Background-In ventricular myocytes, the majority of structures that couple excitation to the systolic rise of Ca 2ϩ are located at the transverse tubular (t-tubule) membrane. In the failing ventricle, disorganization of t-tubules disrupts excitation contraction coupling. The t-tubule membrane is virtually absent in the atria of small mammals resulting in spatiotemporally distinct profiles of intracellular Ca 2ϩ release on stimulation in atrial and ventricular cells. The aims of this study were to determine (i) whether atrial myocytes from a large mammal (sheep) possess t-tubules, (ii) whether these are functionally important, and (iii) whether they are disrupted in heart failure. Methods and Results-Sheep left atrial myocytes were stained with di-4-ANEPPS. Nearly all control cells had an extensive t-tubule network resulting in each voxel in the cell being nearer to a membrane (sarcolemma or t-tubule) than would otherwise be the case. T-tubules decrease the distance of 50% of voxels from a membrane from 3.35Ϯ0.15 to 0.88Ϯ0.04 m. During depolarization, intracellular Ca 2ϩ rises simultaneously at the cell periphery and center. In heart failure induced by rapid ventricular pacing, there was an almost complete loss of atrial t-tubules. The distance of 50% of voxels from a membrane increased to 2.04Ϯ0.08 m, and there was a loss of early Ca 2ϩ release from the cell center. Conclusion-Sheep atrial myocytes possess a substantial t-tubule network that synchronizes the systolic Ca 2ϩ transient. In heart failure, this network is markedly disrupted. This may play an important role in changes of atrial function in heart failure. (Circ Heart Fail. 2009;2:482-489.)

Cardiovascular disease is a leading cause of death worldwide and there is a pressing need for new therapeutic strategies to treat such conditions. The risk of developing cardiovascular disease increases dramatically with age, yet the majority of experimental research is executed using young animals. The cardiac extracellular matrix (ECM), consisting predominantly of fibrillar collagen, preserves myocardial integrity, provides a means of force transmission and supports myocyte geometry. Disruptions to the finely balanced control of collagen synthesis, post-synthetic deposition, post-translational modification and degradation may have detrimental effects on myocardial functionality. It is now well established that the aged heart is characterized by fibrotic remodelling, but the mechanisms responsible for this are incompletely understood. Furthermore, studies using aged animal models suggest that interstitial remodelling with disease may be age-dependent. Thus with the identification of new therapeutic strategies targeting fibrotic remodelling, it may be necessary to consider age-dependent mechanisms. In this review, we discuss remodelling of the cardiac collagen matrix as a function of age, whilst highlighting potential novel mediators of age-dependent fibrotic pathways.

Non-technical summary Heart failure is where the heart is unable to pump sufficient blood in order to meet the requirements of the body. Symptoms of heart failure often first present during exercise. During exercise the blood levels of a hormone, noradrenaline, increase and activate receptors on the muscle cells of the heart known as β-receptors causing the heart to contract more forcefully. We show that in heart failure the response to β-receptor stimulation is reduced and this appears to be due to a failure of the β-receptor to signal correctly to downstream targets inside the cell. However, by-passing the β-receptor and directly activating one of the downstream targets, an enzyme known as adenylyl cyclase, inside the cell restores the function of the muscle cells in failing hearts. These observations provide a number of potential targets for therapies to improve the function of the heart in patients with heart failure.Abstract Reduced inotropic responsiveness is characteristic of heart failure (HF). This study determined the cellular Ca 2+ homeostatic and molecular mechanisms causing the blunted β-adrenergic (β-AR) response in HF. We induced HF by tachypacing in sheep; intracellular Ca 2+ concentration was measured in voltage-clamped ventricular myocytes. In HF, Ca 2+ transient amplitude and peak L-type Ca 2+ current (I Ca-L ) were reduced (to 70 ± 11% and 50 ± 3.7% of control, respectively, P < 0.05) whereas sarcoplasmic reticulum (SR) Ca 2+ content was unchanged. β-AR stimulation with isoprenaline (ISO) increased Ca 2+ transient amplitude, I Ca-L and SR Ca 2+ content in both cell types; however, the response of HF cells was markedly diminished (P < 0.05). Western blotting revealed an increase in protein phosphatase levels (PP1, 158 ± 17% and PP2A, 188 ± 34% of control, P < 0.05) and reduced phosphorylation of phospholamban in HF (Ser16, 30 ± 10% and Thr17, 41 ± 15% of control, P < 0.05). The β-AR receptor kinase GRK-2 was also increased in HF (173 ± 38% of control, P < 0.05). In HF, activation of adenylyl cyclase with forskolin rescued the Ca 2+ transient, SR Ca 2+ content and SR Ca 2+ uptake rate to the same levels as control cells in ISO. In conclusion, the reduced responsiveness of the myocardium to β-AR agonists in HF probably arises as a consequence of impaired phosphorylation of key intracellular proteins responsible for regulating the SR Ca 2+ content and therefore failure of the systolic Ca 2+ transient to increase appropriately during β-AR stimulation.

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