Myofibrillar disruption in hypocontractile myocardium showing perfusion-contraction matches and mismatches

Abstract

Chronically instrumented dogs underwent 2- or 5-h regional reductions in coronary flow that were followed, respectively, by balanced reductions in myocardial contraction and O(2) consumption ("hibernation") and persistently reduced contraction despite normal myocardial O(2) consumption ("stunning"). Previously unidentified myofibrillar disruption developed during flow reduction in both experimental models and persisted throughout the duration of reperfusion (2-24 h). Aberrant perinuclear aggregates that resembled thick filaments and stained positively with a monoclonal myosin antibody were present in 34 +/- 3.8% (SE) and 68 +/- 5.9% of "hibernating" and "stunned" subendocardial myocytes in areas subjected to flow reduction and in 16 +/- 2.5% and 44 +/- 7.4% of subendocardial myocytes in remote areas of the same ventricles. Areas of myofibrillar disruption also showed glycogen accretion and unusual heterochromatin clumping adjacent to the inner nuclear envelope. The degrees of flow reduction employed were sufficient to reduce regional myofibrillar creatine kinase activity by 25-35%, but troponin I degradation was not evident. The observed changes may reflect an early, possibly reversible, phase of the myofibrillar loss characteristic of hypocontractile myocardium in patients undergoing revascularization.

Myosin-Binding Protein C Phosphorylation, Myofibril Structure, and Contractile Function During Low-Flow Ischemia

“…Five-micrometer frozen sections were preserved for 30 minutes in 4% paraformaldehyde-lysine-periodate fixative at 4°C. 2 Sections were rinsed 3 times in PBS plus 10% normal goat serum and incubated for 1 hour at room temperature in a 1:200 dilution of a polyclonal antibody that recognizes the C0C1 epitope of MyBP-C. 14 Sections were rinsed ϫ3 in PBS plus 10% normal goat serum for 1 hour and then incubated in a 1:300 dilution of goat anti-rabbit IgG labeled with FITC and a 1:200 dilution of rhodamine phalloidin to stain MyBP-C and fibrillar actin, respectively. Sections were washed in PBS and mounted with DAPI-labeled medium and viewed in a Zeiss 510 confocal microscope.…”

“…2 Other biopsied samples were rapidly frozen in isopentane cooled to liquid nitrogen temperature and then freezesubstituted in 2% osmium tetroxide in absolute acetone at Ϫ80°C, dehydrated, and embedded in epoxy resin. The biopsied samples were preserved in contracture so that the number of activated cross-bridges could be estimated from high-resolution electron microscopy negatives of longitudinally imaged myofibrils.…”

“…2 Although the subcellular mechanism(s) that regulate(s) contractile function remains unresolved, 3 changes in calcium handling and the properties of myofilament proteins likely are involved in evolving mechanical dysfunction. 4 In large-animal models, contractile dysfunction displayed by ischemically stunned canine and porcine myocardium has been attributed to intrinsic changes in myofilament sensitivity to calcium that may develop as a consequence of the dephosphorylation of phospholamban and troponin I after bouts of low-flow ischemia.…”

“…4 In large-animal models, contractile dysfunction displayed by ischemically stunned canine and porcine myocardium has been attributed to intrinsic changes in myofilament sensitivity to calcium that may develop as a consequence of the dephosphorylation of phospholamban and troponin I after bouts of low-flow ischemia. 5,6 Because proteolysis of troponin I has not been observed consistently in larger animals, 2,[7][8][9] mechanisms that appear to mediate contractile dysfunction in rodents 10,11 may not modulate myocardial function in larger animals during and after low-flow ischemia. 4 Alterations in other cytoskeletal and myofibrillar proteins also are likely to influence contractile function.…”

“…12 Disassembly of myofibrillar thick filaments parallels the reduction of coronary flow and contractile function in a canine model of low-flow ischemia. 2 How individual thick filament disruption evolves and whether it contributes to contractile dysfunction in this model is unclear, but recent experiments implicate myosin-binding protein C (MyBP-C) in regulating both thick filament structure and function and imply that changes in the phosphorylation of MyBP-C may affect contractility. [12][13][14][15] Phosphorylation of a unique aminoterminal sequence in cardiac MyBP-C is believed to have a role in modulating contractile function in the heart.…”