Rapid stimulation causes electrical remodeling in cultured atrial myocytes.
ABSTRACT Rapid stimulation causes electrical remodeling in the intact atrium, with shortening of action potential duration (APD), down-regulation of L-type Ca2+ currents (I(Ca,L)), and increased vulnerability to atrial fibrillation (AF). The essential elements required for this process are currently unknown. We tested the hypothesis that rapid stimulation of cardiomyocytes in vitro is sufficient to recapitulate the remodeling process, and that atrial cells subjected to rapid pacing in culture would display changes similar to those that occur in vivo.
Atrial (HL-1) cells were cultured in the presence of rapid field stimulation (300 beats per min) for 24 h. Action potentials and ionic currents were recorded from stimulated cells, as well as control cells cultured in parallel, using whole-cell voltage-clamp techniques.
Rapid stimulation of atrial cells for 24 h significantly shortened APD. HL-1 cells displayed both I(Ca,L) blocked by nimodipine, and T-type Ca2+ currents (I(Ca,T)) sensitive to mibefradil. Rapid activation in culture caused down-regulation of I(Ca,L), while I(Ca,T) was similarly reduced. Multiple outward currents were present in response to a depolarizing voltage-clamp protocol, and rapid pacing resulted in up-regulation of the rapidly-activating delayed rectifier K+ current, I(Kr).
Rapid stimulation of atrial cells in culture produces electrical remodeling, recapitulating principal phenotypic features of atrial tachycardia remodeling in vivo. Our results demonstrate that an important component of this process is cell autonomous, given that in vivo conditions are not required for the development of electrical remodeling.
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ABSTRACT: In the mammalian embryo, the primitive tubular heart starts beating during the first trimester of gestation. These early heartbeats originate from calcium-induced contractions of the developing heart muscle cells. To explain the initiation of this activity, two ideas have been presented. One hypothesis supports the role of spontaneously activated voltage-gated calcium channels, whereas the other emphasizes the role of Ca(2+) release from intracellular stores initiating spontaneous intracellular calcium oscillations. We show with experiments that both of these mechanisms coexist and operate in mouse cardiomyocytes during embryonic days 9-11. Further, we characterize how inositol-3-phosphate receptors regulate the frequency of the sarcoplasmic reticulum calcium oscillations and thus the heartbeats. This study provides a novel view of the regulation of embryonic cardiomyocyte activity, explaining the functional versatility of developing cardiomyocytes and the origin and regulation of the embryonic heartbeat.The Journal of General Physiology 10/2008; 132(4):397-405. · 3.84 Impact Factor
Article: Inhibition of the cardiac L-type calcium channel current by the TRPM8 agonist, (-)-menthol.[show abstract] [hide abstract]
ABSTRACT: (-)-Menthol and icilin are agonists of the thermoreceptor non-selective cation channel, TRPM8, and are commonly used to investigate TRPM8 function without a full appreciation of their non-specific effects. To investigate the hypothesis that (-)-menthol and icilin inhibit cardiovascular-type L-type Ca(2+) channel currents (I(Ca,L)), the actions of the TRPM8 agonists on rabbit ventricular myocyte I(Ca,L) were examined at near-physiological temperature (≈35°C) using whole-cell recording. Icilin (3-100 μM) did not significantly inhibit I(Ca,L). (3) in contrast, (-)-menthol concentration-dependently inhibited peak I(Ca,L) (IC(50)=74.6 μM; log(10)IC(50)(M)=-4.13±0.14). (-)-Menthol blocked the late I(Ca,L) remaining at the end of depolarising pulses with greater efficacy (96.1±2.4% block at 1 mM) than peak I(Ca,L) (68.9±5.7% block at 1 mM, P<0.01), although there was no difference in potency of block of peak and late currents. Block by (-)-menthol showed no voltage-dependence. The actions of (-)-menthol were compared with those of nimodipine. Nimodipine was a more efficacious (97.3±1.5 % block at 30 μM, P<0.01) and potent (IC(50)=0.74 μM; log(10)IC(50)(M)=-6.13±0.08, P<0.0001) blocker of peak I(Ca,L) than (-)-menthol. In contrast to (-)-menthol, nimodipine showed greater potency (IC(50)=0.056 μM; log(10)IC(50)(M)=-7.25±0.17, P<0.0001), but not greater efficacy, in block of late compared with peak I(Ca,L). In summary, these data demonstrate that, at near-physiological temperature, (-) -menthol blocks cardiac I(Ca,L) at concentrations similar to those reportedly effective in TRPM8-agonism. The data suggest that the mechanism of L-type Ca(2+) channel block by (-)-menthol differs from that of nimodipine.Journal of physiology and pharmacology: an official journal of the Polish Physiological Society 10/2010; 61(5):543-50. · 2.27 Impact Factor
Article: Ca2+-calmodulin-dependent protein kinase II represses cardiac transcription of the L-type calcium channel alpha(1C)-subunit gene (Cacna1c) by DREAM translocation.[show abstract] [hide abstract]
ABSTRACT: Recent studies have demonstrated that changes in the activity of calcium-calmodulin-dependent protein kinase II (CaMKII) induce a unique cardiomyocyte phenotype through the regulation of specific genes involved in excitation-contraction (E-C)-coupling. To explain the transcriptional effects of CaMKII we identified a novel CaMKII-dependent pathway for controlling the expression of the pore-forming α-subunit (Cav1.2) of the L-type calcium channel (LTCC) in cardiac myocytes. We show that overexpression of either cytosolic (δC) or nuclear (δB) CaMKII isoforms selectively downregulate the expression of the Cav1.2. Pharmacological inhibition of CaMKII activity induced measurable changes in LTCC current density and subsequent changes in cardiomyocyte calcium signalling in less than 24 h. The effect of CaMKII on the α1C-subunit gene (Cacna1c) promoter was abolished by deletion of the downstream regulatory element (DRE), which binds transcriptional repressor DREAM/calsenilin/KChIP3. Imaging DREAM-GFP (green fluorescent protein)-expressing cardiomyocytes showed that CaMKII potentiates the calcium-induced nuclear translocation of DREAM. Thereby CaMKII increases DREAM binding to the DRE consensus sequence of the endogenous Cacna1c gene. By mathematical modelling we demonstrate that the LTCC downregulation through the Ca2+-CaMKII-DREAM cascade constitutes a physiological feedback mechanism enabling cardiomyocytes to adjust the calcium intrusion through LTCCs to the amount of intracellular calcium detected by CaMKII.The Journal of Physiology 06/2011; 589(Pt 11):2669-86. · 4.72 Impact Factor