Question: 

Voltage-gated Ca²⁺ channels regulate cardiac automaticity, rhythmicity, and excitation-contraction coupling. L-type (Ca_v1.2 and Ca_v1.3) and T-type (Ca_v3.1 and Ca_v3.2) Ca²⁺ channels are well investigated for their roles, but the so called pharmacoresistant non-L-type Ca²⁺ channel Ca_v2.3 (E-/R-type) is still under discussion for its precise function in cardiac physiology. After gene inactivation, Ca_v2.3-deficient mice exhibit severe alterations in cardiac function. The ablation caused a subsidiary escape rhythm, altered atrial activation patterns, atrioventricular conduction disturbances and alteration in QRS-morphology (Weiergräber et al., 2005: Basic Research Cardiol. 100, 1–13). Administration of atropine/propranolol revealed an increased heart rate due to enhanced sympathetic tone. Escape rhythms, atrial activation disturbances and QRS-dysmorphology remained unaffected, indicating that these are intrinsic features of cardiomyocytes in Ca_v2.3-deficient mice.

Methods: 

The localization of Ca_v2.3 in the murine heart was investigated by immunoblotting of isolated proteins after polyacrylamide gel-electrophoresis (PAGE). As transcripts of at least two different Ca_v2.3 splice variants were detected in cardiomyocytes by single cell RT-PCR, whole cell Ca²⁺ currents were analysed in cardiomyocytes by patch-clamp recordings using a modified protocol, in which the dihydropyridine concentration was adapted to the low sensitivity of Ca_v2.3 towards isradipine. Further, murine ganglia were isolated to perform Ca²⁺ current recordings as well.

Results & Conclusion: 

Immunoblotting of PAGE-separated microsomal proteins from Ca_v2.3-deficient and control mice revealed a faint band of anti-Ca_v2.3 positive proteins in control mice, which leads to the conclusion that Ca_v2.3 expression is low in heart, maybe, because its expression is restricted to pacemaker and ganglion regions only. Pharmacoresistant voltage-gated Ca²⁺ currents were recorded from cardiomyocytes, if the dihydropyridine concentration was kept below 1 mM. Under these conditions, SNX-482 was able to antagonize in some cardiomyocytes the E-/R-type Ca²⁺ current component, leading to the conclusion that Ca_v2.3 may act in both, cardiomyocytes and probably also in ganglia to contribute to cardiac regulation.