High resolution mechanical function in the intact porcine heart: mechanical effects of pacemaker location

Dynamic AFM elastography reveals phase dependent mechanical heterogeneity of beating cardiac myocytes

We developed a novel atomic force microscope (AFM) indentation technique for mapping spatiotemporal stiffness of spontaneously beating neonatal rat cardiac myocytes. Cells were indented at a rate close but unequal to their contractile frequency. Resultant apparent elastic modulus cycled at a predictable envelope frequency between a systolic value of 26.2 +/- 5.1 kPa and a diastolic value of 7.8 +/- 4.1 kPa. In cells probed along their axis, spatial heterogeneity of systolic stiffness correlated with the sarcomeric structure of underlying myofibrils. Treatment with blebbistatin eliminated contractile activity and resulted in a uniform modulus of 6.5 +/- 4.8 kPa. The technique provides a unique means of probing the mechanical effects of disease processes and pharmacological treatments on beating cardiomyocytes at the subcellular level, providing new insights relating myocardial structure and function.

Cross-bridge cycling gives rise to spatiotemporal heterogeneity of dynamic subcellular mechanics in cardiac myocytes probed with atomic force microscopy

To study how the dynamic subcellular mechanical properties of the heart relate to the fundamental underlying process of actin-myosin cross-bridge cycling, we developed a novel atomic force microscope elastography technique for mapping spatiotemporal stiffness of isolated, spontaneously beating neonatal rat cardiomyocytes. Cells were indented repeatedly at a rate close but unequal to their contractile frequency. The resultant changes in pointwise apparent elastic modulus cycled at a predictable envelope frequency between a systolic value of 26.2 +/- 5.1 kPa and a diastolic value of 7.8 +/- 4.1 kPa at a representative depth of 400 nm. In cells probed along their major axis, spatiotemporal changes in systolic stiffness displayed a heterogeneous pattern, reflecting the banded sarcomeric structure of underlying myofibrils. Treatment with blebbistatin eliminated contractile activity and resulted in a uniform apparent modulus of 6.5 +/- 4.8 kPa. This study represents the first quantitative dynamic mechanical mapping of beating cardiomyocytes. The technique provides a means of probing the micromechanical effects of disease processes and pharmacological treatments on beating cardiomyocytes, providing new insights and relating subcellular cardiac structure and function.

Enhanced recovery of mechanical function in the canine heart by seeding an extracellular matrix patch with mesenchymal stem cells committed to a cardiac lineage

The need to regenerate tissue is paramount, especially for the heart that lacks the ability to regenerate after injury. The urinary bladder extracellular matrix (ECM), when used to repair a right ventricular defect, successfully regenerated some mechanical function. The objective of the current study was to determine whether the regenerative effect of ECM could be improved by seeding the patch with human mesenchymal stem cells (hMSCs) enhanced to differentiate down a cardiac linage. hMSCs were used to form three-dimensional spheroids. The expression of cardiac proteins was determined in cells exposed to the spheroid formation and compared with nonmanipulated hMSCs. To determine whether functional calcium channels were present, the cells were patch clamped. To evaluate the ability of these cells to regenerate mechanical function, the spheroids were seeded on ECM and then implanted into the canine heart to repair a full-thickness right ventricular defect. As a result, many of the cells spreading from the spheroids expressed cardiac-specific proteins, including sarcomeric alpha-actinin, cardiotin, and atrial natriuretic peptide, as well as the cell cycle markers cyclin D1 and proliferating cell nuclear antigen. A calcium current similar in amplitude to that of ventricular myocytes was present in 16% of the cells. The cardiogenic cell-seeded scaffolds increased the regional mechanical function in the canine heart compared with the unmanipulated hMSC-seeded scaffolds. In addition, the cells prelabeled with fluorescent markers demonstrated myocyte-specific actinin staining with sarcomere spacing similar to that of normal myocytes. In conclusion, the spheroid-derived cells express cardiac-specific proteins and demonstrate a calcium current similar to adult ventricular myocytes. When these cells are implanted into the canine heart, some of these cells appear striated and mechanical function is improved compared with the unmanipulated hMSCs. Further investigation will be required to determine whether the increased mechanical function is due to a differentiation of the cardiogenic cells to myocytes or to other effects.

