Pharmacological evidence that inducible nitric oxide synthase is a mediator of delayed preconditioning

Brief periods of myocardial ischaemia preceding a subsequent more prolonged ischaemic period 24-72 h later confer protection against myocardial infarction ('delayed preconditioning' or the 'second window' of preconditioning). In the present study, we examined the effects of pharmacological modifiers of inducible nitric oxide synthase (iNOS) induction and activity on delayed protection conferred by ischaemic preconditioning 48 h later in an anaesthetized rabbit model of myocardial infarction. Rabbits underwent a myocardial preconditioning protocol (four 5 min coronary artery occlusions) or were sham-operated. Forty-eight hours later they were subjected to a sustained 30 min coronary occlusion and 120 min reperfusion. Infarct size was determined with triphenyltetrazolium staining. In rabbits receiving no pharmacological intervention, the percentage of myocardium infarcted within the risk zone was 43.9+5.0% in sham-operated animals and this was significantly reduced 48 h after ischaemic preconditioning with four 5 min coronary occlusions to 18.5+5.6% (P<0.01). Administration of the iNOS expression inhibitor dexamethasone (4 mg kg(-1) i.v) 60 min before ischaemic preconditioning completely blocked the infarct-limiting effect of ischaemic preconditioning (infarct size 48.6+/-6.1%). Furthermore, administration of aminoguanidine (300 mg kg(-1), s.c.), a relatively selective inhibitor of iNOS activity, 60 min before sustained ischaemia also abolished the delayed protection afforded by ischaemic preconditioning (infarct size 40.0+/-6.0%). Neither aminoguanidine nor dexamethasone per se had significant effect on myocardial infarct size. Myocardial risk zone volume during coronary ligation, a primary determinant of infarct size in this non-collateralized species, was not significantly different between intervention groups. There were no differences in systolic blood pressure, heart rate, arterial blood pH or rectal temperature between groups throughout the experimental period. These data provide pharmacological evidence that the induction of iNOS, following brief periods of coronary occlusion, is associated with increased myocardial tolerance to infarction 48 h later.