Simulation of Ca2+ release from the sarcoplasmic reticulum with three-dimensional sarcomere model in cardiac muscle

Characterization of junctional sarcoplasmic reticulum Ca2+ content and release by short-term mechanical restitution in cardiac muscle

To study Ca2+ handling by the junctional sarcoplasmic reticulum (JSR), the time course of short-term mechanical restitution after varying magnitudes of twitch contractions was assessed in rat papillary muscle. Mechanical restitution consisted of a pretwitch latency period followed by a rapid and a subsequent much slower restitution of twitch force. The rate of rapid restitution was independent of the magnitude of the preceding twitch, which suggests that the rate of JSR Ca2+ repletion was dependent on the amount of Ca2+ remaining in the JSR after a twitch contraction. Based on this finding, the functions Gt and Ht, representing the time courses of JSR Ca2+ repletion and release, respectively, were derived graphically from a family of the mechanical restitution curves. Gt increased monotonically with time at a decreasing rate, while Ht increased with time in a sigmoid manner. The mechanical alternans were simulated by using experimental values and mathematically predicted values of Gt and Ht. A substitution of extracellular Na+ with Li+ to inhibit Na+/Ca2+ exchange resulted in an augmentation of Gt by approximately 10%, presumably by increasing the tubular SR Ca2+ uptake. The inhibition of tubular SR Ca2+ uptake by thapsigargin (10 microM) reduced mechanical restitution by approximately 13% of the maximal twitch force, independent of the phase of mechanical restitution; the effect was greater at an earlier time point in the mechanical restitution. These results suggest that early JSR Ca2+ replenishment results mainly from the movement of Ca2+ from the tubular SR.

Arterial and Left Ventricular Pressures Illude Transient Alternans of Contractility during Postextrasystolic Potentiation

We have previously found that the postextrasystolic (PES) potentiation (PESP) of the left ventricular (LV) contractility (Emax) decays typically in transient alternans even in the normally ejecting canine heart. This contradicted the general expectation that arterial pressure (AP) and LV pressure (LVP) usually decay exponentially during PESP. We hypothesized this contradiction to be due to the different cardiodynamic behaviors of AP and LVP from LV Emax during PESP. We tested this hypothesis by measuring AP, LVP, LV volume, Emax, effective arterial elastance (Ea) as an index of afterload, and pulse pressure (PP) during PESP in eight anesthetized open-chest dogs by using the conductance catheter system. We changed Ea by changing the total peripheral resistance (TPR) with methoxamine hydrochloride (iv) and repeated the measurements. Although the Emax alternans patterns during PESP were comparable between the normal and high afterloads, LVP and PP were slightly potentiated and alternated under the normal afterload, whereas LVP and PP were obviously potentiated and alternated under the high afterload. We also simulated the effects of Ea/Emax on the transient alternans of AP and LVP on a computer. Despite the same alternans pattern of Emax, a higher Ea/Emax, which is typical in heart failure, caused a larger PP alternans, whereas a lower Ea/Emax, which is typical in normal hearts, almost eliminated it. These results suggest that a transient alternans of LV contractility during PESP could be overlooked when AP and LVP are monitored in in situ normal hearts.