MYOCARDIAL CALCIUM-INDEPENDENT PHOSPHOLIPASE A(2) ACTIVITY DURING GLOBAL-ISCHEMIA IN ISOLATED RABBIT HEARTS

Objectives: To study calcium-independent phospholipast A(2) activity d uring global ischemia in isolated rabbit hearts by measuring the hydro lysis of the endogenous choline phospholipids. Methods: Langendorff pe rfused rabbit hearts were exposed to global ischemia for 15 or 60 min, or control perfusion for the same length of time. The hearts were the n rapidly frozen in liquid nitrogen and lyophilized. Calcium-independe nt phospholipase A(2) activity in the lyophilized tissue was studied b y measuring accumulation of lysophospholipids resulting from hydrolysi s of both the choline diacylphospholipid and the choline plasmalogen p ool. Results: The calcium-independent phospholipase A(2) activity show ed the same pH, temperature and calcium sensitivity in control and isc hemic (15 min of ischemia) lyophilized myocardial tissue. incubation o f control and ischemic tissue showed no difference in the rate of accu mulation of lysophospholipids when the ischemic tissue was obtained fr om hearts exposed to 15 min of ischemia (107+/-4 vs 111+/-7 nmol/g dry wt x min, ischemia versus control, mean+/-s.e.m., n=8), but a signifi cant decrease was noticed in tissue from hearts that had been exposed to 60 min of ischemia (31+/-9 vs 86+/-18 nmol/g dry wt x min, P<0.05, n=4). The decreased phospholipase A(2) activity in tissue exposed to 6 0 min of ischemia was not due to enhanced metabolism of the lysophosph olipids (84+/-15 vs 79+/-8 nmol/g dry wt x min, n=4). The calcium-inde pendent phospholipase A(2) activity was considerably lower in fresh my ocardial tissue compared with lyophilized tissue, but comparison of co ntrol and ischemic fresh tissue gave results comparable to those found using lyophilized tissue. The myocardial calcium-independent phosphol ipase A(2) activity showed no plasmalogen selectivity in either contro l or ischemic myocardium. Conclusions: In isolated perfused rabbit hea rts we found no evidence for activation of calcium-independent phospho lipase A(2) activity during global ischemia. With prolonged time of is chemia there was a significant decrease in calcium-independent phospho lipase A(2) activity.