Myeong-Chan Cho¹, Young-Hwa Kim¹, Jin-Sook Kwon¹, Ji-Youn Kim¹, Kyung-Kuk Hwang¹, Dong-Woon Kim¹, Chang-Soo Hong², Do Han Kim² |
A growing body of evidence, including studies using genetically engineered mice, has shown that Ca2+ cycling and Ca2+-dependent signaling pathways play a pivotal role in cardiac hypertrophy and heart failure. We previously reported that transgenic (TG) mice overexpressing calsequestrin (JBC 1999;274:22251), juntin (FASEB J 2002;16:1310) or junctate-1 (JACC 2003;41:313A) demonstrated cardiac hypertrophy and failure, abnormal calcium handling and arrhythmias. To elucidate the interplay of Ca2+ handling proteins and pathogenetic mechanism of electrical abnormalities (bradycardia, atrial fibrillation, ventricular premature beats, sinus pause) in the junctate-1 TG heart, we evaluated expression levels of excitation-contraction coupling proteins and measured action potential, Ca2+ currents and Ca2+ release from SR in the isolated single cardiomyocytes. Overexpression of junctate-1 led to down-regulation of SR Ca2+-ATPase 2 calsequestrin, calreticulin and ryanodine receptor but to up-regulation of Na+-Ca2+ exchanger and plasma membrane Ca2+-ATPase. The expression levels of triadin, junctin, phospholamban, and dihydropyridine receptor did not change significantly. The density of ryanodine receptor, as determined by Bmax of ryanodine to the receptor, were significantly lower in TG hearts than in wild-type heart (0.53±0.04 vs 0.81±0.02 pmol/mg protein, p<0.05), whereas the Kd of ryanodine binding was not significantly altered (wild-type, 1.14±0.04; TG, 1.07±0.09 nM). The SR Ca2+ content decreased whereas the L-type Ca2+ current density increased in the isolated ventricular cardiomyocytes from junctate-1 transgenic heart. The resting membrane potential (65±5 mV) was not changed (wild-type, 67±5 mV), but the action potential duration at 90% repolarization increased significantly to 174±12 ms (wild-type, 50±9 ms, p<0.05) by the overexpression of junctate-1 to the ventricular muscle. The present work has provided an important example of pathogenesis leading to cardiac hypertrophy and arrhythmia, which was due to impaired Ca2+ handling induced by overexpression of junctate-1 in heart.
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