Propagation of cardiomyocyte hypercontracture by passage of Na+ through gap junctions

Prolonged ischemia increases cytosolic Ca2+ concentration in cardiomyocytes . Cells with severely elevated cytosolic Ca2+ may respond to reperfusion, d eveloping hypercontracture, sarcolemmal disruption, and death. Cardiomyocyt es are efficiently connected through gap junctions (GJs) to form a function al syncytium, and it has been shown that hypercontracture can be propagated to adjacent myocytes through a GJ-mediated mechanism. This study investiga ted the mechanism of propagation of cell injury associated with sarcolemmal rupture in end-to-end connected pairs of isolated rat cardiomyocytes. Micr oinjection of extracellular medium into one of the cells to simulate sarcol emmal disruption induced a marked increase in cytosolic Ca2+ (fura-2) and N a+ (SBFI) in the adjacent cell and its hypercontracture in <30 seconds (22 of 22 cell pairs). This process was not modified when Ca2+ release from the sarcoplasmic reticulum was blocked with 10 mu mol/L ryanodine (5 of 5 cell pairs), but it was fully dependent on the presence of Ca2+ in the extracel lular buffer. Blockade of L-type Ca2+ channels with 10 mu mol/L nifedipine did not alter propagation of hypercontracture, However, the presence of 15 to 20 mu mol/L KB-R7943, a highly selective blocker of reverse Na+/Ca2+ exc hange, 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 Ca2+ chelator EGTA (2 mmol/L) to th e injection solution prevented hypercontracture in the injected cell but no t in the adjacent one (n = 5). These results indicate that passage of Na+ t hrough GJ from hypercontracting myocytes with ruptured sarcolemma to adjace nt cells, and secondary entry of [Ca2+](o) via reverse Na+/Ca2+ exchange, c an contribute to cell-to-cell propagation of hypercontracture, This previou sly unrecognized mechanism could increase myocardial necrosis during ischem ia-reperfusion in vivo and be the target of new treatments aimed to limit i t.