Résumé
Abstract only In cardiac myocytes, Ca2+ release from the sarcoplasmic reticulum (SR) into the cytoplasm via the ryanodine receptor (RyR2) activates cell contraction. During diastole, RyR2 are closed and prevent Ca2+ efflux from the SR. One of the major regulators of RyR2 function is FKBP12.6. Binding of FKBP12.6 stabilizes RyR2 in the closed formation in diastole and contributes to synchronized RyR2 opening in systole. Beta-adrenergic stimulation dissociates FKBP12.6 from RyR2, leading to diastolic Ca2+ leak that can trigger ventricular tachycardias (VT). We tested the hypothesis whether FKBP12.6 overexpression in the myocardium can reduce the susceptibility to VT in stress conditions. We developed a mouse model with conditional cardiac specific overexpression of FKBP12.6 using the Tet-Off system. Transgenic (TG) mice and controls (CT) were examined by echocardiography, PV-catheterization, ECG, and underwent intracardiac stimulation to trigger VT before and after pre-treatment with isoproterenol. In isolated cardiac myocytes, SR Ca2+ load, Ca2+ sparks and Ca2+ transient were measured using confocal microscopy, and L-type Ca2+ current was determined by the patch-clamp technique. Echocardiography, PV-catheterization and ECG recording did not reveal differences between Tg (n=11) and CT (n=13) mice. Burst pacing (figure 1) could induce TV in 4 of 24 controls and in 0 of 14 TG mice (n.s.). Following pre-treatment with isoproterenol, TV could be induced in 10 of 23 controls, but only in 1 of 14 TG animals (figure 2, p<0.05). In isolated myocytes, decreased Ca2+ spark frequency, increased Ca2+ spark size, unchanged SR Ca2+ load and decreased Ca2+ transient were observed in TG cells (n=19) as compared to controls (n=48, p<0.05). L-type Ca2+ channel current was found to be decreased in Tg myocytes (n=29 vs n=32, p<0.01). We conclude that myocardial FKBP12.6 overexpression has a protective effect against VT induced by rapid pacing after pretreatment with catecholamines. This antiarrhythmic effect is probably, at least in part, linked to decreased diastolic SR Ca2+ leak. Our results support the hypothesis that increased FKBP12.6 binding to RyR2 might represent a potential therapeutical target in the prevention and treatment of ventricular arrhythmias.