Cardiac tissue geometry as a determinant of unidirectional conduction block: assessment of microscopic excitation spread by optical mapping in patterned cell cultures and in a computer model

Objective: Unidirectional conduction block (UCB) and reentry may occur as a consequence of an abrupt tissue expansion and a related change in the electrical load. The aim of this study was to evaluate critical dimensions of the tissue necessary for establishing UCB in heart cell culture. Methods: Neonatal rat heart cell cultures with cell strands of variable width emerging into a large cell area were grown using a technique of patterned cell growth. Action potential upstrokes were measured using a voltage sensitive dye (RH-237) and a linear array of 10 photodiodes with a 15 μm resolution. A mathematical model was used to relate action potential wave shapes to underlying ionic currents. Results: UCB (block of a single impulse in anterograde direction — from a strand to a large area — and conduction in the retrograde direction) occurred in narrow cell strands with a width of 15(SD 4) μm (1-2 cells in width, n = 7) and there was no conduction block in strands with a width of 31(8) μm (n = 9, P < 0.001) or larger. The analysis of action potential waveshapes indicated that conduction block was either due to geometrical expansion alone (n = 5) or to additional local depression of conduction (n = 2). In wide strands, action potential upstrokes during anterograde conduction were characterised by multiple rising phases. Mathematical modelling showed that two rising phases were caused by electronic current flow, whereas local ionic current did not coincide with the rising portions of the upstrokes. Conclusions: (1) High resolution optical mapping shows multiphasic action potential upstrokes at the region of abrupt expansion. At the site of the maximum decrement in conduction, these peaks were largely determined by the electrotonus and not by the local ionic current. (2) Unidirectional conduction block occurred in strands with a width of 15(4) μm (1-2 cells

Block of impulse propagation at an abrupt tissue expansion: evaluation of the critical strand diameter in 2- and 3-dimensional computer models

Objective: Unidirectional conduction block in the heart can occur at a site where the impulse is transmitted from a small to a large tissue volume. The aim of this study was to evaluate the occurrence of conduction block in a 2-dimensional and 3-dimensional computer model of cardiac tissue consisting of a narrow strand abruptly emerging into a large area. In this structure, the strand diameter critical for the occurrence of block, hc, was evaluated as a function of changes in the active and passive electrical properties of both the strand and the large medium. Methods: The effects of changes in the following parameters on hc were analysed: (1) maximum sodium conductance (gNamax), (2) longitudinal (Rx) and transverse (Ry) intracellular resistivities, and (3) inhomogeneities in gNamax and Rx and Ry between the strand and the large area. Three ionic models for cardiac excitation described by Beeler-Reuter, Ebihara-Johnson, and Luo-Rudy ionic current kinetics were compared. Results: In the 2-dimensional simulations, hc was 175 μm in Ebihara-Johnson and Beeler-Reuter models and 200 μm in the Luo-Rudy model. At the critical strand diameter, the site of conduction block was located beyond the transition, i.e. a small circular area was activated in the large medium, whereas with narrower strands conduction block occurred within the strands. The decrease of gNamax resulted in a large increase of hc. This increase was mainly due to the change of gNamax in the large area, while hc was almost independent of gNamax in the strand. Changing Rx had no effect on hc, whereas the increase of Ry decreased hc and reversed conduction block. Inhomogeneous changes of Rx and Ry in the strand versus the large medium had opposite effects on hc. When the resistivities of the strand alone were increased, hc also increased. In contrast, the increase of the resistivities in the large area reduced hc. In the 3-dimensional model, hc was 2.7 times larger than the corresponding 2-dimensional values at the various levels of gNamax and resistivity. Conclusions: (1) At physiological values for active and passive electrical properties, hc in the 2D simulations is close to 200 μm in all three ionic models. In the 3-dimensional simulations, hc is 2.7 larger than in the 2-dimensional models. (2) The excitable properties of the large area but not of the strand modify hc. The decrease of intercellular coupling in the large medium facilitates impulse conduction and reduces hc, while the same change in the strand increases hc. (3) Occurrence of conduction block at an abrupt geometrical transition can be explained by both the impedance mismatch at the transition site and the critical curvature beyond the transitio

