Computer modeling of siRNA knockdown effects indicates an essential role of the [Ca.sup.2+] channel [[alpha].sub.2][delta]-1 subunit in cardiac excitation--contraction coupling

L-type [Ca.sup.2+] currents determine the shape of cardiac action potentials (AP) and the magnitude of the myoplasmic [Ca.sup.2+] signal, which regulates the contraction force. The auxiliary [Ca.sup.2+] channel subunits [[alpha].sub.2][delta]-1 and [[beta].sub.2] are important regulators of membrane expression and current properties of the cardiac [Ca.sup.2+] channel ([Ca.sub.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].sub.2][delta]-1 subunit in a muscle expression system with simulation of APs and [Ca.sup.2+] transients by using a quantitative computer model of ventricular myocytes. Reconstitution of dysgenic muscle cells with [Ca.sub.V]1.2 (GFP-[[alpha].sub.1C]) recapitulates key properties of cardiac excitation-contraction coupling. Concomitant depletion of the [[alpha].sub.2][delta]-1 subunit did not perturb membrane expression or targeting of the pore-forming GFP-[alpha].sub.1C] subunit into junctions between the outer membrane and the sarcoplasmic reticulum. However, [[alpha].sub.2][delta]-1 depletion shifted the voltage dependence of [Ca.sup.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.sup.2+] concentration during each contraction. Thus, the [Ca.sup.2+] channel [[alpha].sub.2][delta]-1 subunit is not essential for normal [Ca.sup.2+] channel targeting in muscle but is a key determinant of normal excitation and contraction of cardiac muscle cells, and a reduction of [[alpha].sub.2][delta]-1 function is predicted to severely perturb normal heart function. calcium | heart | action potential | dysgenic myotube