Injury to the Ca2+ ATPase of the sarcoplasmic reticulum in anesthetized dogs contributes to myocardial reperfusion injury.

Objective: Sarcoplasmic reticulum dysfunction may contribute to calcium (Ca2+) overload during myocardial reperfusion. The aim of this study was to investigate its role in reperfusion injury.
Methods: Open chest dogs undergoing 15 min of left anterior descending coronary artery occlusion and 3 h of reperfusion were randomized to intracoronary infusions of 0.9% saline, vehicle, or the Ca2+ channel antagonist, nifedipine (50 micrograms/min from 2 minutes before to 5 minutes after reperfusion). After each experiment, transmural myocardial biopsies were removed from ischemic/reperfused and nonischemic myocardium in the beating state and analyzed for (i) sarcoplasmic reticulum protein content (Ca2+ ATPase, phospholamban, and calsequestrin) by immunoblotting and (ii) Ca2+ uptake by sarcoplasmic reticulum vesicles with and without 300 micromolar ryanodine or the Ca2+ ATPase activator, antiphospholamban (2D12) antibody.
Results: Contractile function did not recover in controls and vehicle-treated dogs after ischemia and reperfusion (mean systolic shortening, -2 +/- 2%), but completely recovered in nifedipine-treated dogs (17 +/- 2%, p = NS vs. baseline, p < 0.01 vs. control). Ventricular fibrillation occurred in 50% of controls and vehicle dogs and 0% of nifedipine-treated dogs (p < 0.01). Ca2+ uptake by the sarcoplasmic reticulum vesicles was severely reduced in ischemic/reperfused myocardium of controls and vehicle dogs (p < 0.01 vs. nonischemic). Ryanodine and the 2D12 antibody improved, but did not reverse the low Ca2+ uptake. Protein content was similar in ischemic/reperfused and nonischemic myocardium. In contrast, Ca2+ uptake and the responses to ryanodine and 2D12 antibody were normal in ischemic/reperfused myocardium from nifedipine-treated dogs.
Conclusion: Dysfunction of the sarcoplasmic reticulum Ca2+ ATPase pump correlates with reperfusion injury. Reactivation of Ca2+ channels at reperfusion contributed to Ca2+ pump dysfunction. Ca2+ pump injury may be a critical event in myocardial reperfusion injury.