Role of wavefront curvature in propagation of cardiac impulse

It is traditionally assumed that impulse propagation in cardiac muscle is determined by the combination of two factors: (1) the active properties of cardiac cell membranes and (2) the passive electrical characteristics of the network formed by cardiac cells. However, advances made recently in the theory of generic excitable media suggest that an additional factor—the geometry of excitation wavefronts—may play an important role. In particular, impulse propagation strongly depends on the wavefront curvature on a small spatial scale. In the heart, excitation wavefronts have pronounced curvatures in several situations including waves initiated by small electrodes, waves emerging from narrow tissue structures, and waves propagating around the sharp edges of anatomical obstacles or around a zone of functional conduction block during spiral wave rotation. In this short review we consider the theoretical background relating impulse propagation to wavefront curvature and we estimate the role of wavefront curvature in electrical stimulation, formation of conduction block, and the dynamic behavior of spiral wave

Block of impulse propagation at an abrupt tissue expansion: evaluation of the critical strand diameter in 2- and 3-dimensional computer models

Objective: Unidirectional conduction block in the heart can occur at a site where the impulse is transmitted from a small to a large tissue volume. The aim of this study was to evaluate the occurrence of conduction block in a 2-dimensional and 3-dimensional computer model of cardiac tissue consisting of a narrow strand abruptly emerging into a large area. In this structure, the strand diameter critical for the occurrence of block, hc, was evaluated as a function of changes in the active and passive electrical properties of both the strand and the large medium. Methods: The effects of changes in the following parameters on hc were analysed: (1) maximum sodium conductance (gNamax), (2) longitudinal (Rx) and transverse (Ry) intracellular resistivities, and (3) inhomogeneities in gNamax and Rx and Ry between the strand and the large area. Three ionic models for cardiac excitation described by Beeler-Reuter, Ebihara-Johnson, and Luo-Rudy ionic current kinetics were compared. Results: In the 2-dimensional simulations, hc was 175 μm in Ebihara-Johnson and Beeler-Reuter models and 200 μm in the Luo-Rudy model. At the critical strand diameter, the site of conduction block was located beyond the transition, i.e. a small circular area was activated in the large medium, whereas with narrower strands conduction block occurred within the strands. The decrease of gNamax resulted in a large increase of hc. This increase was mainly due to the change of gNamax in the large area, while hc was almost independent of gNamax in the strand. Changing Rx had no effect on hc, whereas the increase of Ry decreased hc and reversed conduction block. Inhomogeneous changes of Rx and Ry in the strand versus the large medium had opposite effects on hc. When the resistivities of the strand alone were increased, hc also increased. In contrast, the increase of the resistivities in the large area reduced hc. In the 3-dimensional model, hc was 2.7 times larger than the corresponding 2-dimensional values at the various levels of gNamax and resistivity. Conclusions: (1) At physiological values for active and passive electrical properties, hc in the 2D simulations is close to 200 μm in all three ionic models. In the 3-dimensional simulations, hc is 2.7 larger than in the 2-dimensional models. (2) The excitable properties of the large area but not of the strand modify hc. The decrease of intercellular coupling in the large medium facilitates impulse conduction and reduces hc, while the same change in the strand increases hc. (3) Occurrence of conduction block at an abrupt geometrical transition can be explained by both the impedance mismatch at the transition site and the critical curvature beyond the transition

Bioreactor Suspension Culture: Differentiation and Production of Cardiomyocyte Spheroids From Human Induced Pluripotent Stem Cells

Human induced-pluripotent stem cells (hiPSCs) can be efficiently differentiated into cardiomyocytes (hiPSC-CMs) via the GiWi method, which uses small-molecule inhibitors of glycogen synthase kinase (GSK) and tankyrase to first activate and then suppress Wnt signaling. However, this method is typically conducted in 6-well culture plates with two-dimensional (2D) cell sheets, and consequently, cannot be easily scaled to produce the large numbers of hiPSC-CMs needed for clinical applications. Cell suspensions are more suitable than 2D systems for commercial biomanufacturing, and suspended hiPSCs form free-floating aggregates (i.e., spheroids) that can also be differentiated into hiPSC-CMs. Here, we introduce a protocol for differentiating suspensions of hiPSC spheroids into cardiomyocytes that is based on the GiWi method. After optimization based on cardiac troponin T staining, the purity of hiPSC-CMs differentiated via our novel protocol exceeded 98% with yields of about 1.5 million hiPSC-CMs/mL and less between-batch purity variability than hiPSC-CMs produced in 2D cultures; furthermore, the culture volume could be increased ∼10-fold to 30 mL with no need for re-optimization, which suggests that this method can serve as a framework for large-scale hiPSC-CM production

