Remodeling of T-Tubules and Reduced Synchrony of Ca 2+ Release in Myocytes From Chronically Ischemic Myocardium

Abstract

In ventricular cardiac myocytes, T-tubule density is an important determinant of the synchrony of sarcoplasmic reticulum (SR) Ca 2+ release and could be involved in the reduced SR Ca 2+ release in ischemic cardiomyopathy. We therefore investigated T-tubule density and properties of SR Ca 2+ release in pigs, 6 weeks after inducing severe stenosis of the circumflex coronary artery (91±3%, N=13) with myocardial infarction (8.8±2.0% of total left ventricular mass). Severe dysfunction in the infarct and adjacent myocardium was documented by magnetic resonance and Doppler myocardial velocity imaging. Myocytes isolated from the adjacent myocardium were compared with myocytes from the same region in weight-matched control pigs. T-tubule density quantified from the di-8-ANEPPS (di-8-butyl-amino-naphthyl-ethylene-pyridinium-propyl-sulfonate) sarcolemmal staining was decreased by 27±7% ( P <0.05). Synchrony of SR Ca 2+ release (confocal line scan images during whole-cell voltage clamp) was reduced in myocardium myocytes. Delayed release (ie, half-maximal [Ca 2+ ] i occurring later than 20 ms) occurred at 35.5±6.4% of the scan line in myocardial infarction versus 22.7±2.5% in control pigs ( P <0.05), prolonging the time to peak of the line-averaged [Ca 2+ ] i transient (121±9 versus 102±5 ms in control pigs, P <0.05). Delayed release colocalized with regions of T-tubule rarefaction and could not be suppressed by activation of protein kinase A. The whole-cell averaged [Ca 2+ ] i transient amplitude was reduced, whereas L-type Ca 2+ current density was unchanged and SR content was increased, indicating a reduction in the gain of Ca 2+ -induced Ca 2+ release. In conclusion, reduced T-tubule density during ischemic remodeling is associated with reduced synchrony of Ca 2+ release and reduced efficiency of coupling Ca 2+ influx to Ca 2+ release.