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 <1 micromol/L K201 had no significant effect on 
E-C coupling. K201 at 1 micromol/L decreased Ca(2+) transient amplitude (to 
83+/-7%) without affecting I(Ca,L) or the SR Ca(2+) content. At 3 micromol/L 
K201 caused a larger reduction of Ca(2+) transient amplitude (to 60+/-7%) with 
accompanying reductions in I(Ca,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 micromol/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 beta-escin-permeabilised cardiomyocytes by 
confocal microscopy. K201 (1 micromol/L) reduced the frequency and velocity of 
SR Ca(2+) waves despite no change in SR Ca(2+) content. At 3 micromol/L K201 
completely abolished Ca(2+) waves and reduced the SR Ca(2+) content (to 
approximately 73%). K201 at 1 micromol/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.