Collagen loss in the stunned myocardium

Background. This study was performed to biochemically assess and quantify the previously observed ultrastructural alterations in the collagen matrix of stunned myocardium. Methods and Results. The stunned myocardium was produced in 13 mongrel dogs by a series of 12 coronary artery occlusions of 5 minutes followed by 10-minute reperfusion periods, with a final reperfusion period of 90 minutes. Regional systolic function in the stunned myocardium was 17% of control. Relative end-diastolic length in the stunned region increased up to 8%. There was a nonuniform transmural loss of collagen. Hydroxyproline in the stunned endocardium was not different from control. The stunned midwall and epicardium demonstrated 12.5% (p<0.05) and 14.6% (p<0.005) decreases, respectively. All transmural layers in the stunned myocardium had significant increases in collagenase activity before procollagenase activation, averaging a 73.6% increase (p<0.025). Complete activation of all procollagenase forms with aminophenylmercuric acetate revealed no differences in fully activated collagenase between the stunned and normal regions. The lysosomal enzymes, elastase and cathepsin G, were not different between stunned and normal zone tissue. These results would tend to exclude exogenous sources of protease in the stunned myocardium at the 90-minute final reperfusion time frame. Collagen fibers were isolated from the stunned and normal zone tissue and underwent dansyl chloride reaction. Stunned collagen fibers had 9% greater dansyl labeling, suggesting greater numbers of exposed N-terminal amino acid residues on the fiber and compatible with greater enzymatic cleavage activity on the stunned collagen matrix. Tissue water content was consistently greater in the stunned region compared to the normal: a uniform transmural increase of approximately 1.7%. Conclusions. The stunned myocardium is characterized by both systolic dysfunction and diastolic expansion or dilatation. Endogenous procollagenase is activated by the ischemic process leading to degradation of the extracellular matrix. The underlying mechanisms may be relevant in ischemic enlargement of the heart and cardiomyopathy.