title: Computer modeling of siRNA knockdown effects indicates an essential role of the Ca2+ channel alpha2delta-1 subunit in cardiac excitation-contraction coupling.
abstract: L-type Ca(2+) currents determine the shape of cardiac action potentials (AP) and the magnitude of the myoplasmic Ca(2+) signal, which regulates the contraction force. The auxiliary Ca(2+) channel subunits alpha(2)delta-1 and beta(2) are important regulators of membrane expression and current properties of the cardiac Ca(2+) channel (Ca(V)1.2). However, their role in cardiac excitation-contraction coupling is still elusive. Here we addressed this question by combining siRNA knockdown of the alpha(2)delta-1 subunit in a muscle expression system with simulation of APs and Ca(2+) transients by using a quantitative computer model of ventricular myocytes. Reconstitution of dysgenic muscle cells with Ca(V)1.2 (GFP-alpha(1C)) recapitulates key properties of cardiac excitation-contraction coupling. Concomitant depletion of the alpha(2)delta-1 subunit did not perturb membrane expression or targeting of the pore-forming GFP-alpha(1C) subunit into junctions between the outer membrane and the sarcoplasmic reticulum. However, alpha(2)delta-1 depletion shifted the voltage dependence of Ca(2+) current activation by 9 mV to more positive potentials, and it slowed down activation and inactivation kinetics approximately 2-fold. Computer modeling revealed that the altered voltage dependence and current kinetics exert opposing effects on the function of ventricular myocytes that in total cause a 60% prolongation of the AP and a 2-fold increase of the myoplasmic Ca(2+) concentration during each contraction. Thus, the Ca(2+) channel alpha(2)delta-1 subunit is not essential for normal Ca(2+) channel targeting in muscle but is a key determinant of normal excitation and contraction of cardiac muscle cells, and a reduction of alpha(2)delta-1 function is predicted to severely perturb normal heart function.
authors.affiliation: Department of Physiology and Medical Physics, Division of Physiology, Medical University Innsbruck, Fritz-Pregl-Strasse 3, A-6020 Innsbruck, Austria.
authors.forename: Petronel
authors.initials: P
authors.lastname: Tuluc
authors.forename: Georg
authors.initials: G
authors.lastname: Kern
authors.forename: Gerald J
authors.initials: GJ
authors.lastname: Obermair
authors.forename: Bernhard E
authors.initials: BE
authors.lastname: Flucher
mesh_terms: D000200:Action Potentials; D000818:Animals; D002118:Calcium / Q000378:metabolism; D015220:Calcium Channels / Q000502:physiology*; D002462:Cell Membrane; D003198:Computer Simulation; D004594:Electrophysiology; D006352:Heart Ventricles / Q000166:cytology; D009200:Myocardial Contraction; D032383:Myocytes, Cardiac; D034741:RNA, Small Interfering / Q000494:pharmacology*; D011817:Rabbits; D012519:Sarcoplasmic Reticulum
chemical_list: C425903:CACNA2D1 protein, human; D015220:Calcium Channels; D034741:RNA, Small Interfering; D002118:Calcium