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-...

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Although increased levels of circulating interleukin (IL)-18 have been demonstrated in patients with cardiovascular diseases, the functional consequences of chronically increased circulating IL-18 with respect to myocardial function have not been defined. Thus we aimed to examine the effects of chronic IL-18 exposure on left ventricular (LV) function in healthy mice. Moreover, to clarify whether IL-18 has direct effects on the cardiomyocyte, we examined effects of IL-18 on cardiomyocytes in vitro. After 7 days of daily intraperitoneal injections of 0.5 microg IL-18 in healthy mice, a 40% (P < 0.05) reduction in the LV maximal positive derivative, a 25% (P < 0.05) reduction in the LV maximal rate of pressure decay, and a 2.8-fold (P < 0.001) increase in the LV end-diastolic pressure were measured, consistent with myocardial dysfunction. Furthermore, we measured a 75% (P < 0.05) reduction in beta-adrenergic responsiveness to isoproterenol. IL-18 induced myocardial hypertrophy, and there was a 2.9-fold increase (P < 0.05) in atrial natriuretic peptide mRNA expression in the LV myocardium. In vitro examinations of isolated adult rat cardiomyocytes being stimulated with IL-18 (0.1 microg/ml) exhibited an increase in peak Ca2+ transients (P < 0.05) and in diastolic Ca2+ concentrations (P < 0.05). In conclusion, this study shows that daily administration of IL-18 in healthy mice causes LV myocardial dysfunction and blunted beta-adrenergic responsiveness to isoproterenol. A direct effect of IL-18 on the cardiomyocyte in vitro was demonstrated, suggesting that IL-18 reduces the responsiveness of the myofilaments to Ca2+. Finally, induction of myocardial hypertrophy by IL-18 indicates a role for this cytokine in myocardial remodeling.

Cardiomyocyte contractility is regulated by the extracellular K(+) concentration ([K(+)](o)). Potassium dynamics in the T tubules during the excitation-contraction cycle depends on the diffusion rate of K(+), but this rate is not known. Detubulation of rat cardiomyocytes was induced by osmotic shock using formamide, which separated the surface membrane from the T tubules. Changes in current and membrane potential in voltage-clamped (-80 mV) and current-clamped control and detubulated cardiomyocytes were compared during rapid switches between 5.4 and 8.1 mM [K(+)](o), and the results were simulated in a mathematical model. In the voltage-clamp experiments, the current changed significantly slower in control than in detubulated cardiomyocytes during the switch from 5.4 to 8.1 mM [K(+)](o), as indicated by the times to achieve 25, 50, 90, and 95% of the new steady-state current [control (ms) t(25) = 98 +/- 12, t(50) = 206 +/- 20, t(90) = 570 +/- 72, t(95) = 666 +/- 92; detubulated t(25) = 61 +/- 11, t(50) = 142 +/- 17, t(90) = 352 +/- 52, t(95) = 420 +/- 69]. These time points were not significantly different either during the 8.1 to 5.4 mM [K(+)](o) switch or in current-clamped cardiomyocytes switching from 5.4 to 8.1 mM [K(+)](o). Mathematical simulation of the difference current between control and detubulated cardiomyocytes gave a t-tubular diffusion rate for K(+) of approximately 85 mum(2)/s. We conclude that the diffusion of K(+) in the T tubules is so slow that they constitute a functional compartment. This might play a key role in local regulation of the action potential, and thus in the regulation of cardiomyocyte contractility.

Adrenaline improved haemodynamics during laboratory quality CPR in pigs, but not with quality simulating clinically reported CPR performance.