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.)

T he synchronous rise of the systolic Ca 2+ transient in mammalian ventricular myocytes requires the presence of an extensive and regular transverse (t)-tubular system.1 These t-tubules ensure close apposition of L-type Ca 2+ channels (LTCCs) and sarcoplasmic reticulum (SR) Ca 2+ release channels (ryanodine receptors [RyRs]) forming dyads or couplons where excitation-contraction coupling commences.2,3 The t-tubules are also surrounded by a continuous network of SR, which is thought to assist with amplification of the initial Ca 2+ entry during the action potential and contribute to the synchronous rise of systolic Ca 2+ . 4,5The t-tubule and SR networks are, however, labile with disorganization and loss commonly observed in heart failure (HF). [5][6][7][8][9] In such circumstances the loss of t-tubules leads to dyssynchronous Ca 2+ release patterns, a smaller systolic Ca 2+ transient, and altered β-adrenergic (β-adrenergic receptor) signaling. 6-10 Conversely, recovery from HF is associated with restoration of the t-tubule network along with normalization of β-adrenergic receptor signaling and resynchronization of the systolic Ca 2+ transient. 9,11In This Issue, see p 961More extensive differences in t-tubule organization and density than those occurring in the ventricle during HF are known to exist between the atrium and the ventricle. For example, small mammals (mouse, rat, rabbit, etc) completely lack or possess only a rudimentary, predominantly axially arranged, t-tubule network.2-14 Conversely, some studies have suggested that limited numbers of atrial cells from smaller laboratory species such as the rat have a more ventricular-like t-tubule pattern, 15 although these particular cells may be of different lineage and a feature of the pulmonary vein sleeve region. 16The poorly developed t-tubule network in these atrial myocytes leads to the characteristic early peripheral and delayed central Rationale: Transverse tubules (t-tubules) regulate cardiac excitation-contraction coupling and exhibit interchamber and interspecies differences in expression. In cardiac disease, t-tubule loss occurs and affects the systolic calcium transient. However, the mechanisms controlling t-tubule maintenance and whether these factors differ between species, cardiac chambers, and in a disease setting remain unclear.Objective: To determine the role of the Bin/Amphiphysin/Rvs domain protein amphiphysin II (AmpII) in regulating t-tubule maintenance and the systolic calcium transient. Methods and Results:T-tubule density was assessed by di-4-ANEPPS, FM4-64 or WGA staining using confocal microscopy. In rat, ferret, and sheep hearts t-tubule density and AmpII protein levels were lower in the atrium than in the ventricle. Heart failure (HF) was induced in sheep using right ventricular tachypacing and ferrets by ascending aortic coarctation. In both HF models, AmpII protein and t-tubule density were decreased in the ventricles. In the sheep, atrial t-tubules were also lost in HF and AmpII levels decreased. Conversely, junctophilin 2 leve...

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Richards MA, Clarke JD, Saravanan P, Voigt N, Dobrev D, Eisner DA, Trafford AW, Dibb KM. Transverse tubules are a common feature in large mammalian atrial myocytes including human. Am J Physiol Heart Circ Physiol 301: H1996 -H2005, 2011. First published August 12, 2011; doi:10.1152 doi:10. /ajpheart.00284.2011 tubules are surface membrane invaginations that are present in all mammalian cardiac ventricular cells. The apposition of L-type Ca 2ϩ channels on t tubules with the sarcoplasmic reticulum (SR) constitutes a "calcium release unit" and allows close coupling of excitation to the rise in systolic Ca 2ϩ . T tubules are virtually absent in the atria of small mammals, and therefore Ca 2ϩ release from the SR occurs initially at the periphery of the cell and then propagates into the interior. Recent work has, however, shown the occurrence of t tubules in atrial myocytes from sheep. As in the ventricle, Ca 2ϩ release in these cells occurs simultaneously in central and peripheral regions. T tubules in both the atria and the ventricle are lost in disease, contributing to cellular dysfunction. The aim of this study was to determine if the occurrence of t tubules in the atrium is restricted to sheep or is a more general property of larger mammals including humans. In atrial tissue sections from human, horse, cow, and sheep, membranes were labeled using wheat germ agglutinin. As previously shown in sheep, extensive t-tubule networks were present in horse, cow, and human atrial myocytes. Analysis shows half the volume of the cell lies within 0.64 Ϯ 0.03, 0.77 Ϯ 0.03, 0.84 Ϯ 0.03, and 1.56 Ϯ 0.19 m of t-tubule membrane in horse, cow, sheep, and human atrial myocytes, respectively. The presence of t tubules in the human atria may play an important role in determining the spatio-temporal properties of the systolic Ca 2ϩ transient and how this is perturbed in disease.atria; t-tubule heart T TUBULES ARE INVAGINATIONS of the surface membrane that penetrate deep within the cell. They occur at the z-line and are present in ventricular myocytes of all mammalian species studied to date. Many of the proteins involved in excitation contraction coupling are located on, or in close proximity to, the t-tubule membrane (16). In cardiac muscle, excitation contraction coupling is initiated by opening of L-type Ca 2ϩ channels and subsequent Ca 2ϩ entry (I Ca,L ) triggering release of Ca 2ϩ from the intracellular Ca 2ϩ store, the sarcoplasmic reticulum (SR). T tubules allow close coupling of I Ca,L to ryanodine receptors (RyRs) on the SR membrane resulting in rapid triggered Ca 2ϩ release in the cell interior upon electrical excitation. Thus, in ventricular cells, which have a regular t-tubule network penetrating the entire cell, the rise in intracellular Ca 2ϩ responsible for contraction is both rapid and synchronous throughout the entire cell. Chemically induced t-tubule removal with formamide results in this initial rise in intracellular Ca 2ϩ concentration being localized to the periphery and then propagating to the cell center (46).To ...

