Feb 14, 2017 - Capacitive Memory Suppresses Alternans and Promotes ... Electrical activity in cardiomyocytes is typically modeled using an ideal par-.
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Tuesday, February 14, 2017
inotropic response observed at 1 Hz with beta-adrenergic stimulation. Further analysis of this integrated model would dissect contribution from parameters in the adrenergic signaling cascade to human ventricular electrophysiology. These results will greatly facilitate understanding of human adrenergic regulation on cardiac functions and diseases. 1991-Pos Board B311 Capacitive Memory Suppresses Alternans and Promotes Spontaneous Activity in a Fractional-Order Minimal Cardiomyocyte Model Tien Comlekoglu, Seth H. Weinberg. Biomedical Engineering, Virginia Commonwealth University, Richmond, VA, USA. Electrical activity in cardiomyocytes is typically modeled using an ideal parallel resistor-capacitor circuit. However, studies have suggested that the passive properties of cell membranes may be more appropriately modeled with a non-ideal capacitor, in which the current-voltage relationship is given by a fractional-order derivative. Fractional-order membrane potential dynamics introduces capacitive memory effects, i.e., dynamics are influenced by the prior membrane potential history. We recently showed that fractional-order membrane dynamics alters ionic currents and spiking rates in a neuronal model. Here, we investigate the effects of fractional-order membrane dynamics in a cardiomyocyte model using the minimal 3-variable Fenton-Karma (FK), chosen because the FK model, with first-order derivative membrane dynamics, does not have short-term memory. We performed simulations for fractionalorders between 0.5 and 1 and variable cycle lengths. We found that the action potential duration (APD) was shortened as the fractional-order decreased, for all cycle lengths. As a consequence, the minimum cycle length (MCL) for loss of 1:1 capture decreased as fractional-order decreased. Further, at short cycle lengths at which APD alternans was present in the first-order model, alternans was suppressed, such that the cycle length for alternans onset decreased for decreasing fractional-order. For fractional-order less than ~0.82, alternans was not present at any cycle length. Finally, for fractional-order less than ~0.75, we found that the model produced spontaneous action potentials following pacing. Short-term memory effects were represented by a hypothetical memory ‘‘current,’’ which we found was primarily outward for fractional-order closer to 1, shortening APD, while it was primarily inward for fractional-order closer to 0.5, generating spontaneous action potentials. Collectively, our results suggest the capacitive memory, reproduced by a fractional-order model, may play a role in both alternans formation and suppression and pacemaking. 1992-Pos Board B312 Activation of TRPM3 in Perivascular Sensory Nerves Induces Dilation of Mouse Resistance Arteries Lucia Alonso-Carbajo1,2, Yeranddy A. Alpizar1, Justyna Startek1, Jose Ramo´n Lo´pez-Lo´pez2, Maria Teresa Pe´rez-Garcı´a2, Karel Talavera1. 1 Lab of Ion Channel Research, KU Leuven, Leuven, Belgium, 2Dpto de Bioquı´mica, Biologı´a Molecular y Fisiologı´a, IBGM, Universidad de Valladolid, CSIC, Universidad de Valladolid, Valladolid, Spain. The vascular tone is determined by a complex interplay of vasodilator and vasoconstrictor stimuli that modulate the contractile state of vascular smooth muscle cells (VSMCs). Activation of the cation channel Transient Receptor Potential Melastatin 3 (TRPM3) has been shown to induce contraction of VSMCs in aorta. However, the contribution of this channel to the vascular tone in resistance arteries remains unknown. Real-time qPCR and immunochemistry experiments showed Trpm3 expression in mesenteric arteries isolated from wild type (WT) C57BL/6J mice. Myography experiments carried out in intact pressurized mesenteric arteries from WT mice showed that the TRPM3 agonist PS induces vasodilation at concentrations above ~5 mM, with a concentration-dependency featuring two distinct increasing phases. In contrast, PS only induced vasodilation above 10 mM following a single Hilltype behavior in preparations from Trpm3 knockout (KO) mice. Recordings in WT arteries in the presence of the CGRP receptor antagonist BIBN 4096 recapitulated the results of Trpm3 KO preparations, indicating that the TRPM3-mediated effect of PS entails CGRP release from perivascular nerve endings. The effect of 10 mM PS was inhibited to about 50% by the combination of potassium channel blockers (500 nM paxilline, 10 mM correolide and 50 nM stromatoxin). Electrophysiological recordings in freshly isolated mesenteric VSMCs, revealed that basal currents are not sensitive to PS (10 mM and 30 mM) and that PS has no effect on potassium currents. Our data indicates that activation of TRPM3 channels in perivascular sensory nerves induces CGRP release, which leads to activation of potassium channels in smooth muscle cells, resulting in dilation of mesenteric arteries. These findings reveal a potential role of TRPM3 in vascular tone regulation, and support the recent notion that this channel may play roles in neurogenic inflammation.
