Intracellular
calcium (Cai2+) and left ventricular (LV) function were determined in the coronary-perfused mouse heart to study Cai2+-related mechanisms of injury from
myocardial ischemia and reperfusion. Specifics for loading of the
photoprotein aequorin into isovolumically contracting mouse hearts under constant-flow conditions are provided. The method allows detection of changes in Cai2+ on a beat-to-beat basis in a model of
myocardial stunning and permits correlation of interventions that regulate Ca2+ exchange with functional alterations. Twenty-three coronary-perfused mouse hearts were subjected to 15 min of
ischemia followed by 20 min of reperfusion. In 13 hearts, the perfusate included the
calmodulin antagonist
W7 (10 microM) to inhibit Ca(2+)-
calmodulin-regulated mechanisms. Peak Cai2+ was 0.77 +/- 0.03 microM in the control group and was unaffected by
W7 at baseline.
Ischemia was characterized by a rapid decline in LV function, followed by
ischemic contracture, accompanied by a gradual rise in Cai2+. Reperfusion was characterized by an initial burst of Cai2+ and a gradual recovery to nearly normal systolic Cai2+ while LV pressure recovered to 55% after 20 min of reperfusion (
stunned myocardium). These results in the mouse heart confirm that stunning does not result from deficiency of Cai2+ but rather from a decreased myofilament responsiveness to Cai2+ due to changes in the myofilaments themselves. In hearts perfused with
W7, the rise in Cai2+ during
ischemia was significantly attenuated, as was the magnitude of mean Cai2+ during early reflow.
Ischemic contracture was abolished or delayed. Hearts perfused with
W7 showed significantly improved recovery of LV pressure, rate of contraction, and rate of relaxation. Diastolic Cai2+ was increased in control hearts during stunning but returned to baseline in hearts perfused with
W7. Simultaneous assessment of Cai2+ and LV function demonstrates that
calmodulin-regulated mechanisms may contribute to the pathogenesis of
myocardial stunning in the mouse heart.