Effects of acidic reperfusion on arrhythmias and Na(+)-K(+)-ATPase activity in regionally ischemic rat hearts.

We studied the effects of acidic reperfusion on 1) the incidence of ventricular fibrillation (VF) and 2) sarcolemmal Na(+)-K(+)-adenosinetriphosphatase (ATPase) activity. Isolated rat hearts (n = 12/group) were subjected to independent perfusion (15 min) of left and right coronary beds with pH 7.4 buffer followed by zero-flow ischemia (10 min) of the former bed. This was then reperfused for 5 min, with acidic (pH 6.6) buffer for the first 0 (control), 0.5,1,2, or 4 min and with pH 7.4 buffer thereafter. In the control group, 92% of hearts developed VF within 20 s of reperfusion and remained in VF. In the 0.5-, 1-, 2-, and 4-min acidic reperfusion groups, only 17, 17, 42, and 25% of hearts (P < 0.05 vs. control for all groups), respectively, exhibited VF during acidic reperfusion. However, on switching to pH 7.4, VF occurred in a further 50, 58, 0, and 0% of hearts, respectively; thus the overall incidences of VF were 67, 75, 42 (P < 0.05 vs. control), and 25% (P < 0.05 vs. control), respectively. Additional hearts (n = 8/group) were used for cytochemical determination of sarcolemmal Na(+)-K(+)-ATPase activity in both the ischemic/reperfused left ventricular (LV) and the nonischemic right ventricular (RV) free walls. Ischemia (10 min) reduced LV Na2(+)-K(+)-ATPase activity from 110 +/- 8 to 25 +/- 3% of the RV value. After 0.5, 1, 2, 3, and 4 min of acidic reperfusion, LV Na(+)-K(+)-ATPase activity was 24 +/- 3, 29 +/- 3, 37 +/- 5, 55 +/- 6, and 70 +/- 4, respectively (P < 0.05 vs. 10-min ischemia). No significant recovery of LV Na(+)-K(+)-ATPase activity occurred following up to 4 min of pH 7.4 reperfusion. In conclusion, 1) at least 2 min of acidic reperfusion is required to achieve sustained protection against VF and 2) the protective mechanism may involve enhanced recovery of Na(+)-K(+)-ATPase activity as well as inhibition of Na+ influx.