Propagation of Cardiomyocyte Hypercontracture by Passage of Na + Through Gap Junctions

Abstract

Abstract —Prolonged ischemia increases cytosolic Ca 2+ concentration in cardiomyocytes. Cells with severely elevated cytosolic Ca 2+ may respond to reperfusion, developing hypercontracture, sarcolemmal disruption, and death. Cardiomyocytes are efficiently connected through gap junctions (GJs) to form a functional syncytium, and it has been shown that hypercontracture can be propagated to adjacent myocytes through a GJ-mediated mechanism. This study investigated the mechanism of propagation of cell injury associated with sarcolemmal rupture in end-to-end connected pairs of isolated rat cardiomyocytes. Microinjection of extracellular medium into one of the cells to simulate sarcolemmal disruption induced a marked increase in cytosolic Ca 2+ (fura-2) and Na + (SBFI) in the adjacent cell and its hypercontracture in <30 seconds (22 of 22 cell pairs). This process was not modified when Ca 2+ release from the sarcoplasmic reticulum was blocked with 10 μmol/L ryanodine (5 of 5 cell pairs), but it was fully dependent on the presence of Ca 2+ in the extracellular buffer. Blockade of L-type Ca 2+ channels with 10 μmol/L nifedipine did not alter propagation of hypercontracture. However, the presence of 15 to 20 μmol/L KB-R7943, a highly selective blocker of reverse Na + /Ca 2+ exchange, prevented propagation of hypercontracture in 16 of 20 cell pairs ( P <0.01) without interfering with GJ permeability, as assessed by the Lucifer Yellow transfer method. Addition of the Ca 2+ chelator EGTA (2 mmol/L) to the injection solution prevented hypercontracture in the injected cell but not in the adjacent one (n=5). These results indicate that passage of Na + through GJ from hypercontracting myocytes with ruptured sarcolemma to adjacent cells, and secondary entry of [Ca 2+ ] o via reverse Na + /Ca 2+ exchange, can contribute to cell-to-cell propagation of hypercontracture. This previously unrecognized mechanism could increase myocardial necrosis during ischemia-reperfusion in vivo and be the target of new treatments aimed to limit it.