Susceptibility of the heart to ischaemia-reperfusion injury and exercise-induced cardioprotection are sex-dependent in the rat

Abstract

The cardioprotective effects of short-term exercise against myocardial ischaemia-reperfusion injury in male and female rats were examined. We subjected male and female rats to 0 (Sed; n = 8 males and 8 females), 1 (1 day; n = 10 males and 8 females), or 5 (5 day; n = 6 males and 6 females) days of treadmill running. Langendorff-perfused hearts underwent 1 h of regional ischaemia and 2 h of reperfusion, and infarct size (expressed as a percentage of the zone at risk; ZAR), left ventricular pressure development, and coronary flow were measured for each heart. Preischaemic pressure development and coronary flow did not differ between the sexes nor were they influenced by exercise. Sed females had significantly smaller infarct sizes (25 +/- 3%) than Sed male hearts (37 +/- 3%; P < 0.001). Short-term running significantly reduced infarct size following 1 day (27 +/- 3%; P < 0.05) and 5 days (30 +/- 4%; P < 0.10) of exercise in males. One day of running did not reduce infarct size in females (19 +/- 3%; P = NS), but 5 day females did show a significant reduction in infarct size (13 +/- 2%; P < 0.05). There was no relationship between postischaemic coronary vascular hyperaemia and infarct size across sexes or exercise training groups. Hearts from Sed females exhibited significantly higher manganese superoxide dismutase (MnSOD) protein expression than hearts from Sed males, but short-term exercise (neither 1 nor 5 days) did not alter MnSOD protein in either sex. Increased sarcolemmal ATP-sensitive K(+) (K(ATP)) channel subunit protein expression (SUR2A and/or K(ir)6.2) correlated closely with sex-dependent and exercise-acquired protection against myocardial infarction. These data indicate that: (1) sex-dependent and exercise-induced differences in the susceptibility of the heart to ischaemia-reperfusion injury are not associated with improved coronary flow or postischaemic hyperaemia; (2) increased MnSOD protein expression is not necessary for exercise-induced protection from infarction; and (3) one possible mechanism for sex-dependent and exercise-mediated reductions in infarct size involves an increased protein expression of cardiac sarcolemmal K(ATP) channels.

Figure 1. Left ventricular infarct size following ischaemia–reperfusion

Infarct size is expressed as a percentage of the zone at risk (ZAR). Bars represent means ± s.e.m. for sedentary (n = 8 males and 8 females, black bars), 1 day (n = 10 males and 8 females, grey bars), and 5 day (n = 6 males and 6 females, white bars) groups (*P < 0.05 versus Sed male; †P < 0.1 versus Sed male; ‡P < 0.01 versus Sed male; **P < 0.05 versus Sed female).

Figure 2. Hyperaemic response for male and female hearts at the onset of reperfusion

Mean increases in coronary flow from the end of ischaemia (minute 60) through the first minute of reperfusion (minute 61) are plotted as means ± s.e.m. Black bars represent Sed (n = 8 males and 8 females), grey bars represent 1 day (n = 10 males and 8 females), and white bars represent 5 day (n = 6 males and 6 females) hearts. No statistically significant differences were observed in the hyperaemic response from hearts in the study.

Figure 3. Manganese superoxide dismutase (MnSOD) protein expression in rat heart

Representative Western blots for MnSOD protein expression and band density quantification are presented in 3 panels. A, MnSOD expression from Sed male (M) and Sed female (F) hearts. Band density for all hearts (n = 4 males and 4 females) is quantified and expressed as a function of Sed male density (*P < 0.05 versus Sed male). To ensure accurate data collection, pellet fractions from the clarifying spin were tested for the presence of MnSOD. Pellets were obtained from homogenizing buffer containing either 1% NP-40 (P1) or no NP-40 (P2) and each band is expressed alongside a positive control (+ control) for MnSOD protein. B, MnSOD expression from male hearts exposed to 0 (Sed, n = 4), 1 (n = 5), or 5 (n = 5) days of treadmill running. Band density is presented as a function of Sed males. C, MnSOD expression from female hearts exposed to 0 (Sed, n = 4), 1 (n = 5), or 5 (n = 5) days of treadmill running. Band density is expressed as a function of Sed females.

Figure 4. Myocardial protein expression of the pore-forming subunit of the sarcolemmal ATP-sensitive K⁺ channel (K_ir6.2)

Representative Western blots and band density for K_ir6.2 from male and female hearts. A, K_ir6.2 protein from Sed male (M, n = 4) and Sed female (F, n = 4) hearts. Data are expressed as a function of Sed male (*P < 0.05 versus Sed male) band density. B, myocardial K_ir6.2 protein from male rats exposed to 0 (Sed, n = 4), 1 (n = 5) or 5 (n = 5) days of exercise (†P < 0.1 versus Sed male). C, myocardial K_ir6.2 protein from female rats exposed to 0 (Sed, n = 4), 1 (n = 5) or 5 (n = 5) days of exercise.

Figure 5. Myocardial protein expression of the sulphonylurea receptor (SUR)

Representative Western blots and band density for SUR from male and female hearts. A, SUR protein from Sed male (n = 4) and Sed female (n = 4) hearts. Band density is presented as a function of Sed male (*P < 0.05 versus Sed male). B, myocardial SUR protein expression from male rats exercised for 0 (Sed, n = 4), 1 (n = 4), or 5 (n = 5) days (*P < 0.05 versus Sed male; **P < 0.1 versus Sed male). C, myocardial SUR protein expression from female rats exercised for 0 (Sed, n = 4), 1 (n = 5), or 5 (n = 5) days (*P < 0.05 versus Sed female).