Abstract: AIM: Changes of endoxin level, ATPase activities, intramitochondrial Ca²⁺ concentration, and gene expression of Na⁺-K⁺-ATPase isoforms in myocardium of rats with MIR and effect of verapamil were observed, in order to investigate mechanism of endoxinmediating intracellular calcium overload of myocytes.METHODS: Twenty four male Sprauge Dawley rats were randomized into 3 groups.Sham operation group:silk suture was threaded the left anterior descending coronary artery without ligature;MIR group (MIR):left anterior descending coronary artery was subjected to 30 min ligation followed by 45 min reperfusion;verapamil group:MIR model was given 5 mg/kg verapamil.Verapamil was injected via femoral vein 5 min before reperfusion.Left ventricle myocardium samples were processed immediately after reperfusion in order to measure the activities of Na⁺-K⁺-ATPase and Ca²⁺-²⁺-ATPase, endoxin level, and intramitochondrial Ca²⁺ concentration.The levels of α₁, α₂, α₃ and β₁ isoforms of Na⁺-K⁺-ATPase were measured by immunohistochemical assay.RESULTS: After MIR, the level of endoxin in myocardium was substantially increased; the activities of Na⁺-K⁺-ATPase and Ca²⁺-Mg²⁺-ATPase in myocardial membrane were significantly decreased while the concentration of intramitochondrial Ca²⁺ was increased;the levels of the α₁, α₂, α₃ and β₁ isoforms of Na⁺-K⁺-ATPase were reduced markedly. Verapamil had only effect on reducing the concentration of intramitochondrial Ca²⁺.CONCLUSION: MIR increases endoxin secretion.The latter may depress the activity of Na⁺-K⁺-ATPase by changing the gene expression of α₁, α₂, α₃ and β₁ isoforms of Na⁺-K⁺-ATPase in myocardial membrane, inducing intramitochondrial Ca²⁺ overload.

Key words: endoxin, ischemia reperfusion injury, myocardium, Na⁺-K⁺ exchanging ATPase isoforms, immunohistochemical assay