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

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.