Infarct size reduction after ischemic preconditioning has been reported as an all-or-nothing response, with a threshold of 2 to 5 min of preconditioning ischemia time. Furthermore, infarct size reduction is associated with glycogen depletion before prolonged ischemia. We sought to determine whether increasing preconditioning ischemia time results in progressive myocardial glycogen depletion and infarct size reduction and if so, whether infarct size reduction correlates with glycogen depletion before prolonged ischemia. Therefore, ventilated rats underwent a single episode of preconditioning ischemia lasting 1.25 (n = 18), 2.5 (n = 20), 5 (n = 9), or 10 (n = 16) min, or multiple episodes cumulating in 10 (2 × 5 min, n = 17) or 20 (4 × 5 min, n = 13 or 2 × 10 min, n = 14) min of preconditioning ischemia time, all followed by 5 min of reperfusion. Control rats (n = 22) were not preconditioned. In separate groups, glycogen was determined before whereas infarct size was determined after prolonged ischemia (45 min) and reperfusion 120 min). Myocardial glycogen levels and infarct size are shown as a function of preconditioning ischemia time. Download : Download high-res image (103KB) Download : Download full-size image In addition, infarct size reduction correlated with glycogen depletion before prolonged ischemia (r = 0.98; 19 P l 0.001). Thus, increasing preconditioning ischemia time results in progressive myocardial glycogen depletion and infarct size reduction, reaching a maximal effect when the preconditioning ischemia time is ≥5 min. This maximal effect may be due to marked depletion of myocardial glycogen stores before prolonged ischemia