Early in reperfusion following myocardial ischemia, leukocyte activation is necessary for venular adhesion but not capillary retention.

OBJECTIVE

The pathobiology of leukocyte sequestration in the coronary microcirculation following ischemia is unclear. We examined the location(s) and persistence of leukocyte sequestration of unactivated and preactivated blood in the coronary microcirculation early during reperfusion following ischemia.

METHODS

Isolated rat hearts were subjected to 30 min of 37 degrees C, no-flow ischemia. Hearts were initially reperfused with diluted whole blood containing fluorescent leukocytes (DWB*). At 5, 20, and 35 min of reperfusion (R), the deposition of leukocytes in the coronary capillaries and venules was observed directly using intravital fluorescence microscopy. Four groups were studied: a nonischemic control group (Gr I), and postischemic groups reperfused with DWB* treated with vehicle (Gr II) or preactivated with 10(-8) M N-formylmethionyl-leucyl-phenylalanine (fMLP) (Gr III) or 10(-6) M fMLP (Gr IV).

RESULTS

At R5, postischemic reperfusion with unactivated blood caused a significant trapping of leukocytes in coronary capillaries (Gr I = 2.2 +/- 0.4 versus Gr II = 5.6 +/- 0.6 leukocytes per capillary field, P < 0.05). Hearts in Gr IV exhibited significantly greater leukocyte retention in capillaries compared to all other groups at R5 (R5, Gr IV = 8.8 leukocytes per capillary field, P < 0.05) and at R35. At R5, although more leukocytes were observed adhered to the venules in Gr II compared to Gr I, the difference was not statistically significant (Gr I = 1.7 +/- 0.7 versus Gr II = 3.4 +/- 0.5 leukocytes per 100 microns venule, P = 0.23). DWB* preactivated with the lower concentration of fMLP (10(-8) M) resulted in a significant increase in venular leukocyte adhesion at R5 compared to Gr I and Gr II (Gr III 6.1 +/- 0.5, P < 0.05). After 35 min of reperfusion, a greater percentage of leukocytes remained in the capillaries than in the venules.

CONCLUSIONS

These direct observations suggest that early in reperfusion after ischemia, both leukocyte and endothelial activation are necessary for venular adhesion, but that ischemia-induced coronary microvascular alterations are sufficient to promote leukocyte retention in coronary capillaries. These results also indicate that during 35 min of reperfusion, the degree of leukocyte washout is greater in the venules than in the capillaries. These results suggest that the mechanisms contributing to leukocyte retention early in reperfusion following myocardial ischemia are, indeed, different in the capillaries and venules and that the mechanisms affecting retention in capillaries are more persistent than those in the venules.