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Calpain is an intracellular Ca 2+  -activated protease that is involved in numerous Ca 2+  dependent regulation of protein function in many cell types. This paper tests a hypothesis that calpains are involved in Ca 2+  -dependent increase of the late sodium current (I NaL ) in failing heart. Chronic heart failure (HF) was induced in 2 dogs by multiple coronary artery embolization. Using a conventional patch-clamp technique, the whole-cell I NaL  was recorded in enzymatically isolated ventricular cardiomyocytes (VCMs) in which I NaL  was activated by the presence of a higher (1μM) intracellular [Ca 2+ ] in the patch pipette. Cell suspensions were exposed to a cell- permeant calpain inhibitor MDL-28170 for 1–2 h before I NaL  recordings. The numerical excitation-contraction coupling (ECC) model was used to evaluate electrophysiological effects of calpain inhibition in silico. MDL caused acceleration of I NaL  decay evaluated by the two-exponential fit (τ 1  = 42±3.0 ms τ 2  = 435±27 ms, n = 6, in MDL vs. τ 1  = 52±2.1 ms τ 2  = 605±26 control no vehicle, n = 11, and vs. τ 1  = 52±2.8 ms τ 2  = 583±37 ms n = 7, control with vehicle, P<0.05 ANOVA). MDL significantly reduced I NaL  density recorded at –30 mV (0.488±0.03, n = 12, in control no vehicle, 0.4502±0.0210, n = 9 in vehicle vs. 0.166±0.05pA/pF, n = 5, in MDL). Our measurements of current-voltage relationships demonstrated that the I NaL  density was decreased by MDL in a wide range of potentials, including that for the action potential plateau. At the same time the membrane potential dependency of the steady-state activation and inactivation remained unchanged in the MDL-treated VCMs. Our ECC model predicted that calpain inhibition greatly improves myocyte function by reducing the action potential duration and intracellular diastolic Ca 2+  accumulation in the pulse train.   Conclusions  Calpain inhibition reverses I NaL  changes in failing dog ventricular cardiomyocytes in the presence of high intracellular Ca 2+ . Specifically it decreases I NaL  density and accelerates I NaL  kinetics resulting in improvement of myocyte electrical response and Ca 2+  handling as predicted by our in silico simulations.

Calpain Inhibition Reduces Amplitude and Accelerates Decay of the Late Sodium Current in Ventricular Myocytes from Dogs with Chronic Heart Failure