Background: Hyperosmotic solutions have been used successfully in different shock resuscitations with cardioprotection. This study was to examine the effects of hyperosmotic sodium chloride on isolated heart function and heart responses to ischemia/reperfusion in normotensive and hypertensive rats. The roles of hyperosmolarity-induced antioxidants including hyperosmolarity-relevant heat shock proteins as well as vasodilating endothelial nitric oxide synthase (eNOS) and vasoactive catecholamines were investigated.Methods: Hearts of normal rats and stroke-prone spontaneously hypertensive rats were isolated and perfused for 30 min with control Krebs-Henseleit buffer (osmolarity 300 mOsm/L) or hyperosmotic buffer of different sodium chloride concentrations (320, 350 and 400 mOsm/L) before subjected to 40-min global ischemia followed by 10-min hyperosmotic reperfusion and 30-min normal buffer reperfusion. Heart function, creatine phosphokinase leakage and myocardial antioxidants were examined. Myocardial antioxidants after hyperosmotic perfusion with different osmolytes were assayed with Western blotting.Results: Pre-ischemic hyperosmotic sodium chloride perfusion enhanced heart contractility and diastole function and reduced coronary vascular resistance in both normal and hypertensive hearts. Post-ischemic recoveries of heart function were improved in hyperosmotic perfused hearts, associated with lower creatine phosphokinase leakage, higher coronary flow, reduced coronary resistance and lower norepinephrine overflow. At the end of reperfusion, the myocardial activities of total superoxide dismutase and catalase, glutathione content as well as osmosis-relevant heat shock protein 32 and 90 were increased in hyperosmotic hearts. In addition to sodium chloride, in vitro hyperosmotic mannitol, glucose and raffinose also increased protein expressions of antioxidants including superoxide dismutase, catalase, heat shock protein 32 and 90 and vasodilating eNOS. Conclusion:Hyperosmotic perfusion enhanced heart function and preconditioned normal and hypertensive hearts against ischemia/reperfusion injury. The hyperosmolarity-induced up-regulations in myocardial antioxidants including heat shock proteins and eNOS may play an important role in the hyperosmolarity-induced cardioprotection.