Contractile heterogeneity in ventricular myocardium

Abstract

The transmural heterogeneity of the contractility in ventricular muscle has not been well-studied. Here, we investigated the calcium transient and sarcomere contraction/relaxation in the endocardial (Endo) and epicardial (Epi) myocytes. Endo and Epi myocytes were isolated from C57/BL6 mice by Langendorff perfusion. Ca transient and sarcomere contraction/relaxation were recorded simultaneously at different stimulation frequencies using a dual excitation fluorescence photomultiplier system. We found that the Endo myocytes have higher baseline diastolic calcium, significantly larger calcium transient and stronger sarcomere shortening than Epi myocytes. However, both the rising and decline phases for calcium transient and sarcomere shortening were slower in Endo than in Epi myocytes. When simulation frequency was increased from 1 to 3 Hz, a greater percent increase in the diastole calcium level, Ca transient and sarcomere shortening amplitude has been observed in the Endo myocytes. Accordingly, the frequency-dependent acceleration in the decay rate of calcium transient and sarcomere relaxation was more profound in the Endo than in Epi myocytes. Western blot analysis showed that CaMKII activity was significantly higher in Epi than in Endo myocardium before stimulation. However, this transmural heterogeneity was reversed by rapid pacing. CaMKII inhibition by KN93 diminished the frequency-dependent alterations of Ca transient and sarcomere contraction. Our results suggest that the contractility of ventricular myocytes is heterogeneous. The Endo-myocardium is the major force generating layer in the heart, both at slow and fast heart rate, and the transmural heterogeneity of CaMKII activation plays an important role in the frequency-dependent alterations.

The Effects of Mechanical Preload on Transmural Differences in Mechano-Calcium-Electric Feedback in Single Cardiomyocytes: Experiments and Mathematical Models

“…Consistent with findings on mouse unloaded single-cell shortening (Kondo et al, 2006;Pan et al, 2018), our data showed that the amplitude of auxotonic tension was higher and T max was greater in single Endo cells compared with Epi ones. The Endo and Epi mathematical models that we developed for the study reproduced the experimental data on transmural differences in contraction and literature data on the transmural gradient in AP and Ca 2+ parameters ( Table 2).…”

“…Endo cardiomyocytes also showed a slower Ca 2+ transient. Consistently with literature data (Dilly et al, 2006;Rossow et al, 2006;Pan et al, 2018), the Endo model had a higher Ca 2+ transient amplitude (15% relative to the Endo model) and a higher diastolic Ca 2+ level (12% relative to the Epi model) compared with the Epi model ( Table 2).…”

“…On the other hand, regional heterogeneity in the expression and function of Ca 2+ cycling and sarcomere proteins, including L-type Ca 2+ channels (Wang and Cohen, 2003;Wan et al, 2005), SERCA2a (Lou et al, 2011), Na + -Ca 2+ exchanger (Dilly et al, 2006), and myosin isoform (Cazorla et al, 2005), also contribute to regional variations in the Ca 2+ transient and sarcomere shortening. Experiments on single cells showed that Endo myocytes had a higher diastolic Ca 2+ level, greater amplitude of the Ca 2+ transient and sarcomere shortening, and slower rising and decline phases of the Ca 2+ transient and sarcomere shortening (Cordeiro et al, 2004;Pan et al, 2018). Moreover, single studies on mechanically loaded cardiomyocytes demonstrated that the Frank-Starling mechanisms depended on cell location with a steeper sarcomere length (SL)-active tension relationship (Cazorla et al, 1997(Cazorla et al, , 2000 and a larger increase in Ca 2+ sensitivity following stretch (Cazorla et al, 2005) in endocardium than in epicardium.…”

ER–Mitochondria Microdomains in Cardiac Ischemia–Reperfusion Injury: A Fresh Perspective

“…Many researchers have attempted to demonstrate the causal role of ER–mitochondrial microdomain in modifying I/R-mediated cell death. First, it is well documented that the sensitivity of cardiomyocyte to death (regardless of apoptosis and necroptosis) is fine-tuned by cellular calcium concentration ( Mofid et al, 2017 ; Zhu et al, 2017 ; Pan et al, 2018 ) which drastically is affected by the ER–mitochondrial microdomain. Based on previous studies ( Zhou et al, 2018d ; Zhu H. et al, 2018 ), IP3R expression is upregulated in response to I/R stress, leading to the calcium overload in mitochondria.…”

Contractile State Dependent Sarcomere Length Variability in Isolated Guinea-Pig Cardiomyocytes

“…The averaged SL at end-systole was ∼9% smaller than at end-diastole. The amount of SL shortening is slightly lower compared to the data obtained in mouse cardiomyocytes in vivo under a natural beat frequency ( Kobirumaki-Shimozawa et al, 2016 ), but is of the same magnitude or even higher compared to the averaged fractional shortening of isolated rat and guinea-pig LV cardiomyocytes, which were mechanically unloaded and paced at a relatively low rate ( Natali et al, 2002 ; Wan et al, 2003 ; Carneiro-Júnior et al, 2013 ; Chung and Campbell 2013 ; Pan et al, 2018 ).…”