Subcellular heterogeneity of ryanodine receptor properties in ventricular myocytes with low T-tubule density.
BACKGROUND
In ventricular myocytes of large mammals, not all ryanodine receptor (RyR) clusters are associated with T-tubules (TTs); this fraction increases with cellular remodeling after myocardial infarction (MI).


OBJECTIVE
To characterize RyR functional properties in relation to TT proximity, at baseline and after MI.


METHODS
Myocytes were isolated from left ventricle of healthy pigs (CTRL) or from the area adjacent to a myocardial infarction (MI). Ca(2+) transients were measured under whole-cell voltage clamp during confocal linescan imaging (fluo-3) and segmented according to proximity of TTs (sites of early Ca(2+) release, F>F(50) within 20 ms) or their absence (delayed areas). Spontaneous Ca(2+) release events during diastole, Ca(2+) sparks, reflecting RyR activity and properties, were subsequently assigned to either category.


RESULTS
In CTRL, spark frequency was higher in proximity of TTs, but spark duration was significantly shorter. Block of Na(+)/Ca(2+) exchanger (NCX) prolonged spark duration selectively near TTs, while block of Ca(2+) influx via Ca(2+) channels did not affect sparks properties. In MI, total spark mass was increased in line with higher SR Ca(2+) content. Extremely long sparks (>47.6 ms) occurred more frequently. The fraction of near-TT sparks was reduced; frequency increased mainly in delayed sites. Increased duration was seen in near-TT sparks only; Ca(2+) removal by NCX at the membrane was significantly lower in MI.


CONCLUSIONS
TT proximity modulates RyR cluster properties resulting in intracellular heterogeneity of diastolic spark activity. Remodeling in the area adjacent to MI differentially affects these RyR subpopulations. Reduction of the number of sparks near TTs and reduced local NCX removal limit cellular Ca(2+) loss and raise SR Ca(2+) content, but may promote Ca(2+) waves.
Laboratory of Experimental Cardiology, University of Leuven, Leuven, Belgium.
Liesbeth
L
Biesmans
Niall
N
Macquaide
Frank R
FR
Heinzel
Virginie
V
Bito
Godfrey L
GL
Smith
Karin R
KR
Sipido
D000818:Animals; D002110:Caffeine / Q000494:pharmacology; D020013:Calcium Signaling / Q000187:drug effects; D002478:Cells, Cultured; D006352:Heart Ventricles / Q000187:drug effects / Q000473:pathology* / Q000503:physiopathology; D008954:Models, Biological; D009203:Myocardial Infarction / Q000378:metabolism / Q000473:pathology / Q000503:physiopathology; D032383:Myocytes, Cardiac / Q000187:drug effects / Q000378:metabolism*; D019837:Ryanodine Receptor Calcium Release Channel / Q000378:metabolism*; D012508:Sarcolemma / Q000187:drug effects / Q000378:metabolism; D012519:Sarcoplasmic Reticulum / Q000187:drug effects / Q000378:metabolism* / Q000473:pathology; D019831:Sodium-Calcium Exchanger / Q000378:metabolism; D013347:Subcellular Fractions / Q000187:drug effects / Q000378:metabolism; D034421:Sus scrofa; D013997:Time Factors; D020257:Ventricular Remodeling / Q000187:drug effects
D019837:Ryanodine Receptor Calcium Release Channel; D019831:Sodium-Calcium Exchanger; D002110:Caffeine