INHIBITION OF NA+ H+ EXCHANGE PRESERVES VIABILITY, RESTORES MECHANICAL FUNCTION, AND PREVENTS THE PH PARADOX IN REPERFUSION INJURY TO RAT NEONATAL MYOCYTES/

Rat neonatal myocytes exposed to 2.5 mM CaCN and 20 mM 2-deoxyglucose at pH 6.2 (chemical hypoxia) quickly lose viability when pH is increas ed to 7.4, with or without washout Of inhibitors - a 'pH paradox'. In this study, we evaluated the effect of two Na+/H+ exchange inhibitors (dimethylamiloride and HOE694) and a Na+/Ca2+ exchange inhibitor (dich lorobenzamil) on pH-dependent reperfusion injury. Intracellular free C a2+ and electrical potential were monitored by laser scanning confocal microscopy of rat neonatal cardiac myocytes grown on coverslips and c o-loaded with Fluo-3 and tetramethylrhodamine methylester. After 30-mi n of chemical hypoxia at pH 6.2, mitochondria depolarized and Ca2+ beg an to increase uniformly throughout the cell. Free Ca2+ reached levels estimated to exceed 2 muM by 4 h. Washout of inhibitors at pH 7.4 (re perfusion), with or without dichlorobenzamil, killed most cells within 60 min, despite a marked reduction of Ca2+ in dichlorobenzamil-treate d cells. Reperfusion at pH 7.4 in the presence of 75 muM dimethylamilo ride or 20 muM HOE694, or at pH 6.2, prevented cell death. HOE694-trea ted cells placed into culture medium recovered mitochondrial membrane potential. In most cells, this occurred before normal Ca2+ was restore d. Contracted myocytes re-extended over a 24-h-period. By 48 hours, mo st cells contracted spontaneously and showed normal Ca2+ transients. O ur results indicate that Na+/H+ exchange inhibition protects against p H-dependent reperfusion injury and facilitates full recovery of cell f unction.