Cardioprotective effects of chronic hypoxia and ischaemic preconditioning are not additive

The objective of the work was to examine whether adaptation to intermittent high altitude hypoxia and ischaemic preconditioning provide additive protection of the heart against subsequent acute ischaemic injury. Adult male rats were exposed to hypoxia (7000 m, 8 h/day, 24-30 exposures) in a hypobaric chamber. Susceptibility of their hearts to ischaemia-induced ventricular arrhythmias and infarction was evaluated in open-chest animals subjected to 30-min coronary artery occlusion and 4-h reperfusion. Preconditioning was induced by either two (PC1) or five (PC2) occlusions of the same artery for 5 min, each followed by 5-min reperfusion. Adaptation to hypoxia decreased the arrhythmia score from 2.75 +/- 0.13 in normoxic controls to 2.17 +/- 0.18. Both PC1 and PC2 reduced the arrhythmia score in the controls (0.15 +/- 0.10 and 0.71 +/- 0.24, respectively), as well as in the hypoxic groups (0.40 +/- 0.15 and 0.27 +/- 0.15, respectively). The infarct size was reduced from 66.6 +/- 2.3% of the area at risk in the controls to 50.2 +/- 1.9% in the adapted rats. PC1 conferred further protection in adapted animals (38.4 +/- 2.8%) but this combined effect was of the same magnitude as that of preconditioning in the controls (37.5 +/- 1.6%). Similar results were obtained using PC2. It is concluded that adaptation to hypoxia decreases the efficiency of ischaemic preconditioning; cardioprotective effects of these two phenomena are not additive. The results are consistent with the view that the mechanisms of protection conferred by chronic hypoxia and preconditioning may share the same signalling pathway.