Hypoxic Preconditioning Preserves Cardiac Contractility and Reduces Infarct Size In Vivo

Background: Preconditioning is a powerful endogenous mechanism to protect the heart against ischemic damage. The second window of preconditioning (SWOP) is therapeutically the most attractive, but is hard to achieve by local cardiac ischemia 24 hrs before the index ischemia in the many mice models for cardiovascular pathology, because of excessive mortality during the two procedures or the period in between. Hypoxic preconditioning is an attractive alternative preconditioning stimulus. To date, the SWOP after hypoxic preconditioning has not been studied in mice concerning infarct size reduction and preservation of left ventricular contractility in vivo. Aim: To determine whether transient hypoxia can induce a SWOP in vivo in mice. Methods: Hypoxic preconditioning was induced by 5 cycles of 6 minutes of 6% oxygen in 24-week-old wild type mice. Twenty-four hours later, a 30 minutes coronary occlusion was performed. After 1 hour of reperfusion, in vivo cardiac pressure-conductance catheterization was performed with determination of the load-dependent and load-independent parameters. Infarct size was determined by TTC-staining. Sham procedures were used to obtain non-preconditioned controls. Results: There was no mortality with the hypoxic preconditioning protocol. The left ventricular contractile parameters ejection fraction, end-systolic elastance and preload recruitable stroke work were significantly better preserved after ischemia in the preconditioned group. Diastolic relaxation (tau) was also significantly better preserved. Infarct size was reduced to half that of the non-preconditioned group. Conclusion: Hypoxic preconditioning is a feasible stimulus to induce in vivo a second window of preconditioning in mice. Infarct size is reduced and cardiac contractility better preserved after 30 min regional ischemia in vivo by hypoxic preconditioning 24 hrs earlier.