Aftercontractions and excitation–contraction coupling in rat cardiac muscle

Abstract

Calcium loading in cardiac muscle may cause spontaneous contractions (SC). We observed that SC move at a constant rate (Vsc) through isolated rat myocytes and trabeculae. Factors that influence the properties of SC were studied with Nomarsky microscopy and laser diffraction techniques. Myocytes and trabeculae were superfused with Krebs–Henseleit solution (21 °C, pH 7.35; Ca2+, 0.5–7 mM). Vsc in myocytes and within cells of trabeculae ranged between 50 and 150 μm/s. After a train of 3–25 stimuli at 2 Hz, SC in trabeculae started at a site of damage in a region 250 μm in length throughout the muscle. This regional contraction then moved at a constant rate (Vsc) along the length of the muscle. Vsc increased from 0.1 to 15 mm/s with stimulation and Ca2+. Under conditions of calcium loading, spontaneous twitches also occurred throughout the trabeculae, often as triggered arrhythmias. These twitches were always preceded by SC. The range of observed Vsc could be predicted by the Ca2+-induced Ca2+ release hypothesis. We postulated that the contraction propagates by virtue of focal calcium release from the sarcoplasmic reticulum (SR) and we simulated this process together with the processes of diffusion into the cytosol, binding to calmodulin and troponin, sequestration by the SR, and subsequent induction of Ca2+ release from the adjacent SR. The parameters used for the kinetics of binding, release, and sequestration were obtained from those reported in the literature. The minimal and maximal velocities derived from the simulation were 0.09 and 25 mm/s, respectively. The method of solution involved writing the diffusion equation as a difference equation in the spatial coordinates. Thus, bounded, coupled, and ordinary differential equations in time were generated. The coupled equations were solved by using Gear's sixth order predictor–corrector algorithm for stiff equations along with reflective boundaries. These calculations were performed on the Cyber 205 at the University of Calgary. Our results are consistent with the assumptions that Ca2+ loading causes an increase in intracellular Ca2+ concentration, a decrease in the relative threshold for induction of release, and an increase in the amount and rate of Ca2+ released, and hence causes a higher propagation velocity.