Cardioprotection by postconditioning in conscious rats is limited to coronary occlusions <45 min

Abstract

Objectives: Brief episodes of ischemia and reperfusion after a lethal ischemic insult confer cardioprotection, a phenomenon termed "ischemic postconditioning." However, all studies reported to date have been conducted in open-chest animal models. We sought to determine whether postconditioning occurs in conscious animals and whether it protects against severe myocardial injury.

Methods: Chronically instrumented rats were assigned to a 30- (Subset 1), 45- (Subset 2), or 60-min (Subset 3) coronary occlusion followed by 24 h of reperfusion. In each subset, rats received no further intervention (control), were preconditioned with 12 cycles of 2-min occlusion/2-min reperfusion immediately (early preconditioning; EPC) or 24 h (late preconditioning; LPC) before myocardial infarction, or were postconditioned with 20 cycles of 10-s occlusion/10-s reperfusion immediately after myocardial infarction (20-10 PostC).

Results: With a 30-min occlusion, infarct size (54.4 +/- 2.3% of risk region in control-30) was significantly reduced in EPC-30, LPC-30, and 20-10 PostC-30 groups (by 72, 70, and 47%, respectively; all P < 0.05 vs. control-30). With a 45-min occlusion, infarct size (62.2 +/- 2.4% in control-45) was reduced in EPC-45 and LPC-45 groups (by 47 and 41%, respectively; all P < 0.05 vs. control-45) but not in the 20-10 PostC-45 group [55.4 +/- 2.3%, P = not significant (NS) vs. control-45]. With a 60-min occlusion, infarct size (72.7 +/- 2.2% in control-60) was reduced in the EPC-60 (by 20%, P < 0.05) but not in the LPC-60 (63.6 +/- 2.5%, P = NS) or in the 20-20 PostC group (71.5 +/- 3.4%, P = NS).

Conclusions: Both early and late ischemic preconditioning as well as ischemic postconditioning confer protection in conscious rats; however, unlike early preconditioning, postconditioning protects only against coronary occlusions <45 min. In the conscious rat, the cardioprotection afforded by postconditioning is limited to mild to moderate myocardial injury.

Fig. 1

Experimental protocols for studies of rats exposed to a 30-min coronary artery occlusion (Subset 1). O, occlusion; R, reperfusion; EPC, early preconditioning; LPC, late preconditioning; 6–30 PostC, six 30-s cycles of postconditioning; 6–10 PostC, six 10-s cycles of postconditioning; 60–10 PostC, sixty 10-s cycles of postconditioning.

Fig. 2

Experimental protocols for studies of rats exposed to a 45-min (Subset 2) or a 60-min (Subset 3) coronary artery occlusion.

Fig. 3

Infarct size after a 30-min coronary occlusion and 24 h of reperfusion. ○, Individual hearts; ●, group means ± SE.

Fig. 4

Relationship between size of the region at risk and size of myocardial infarction in rats exposed to a 30-min coronary occlusion. Both individual values and the regression lines obtained by linear regression analysis for groups I, II, III, IV, V, VI, and VII are illustrated. In all groups, infarct size was positively and linearly related to the size of region at risk. The linear regression equations were as follows: group I, y = 0.56x − 0.002 (r = 0.88); group II, y = 0.21x − 0.011 (r = 0.60); group III, y = 0.20x − 0.001 (r = 0.35); group IV, y = 0.65x − 0.011 (r = 0.95); group V, y = 0.48x − 0.015 (r = 0.63); group VI, y = 0.38x − 0.014 (r = 0.57); group VII, y = 0.79x − 0.026 (r = 0.96). Analysis of covariance (ANCOVA) demonstrated that the regression lines for groups II, III, V, and VI were significantly different from that for group I (P < 0.05 for each comparison), indicating that for any given size of the region at risk, infarct size was smaller in rats that received either the 2 preconditioning protocols or the 2 appropriate postconditioning protocols compared with the controls; in contrast, the lines for groups IV and VII were similar to that for group I.

Fig. 5

Plasma creatine kinase (CK) activity in rats exposed to a 30-min coronary occlusion at 1, 4, and 24 h of reperfusion. Inset: cumulative CK activity over the 24-h reperfusion period in groups I, II, III, IV, V, VI, and VII. Data are means ± SE.

Fig. 6

Infarct size after a 45-min coronary occlusion and 24 h of reperfusion. ○, Individual hearts; ●, group means ± SE.

Fig. 7

Relationship between size of the region at risk and size of myocardial infarction in rats exposed to a 45-min coronary occlusion. Both individual values and the regression lines obtained by linear regression analysis for groups VIII, IX, X, and XI are shown. In all groups, infarct size was positively and linearly related to the size of region at risk. The linear regression equations were as follows: group VIII, y = 0.77x− 0.025 (r = 0.89); group IX, y = 0.38x− 0.010 (r = 0.36); group X, y = 0.55x− 0.028 (r = 0.70); group XI, y = 0.85x− 0.049 (r = 0.95). ANCOVA demonstrated that the regression lines for groups IX and X were significantly shifted downward and to the right compared with that for group VIII (P < 0.05 for each comparison), indicating that, for any given size of the region at risk, infarct size was smaller in rats that received either the early or the late preconditioning protocol; in contrast, the line for group XI was similar to that for group VIII, indicating that no protective effect is exerted by postconditioning.

Fig. 8

Plasma CK activity in rats exposed to a 45-min coronary occlusion at 1, 4, and 24 h of reperfusion. Inset: cumulative CK activity over the 24-h reperfusion period in groups VIII, IX, X, and XI. Data are means ± SE.

Fig. 9

Infarct size after a 60-min coronary occlusion and 24 h of reperfusion. ○, Individual hearts; ●, group means ± SE.

Fig. 10

Relationship between size of the region at risk and size of myocardial infarction in rats exposed to a 60-min coronary occlusion. Both individual values and the regression lines obtained by linear regression analysis for groups XII, XIII, XIV, and XV are illustrated. In all groups, infarct size was positively and linearly related to the size of region at risk. The linear regression equations were as follows: group XII, y = 0.83x− 0.017 (r = 0.88); group XIII, y = 0.57x− 0.001 (r = 0.70); group XIV, y = 0.79x− 0.029 (r = 0.92); group XV, y = 0.89x− 0.034 (r = 0.89). ANCOVA demonstrated that the regression line for group XIII was significantly shifted downward and to the right compared with that for group XII (P < 0.05), indicating that, for any given size of region at risk, infarct size was smaller in rats that received the early preconditioning protocol; in contrast, the lines for groups XIV and XV were not significantly different from that for group XII.

Fig. 11

Plasma CK activity in rats exposed to a 60-min coronary occlusion at 1, 4, and 24 h of reperfusion. Inset: cumulative CK activity over the 24-h reperfusion period in groups XII, XIII, XIV, and XV. Data are means ± SE.

Fig. 12

Myocardial infarct size after a 30-, 45-, or 60-min coronary occlusion and the effects of ischemic preconditioning and postconditioning. Data are means ± SE.