Modeling effects of L-type Ca 2+ current and Na +-Ca 2+

Abstract

The transverse tubular system of rabbit ventricular myocytes consists of cell membrane invaginations (t-tubules) that are essential for efficient cardiac excitation-contraction coupling. In this study, we investigate how t-tubule micro-anatomy, L-type Ca 2+ channel (LCC) clustering, and allosteric activation of Na +/Ca 2+ exchanger by L-type Ca 2+ current affects intracellular Ca 2+ dynamics. Our model includes a realistic 3D geometry of a single ttubule and its surrounding half-sarcomeres for rabbit ventricular myocytes. The effects of spatially distributed membrane ion-transporters (LCC, Na +/Ca 2+ exchanger, sarcolemmal Ca 2+ pump, and sarcolemmal Ca 2+ leak), and stationary and mobile Ca 2+ buffers (troponin C, ATP, calmodulin, and Fluo-3) are also considered. We used a coupled reaction-diffusion system to describe the spatio-temporal concentration profiles of free and buffered intracellular Ca 2+. We obtained parameters from voltage-clamp protocols of L-type Ca 2+ current and line-scan recordings of Ca 2+ concentration profiles in rabbit cells, in which the sarcoplasmic reticulum is disabled. Our model results agree with experimental measurements of global Ca 2+ transient in myocytes loaded with 50 μM Fluo-3. We found that local Ca 2+ concentrations within the cytosol and sub-sarcolemma, as well as the local trigger fluxes of Ca 2+ crossing the cell membrane, are sensitive to details of t-tubule micro-structure and membrane Ca 2+ flux distribution. The model additionally predicts that local Ca 2+ trigger fluxes are at least threefold to eightfold higher than the whole-cell Ca 2+ trigger flux. We found also that the activation of allosteric Ca 2+-binding sites on the Na +/Ca 2+ exchanger could provide a mechanism for regulating global and local Ca 2+ trigger fluxes in vivo. Our studies indicate that improved structural and functional models could improve our understanding of the contributions of L-type and Na +/Ca 2+ exchanger fluxes to intracellular Ca 2+ dynamics. © 2012 Kekenes-Huskey, Cheng, Hake, Sachse, Bridge, Holst, McCammon, McCulloch and Michailova.