Protective effect of the protease inhibitor leupeptin against myocardial stunning

To elucidate whether activation of intracellular protease causes the contractile dysfunction of postishemic reperfused heart (stunned myocardium), the effect of leupeptin, a cysteine-protease inhibitor, was evaluated in isolated guinea pig hearts. Left ventricular (LV) isovolumic pressure was measured in hearts reperfused after global ischemia (15 min, 37°C). Recovery of developed pressure during reperfusion in hearts treated with 50 μM leupeptin was significantly greater than that in untreated hearts [94.3 ± 3.2% of control, n = 11 (mean ± SEM] vs. 78.1 ± 3.1%, n = 14), and was almost identical to that in nonischemic control (93.5 ± 1.6%, n = 11). Maximal Ca²⁺-activated pressure, the intact-heart correlate of maximal Ca²⁺-activated force, was also evaluated at the end of experiments during tetani elicited by rapid pacing after exposure to ryanodine. Maximal Ca²⁺-activated pressure in hearts treated with leupeptin (168 ± 4.6 mm Hg) was significantly higher than in untreated stunned hearts (144.5 ± 5.7 mm Hg), but significantly lower than in nonischemic control (198.4 ± 5.5 mm Hg). These results indicate that leupeptin has a protective effect against myocardial stunning. In coupling with previous reports of transient increase in intracellular [Ca²⁺] during ischemia and/or reperfusion, activation of proteases by Ca²⁺overload is suggested to play a significant role in myocardial stunning.