Background: Cardiac remodeling can
be driven by either septic or aseptic inflammation initialized by the
activation of Toll-Like-Receptor 4 (TLR4). TLR4 activation by injection of Lipopolysaccharide
(LPS) is known to decrease contractile function via nitric oxide signaling and increase the probability of atrial and ventricular
arrhythmia, however the exact mechanism is not fully explored. TLR4 can be
found mainly on cells of innate immune system but also on highly specialized
cells like cardiomyocytes. The difference in indirect effects mediated by TLR4
function on circulating cells or
direct function on cardiomyocytes remains unclear. Therefore, in this study we
aim to identify the cell type responsible for TLR4 dependent cardiac remodeling
to suggest novel therapeutic principles.
Methods: A acute septic mouse model
was generated by injecting the TLR4 activator LPS (2 mg/kg BW) or NaCl intraperitoneal and 3.5 hours later animals were sacrificed for further experiments.
Wildtype C57/Bl6J mice and ubiquitous TLR4-/- mice were compared in explanted
Langendorff-perfused hearts by optical epicardial voltage-mapping using the voltage
sensitive dye Di-4-ANEPPS and a highly light sensitive sCMOS camera. Conduction
velocity (CV) as well as action potential shape were analyzed from atria and
ventricles with up to 10.000 frames per second. In addition, single cardiomyocytes were dissociated from atria of
untreated mice and exposed to LPS for 3 h in
vitro before recording of action potentials by patchclamp analyses.
Results: In atria of LPS injected wildtype mice, voltage-mapping
showed decreased conduction velocity (+LPS: 43.1 ± 3.1 cm/s, n = 5; +NaCl: 72.6
± 9.8 cm/s, n = 10, p = 0.04). Analysis of syncytial action potential shape
showed a tendency to decreased upstroke velocity (max. dV/dt: +LPS: 38.4 ± 2.9
mV/ms, n = 5; +NaCl: 54.8 ± 5.9 mV/ms, n = 10, p = 0.06) but action potential
duration was unaltered (APD70: +LPS: 25.6 ± 4.1 ms, n = 5; +NaCl: 28.7 ± 3.2
ms, n = 10, p = 0.57). Decreased CV and max. dV/dt, both indicators of
decreased Na+ channel availability, were not observed in TLR4-/-
mice. Interestingly, LPS stimulation of isolated atrial cardiomyocytes also lead
to decreased max.dV/dt in patchclamp experiments (+LPS: 45.7 ± 4.1 mV/ms, n =
5; + NaCl: 156.3 ± 11.6 mV/ms, n = 4, p < 0.001), indicating a cardiomyocyte
specific effect of TLR4 on Na+ channels. In addition we found proarrhythmogenic
shortened action potential durations (APD70: +LPS: 10.3 ± 2.5 ms, n = 5; +NaCl:
19.1 ± 2.3 ms, n = 6, p = 0.003). In the left and right ventricle we observed significantly
decreased CV (+LPS: 50.2 ± 2.2 cm/s, n = 6; + NaCl: 67.7 ± 5.0 cm/s, n = 10, p
= 0.02), decreased upstroke velocity (max. dV/dt: +LPS: 32.8 ± 1.6 mV/ms, n =
6; +NaCl: 46.3 ± 0.9 mV/ms, n = 10, p < 0.0001) and shortened APD70 (+LPS:
41.5 ± 2.7 ms, n = 6; +NaCl: 67.7 ± 7.8 ms, n = 10, p =
0.02).
Conclusion: Herein we report for the first time that a septic condition
induced by LPS impaired cardiac depolarization and conduction in a
TLR4-depentent mechanism. These pro-arrhythmogenic changes can already be
observed shortly (3.5h) after sepsis induction, suggesting a non-structural remodeling
as underlying mechanism. The comparison decreased upstroke velocity after in vivo LPS stimulation and LPS exposure
to isolated cardiomyocytes in vitro
imply a cardiomyocyte specific effect of TLR4 activation, which could become a
promising target of immunomodulatory antiarrhythmic therapies.