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The article presents a study which discloses the fundamental mechanisms diluted sarcoplasmic reticulum (SR) Ca2+ discharge in persistent atrial fibrillation (AF). The study examines the membrane currents and cell shortening in right atrial myocytes separated from sheep with persistent AF and compared it to matched control animals. Conclusion shows that in persistent AF, diluted SR Ca2+ discharge is a primary factor in contractile dysfunction.

The article discusses a study which examines the effect of the pharmacological inhibition of Na/Ca exchange (NCX) on cellular Ca2+. The study supports the hypothesis that in normal myocytes and physiological conditions, the drug SEA-0400 would not exhibit mode dependence. Even partial inhibition of NCX would lead to a net gain of Ca2+. The study also confirms the potency of SEA-0400 to reduce potentially arrhythmogenic inward NCX currents.

Rationale: Persistent atrial fibrillation (AF) has been associated with structural and electric remodeling and reduced contractile function., Objective: To unravel mechanisms underlying reduced sarcoplasmic reticulum (SR) Ca(2+) release in persistent AF., Methods: We studied cell shortening, membrane currents, and [Ca(2+)](i) in right atrial myocytes isolated from sheep with persistent AF (duration 129+/-39 days, N=16), compared to matched control animals (N=21). T-tubule density, ryanodine receptor (RyR) distribution, and local [Ca(2+)](i) transients were examined in confocal imaging., Results: Myocyte shortening and underlying [Ca(2+)](i) transients were profoundly reduced in AF (by 54.8% and 62%, P<0.01). This reduced cell shortening could be corrected by increasing [Ca(2+)](i). SR Ca(2+) content was not different. Calculated fractional SR Ca(2+) release was reduced in AF (by 20.6%, P<0.05). Peak Ca(2+) current density was modestly decreased (by 23.9%, P<0.01). T-tubules were present in the control atrial myocytes at low density and strongly reduced in AF (by 45%, P<0.01), whereas the regular distribution of RyR was unchanged. Synchrony of SR Ca(2+) release in AF was significantly reduced with increased areas of delayed Ca(2+) release. Propagation between RyR was unaffected but Ca(2+) release at subsarcolemmal sites was reduced. Rate of Ca(2+) extrusion by Na(+)/Ca(2+) exchanger was increased., Conclusions: In persistent AF, reduced SR Ca(2+) release despite preserved SR Ca(2+) content is a major factor in contractile dysfunction. Fewer Ca(2+) channel-RyR couplings and reduced efficiency of the coupling at subsarcolemmal sites, possibly related to increased Na(+)/Ca(2+) exchanger, underlie the reduction in Ca(2+) release.

Block of Na/Ca exchange (NCX) has potential therapeutic applications, in particular, if a mode-selective block could be achieved, but also carries serious risks for disturbing the normal Ca2+ balance maintained by NCX. We have examined the effects of partial inhibition of NCX by SEA-0400 (1 or 0.3 micromol/L) in left ventricular myocytes from healthy pigs or mice and from mice with heart failure (MLP-/-). During voltage clamp ramps with [Ca2+](i) buffering, block of reverse mode block was slightly larger than of forward mode (by 25+/-5%, P<0.05). In the absence of [Ca2+](i) buffering and with sarcoplasmic reticulum (SR) fluxes blocked, rate constants for Ca2+ influx and Ca2+ efflux were reduced to the same extent (to 36+/-6% and 32+/-4%, respectively). With normal SR function the reduction of inward NCX current (I(NCX)) was 57+/-10% (n=10); during large caffeine-induced Ca2+ transients, it was larger (82+/-3%). [Ca2+](i) transients evoked during depolarizing steps increased (from 424+/-27 to 994+/-127 nmol/L at +10 mV, P<0.05), despite a reduction of I(CaL) by 27%. Resting [Ca2+](i) increased; there was a small decrease in the rate of decline of [Ca2+](i). SR Ca2+) content increased more than 2-fold. Contraction amplitude of field-stimulated myocytes increased in healthy myocytes but not in myocytes from MLP-/- mice, in which SR Ca2+ content remained unchanged. These data provide proof-of-principle that even partial inhibition of NCX results in a net gain of Ca2+. Further development of NCX blockers, in particular, for heart failure, must balance potential benefits of I(NCX) reduction against effects on Ca2+ handling by refining mode dependence and/or including additional targets.

