title: Modeling short-term interval-force relations in cardiac muscle.
abstract: This study employs two modeling approaches to investigate short-term interval-force relations. The first approach is to develop a low-order, discrete-time model of excitation-contraction coupling to determine which parameter combinations produce the degree of postextrasystolic potentiation seen experimentally. Potentiation is found to increase 1) for low recirculation fraction, 2) for high releasable fraction, i.e., the maximum fraction of Ca(2+) released from the sarcoplasmic reticulum (SR) given full restitution, and 3) for strong negative feedback of the SR release on sarcolemmal Ca(2+) influx. The second modeling approach is to develop a more detailed single ventricular cell model that simulates action potentials, Ca(2+)-handling mechanisms, and isometric force generation by the myofilaments. A slow transition from the adapted state of the ryanodine receptor produces a gradual recovery of the SR release and restitution behavior. For potentiation, a small extrasystolic release leaves more Ca(2+) in the SR but also increases the SR loading by two mechanisms: 1) less Ca(2+)-induced inactivation of L-type channels and 2) reduction of action potential height by residual activation of the time-dependent delayed rectifier K(+) current, which increases Ca(2+) influx. The cooperativity of the myofilaments amplifies the relatively small changes in the Ca(2+) transient amplitude to produce larger changes in isometric force. These findings suggest that short-term interval-force relations result mainly from the interplay of the ryanodine receptor adaptation and the SR Ca(2+) loading, with additional contributions from membrane currents and myofilament activation.
authors.affiliation: Department of Biomedical Engineering, The Johns Hopkins University School of Medicine, Baltimore, Maryland 21205, USA. jrice@bme.jhu.edu
authors.forename: J J
authors.initials: JJ
authors.lastname: Rice
authors.forename: M S
authors.initials: MS
authors.lastname: Jafri
authors.forename: R L
authors.initials: RL
authors.lastname: Winslow
mesh_terms: D000200:Action Potentials; D002118:Calcium / Q000378:metabolism; D020746:Calcium Channels, L-Type / Q000502:physiology; D004553:Electric Conductivity; D005246:Feedback; D006321:Heart / Q000502:physiology*; D006352:Heart Ventricles / Q000166:cytology; D008954:Models, Biological; D009200:Myocardial Contraction / Q000502:physiology*; D015221:Potassium Channels / Q000502:physiology; D019837:Ryanodine Receptor Calcium Release Channel / Q000502:physiology; D012519:Sarcoplasmic Reticulum / Q000378:metabolism; D013599:Systole; D016276:Ventricular Function
chemical_list: D020746:Calcium Channels, L-Type; D015221:Potassium Channels; D019837:Ryanodine Receptor Calcium Release Channel; D002118:Calcium