Catecholaminergic polymorphic ventricular tachycardia (CPVT) is an inherited 
disease characterized by life threatening arrhythmias and mutations in the gene 
encoding the ryanodine receptor (RyR2). Disagreement exists on whether (1) RyR2 
mutations induce abnormal calcium transients in the absence of adrenergic 
stimulation; (2) decreased affinity of mutant RyR2 for FKBP12.6 causes CPVT; (3) 
K201 prevent arrhythmias by normalizing the FKBP12.6-RyR2 binding. We studied 
ventricular myocytes isolated from wild-type (WT) and knock-in mice harboring 
the R4496C mutation (RyR2(R4496C+/-)). Pacing protocols did not elicit delayed 
afterdepolarizations (DADs) (n=20) in WT but induced DADs in 21 of 33 (63%) 
RyR2(R4496C+/-) myocytes (P=0.001). Superfusion with isoproterenol (30 nmol/L) 
induced small DADs (45%) and no triggered activity in WT myocytes, whereas it 
elicited DADs in 87% and triggered activity in 60% of RyR2(R4496C+/-) myocytes 
(P=0.001). DADs and triggered activity were abolished by ryanodine (10 
micromol/L) but not by K201 (1 micromol/L or 10 micromol/L). In vivo 
administration of K201 failed to prevent induction of polymorphic ventricular 
tachycardia (VT) in RyR2(R4496C+/-) mice. Measurement of the FKBP12.6/RyR2 ratio 
in the heavy sarcoplasmic reticulum membrane showed normal RyR2-FKBP12.6 
interaction both in WT and RyR2(R4496C+/-) either before and after treatment 
with caffeine and epinephrine. We suggest that (1) triggered activity is the 
likely arrhythmogenic mechanism of CPVT; (2) K201 fails to prevent DADs in 
RyR2(R4496C+/-) myocytes and ventricular arrhythmias in RyR2(R4496C+/-) mice; 
and (3) RyR2-FKBP12.6 interaction in RyR2(R4496C+/-) is identical to that of WT 
both before and after epinephrine and caffeine, thus suggesting that it is 
unlikely that the R4496C mutation interferes with the RyR2/FKBP12.6 complex.