title: Dynamical mechanisms of pacemaker generation in IK1-downregulated human ventricular myocytes: insights from bifurcation analyses of a mathematical model.
abstract: Dynamical mechanisms of the biological pacemaker (BP) generation in human ventricular myocytes were investigated by bifurcation analyses of a mathematical model. Equilibrium points (EPs), periodic orbits, stability of EPs, and bifurcation points were determined as functions of bifurcation parameters, such as the maximum conductance of inward-rectifier K+ current (I(K1)), for constructing bifurcation diagrams. Stable limit cycles (BP activity) abruptly appeared around an unstable EP via a saddle-node bifurcation when I(K1) was suppressed by 84.6%. After the bifurcation at which a stable EP disappears, the I(K1)-reduced system has an unstable EP only, which is essentially important for stable pacemaking. To elucidate how individual sarcolemmal currents contribute to EP instability and BP generation, we further explored the bifurcation structures of the system during changes in L-type Ca2+ channel current (I(Ca,L)), delayed-rectifier K+ currents (I(K)), or Na(+)/Ca2+ exchanger current (I(NaCa)). Our results suggest that 1), I(Ca,L) is, but I(K) or I(NaCa) is not, responsible for EP instability as a requisite to stable BP generation; 2), I(K) is indispensable for robust pacemaking with large amplitude, high upstroke velocity, and stable frequency; and 3), I(NaCa) is the dominant pacemaker current but is not necessarily required for the generation of spontaneous oscillations.
authors.affiliation: Department of Physiology, Kanazawa Medical University, Ishikawa 920-0293, Japan. yasu@kanazawa-med.ac.jp
authors.forename: Yasutaka
authors.initials: Y
authors.lastname: Kurata
authors.forename: Ichiro
authors.initials: I
authors.lastname: Hisatome
authors.forename: Hiroyuki
authors.initials: H
authors.lastname: Matsuda
authors.forename: Toshishige
authors.initials: T
authors.lastname: Shibamoto
mesh_terms: D001683:Biological Clocks / Q000502:physiology*; D020013:Calcium Signaling / Q000502:physiology; D002478:Cells, Cultured; D003198:Computer Simulation; D005246:Feedback / Q000502:physiology; D006329:Heart Conduction System / Q000502:physiology*; D006801:Humans; D051740:Ikaros Transcription Factor / Q000172:deficiency / Q000378:metabolism*; D008564:Membrane Potentials / Q000502:physiology*; D008955:Models, Cardiovascular; D008959:Models, Neurological; D032383:Myocytes, Cardiac / Q000502:physiology*; D017711:Nonlinear Dynamics; D016276:Ventricular Function
chemical_list: C497239:IKZF1 protein, human; D051740:Ikaros Transcription Factor