Voltage-gated K Channels and Mechanisms of Voltage Sensing and Gating III 1993-Pos Board B313 KV2.1/KV6.4 Heterotetramers are Functional in Two Stoichiometric Configurations Glenn Regnier, Dirk J. Snyders. Biomedical Sciences, University of Antwerp, Antwerpen, Belgium. Members of the voltage-gated Kþ (Kv) subfamilies Kv5, Kv6, Kv8, and Kv9, which are collectively designated as electrically silent (KvS) subunits, selectively modulate the biophysical properties of Kv2 channels by forming heterotetrameric Kv2/KvS channel complexes: e.g. Kv2.1/Kv6.4 heterotetramers display a 40 mV hyperpolarizing shift in the voltage-dependence of C-type inactivation. Fo¨ster resonance energy transfer (FRET) analysis has previously demonstrated that Kv2.1/Kv9.3 channels are composed of three Kv2.1 subunits and one Kv9.3 subunit. However, it remains unknown whether Kv2.1/KvS channels can have other stoichiometries. We investigated this specifically for the Kv2.1/Kv6.4 combination by comparing the biophysical properties of different concatemeric constructs to these of the corresponding monomers. Kv2.1-Kv2.1 (2_2) dimers yielded delayed rectifier currents with biophysical properties similar to those of channels composed of individual Kv2.1 subunits. Kv2.1-Kv6.4 (2_6) or Kv6.4-Kv2.1 (6_2) dimers displayed a midpoint of inactivation of 5953 and 5851 mV (n=6), respectively, similar to that of Kv2.1/Kv6.4 channels obtained from cotransfection the individual subunits (6252 mV). This suggested that these channels are functional with a 2:2 stoichiometry. Analysis of tetrameric concatemers revealed that the positional arrangement of the Kv6.4 subunits was crucial. Indeed, we could not detect any functional channels when two Kv6.4 subunits were positioned side by side (2_6_6_2), but the tetramer with alternating subunits (2_6_2_6) displayed a midpoint of inactivation of 62 52 mV (n=6). Additionally, tetramers that represented a 3:1 stoichiometry (2_6_2_2) also displayed the hyperpolarizing shift of the inactivation. Interestingly, the voltage-dependence of activation became shallower with incorporation of more Kv6.4 subunits: slope factors were 9 mV for 2_2_2_2, 15 mV for 2_6_2_2 and 22 mV for 2_6_2_6. Taken together, our data suggest that the Kv2.1:Kv6.4 stoichiometry in heteroteramers can be either 3:1 or 2:2 with the restriction that Kv2.1 and Kv6.4 have to alternate in the channel complex in the 2:2 configuration. 1994-Pos Board B314 Lipid-Dependent Gating of Kv Channels and Excitability Change of Cerebellar Purkinje Neurons in an NPC1 Model Mouse Qiu-Xing Jiang1, Hui Zheng Zheng2, Hong Xing3, Gaya Yadav1, Yuqing Li3. 1 Department of Microbiology and Cell Science, University of Florida, Gainesville, FL, USA, 2Department of Physiology, UT Southwestern Medical Center, Dallas, TX, USA, 3Department of Neurology, University of Florida, Gainesville, FL, USA. Recent studies of Kv channels in different membrane systems with altered ratios between phospholipids and nonphospholipids support our lipid-dependent gating hypothesis that the nonphospholipids favor the conformational switch of the voltage sensor domains of a Kv channel to a resting state. To enable more precise control of lipid composition in an artificial membrane and avoid possible solvent contamination or phase separation of lipids in bilayer membranes, we constructed a bead-supported unilamellar membrane (bSUM) and used it to define a more quantitative relationship between nonphospholipid content and change in the gating property of a voltage-gated potassium channel. In the bSUMs, we were able to show for the first time that a small amount of cholesterol (~10 molar %) in a phospholipid membrane in a fluidic phase exerts a strong inhibitory effect on a KvAP channel, suggesting that the Kv channels might be highly sensitive to change in membrane cholesterol content. To test this prediction in a physiological system where lipid metabolic defects cause significant neurological disorders, we analyzed the neuronal excitability in a mouse model for the Niemann-Pick disease type C, an NPC1-I1061T knockin mice. Systematic comparison of five different groups of cerebellar Purkinje neurons from both knockin mice and control animals revealed a significant decrease in the firing frequency of action potentials in the tonic-burst firing Purkinje neurons of the NPC1 mice, which appears to be consistent with the predicted cholesterol effects on Kv channels. Detailed studies of the gating properties in these Purkinje neurons are being conducted to understand the underlying mechanism. Our data support a potential connection among cholesterol content in cell membranes, cholesterol-dependent gating of voltage-gated K channels and change in neuronal excitability in CNS neurons.
