In cardiac myocytes, initiation of excitation-contraction coupling is highly localized near the T-tubule network. Myocytes with a dense T-tubule network exhibit rapid and homogeneous sarcoplasmic reticulum (SR) Ca2+ release throughout the cell. We examined whether progressive changes in T-tubule organization and Ca 2+ release synchrony occur in a murine model of congestive heart failure (CHF). Myocardial infarction (MI) was induced by ligation of the left coronary artery, and CHF was diagnosed by echocardiography (left atrial diameter >2.0 mm). CHF mice were killed at 1 or 3 weeks following MI (1-week CHF, 3-week CHF) and cardiomyocytes were isolated from viable regions of the septum, excluding the MI border zone. Septal myocytes from SHAM-operated mice served as controls. T-tubules were visualized by confocal microscopy in cells stained with di-8-ANEPPS. SHAM cells exhibited a regular striated T-tubule pattern. However, 1-week CHF cells showed slightly disorganized T-tubule structure, and more profound disorganization occurred in 3-week CHF with irregular gaps between adjacent T-tubules. The authors are indebted to Dr Gregory R. Ferrier who contributed immeasurably to the inception of this project during his sabbatical in Oslo. Sadly, he passed away before its completion. This manuscript is dedicated to his memory. channels (ryanodine receptors) are in close proximity (Flucher & Franzini-Armstrong, 1996). Thus, initiation of excitation-contraction coupling is highly localized near the T-tubule network (Shacklock et al. 1995).In myocytes with a high density of T-tubules, such as in rats and mice, SR Ca 2+ release occurs almost simultaneously throughout the cell (Berlin, 1995; Shacklock et al. 1995;Heinzel et al. 2002). However, myocytes with less-dense T-tubule networks exhibit less synchronous Ca 2+ transients, with regions of delayed Ca 2+ release occurring where T-tubules are not present (Heinzel et al. 2002) sarcolemma, but more slowly in the cell interior following propagation of Ca 2+ (Berlin, 1995;Cordeiro et al. 2001). Experimentally promoting loss of T-tubules by cell culture or de-tubulation techniques has also been shown to reduce the synchrony of Ca 2+ transients, which results in slower spatially averaged Ca 2+ release (Lipp et al. 1996;Yang et al. 2002;Louch et al. 2004). Thus, there is considerable evidence to suggest that a dense and intact T-tubular network is required for rapid and homogeneous SR Ca 2+ release.Several reports have suggested that the T-tubular network may be altered in heart failure. A marked loss of T-tubules has been observed in failing canine ventricular myocytes (He et al. 2001;Balijepalli et al. 2003), although it is unclear whether such changes occur in human heart failure (Kaprielian et al. 2000;Wong et al. 2001;Ohler et al. 2001). However, the structural organization of T-tubules may be altered in failing human cardiomyocytes (Kostin et al. 1998;Kaprielian et al. 2000;Wong et al. 2001;Louch et al. 2004). It is not known how such disorganization may influence excitation-...

Alterations in trans-sarcolemmal and sarcoplasmic reticulum (SR) Ca2+ fluxes may contribute to impaired cardiomyocyte contraction and relaxation in heart failure. We investigated the mechanisms underlying heart failure progression in mice with conditional, cardiomyocyte-specific excision of the SR Ca 2+ -ATPase (SERCA) gene. At 4 weeks following gene deletion (4-week KO) cardiac function remained near normal values. However, end-stage heart failure developed by 7 weeks (7-week KO) as systolic and diastolic performance declined. Contractions in isolated myocytes were reduced between 4-and 7-week KO, and relaxation was slowed.

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Deterioration of cardiac contractility during congestive heart failure (CHF) is believed to involve decreased function of individual cardiomyocytes and may include reductions in contraction magnitude and/or kinetics. We examined the progression of in vivo and in vitro alterations in contractile function in CHF mice and investigated underlying alterations in Ca2+ homeostasis. Following induction of myocardial infarction (MI), mice with CHF were examined at early (1 wk post-MI) and chronic (10 wk post-MI) stages of disease development. Sham-operated mice served as controls. Global and local left ventricle function were assessed by echocardiography in sedated animals (∼2% isoflurane). Excitation-contraction coupling was examined in cardiomyocytes isolated from the viable septum. CHF progression between 1 and 10 wk post-MI resulted in increased mortality, development of hypertrophy, and deterioration of global left ventricular function. Local function in the noninfarcted myocardium also declined, as posterior wall shortening velocity was reduced in chronic CHF (1.2 ± 0.1 vs. 1.9 ± 0.2 cm/s in sham). Parallel alterations occurred in isolated cardiomyocytes since contraction and Ca2+ transient time to peak values were prolonged in chronic CHF (115 ± 6 and 158 ± 11% sham values, respectively). Surprisingly, contraction and Ca2+ transient magnitudes in CHF were larger than sham values at both time points, resulting from increased sarcoplasmic reticulum Ca2+ content and greater Ca2+ influx via L-type channels. We conclude that, in mice with CHF following myocardial infarction, declining myocardial function involves slowing of cardiomyocyte contraction without reduction in contraction magnitude. Corresponding alterations in Ca2+ transients suggest that slowing of Ca2+ release is a critical mediator of CHF progression.