Previous studies have revealed that the regional accumulation of ischemic metabolites including hydrogen ion (H+) and PcO2diminish after repeated occlusions. We postulated that tins diminution reflects a blunted metabolic response that is related to the severity of ischemic injury and, hence, may be most pronounced in subendocardial (ENDO) regions. To investigate tins hypothesis, the left anterior descending coronary artery was serially occluded three times in 51 dogs for a period of either 3 minutes (n=15), 5 minutes (n=18), or 15 minutes (n=18). Each occlusion was separated by 45 minutes of reperfusion. Myocardial [H+] was measured in the endomyocardium and in the epimyocardium of the ischemic anterior wall by use of miniature pH glass electrodes. Accumulation of H+during occlusion (A[H+]) in the ENDO region was significantly less during the second occlusion when compared with the first occlusion (3-minute occlusions: 28.2±3.7 nM/l vs. 39.4±5.4 nM/l, p<0.002; 5-minute occlusions: 49.8±5.0 nM/l vs. 72.1±6.5 nM/l, p<0.0002; 15-minute occlusions: 132.3±14.6 nM/1 vs. 225.6±27.7 nM/1, p<0.0003). A similar trend was noted for A[H+] in the subepicardial (EPI) regions. During occlusion, the rise in [H+] occurred sooner, and A[H+] was consistently greater in the ENDO when compared with the EPI regions (p<0.05). Regional myocardial blood flow did not change during the three occlusions, indicating that the diminution in H+accumulation stemmed from a decrease in H+production and not from an increase in collateral flow. The decrement in H+accumulation between the first and second occlusions (±[H+]|-±A[H+]2)1 was greater in the ENDO than in the EPI regions (p<0.05); 2) correlated with the duration of occlusion (ENDO: r=0.66, p<0.001; EPI: r=0.82, p<0.0001); and 3) was related to the impairment of anterior wall systolic shortening after the first reperfusion period. These findings suggest that the diminution in H+production that follows serial coronary occlusions reflects a blunted metabolic response that is related to both the duration of ischemia and the degree of systolic dysfunction. Moreover±, though attenuation of ischemic metabolite production occurs transmurally, it is most pronounced in the deep ENDO regions.