LOW-FLOW REPERFUSION AFTER MYOCARDIAL-ISCHEMIA ENHANCES LEUKOCYTE ACCUMULATION IN CORONARY MICROCIRCULATION

During early reperfusion after myocardial ischemia, the mechanisms res ponsible for leukocyte accumulation in the heart are unclear. We exami ned the effects of reducing coronary blood flow during reperfusion on leukocyte accumulation in coronary capillaries and postcapillary venul es. Isolated rat hearts were perfused for 30 min and then subjected to 30 min of 37 degrees C, no-flow ischemia. The deposition of fluoresce ntly labeled leukocytes was observed directly in coronary capillaries and venules using intravital microscopy after 5, 20, and 35 min of rep erfusion. Blood cell velocity was measured in venules after 5 min of r eperfusion (R-5), and shear rate (s(-1)) was calculated. Four groups w ere studied: nonischemic control (NIC) hearts and postischemic hearts reperfused at full flow (I/R-100) and at 50 and 10% of full flow (I/R- 50 and I/R-10, respectively). In I/R-100 hearts, there was a significa nt increase in leukocyte trapping in capillaries compared with the NIC group (R-5: 5.7 +/- 0.6 vs. 2.0 +/- 0.4 leukocytes/capillary field, r espectively; P < 0.05). However, the increase in leukocyte adhesion to venules was not statistically significant compared with NIC (R-5: 3.2 +/- 0.4 vs. 1.5 +/- 0.6 leukocytes/100-mu m venule, respectively; P < 0.2). In I/R-50 hearts, a further increase in leukocyte accumulation occurred in the capillaries but not in the venules. However, in I/R-10 hearts, there was a statistically significant increase in both capill aries (R-5: 9.2 +/- 0.8; P < 0.05) and venules (R-5: 4.4 +/- 0.5; P < 0.05). When leukocyte margination in coronary venules was examined as a function of venular shear rate, a significant correlation (r = 0.99, P < 0.05) was found. These results suggest that, after ischemia, a re duction in reflow enhances leukocyte trapping in capillaries and that leukocyte adhesion in venules is inversely related to shear rate. Enha nced leukocyte accumulation may in turn increase the leukocyte contrib ution to early reperfusion injury in the heart.