Accuracy and reproducibility of a subpixel extended phase correlation method to determine micron level displacements in the heart

Future treatment of heart disease may involve local perturbations of mechanical function, such as intramyocardial injections of angiogenic growth factors or progenitor cells. This necessitates an accurate measurement technique to determine regional heart function. We have previously developed a method to determine regional heart function using a phase correlation algorithm. However, in determining regional function over a single heartbeat it is necessary to sum displacements between many images. We have therefore incorporated a subpixel algorithm that models the result of phase correlation as a sinc function in order to increase the accuracy of our technique. This method, which we have named high density mapping (HDM), determines the subpixel displacement of 64 x 64 pixel regions from images of the heart. To determine the accuracy and precision of the technique, a high contrast image of a heart was digitally shifted 1, 2 or 3 pixels. The original and shifted images were then downsampled four times resulting in 0.25, 0.50 or 0.75 pixel shifts between the original and shifted images. The average accuracy of HDM in the digitally shifted images was 0.06 pixels, with a precision of 0.08 pixels. Effectiveness of HDM in characterization of deformation was also assessed in digitally stretched images. Error in quantification of strain was found to be less than 3.5% of the calculated strain. In an additional set of experiments, in which accuracy was determined using physical motion instead of digital shifting and downsampling, a speckle pattern was displaced by known distances using a micromanipulator, such that the displacement between the captured images was 0.5 pixels. These data demonstrated an accuracy of 0.09 pixels and a precision of 0.02 pixels. Finally, as HDM is used to determine the regional stroke work index (RSW) in beating hearts, the repeatability of using this method to compute RSW was assessed. RSW, the integral of intraventricular pressure with respect to unitless regional area, where end diastolic area was normalized to unity, was assessed in consecutive beats from four different hearts. The average standard deviation of RSW was 0.098 mmHg. Uncertainty analysis determined the maximum error of RSW to be +/-0.41 mmHg, approximately two-thirds of the measured biologic variability. These data demonstrate the ability of HDM to accurately and reproducibly measure displacement and regional function in the beating heart.

The use of extracellular matrix as an inductive scaffold for the partial replacement of functional myocardium

Regenerative medicine approaches for the treatment of damaged or missing myocardial tissue include cell-based therapies, scaffold-based therapies, and/or the use of specific growth factors and cytokines. The present study evaluated the ability of extracellular matrix (ECM) derived from porcine urinary bladder to serve as an inductive scaffold for myocardial repair. ECM scaffolds have been shown to support constructive remodeling of other tissue types including the lower urinary tract, the dermis, the esophagus, and dura mater by mechanisms that include the recruitment of bone marrow-derived progenitor cells, angiogenesis, and the generation of bioactive molecules that result from degradation of the ECM. ECM derived from the urinary bladder matrix, identified as UBM, was configured as a single layer sheet and used as a biologic scaffold for a surgically created 2 cm2 full-thickness defect in the right ventricular free wall. Sixteen dogs were divided into two equal groups of eight each. The defect in one group was repaired with a UBM scaffold and the defect in the second group was repaired with a Dacron patch. Each group was divided into two equal subgroups (n = 4), one of which was sacrificed 15 min after surgical repair and the other of which was sacrificed after 8 weeks. Global right ventricular contractility was similar in all four subgroups groups at the time of sacrifice. However, 8 weeks after implantation the UBM-treated defect area showed significantly greater (p < 0.05) regional systolic contraction compared to the myocardial defects repaired with by Dacron (3.3 +/- 1.3% vs. -1.8 +/- 1.1%; respectively). Unlike the Dacron-repaired region, the UBM-repaired region showed an increase in systolic contraction over the 8-week implantation period (-4.2 +/- 1.7% at the time of implantation vs. 3.3 +/- 1.3% at 8 weeks). Histological analysis showed the expected fibrotic reaction surrounding the embedded Dacron material with no evidence for myocardial regeneration. Histologic examination of the UBM scaffold site showed cardiomyocytes accounting for approximately 30% of the remodeled tissue. The cardiomyocytes were arranged in an apparently randomly dispersed pattern throughout the entire tissue specimen and stained positive for alpha- sarcomeric actinin and Connexin 43. The thickness of the UBM graft site increased greatly from the time of implantation to the 8-week sacrifice time point when it was approximately the thickness of the normal right ventricular wall. Histologic examination suggested complete degradation of the originally implanted ECM scaffold and replacement by host tissues. We conclude that UBM facilitates a constructive remodeling of myocardial tissue when used as replacement scaffold for excisional defects.