Modeling autoregulation of cardiac excitation-Ca-contraction and arrhythmogenic activities in response to mechanical load changes

Figures

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  Figure 1. EC coupling with MCT feedback model The electrophysiology model by Shannon et al.28 and cross-bridge force development model by Negroni et al.29 shown at the bottom were implemented, referring to length and velocity calculated in the mechanical model and MCT-Ca2+ gain for LCC upregulation. The mechanical model shown in the blue box is simplified to 1° of freedom, to calculate the equilibrium between cross-bridge force from the EC coupling model, nonlinear cell-passive force, and gel-viscoelastic force expressed by the generalized Maxwell model. Calculated stress feedback to EC coupling model through Mechano-chemo-transduction (MCT) model shown in red. The MCT-Ca2+ gain function relates the cell-edge tension loaded by the gel to the L-type Ca2+ Chennel upregulation coefficient. This MCT feedback autoregulates contraction in our proposed system.

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  Figure 2. Validation of the new model on mechanical load effect on EC coupling Time courses of simulated electrophysiological and mechanical responses under load-free (black), in-gel (red), and in-gel without MCT (blue) conditions (left panels) are compared with experimentally reported time courses (right panels in the green background). (D) is adapted with permission from Shimkunus et al. (2021)20; copyright 2021 Wolters Kluwer Health. (E and F) are reproduced from Hegyi et al. (2021)24 under a CC BY-NC-ND license. (Left panels) (A–C) The steady-state under 0.5Hz pacing is shown: (A) bulk cytosolic [Ca2+], (B) action potential, (C) normalized length of the cell. In B and C, in-gel without MCT (blue dashed) lines coincide with load-free (black) lines. (Right panels) Whole-cell patch-clamp experiments in isolated rabbit ventricular cardiomyocytes embedded in PVA gel and Tyrode’s solution paced at 0.5 Hz. (D) Increased Ca2+ transient was observed with the in-gel condition. (E and F) Action potential was significantly prolonged under in-gel conditions (red) and was attenuated by nitric oxide synthase (which mediates mechano-chemo-transduction effect) inhibitor NG-nitro-L-arginine methyl ester…

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  Figure 3. Autoregulation of EC coupling enabled by MCT feedback The color indicates CL concentration, black: 0% (=load free), gray: 2.5%, blue: 5%, orange:7.5%, red: 10%, pink: 12.5%, and green: 15%. (A) Time courses of simulated [Ca2+]i. The insets in (A) and (C) are enlarged views of the first 300 ms. (B) Time courses of the simulated normalized length of the cell. (C) Time courses of L-type Ca2+ channel current (ICa,L). (D) Maximum intracellular [Ca2+]i upon changing CL concentration. Diagonal striped boxes in (D) and (E) exhibit the results without MCT feedback. (E) Fractional shortening on changing CL concentration. (F) Experimental maximum intracellular [Ca2+] depending on CL concentration under 0.5Hz pacing. (G) Experimental FS depending on CL concentration under 0.5Hz pacing. (F) and (G) on a green background are adapted with permission from Shimkunas et al.24; copyright 2021 Wolters Kluwer Health. Data are represented as mean ± SEM.

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  Figure 4. APD prolongation and alternans development under increasing mechanical load (A–C) Representative time courses of action potential (A and B) and time development of ΔAPD90 (C) in the whole-cell patch-clamp experiments in isolated rabbit ventricular cardiomyocytes embedded in PVA under 0.5 Hz pacing at room temperature. The panels A, B, and C are reproduced from Hegyi et al. (2021)20 under a CC BY-NC-ND license. After adding a cross-linker, a hydrogel formed around the cell to prolong APD (A, red) compared to load-free condition (black). Further gel stiffening often causes alternans, with longer APD significantly larger than that of load-free condition (B, blue). (D) Suppositional effective cross-linker concentration for simulating responses on the application of the cross-linker. Exponential curve which reaches 9% CL (90% of the target concentration of 10% CL) after 60 s. (E) Simulated ΔAPD90 in response to CL concentration change under MCT1 (solid) and MCT1.5 (open) are shown in D. APD90 prolongation occurred at first and subsequently, alternans were triggered under MCT1.5. To replicate alternans development, 1.5 times amplified MCT-Ca2+ gain function MCT1.5 was used (see Figure…

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  Figure 5. The effect of MCT-Ca2+ gain and functional form on EC coupling stability (A) Three curves of the MCT function were tested. MCTgauss (light blue x-mark) has the same peak as MCT1.5 without downregulation of LCC (MCTgauss > 1). MCTatan traces the rising curve of MCT1.5 and saturates without a negative slope (blue circle). (B) Action potential duration (APD) in response to varying pacing cycle length (PCL) between 3000 and 300 ms with MCT functions in (A). (C) The original MCT-Ca2+ gain function MCT1 (red circle), amplified by 1.5 times (orange triangle) and 2.0 times (yellow plus). (D) Action potentials with amplified MCT-Ca2+ gains shown in (C).

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  Figure 6. Mechanical load induced discordant alternans and mechanistic explanation (A) Simulated intra-SR [Ca2+], cytosolic [Ca2+], normalized sarcomere length (SL), and action potential. The simulation was performed under 10% CL concentration with MCT1.5 at 0.5Hz pacing. (B) Experimentally reported alternans at 0.5 Hz pacing under cell-in-gel conditions. Figures are reproduced from Hegyi et al. (2021),20 under a CC BY-NC-ND license. (C) Comparison of time courses of two consecutive beats in alternans. Stress ranges in red and blue colored beats are indicated by red and blue arrows in the MCT-Ca2+ gain diagram. Lower intra-SR [Ca2+] at the initiation of red-colored beat starting at 2 s leads to smaller Ca2+ transient and gel force exerted on the cell. Since this smaller force falls within the range of positive slope of MCT-Ca2+ gain, maintaining the gain high during the contraction. As a result, upregulated ICa,L leads to longer APD. A larger Ca2+ influx and smaller release from SR lead to a higher SR load for the next beat colored in blue. The succeeding beat exhibits larger Ca2+ transient and gel force. Then gel force reaches the range of suppressive MCT-Ca2+ gain function, resulting in…