Voltage-Gated Ca²⁺ Channels in the Cardiovascular System

Voltage-gated Ca²⁺ channels play a pivotal role in the regulation of a wide range of cellular processes, including membrane excitability, Ca²⁺ homeostasis, protein phosphorylation, gene regulation, muscle contraction and vesicular secretion. The L-type voltage-gated Ca²⁺ channels are the most common. This brief review aims at demonstrating advances made in research regarding voltage-gated Ca²⁺ channels in the cardiovascular system and the contributions of Alomone Labs in the field.

In cardiac and smooth muscle cells the predominant voltage-gated Ca²⁺ channels (Ca_V) are the dihydropyridine-sensitive Ca_V1.2 (α_1C) channels (also called dihydropyridine receptor or L-type channel). These channels mediate long-lasting Ca²⁺ currents as a criticalstep in excitation-contraction coupling and have a central rolein controlling cardiac function^2,3. The Ca_V1.2 channels provide a trigger for intracellular Ca²⁺ release during excitation-contraction coupling in a processknown as Ca²⁺-induced Ca²⁺ release. Dysregulationof Ca²⁺ channels in cardiac muscles leads to Ca²⁺ overload that influences the plateau phase of the cardiac action potential, and disrupts pacemaker activity in the sinoatrial node. The α₁ subunit (the pore forming subunit) is a targetfor calcium channel blockers such as dihydropyridines (DHPs), phenylalkylamines, and benzothiazepines, which are widely used for treating hypertension, angina pectoris, and cardiac arrhythmias^4,5.

In the cardiac muscle, a distinct α₁ subunit (α₁1.2a cardiac isoform), an α₂δ subunit, and several isoforms of β subunits (β_1b and β_2a-d) have been identified and implicated to form the Ca_V1.2 channel. The Ca_V1.2a α_1C subunit is an alternative splice variant of exon 1 which differs from the vascular smooth muscle isoform at the N-terminus.

Two forms of the Ca_V1.2 α_1C subunit (≈240 and 210 kDa) are expressed in the cardiac muscle which differ in a truncation at the C-terminus. Whereas the majority of α_1C subunits isolated from cardiac muscle are truncated (demonstrated using Anti-Ca_V1.2a (CACNA1C) Antibody, (#ACC-013))⁶, the cleaved distal C-terminus remains associated with the truncated α₁ subunit of Ca_V1.2a following proteolytic processing, and peptides derived from it can control the channel’s activity. The cardiac Ca_V1.2a activity is highly regulated by a variety of stimulus. The C-terminal region is a target site for β-Adrenergic regulation and for phosphorylation by PKA⁶. Stimulation of cardiac cells by Recombinant human LIF protein (human Leukemia Inhibitory Factor) (#L-200), enhances Ca_V1.2a currents through phosphorylation by ERK at S1829 of the α_1C subunit^6,7.

Although physiologically essential for normal cardiac function, increased level of Ca_V1.2 channels or activity have been implicated in pathological processesinvolving Ca²⁺ disturbance, including cardiac ischemia and myocardial stunning⁸. The expression level of Ca_V1.2 in the heart was demonstrated using Anti-Ca_V1.2 (CACNA1C) Antibody (#ACC-003) and its expression was up-regulated when PI3K and Ca²⁺/calmodulindependent protein kinase II (CamK II) were overexpressed^9,10 (Figure 1).

In hypertension diseases, the level of vascular voltage-gated L-type calcium channel currents and vascular tone are increased, as a result of the Ca_V1.2 non-cardiac form overexpression (demonstrated in different rodent models of hypertension using Anti-Ca_V1.2 (CACNA1C) Antibody¹¹.

The physiological importance of the β3 subunit in the cardiovascular system was investigated using gene targeted mutant mice. In this system the lack of β3 subunit in β3-nullmice was confirmed using Anti-CACNB3 Antibody (#ACC-008). Western blot analysis using Anti-Ca_V1.2a (CACNA1C) Antibody demonstrated a remarkable reduction in the membrane expression of α_1C subunit in these mice. The reduction of α_1C was accompanied by a significant decrease in Ca²⁺ current density, a slower inactivation rate of Ca²⁺ channel, and reduced dihydropyridine-sensitivity¹².

Ca_V1.3 channels play a pivotal role in the generation of cardiac pacemaker activity by contributing to diastolic depolarization in the sinoatrial node (SAN). Abnormal Ca_V1.3 trafficking caused by ankyrin-B dysfunction leads to abnormal sinoatrial node (SAN) excitability and causes SAN disease. In this syndrome the cardiac distribution of Ca_V1.3 was demonstrated by immunocytochemistry staining using the Anti-Ca_V1.3 (CACNA1D) Antibody (#ACC-005) (Figure 2)¹³.

Ca²⁺ currents, through voltage-gated Ca²⁺ channels play a key role during excitation contraction coupling in smooth muscle cells (SMCs) of the vascular system. Using vascular smooth muscles cells, L-, P- and Q-type voltage-gated Ca²⁺ channels were found to be expressed using Anti-CACNA1A (Ca_V2.1) Antibody (#ACC-001) as well as Anti-Ca_V1.2 (CACNA1C) Antibody. Additional information about the identity of these channels was confirmed using Calciseptine (#SPC-500) known to inhibit the L-Type Ca²⁺ channels and ω-Agatoxin IVA (#STA-500) which inhibits P and Q-type Ca²⁺ channels¹⁴. Figure 3 shows smooth muscle cells treated with Calciseptine, showing that L-type Ca²⁺ channels are indeed involved Ca²⁺ currents in these cells¹⁵.

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