Voltage-gated Ca(2+) channels mediate Ca(2+) entry into cells in response to membrane depolarization. Electrophysiological studies reveal different Ca(2+) currents designated L-, N-, P-, Q-, R-, and T-type. The high-voltage-activated Ca(2+) channels that have been characterized biochemically are complexes of a pore-forming alpha1 subunit of approximately 190-250 kDa; a transmembrane, disulfide-linked complex of alpha2 and delta subunits; an intracellular beta subunit; and in some cases a transmembrane gamma subunit. Ten alpha1 subunits, four alpha2delta complexes, four beta subunits, and two gamma subunits are known. The Cav1 family of alpha1 subunits conduct L-type Ca(2+) currents, which initiate muscle contraction, endocrine secretion, and gene transcription, and are regulated primarily by second messenger-activated protein phosphorylation pathways. The Cav2 family of alpha1 subunits conduct N-type, P/Q-type, and R-type Ca(2+) currents, which initiate rapid synaptic transmission and are regulated primarily by direct interaction with G proteins and SNARE proteins and secondarily by protein phosphorylation. The Cav3 family of alpha1 subunits conduct T-type Ca(2+) currents, which are activated and inactivated more rapidly and at more negative membrane potentials than other Ca(2+) current types. The distinct structures and patterns of regulation of these three families of Ca(2+) channels provide a flexible array of Ca(2+) entry pathways in response to changes in membrane potential and a range of possibilities for regulation of Ca(2+) entry by second messenger pathways and interacting proteins.

Structure and regulation of voltage-gated Ca2+ channels.

Voltage-sensitive ion channels mediate action potentials in electrically excitable cells and play important roles in signal transduction in other cell types. In the past several years, their protein components have been identified, isolated, and restored to functional form in the purified state. Na+ and Ca2+ channels consist of a principal transmembrane subunit, which forms the ion-conducting pore and is expressed with a variable number of associated subunits in different cell types. The principal subunits of voltage-sensitive Na+, Ca2+, and K+ channels are homologous members of a gene family. Models relating the primary structures of these principal subunits to their functional properties have been proposed, and experimental results have begun to define a functional map of these proteins. Coordinated application of biochemical, biophysical, and molecular genetic methods should lead to a clear understanding of the molecular basis of electrical excitability.

http://science.sciencemag.org/content/sci/242/4875/50.full.pdf

Structure and function of voltage-sensitive ion channels

Microinjection of an expression plasmid that carries complementary DNA encoding the receptor for dihydropyridine calcium channel blockers of skeletal muscle restores both excitation-contraction coupling and slow calcium current in cultured skeletal muscle cells from mice with muscular dysgenesis. This suggests that the dihydropyridine receptor in the transverse tubule membrane of skeletal muscle functions both as the voltage sensor for excitation-contraction coupling and as the slow calcium channel.

Restoration of excitation—contraction coupling and slow calcium current in dysgenic muscle by dihydropyridine receptor complementary DNA

Calcium Release from Sarcoplasmic Reticulum

I Basics- 1 The Physical Nature of Planar Bilayer Membranes- 2 Ion Channel Electrostatics and the Shapes of Channel Proteins- 3 Superoxide Dismutase as a Model Ion Channel- 4 Single-Channel Enzymology- 5 How to Set Up a Bilayer System- 6 Fusion of Liposomes to Planar Bilayers- 7 Incorporation of Ion Channels by Fusion- II Nicotinic Acetylcholine Receptor- 8 The Reconstituted Acetylcholine Receptor- 9 Immunologic Analysis of the Acetylcholine Receptor- 10 Function of Acetylcholine Receptors in Reconstituted Liposomes- III Sodium Channel- 11 Skeletal Muscle Sodium Channels: Isolation and Reconstitution- 12 Reconstitution of the Sodium Channel from Electrophorus electricus- 13 The Reconstituted Sodium Channel from Brain- 14 Gating of Batrachotoxin-Activated Sodium Channels in Lipid Bilayers- 15 Ion Conduction Through Sodium Channels in Planar Lipid Bilayers- 16 Blocking Pharmacology of Batrachotoxin-Activated Sodium Channels- IV Other Channels in Model Membranes- 17 The Large Calcium-Activated Potassium Channel- 18 The Sarcoplasmic Reticulum Potassium Channel: Lipid Effects- 19 Characterization of Dihydropyridine-Sensitive Calcium Channels from Purified Skeletal Muscle Transverse Tubules- 20 Calcium Channels- 21 Phosphorylation of a Reconstituted Potassium Channel- 22 Voltage Gating in VDAC: Toward a Molecular Mechanism- 23 Analysis and Chemical Modification of Bacterial Porins

Ion Channel Reconstitution

1. Voltage clamp experiments using the three micro-electrode voltage clamp technique were performed on sartorius muscles of the frog. 2. By blocking potassium currents with tetraethylammonium and replacing chloride ions with sulphate a slow inward current was detected. 3. The slow inward current is mainly carried by calcium, since it is abolished by cobalt and D-600, it depends on external calcium, and is not affected by removing external sodium or by tetrodotoxin (TTX). 4. The slow inward current has a mean threshold of -40 mV, reaches a mean maximum value at ca. 0 mV of 81 microamperemetercm-2 and has a mean reversal potential of +38 mV. 5. The calcium current is inactivated by the application of 2 sec conditioning prepulses according to a sigmoid curve with V(h) = -42 mV and k = 6.2 mV. 6. The slow time course of this calcium current makes it rather unlikely that it participates in contraction during a twitch, but it might be activated during long depolarizations as potassium contractures.

https://physoc.onlinelibrary.wiley.com/doi/pdf/10.1113/jphysiol.1978.sp012496

Inward calcium current in twitch muscle fibres of the frog.

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Inward calcium current in twitch muscle fibres of the frog.