We recently demonstrated that exogenous nitric oxide (NO) acts as a trigger for preconditioning in the isolated rat heart model. There is however little data concerning the effects of elevated cardiac endothelial nitric oxide synthase (eNOS) expression on myocardial tolerance to ischaemia. Similarly, the effects of gender and eNOS overexpression on ischaemic preconditioning is unknown. We hypothesized that: 1) eNOS overexpression increases myocardial tolerance to ischaemia, and, 2) eNOS overexpressed hearts cannot be preconditioned, since the hearts are already maximally protected.
Male and female wild-type and transgenic mice that overexpress eNOS exclusively in cardiac myocytes were perfused in the working heart mode with a modified Krebs–Henseleit buffer at a pre-load of 12.5 mm Hg and afterload of 50 mm Hg. Cardiac output, coronary flow, peak aortic systolic pressure and total work were determined before hearts were preconditioned by 4 × 5 min cycles of ischaemia/reperfusion, and then subjected to 20 min total global ischaemia, followed by reperfusion. Reperfusion function and myocardial infarct size were used as endpoints. Pre-ischaemic mechanical function (rate pressure product and cardiac output) was similar for wild-type and transgenic mice of both sexes. The eNOS overexpressed hearts had smaller infarcts than the hearts from their wild-type littermates (26.9 ± 1.4% vs. 37.0 ± 2.1% for controls, P < 0.05). Preconditioning the eNOS overexpressed hearts resulted in infarct sizes comparable with control non-preconditioned hearts (27.5 ± 2.0% vs. 26.9 ± 1.4% for controls). Myocardial cGMP levels were elevated during sustained ischaemia in the transgenic hearts when compared with wild-type hearts (22.43 ± 1.63 pmol/g ww vs 16.54 ± 1.48 pmol/g ww, P < 0.05). Preconditioning also elevated myocardial cGMP levels during sustained ischaemia in the wild-type hearts (26.77 ± 2.81 pmol/g ww, P < 0.05). We conclude that: 1) basal mechanical function is similar for both wild-type and transgenic mice of both sexes, 2) reperfusion function and infarct size was also similar for both sexes under both control conditions and after preconditioning, 3) the transgenic mice are more tolerant of ischaemia as reflected by their smaller myocardial infarcts, and, 4) the eNOS overexpressed mouse heart cannot be preconditioned regardless of whether mechanical function or infarct size is used as an end-point. These hearts may be maximally protected against ischaemia/reperfusion injury by their elevated endogenous NO levels.