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

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 micromol/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 micromol/L nifedipine did not alter propagation of hypercontracture. However, the presence of 15 to 20 micromol/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.