https://ogma.newcastle.edu.au/vital/access/ /manager/Index ${session.getAttribute("locale")} 5 https://ogma.newcastle.edu.au/vital/access/ /manager/Repository/uon:55548 Wed 05 Jun 2024 09:44:59 AEST ]]> https://ogma.newcastle.edu.au/vital/access/ /manager/Repository/uon:39630 N-methyl flecainide) and showed that N-methylation reduces flecainide’s inhibitory potency on RyR2 channels incorporated into artificial lipid bilayers. N-methylation did not alter flecainide’s inhibitory activity on human cardiac sodium channels expressed in HEK293T cells. Antiarrhythmic efficacy was tested utilizing a Casq2 (cardiac calsequestrin) knockout (Casq2−/−) CPVT mouse model. In membrane-permeabilized Casq2−/− cardiomyocytes—lacking intact sarcolemma and devoid of sodium channel contribution—flecainide, but not its analogues, suppressed RyR2-mediated Ca release at clinically relevant concentrations. In voltage-clamped, intact Casq2−/− cardiomyocytes pretreated with tetrodotoxin to inhibit sodium channels and isolate the effect of flecainide on RyR2, flecainide significantly reduced the frequency of spontaneous sarcoplasmic reticulum Ca release, while QX-flecainide and N-methyl flecainide did not. In vivo, flecainide effectively suppressed catecholamine-induced ventricular tachyarrhythmias in Casq2−/− mice, whereas N-methyl flecainide had no significant effect on arrhythmia burden, despite comparable sodium channel block. Conclusions: Flecainide remains an effective inhibitor of RyR2-mediated arrhythmogenic Ca release even when cardiac sodium channels are blocked. In mice with CPVT, sodium channel block alone did not prevent ventricular tachycardia. Hence, RyR2 channel inhibition likely constitutes the principal mechanism of antiarrhythmic action of flecainide in CPVT.]]> Thu 16 Jun 2022 11:46:05 AEST ]]> https://ogma.newcastle.edu.au/vital/access/ /manager/Repository/uon:22802 2+ release. Although dantrolene inhibits Ca²⁺ release from the sarcoplasmic reticulum of skeletal and cardiac muscle preparations, its mechanism of action has remained controversial, because dantrolene does not inhibit single ryanodine receptor (RyR) Ca²⁺ release channels in lipid bilayers. Here we test the hypothesis that calmodulin (CaM), a physiologic RyR binding partner that is lost during incorporation into lipid bilayers, is required for dantrolene inhibition of RyR channels. In single channel recordings (100 nM cytoplasmic [Ca²⁺] + 2 mM ATP), dantrolene caused inhibition of RyR1 (rabbit skeletal muscle) and RyR2 (sheep) with a maximal inhibition of P_o (E_max) to 52 ± 4% of control only after adding physiologic [CaM] = 100 nM. Dantrolene inhibited RyR2 with an IC₅₀ of 0.16 ± 0.03 µM. Mutant N98S-CaM facilitated dantrolene inhibition with an IC₅₀ = 5.9 ± 0.3 nM. In mouse cardiomyocytes, dantrolene had no effect on cardiac Ca²⁺ release in the absence of CaM, but reduced Ca²⁺ wave frequency (IC₅₀ = 0.42 ± 0.18 µM, E_max = 47 ± 4%) and amplitude (IC₅₀ = 0.19 ± 0.04 µM, E_max = 66 ± 4%) in the presence of 100 nM CaM. We conclude that CaM is essential for dantrolene inhibition of RyR1 and RyR2. Its absence explains why dantrolene inhibition of single RyR channels has not been previously observed.]]> Sat 24 Mar 2018 07:12:18 AEDT ]]>