INTRACELLULAR CALCIUM DYNAMICS IN MOUSE MODEL OF MYOCARDIAL STUNNING

Intracellular calcium (Ca-i(2+)) and left ventricular (LV) function we re determined in the coronary-perfused mouse heart to study Ca-i(2+)-r elated 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 int erventions that regulate Ca2+ exchange with functional alterations. Tw enty-three coronary-perfused mouse hearts were subjected to 15 min of ischemia followed by 20 min of reperfusion. In 13 hearts, the perfusat e included the calmodulin antagonist W7 (10 mu M) to inhibit Ca2+-calm odulin-regulated mechanisms. Peak Ca-i(2+) was 0.77 +/- 0.03 mu M in t he control group and was unaffected by W7 at baseline. Ischemia was ch aracterized by a rapid decline in LV function, followed by ischemic co ntracture, accompanied by a gradual rise in Ca-i(2+) Reperfusion was c haracterized 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% af ter 20 min of reperfusion (stunned myocardium). These results in the m ouse 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 attenu ated, as was the magnitude of mean Ca-i(2+) during early reflow. Ische mic contracture was abolished or delayed. Hearts perfused with W7 show ed 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 wit h W7. Simultaneous assessment of Ca-i(2+) and LV function demonstrates that calmodulin-regulated mechanisms may contribute to the pathogenes is of myocardial stunning in the mouse heart.