Conditional FKBP12.6 overexpression in mouse cardiac myocytes prevents triggered ventricular tachycardia through specific alterations in excitation-contraction coupling

BACKGROUND

Ca(2+) release from the sarcoplasmic reticulum via the ryanodine receptor (RyR2) activates cardiac myocyte contraction. An important regulator of RyR2 function is FKBP12.6, which stabilizes RyR2 in the closed state during diastole. Beta-adrenergic stimulation has been suggested to dissociate FKBP12.6 from RyR2, leading to diastolic sarcoplasmic reticulum Ca(2+) leakage and ventricular tachycardia (VT). We tested the hypothesis that FKBP12.6 overexpression in cardiac myocytes can reduce susceptibility to VT in stress conditions.

METHODS AND RESULTS

We developed a mouse model with conditional cardiac-specific overexpression of FKBP12.6. Transgenic mouse hearts showed a marked increase in FKBP12.6 binding to RyR2 compared with controls both at baseline and on isoproterenol stimulation (0.2 mg/kg i.p.). After pretreatment with isoproterenol, burst pacing induced VT in 10 of 23 control mice but in only 1 of 14 transgenic mice (P<0.05). In isolated transgenic myocytes, Ca(2+) spark frequency was reduced by 50% (P<0.01), a reduction that persisted under isoproterenol stimulation, whereas the sarcoplasmic reticulum Ca(2+) load remained unchanged. In parallel, peak I(Ca,L) density decreased by 15% (P<0.01), and the Ca(2+) transient peak amplitude decreased by 30% (P<0.001). A 33.5% prolongation of the caffeine-evoked Ca(2+) transient decay was associated with an 18% reduction in the Na(+)-Ca(2+) exchanger protein level (P<0.05).

CONCLUSIONS

Increased FKBP12.6 binding to RyR2 prevents triggered VT in normal hearts in stress conditions, probably by reducing diastolic sarcoplasmic reticulum Ca(2+) leak. This indicates that the FKBP12.6-RyR2 complex is an important candidate target for pharmacological prevention of VT.

Recurrence of dilated cardiomyopathy after re-introduction of a tricyclic antidepressant

A causal relationship between treatment with tricyclic antidepressant (TCA) at therapeutic doses and cases of dilated cardiomyopathy has been suspected, but not definitely evidenced. We present a case of a dilated cardiomyopathy, which seemed idiopathic, but occurred during treatment with imipramine and recovered after its withdrawal. For the first recorded time, the cardiomyopathy recurred 9 years later, after a 2-year period of treatment with amitriptyline and lithium, and recovered again 1 year after both psychotropic drugs withdrawal. New arguments for the existence of TCA-induced dilated cardiomyopathy are brought.

[A rare cause of myocardial infarction: papillary fibroelastoma of the aortic valve]

Papillary fibroelastoma is a rare, benign endocardial tumour usually located on the cardiac valves. Before echocardiography, these tumours were chance findings either at surgery or at autopsy. With the advent of echocardiography, the diagnosis has become commoner and they are often the cause of systemic embolism justifying surgical ablation. In this case, an aortic valve papillary fibroelastoma presented with myocardial infarction in a 78 year old woman with normal coronary angiography. The diagnosis was strongly suspected at echocardiography and confirmed by histological analysis of the surgically excised tumour.