In ventricular cardiac myocytes, T-tubule density is an important determinant of the synchrony of sarcoplasmic reticulum (SR) Ca2+ release and could be involved in the reduced SR Ca2+ release in ischemic cardiomyopathy. We therefore investigated T-tubule density and properties of SR Ca2+ release in pigs, 6 weeks after inducing severe stenosis of the circumflex coronary artery (91+/-3%, N=13) with myocardial infarction (8.8+/-2.0% of total left ventricular mass). Severe dysfunction in the infarct and adjacent myocardium was documented by magnetic resonance and Doppler myocardial velocity imaging. Myocytes isolated from the adjacent myocardium were compared with myocytes from the same region in weight-matched control pigs. T-tubule density quantified from the di-8-ANEPPS (di-8-butyl-amino-naphthyl-ethylene-pyridinium-propyl-sulfonate) sarcolemmal staining was decreased by 27+/-7% (P<0.05). Synchrony of SR Ca2+ release (confocal line scan images during whole-cell voltage clamp) was reduced in myocardium myocytes. Delayed release (ie, half-maximal [Ca2+]i occurring later than 20 ms) occurred at 35.5+/-6.4% of the scan line in myocardial infarction versus 22.7+/-2.5% in control pigs (P<0.05), prolonging the time to peak of the line-averaged [Ca2+]i transient (121+/-9 versus 102+/-5 ms in control pigs, P<0.05). Delayed release colocalized with regions of T-tubule rarefaction and could not be suppressed by activation of protein kinase A. The whole-cell averaged [Ca2+]i transient amplitude was reduced, whereas L-type Ca2+ current density was unchanged and SR content was increased, indicating a reduction in the gain of Ca2+-induced Ca2+ release. In conclusion, reduced T-tubule density during ischemic remodeling is associated with reduced synchrony of Ca2+ release and reduced efficiency of coupling Ca2+ influx to Ca2+ release.

The [Ca2+]i transient of ventricular myocytes during normal excitation-contraction coupling is the summation of primary Ca2+ release events, which originate at the junction of the sarcoplasmic reticulum (SR) and the T-tubular system. Studies in small mammals have shown a high density of release sites, but little is known of larger mammals. We have studied the spatial distribution of SR Ca2+ release in pig ventricular myocytes using a confocal microscopy. In 69 of 107 cells, large inhomogeneities of Ca2+ release were observed along the longitudinal scan line. Areas where the increase of [Ca2+]i was delayed (time to 50% of peak F/F0 [where F indicates fluorescence intensity, and F0 indicates F at rest] was 26+/-1 ms in delayed areas versus 11+/-2 ms in early areas) and smaller (peak F/F0 was 2.27+/-0.10 for delayed areas versus 2.69+/-0.13 for early areas; n=13 cells, P<0.05) could be up to 26 microm wide. The sum of all delayed areas could make up to 55% of the line scan. The spatial pattern was constant during steady-state stimulation and was not altered by enhancing Ca2+ channel opening or SR Ca2+ content (Bay K8644, isoproterenol). Imaging of sarcolemmal membranes revealed several areas devoid of T tubules, but SR Ca2+ release channels were homogeneously distributed. In contrast, compared with pig myocytes, mouse myocytes had a very dense T-tubular network, no large inhomogeneities of release, and a faster rate of rise of [Ca2+]i. In conclusion, in pig ventricular myocytes, areas of delayed release are related to regional absence of T tubules but not ryanodine receptors. This lower number of functional couplons contributes to a slower overall rate of rise of [Ca2+]i.