Calpain is an intracellular Ca²⁺-activated protease that is involved in numerous 
Ca²⁺ dependent regulation of protein function in many cell types. This paper 
tests a hypothesis that calpains are involved in Ca²⁺-dependent increase of the 
late sodium current (INaL) 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 INaL was recorded in enzymatically 
isolated ventricular cardiomyocytes (VCMs) in which INaL was activated by the 
presence of a higher (1 μM) intracellular [Ca²⁺] in the patch pipette. Cell 
suspensions were exposed to a cell- permeant calpain inhibitor MDL-28170 for 1-2 
h before INaL recordings. The numerical excitation-contraction coupling (ECC) 
model was used to evaluate electrophysiological effects of calpain inhibition in 
silico. MDL caused acceleration of INaL decay evaluated by the two-exponential 
fit (τ₁ = 42±3.0 ms τ₂ = 435±27 ms, n = 6, in MDL vs. τ₁ = 52±2.1 ms τ₂ = 605±26 
control no vehicle, n = 11, and vs. τ₁ = 52±2.8 ms τ₂ = 583±37 ms n = 7, control 
with vehicle, P<0.05 ANOVA). MDL significantly reduced INaL 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 INaL 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 Ca²⁺ accumulation in the pulse 
train.
CONCLUSIONS: Calpain inhibition reverses INaL changes in failing dog ventricular 
cardiomyocytes in the presence of high intracellular Ca²⁺. Specifically it 
decreases INaL density and accelerates INaL kinetics resulting in improvement of 
myocyte electrical response and Ca²⁺ handling as predicted by our in silico 
simulations.