Excitation-contraction coupling in hibernating myocardium

Calcium responsiveness in regional myocardial short-term hibernation and stunning in the in situ porcine heart. Inotropic responses to postextrasystolic potentiation and intracoronary calcium

BACKGROUND

We tested the hypothesis that decreased calcium responsiveness is responsible for the reduction in contractile function in regional hibernating and stunned myocardium in situ.

METHODS AND RESULTS

In 19 anesthetized swine, the left anterior descending coronary artery flow was reduced to decrease anterior myocardial work index (sonomicrometry) by approximately 60%. During 90 minutes of hypoperfusion, creatine phosphate recovered (as determined by biopsy specimens and bioluminescence) and no necrosis developed (as determined by staining with triphenyl tetrazolium chloride). In 10 swine, changes in the intracellular calcium concentration were induced by systematic variation of the postextrasystolic time interval at a constant prematurity. In 9 additional swine, a graded IC calcium infusion was performed. Under control conditions, anterior myocardial work increased with a fully compensated postextrasystolic time interval from 380+/-93 (mean+/-SD) to 523+/-98 mm Hg . mm. IC calcium infusion increased anterior myocardial work under control conditions from 356+/-85 to a maximum of 428+/-93 mm Hg . mm. Although the maximal responses were decreased during postextrasystolic potentiation (222+/-68 versus 523+/-98 mm Hg . mm) and calcium infusion (176+/-32 versus 428+/-93 mm Hg . mm) after 90 minutes of ischemia, the relationships between increases in anterior myocardial work and, respectively, postextrasystolic time interval and IC calcium were not different. The same was true after 30 minutes of reperfusion.

CONCLUSIONS

Both regional hibernating myocardium and stunned myocardium in situ are characterized by a decrease in overall myocardial calcium responsiveness; however, there appears to be no significant myocardial desensitization to calcium.