K201 modulates excitation-contraction coupling and spontaneous Ca^2^+ release in normal adult rabbit ventricular cardiomyocytes
Objectives: The drug K201 (JTV-519) increases inotropy and suppresses arrhythmias in failing hearts, but the effects of K201 on normal hearts is unknown. Methods: The effect of K201 on excitation-contraction (E-C) coupling in normal myocardium was studied by using voltage-clamp and intracellular Ca^2^+ measurements in intact cells. Sarcoplasmic reticulum (SR) function was assessed using permeabilised cardiomyocytes. Results: Acute application of L amplitude (to 66+/-8%) and SR Ca^2^+ content (74+/-9%). Spontaneous SR Ca^2^+ release during diastole was induced by increasing intracellular [Ca^2^+]. At 1 @mmol/L K201 reduced the frequency of spontaneous Ca^2^+ release. The effect of K201 on SR-mediated Ca^2^+ waves and Ca^2^+ sparks was examined in @b-escin-permeabilised cardiomyocytes by confocal microscopy. K201 (1 @mmol/L) reduced the frequency and velocity of SR Ca^2^+ waves despite no change in SR Ca^2^+ content. At 3 @mmol/L K201 completely abolished Ca^2^+ waves and reduced the SR Ca^2^+ content (to ~73%). K201 at 1 @mmol/L reduced Ca^2^+ spark amplitude and frequency. Assays specific to SR Ca^2^+-ATPase and RyR2 activity indicated that K201 inhibited both SR Ca^2^+ uptake and release. Conclusions: K201 modifies E-C coupling in normal cardiomyocytes. A dual inhibitory action on SERCA and RyR2 explains the ability of K201 to suppress spontaneous diastolic Ca^2^+ release during Ca^2^+ overload without significantly affecting Ca^2^+ transient amplitude.
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