Intracellular calcium dynamics in mouse model of myocardial stunning

Intracellular calcium (Ca-i(2+)) and left ventricular (LV) function were determined in the coronary-perfused mouse heart to study Ca-i(2+)-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 Ca-i(2+) 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 mu M) to inhibit Ca2+-calmodulin-regulated mechanisms. Peak Ca-i(2+) was 0.77 +/- 0.03 mu M 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 Ca-i(2+) Reperfusion was characterized by an initial burst of Ca-i(2+) and a gradual recovery to nearly normal systolic Ca-i(2+) 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 Ca-i(2+) but rather from a decreased myofilament responsiveness to Ca-i(2+) due to changes in the myofilaments themselves. In hearts perfused with W7, the rise in Ca-i(2+) during ischemia was significantly attenuated, as was the magnitude of mean Ca-i(2+) 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 Ca-i(2+) was increased in control hearts during stunning but returned to baseline in hearts perfused with W7. Simultaneous assessment of Ca-i(2+) and LV function demonstrates that calmodulin-regulated mechanisms may contribute to the pathogenesis of myocardial stunning in the mouse heart.