling

Two Types of VGCCs Are Expressed in Cardiomyocytes

Bean (1985) J Gen Physiol 86:1

Canine atrial myocytes

VGCCs in the Heart

VGCCs and Neurotransmissi

on

Synaptotagmin (突触结合蛋白 ) functions as a calcium sensor

1. Neurotransmitter release relies most on P-/Q- (Cav2.1) and N-type (Cav2.2) Ca2+ channels in the vast majority of neurons (Trends Neurosci 2003; 26 (12))

2. P/Q-type channels are more frequently linked to excitatory transmission (Handbook of Experimental Pharmacology, volume 184)

3. In inhibitory synapses, P-/Q-type channels but not N-type channels are involved (Rev Neurosci 2012;23:179–190)

In fast inhibitory synapses, P-/Q-type channels but not N-type channels are involved

Modified from J Neurophysiol 97:3567-3573, 2007.

Distribution and Functions of T-Type Calcium Channels

• Increase in fidelity between presynaptic stimulation and post-synaptic response

• Repetetive firing • Burst firing

Journal of Medicine and Life Vol. 4, 2011, pp.126-138

Cav3.1, Cav3.2 and Cav3.3 mRNA Levels in the Rat Brain

European Journal of Neuroscience, Vol. 24, pp. 2581–2594, 2006

T-Type Calcium Channels Support Burst Firing

Neuron, Vol. 31, 35–45, July 19, 2001

Journal of Medicine and Life Vol. 4, 2011, pp.126-138

L-cys – L-cysteineL-Ncys – L-nitrosocysteine

T-Type Calcium Channels amd Diseases

• Mutation of T-type calcium channnels is associated with epilepsy in humans

• Increased T-type calcium channel activity has also been linked to neuropathic and inflammatory pain states

• Increases in T-type calcium channel expression are associated the development of epilepsy

Relatively Specific VGCC BlockersP/Q type: ω-agatoxin IVA (funnel-web spider, Agelenopsis aperta) N-type: ω-Conotoxins (MVIIA, CVID and GVIA) (cone snail Conus geographus)R-type: SNX-482 (tarantula)L-type: dihydropyridines

Tarantula (毛蜘蛛 )

cone snail Conus geographus( 锥形螺）

funnel-web spider, Agelenopsis aperta

Where to Buy Calcium Channel Blockers

• http://www.tocris.com• http://pepnet.com/• http://www.alomone.com/

Big Questions

• How does a channel selectively permit one ionic species to cross the membrane while excluding another?

• How do VGCCs sense voltage?• How are ion channels regulated?