Reduced synchrony of Ca2+ release with loss of T-tubules—a comparison to Ca2+ release in human failing cardiomyocytes

OBJECTIVES: During cardiac excitation-contraction coupling, Ca2+ release from the sarcoplasmic reticulum (SR) occurs at the junctional complex with the T-tubules, containing the L-type Ca2+ channels. A partial loss of T-tubules has been described in myocytes from failing canine and human hearts. We examined how graded reduction of T-tubule density would affect the synchrony of Ca2+ release. METHODS: Adult pig ventricular myocytes were isolated and cultured for 24 and 72 h. T-tubules, visualized with di-8-ANEPPS, and Ca2+i transients (Fluo-3) were recorded during confocal line scan imaging. RESULTS: Cultured cardiomyocytes exhibited a progressive reduction in T-tubule density. Ca2+i transients showed small areas of delayed Ca2+ release which gradually increased in number and size with loss of T-tubules. Local Ca2+i transients in the delayed regions were reduced. Due to these changes, loss of T-tubules resulted in an overall slowing of the rise of Ca2+ along the entire line scan and transient magnitude tended to be reduced, but there was no change in SR Ca2+ content. Human myocytes isolated from failing hearts had a T-tubule density comparable to that of freshly isolated pig myocytes. The size, but not the number, of delayed release areas tended to be larger. The overall rate of rise of Ca2+i was significantly faster than in pig myocytes with low T-tubule density. CONCLUSIONS: Loss of T-tubules reduces the synchrony of SR Ca2+ release. This could contribute to reduced efficiency of excitation-contraction coupling in heart failure, though dyssynchrony in human failing cells appears to be modest.