Evidence of Υ(1S)→J/ψ+χc1\Upsilon(1S) \to J/\psi+\chi_{c1} and search for double-charmonium production in Υ(1S)\Upsilon(1S) and Υ(2S)\Upsilon(2S) decays

Using data samples of $102\times10^6$ $\Upsilon(1S)$ and $158\times10^6$ $\Upsilon(2S)$ events collected with the Belle detector, a first experimental search has been made for double-charmonium production in the exclusive decays $\Upsilon(1S,2S)\rightarrow J/\psi(\psi')+X$, where $X=\eta_c$, $\chi_{cJ} (J=~0,~1,~2)$, $\eta_c(2S)$, $X(3940)$, and $X(4160)$. No significant signal is observed in the spectra of the mass recoiling against the reconstructed $J/\psi$ or $\psi'$ except for the evidence of $\chi_{c1}$ production with a significance of $4.6\sigma$ for $\Upsilon(1S)\rightarrow J/\psi+\chi_{c1}$. The measured branching fraction \BR(\Upsilon(1S)\rightarrow J/\psi+\chi_{c1}) is $(3.90\pm1.21(\rm stat.)\pm0.23 (\rm syst.))\times10^{-6}$. The $90\%$ confidence level upper limits on the branching fractions of the other modes having a significance of less than $3\sigma$ are determined. These results are consistent with theoretical calculations using the nonrelativistic QCD factorization approach.Comment: 12 pages, 4 figures, 1 table. The fit range was extended to include X(4160) signal according to referee's suggestions. Other results unchanged. Paper was accepted for publication as a regular article in Physical Review

Cardiac tissue geometry as a determinant of unidirectional conduction block: assessment of microscopic excitation spread by optical mapping in patterned cell cultures and in a computer model

Objective: Unidirectional conduction block (UCB) and reentry may occur as a consequence of an abrupt tissue expansion and a related change in the electrical load. The aim of this study was to evaluate critical dimensions of the tissue necessary for establishing UCB in heart cell culture. Methods: Neonatal rat heart cell cultures with cell strands of variable width emerging into a large cell area were grown using a technique of patterned cell growth. Action potential upstrokes were measured using a voltage sensitive dye (RH-237) and a linear array of 10 photodiodes with a 15 μm resolution. A mathematical model was used to relate action potential wave shapes to underlying ionic currents. Results: UCB (block of a single impulse in anterograde direction — from a strand to a large area — and conduction in the retrograde direction) occurred in narrow cell strands with a width of 15(SD 4) μm (1–2 cells in width, n = 7) and there was no conduction block in strands with a width of 31(8) μm (n = 9, P < 0.001) or larger. The analysis of action potential waveshapes indicated that conduction block was either due to geometrical expansion alone (n = 5) or to additional local depression of conduction (n = 2). In wide strands, action potential upstrokes during anterograde conduction were characterised by multiple rising phases. Mathematical modelling showed that two rising phases were caused by electronic current flow, whereas local ionic current did not coincide with the rising portions of the upstrokes. Conclusions: (1) High resolution optical mapping shows multiphasic action potential upstrokes at the region of abrupt expansion. At the site of the maximum decrement in conduction, these peaks were largely determined by the electrotonus and not by the local ionic current. (2) Unidirectional conduction block occurred in strands with a width of 15(4) μm (1–2 cells)

Role of wavefront curvature in propagation of cardiac impulse

It is traditionally assumed that impulse propagation in cardiac muscle is determined by the combination of two factors: (1) the active properties of cardiac cell membranes and (2) the passive electrical characteristics of the network formed by cardiac cells. However, advances made recently in the theory of generic excitable media suggest that an additional factor—the geometry of excitation wavefronts—may play an important role. In particular, impulse propagation strongly depends on the wavefront curvature on a small spatial scale. In the heart, excitation wavefronts have pronounced curvatures in several situations including waves initiated by small electrodes, waves emerging from narrow tissue structures, and waves propagating around the sharp edges of anatomical obstacles or around a zone of functional conduction block during spiral wave rotation. In this short review we consider the theoretical background relating impulse propagation to wavefront curvature and we estimate the role of wavefront curvature in electrical stimulation, formation of conduction block, and the dynamic behavior of spiral waves