Heart block in mice overexpressing calcineurin but not NF-AT3.
OBJECTIVE
Overexpression of calcineurin causes cardiac hypertrophy and arrhythmic deaths. During disease development, sinus bradycardia followed by high degree atrioventricular (AV) block finally culminating in ventricular asystole has been observed over time in calcineurin hearts. AV block is associated with the development of pleomorphic ventricular tachycardia in mice and downregulation of potassium currents in ventricular myocytes. We tested the hypothesis that the abnormalities of AV block and propensity to ventricular tachycardia relate to overexpression of calcineurin independent of the development of hypertrophy.


METHODS
Cardiac electrophysiologic properties were compared in isolated perfused hearts with ventricular hypertrophy due to overexpression of calcineurin or NF-AT3 and in their corresponding wild types at 15 or 30 days of age.


RESULTS
Compared to wild-type hearts, significant prolongation of sinus node recovery times was noted in both NF-AT3 and calcineurin hearts. Compared to wild-type hearts, Wenckebach cycle length (WCL) and the left ventricular effective refractory period (LVERP) were significantly prolonged in the calcineurin hearts (p<0.05) but not NF-AT3 hearts. In calcineurin mice, left ventricular effective refractory period impinged on Wenckebach cycle length resulting in a significant correlation between left ventricular effective refractory period and Wenckebach cycle length (r(2)=0.56). No such correlation was observed for wild type or NF-AT3 hearts. At 30 days of development, ventricular tachycardia developed in 70% of calcineurin hearts compared to 0% wild-type hearts (p=0.003), whereas ventricular tachycardia was observed in 33% of NF-AT3 hearts and 10% of corresponding wild-type hearts (p=NS).


CONCLUSIONS
The prolonged ventricular refractoriness, seen only in calcineurin hearts, impinges on Wenckebach cycle length resulting in heart block and is associated with propensity to ventricular tachycardia.
Gillis
Anne M
AM
The Cardiovascular Research Group and the Division of Cardiology, The University of Calgary, Health Sciences Hospital 1634, 3330 Hospital Drive NW, Calgary, Alberta, Canada T2N 4N1. amgillis@ucalgary.ca
Kavanagh
Katherine M
KM
Mathison
Heather J
HJ
Somers
Julie R
JR
Zhan
Shu
S
Duff
Henry J
HJ
D000818:Animals; D019703:Calcineurin / Q000235:genetics* / Q000378:metabolism; D004268:DNA-Binding Proteins / Q000235:genetics*; D004562:Electrocardiography; D015870:Gene Expression; D006327:Heart Block / Q000378:metabolism*; D006329:Heart Conduction System; D017379:Hypertrophy, Left Ventricular / Q000196:embryology / Q000378:metabolism*; D051379:Mice; D008822:Mice, Transgenic; D050778:NFATC Transcription Factors; D009687:Nuclear Proteins / Q000235:genetics*; D010477:Perfusion; D014157:Transcription Factors / Q000235:genetics*
D004268:DNA-Binding Proteins; D050778:NFATC Transcription Factors; D009687:Nuclear Proteins; D014157:Transcription Factors; D019703:Calcineurin