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Cardiovascular disease is the main cause of death globally, accounting for over 17 million deaths each year. As the incidence of cardiovascular disease rises markedly with age, the overall risk of cardiovascular disease is expected to increase dramatically with the aging of the population such that by 2030 it could account for over 23 million deaths per year. It is therefore vitally important to understand how the heart remodels in response to normal aging for at least two reasons: i) to understand why the aged heart is increasingly susceptible to disease; and ii) since it may be possible to modify treatment of disease in older adults if the underlying substrate upon which the disease first develops is fully understood. It is well known that age modulates cardiac function at the level of the individual cardiomyocyte. Generally, in males, aging reduces cell shortening, which is associated with a decrease in the amplitude of the systolic Ca(2+) transient. This may arise due to a decrease in peak L-type Ca(2+) current. Sarcoplasmic reticulum (SR) Ca(2+) load appears to be maintained during normal aging but evidence suggests that SR function is disrupted, such that the rate of sarco/endoplasmic reticulum Ca(2+)-ATPase (SERCA)-mediated Ca(2+) removal is reduced and the properties of SR Ca(2+) release in terms of Ca(2+) sparks are altered. Interestingly, Ca(2+) handling is modulated by age to a lesser degree in females. Here we review how cellular contraction is altered as a result of the aging process by considering expression levels and functional properties of key proteins involved in controlling intracellular Ca(2+). We consider how changes in both electrical properties and intracellular Ca(2+) handling may interact to modulate cardiomyocyte contraction. We also reflect on why cardiovascular risk may differ between the sexes by highlighting sex-specific variation in the age-associated remodeling process. This article is part of a Special Issue entitled CV Aging.

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The glycine-tyrosine-glycine (GYG) sequence in the p-loop of K+ channel subunits lines a narrow pore through which K+ ions pass in single file intercalated by water molecules. Mutation of the motif can give rise to non-selective channels, but it is clear that other structural features are also required for selectivity because, for instance, a recently identified class of cyclic nucleotide-gated pacemaker channels has the GYG motif but are poorly K+ selective. We show that mutation of charged glutamate and arginine residues behind the selectivity filter in the Kir3.1/Kir3.4 K+ channel reduces or abolishes K+ selectivity, comparable with previously reported effects in the Kir2.1 K+ channel. It has been suggested that a salt bridge exists between the glutamate-arginine residue pair. Molecular modeling indicates that the salt bridge does exist, and that it acts as a "bowstring" to maintain the rigid bow-like structure of the selectivity filter and restrict selectivity to K+. The modeling shows that relaxation of the bowstring by mutation of the residue pair leads to enhanced flexibility of the p-loop, allowing permeation of other cations, including polyamines. In experiments, mutation of the residue pair can also abolish polyamine-induced inward rectification. The latter effect occurs because polyamines now permeate rather than block the channel, to the remarkable extent that large polyamine currents can be measured.