4.3 Effects of isorhamnetin on electrical remodeling
Electrical remodeling involves cellular Ca2+-handling
abnormalities that can cause
arrhythmogenic
post-depolarization and
spontaneous
ectopic beats while promoting the development of arrhythmogenic
substrates that initiate and maintain re-entry (Nattel et al., 2021).
CaMKII activation and enhanced reactive oxygen species (ROS) are widely
known to contribute to cardiac arrhythmias (Anderson, 2015). Among ROS,
the p47(phox)-mediated activation of nicotinamide adenine dinucleotide
phosphate (NADPH) oxidase plays a crucial role in CaMKII oxidation (Wang
et al., 2018). Previous reports have demonstrated that CaMKII activation
is initiated by the oxidation of methionine 281/282 under AngII
infusion. Additionally, when CaMKII is oxidized, it promotes the
phosphorylation of ryanodine receptor type 2 (RyR2) serine-2814, which
is a key downstream molecular target for the
arrhythmogenesis
of AngII (Wang et al., 2018). In short, through CaMKII oxidation, a
diastolic
SR Ca2+ leak from the RyR2 can be observed.
Furthermore, it has been reported that CaMKII activation enhances the
Ca2+ sensitivity of RyR2, which lowers the threshold
for spontaneous Ca2+ release
and
predisposes the heart to DAD-induced arrhythmias (Wehrens, 2011). In
addition, CaMKII activation affects AP morphology by changing various
ionic currents and ionic channel activities related to calcium, sodium,
and potassium (Thompson et al., 2017; Wagner et al., 2009). On the other
hand, previous reports have indicated that isorhamnetin treatment
prevents ROS production by suppressing the overexpression of p47 (phox),
a major component of CaMKII oxidation (Romero et al., 2009; Sanchez et
al., 2007). Furthermore, a previous report demonstrated that
isorhamnetin could modulate
Ca2+channels (Cav1.2) and Ca2+ currents, which play an
important role in AP morphology (Saponara et al., 2011). In our study,
AngII induced
diastolic
SR Ca2+ sparks and CaMKII oxidation. However,
isorhamnetin diminished the occurrence of diastolic SR
Ca2+ sparks, the phosphorylation of RyR2 at serin
2814, and the oxidation of CaMKII in the atrium (Fig. 2B, Fig. 7.1E-F).
In addition, although AngII promoted APD prolongation and DAD formation,
isorhamnetin brought the wave morphology closer to that of the control
group and diminished the occurrence of DADs (Fig. 4K).
Aberrant
spontaneous Ca2+ waves were observed more frequently
in the AngII group than in the control and isorhamnetin groups (Fig.
3F). These spontaneous abnormal Ca2+ waves may be
caused by
arrhythmogenic
depolarizations, such as DADs. Interestingly, isorhamnetin also
normalized the elevated expression of CACNA1C, which encodes Cav1.2
(Fig. 7.1C, G). This may have contributed to the normalization of wave
morphology (Fig. 4G). Collectively, these results suggest that
isorhamnetin reduces abnormal diastolic intracellular
Ca2+ activity and the occurrence of DADs by abrogating
CaMKII oxidation and RyR2 phosphorylation, while isorhamnetin normalizes
AP morphology by modulating Ca2+ channels and
currents.