Abstract 412: Increased Spontaneous Ca2+ Release in Arrhythmogenic Purkinje Cells from Infarcted Canine Heart is Due to a Lowered Threshold of Ca2+ Release Channels
Spontaneous Ca2+ release in Purkinje cells from the endocardial border zone of 48hr infarcted canine heart (IZPCs) is the source of arrhythmias.
Purpose: to study the characteristics of Ca2+ release events in subendocardial Purkinje cells from normal (NZPCs) and IZPCs.
Methods: Ca2+ release events (elevations of basal [Ca2+] equivalent to F/F0 3.4 SD over F0) in intact and saponin-permeabilized cells were imaged using 2D confocal microscopy (16.6Hz) with Fluo-4 and analyzed using a custom IDL program. Ca2+ dependence of Ca2+ release events was determined in permeabilized cells at various [Ca2+] ([Ca2+]cyto from 25 to150nM).
Results: In intact cells, the majority (94% NZPCs; 92% IZPCs) of spontaneous Ca2+ release events (typical events, TE) were detected within a single frame. Other events lasted over several frames and had a greater spatial extent (large events, LE). TE event rate in NZPCs was 6x higher in Subsarcolemmal region (SSL, a layer of ~5μm below the membrane; 0.0089±0.018 ev/μm2/sec, 747events, n=4) than in Core (0.0015±0.004 ev/μm2/sec, 275events, n=34, p<0.01). In IZPCs, TE event rate increased in SSL (0.0161±0.031 ev/μm2/sec, 1198events, n=30, p<0.05) but not in Core. TE amplitude in IZPCs was significantly higher than NZPCs in both SSL (1.32±0.004 vs 1.210.004 F/F0) and Core (1.300.±006 vs 1.185±0.004 F/F0) whereas spatial extent did not differ (SSL: 9.35±0.35 vs 9.04±0.40μm2, Core: 9.11±0.59 vs 8.99±0.68 μm2). The event rate in permeabilized cells was related to [Ca2+]cyto 25–150nM in both SSL and Core (n=104; NZPCs, n=98; IZPCs). In IZPCs, the event rate was significantly increased at each [Ca2+ 100nM), SRcyto. In both intact and permeabilized cells ([Ca2+]cyto SR[Ca2+] content in IZPCs estimated by rapid caffeine (20mM) application did not differ from NZPCs. In both SSL and Core, LEs in intact IZPCs were more frequent (p<0.05), had higher amplitude (p<0.05) and faster decay (p<0.05) than in NZPCs.
Conclusion: Increased spontaneous Ca2+ release events in both SSL and Core of IZPCs are due to uniform regionally increased Ca2+ release channel sensitivity without a change in SR content. These properties would lower threshold of Ca2+ release channels, setting the stage for a high frequency of arrhythmogenic Cell wide Ca2+ waves observed in IZPCs.