Modeling short-term interval-force relations in cardiac muscle

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 Ca²⁺ released from the sarcoplasmic reticulum (SR) given full restitution, and 3) for strong negative feedback of the SR release on sarcolemmal Ca²⁺ influx. The second modeling approach is to develop a more detailed single ventricular cell model that simulates action potentials, Ca²⁺-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 Ca²⁺ in the SR but also increases the SR loading by two mechanisms: 1) less Ca²⁺-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 Ca²⁺ influx. The cooperativity of the myofilaments amplifies the relatively small changes in the Ca²⁺ 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 Ca²⁺ loading, with additional contributions from membrane currents and myofilament activation.