T-Tubule Proximity Modulates RyR Properties Figure 7. Ca2+ removal by NCX during caffeine application. A. Example of current and [Ca2+]i transient recording obtained during the last conditioning pulse from -70 to +10 mV and caffeine application (left). The decay of the current was fit by a 1- or 2-exponential according to the goodness of fit (R2.0.95) and with the 2 amplitudes being negative. The right panel is a typical example of a 2-exponential decay in a CTRL myocyte. B. Example of monophasic INCX decay (left) and mean data on incidence of biphasic decay (middle). The percentage of cells better fit by a biphasic exponential was significantly higher in CTRL than in MI (CTRL, Npigs = 4, ncells = 17; MI, Npigs = 4, ncells = 12, P,0.05). In addition, Tau of fast component of INCX decay tended to be faster in CTRL than in MI (right, CTRL, Npigs = 4, ncells = 8, vs. MI, Npigs = 2, ncells = 4, P = NS). * denotes P,0.05. doi:10.1371/journal.pone.0025100.g007 modeling has shown that when inter-RyR coupling is decreased spark duration is prolonged [47]. Pharmacological reduction of RyR Po in combination with increased SR load leads to prolonged spark events [48]. In MI the increased SR Ca2+ load may enhance the occurrence of long sparks, although changes in RyR regulation may also contribute. Perspectives for myocyte function after MI The current data were obtained from pigs with a moderate size infarct in the posterior wall that did not lead to global heart failure; myocytes were obtained from an area with reduced perfusion adjacent to the MI [18]. This model represents a stage and aspect of ischemic cardiomyopathy in which NCX is not upregulated and SR content not reduced, as typically described in models of heart failure. It should be noted that NCX upregulation is not a universal feature of cardiac remodeling [49]. The combination of loss of TTs, high SR content and low NCX set the stage for a specific myocyte phenotype. The redistribution of spark sites with a reduced fraction of sites near-TT will by itself reduce the extrusion of released Ca2+ from the cell by NCX. Indeed, at delayed release areas, which represent the largest fraction of the cell, spark frequency is increased but the distance to NCX reduces Ca2+ removal. In addition, at early sites, NCX density is reduced, so cellular Ca2+ loss will be small. Therefore, although total spark mass is increased, actual spark- mediated cellular Ca2+ efflux is minimized. This mechanism would preserve SR Ca2+ content. Reduced removal of spontaneous Ca2+ release and higher SR Ca2+ content may on the other hand promote transition from PLoS ONE | www.plosone.org 9 October 2011 | Volume 6 | Issue 10 | e25100