Role of slowed Ca²⁺ transient decline in slowed relaxation during myocardial ischemia

The goal of this study was to test the hypothesis that during myocardial ischemia, slowing of the Ca²⁺ transient decline causes slowed relaxation. Our approach was to monitor pressure and Ca²⁺ transients in isovolumic rat hearts during control and low flow ischemia conditions. In addition, we experimentally slowed the decline of the Ca²⁺ transient using cyclopiazonic acid (CPA) to inhibit the sarcoplasmic reticulum Ca²⁺-ATPase (SERCA, the most important pump for rapidly transporting Ca²⁺ out of the cytosol). Using 9 μM CPA during normoxia, we were able to reproduce the slowed Ca²⁺ transient decline and slowed relaxation found during low flow ischemia. The time constants of cytosolic [Ca²⁺] decline and pressure decline (τ(Ca) and τ(p) respectively) with CPA (78 ± 5 ms and 64 ± 3 ms) were similar to those found with ischemia (89 ± 12 ms and 72 ± 10 ms, mean ± SEM, n = 7) and were considerably greater than for controls (41 ± 3 and 25 ± 2 ms, mean ± SEM, n = 14, P < 0.01). Furthermore, the relationship of τ(p) v τ(Ca) with CPA was similar to that found with ischemia. These findings are consistent with the hypothesis that the slowed Ca²⁺ transient decline with both CPA and ischemia causes slowed relaxation. Consistent with this conclusion, a simple mathematical model to relate cytosolic [Ca²⁺] and pressure also suggests that slowed pressure relaxation can be explained by slowing of the Ca²⁺ transient decline. This study suggests that impaired Ca²⁺ uptake is a major injury causing slowed relaxation during ischemia.