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 Ca2+ channel (LCC) clustering, and allosteric activation of Na+/Ca2+ exchanger by L-type Ca2+ current affects intracellular Ca2+ dynamics. Our model includes a realistic 3D geometry of a single t-tubule and its surrounding half-sarcomeres for rabbit ventricular myocytes. The effects of spatially distributed membrane ion-transporters (LCC, Na+/Ca2+ exchanger, sarcolemmal Ca2+ pump, and sarcolemmal Ca2+ leak), and stationary and mobile Ca2+ 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 Ca2+. We obtained parameters from voltage-clamp protocols of L-type Ca2+ current and line-scan recordings of Ca2+ concentration profiles in rabbit cells, in which the sarcoplasmic reticulum is disabled. Our model results agree with experimental measurements of global Ca2+ transient in myocytes loaded with 50 μM Fluo-3. We found that local Ca2+ concentrations within the cytosol and sub-sarcolemma, as well as the local trigger fluxes of Ca2+ crossing the cell membrane, are sensitive to details of t-tubule micro-structure and membrane Ca2+ flux distribution. The model additionally predicts that local Ca2+ trigger fluxes are at least threefold to eightfold higher than the whole-cell Ca2+ trigger flux. We found also that the activation of allosteric Ca2+-binding sites on the Na+/Ca2+ exchanger could provide a mechanism for regulating global and local Ca2+ 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+/Ca2+ exchanger fluxes to intracellular Ca2+ dynamics.
Highlights
In cardiac ventricular myocytes, invaginations of the cell membrane, known as t-tubules, promote rapid propagation of the action potential (AP) in the cell interior (Savio-Galimberti et al., 2008; Orchard et al, 2009; Smyrnias et al, 2010)
At +50 mV, a more gradual accumulation of Ca2+ is predicted until converging to ∼0.2 vs. 0.19 μM experimentally. These results indicate that our model is a reliable representation of whole-cell Ca2+ dynamics as measured in rabbit ventricular myocytes (Sobie et al, 2008)
In the presence of 10 mM [Na+]i, our results demonstrate that: (1) the overall scale of I NCXsarc was ∼5.6-fold greater than I NCXglobal; (2) the increase in I NCXsarc during I L-type Ca2+ channels (LCC) upstroke relative to I NCXglobal was much faster; (3) while I NCXglobal monotonically increased over the entire simulation, local peaks in I NCXsarc are predicted at 15 ms
Summary
Invaginations of the cell membrane, known as t-tubules, promote rapid propagation of the action potential (AP) in the cell interior (Savio-Galimberti et al., 2008; Orchard et al, 2009; Smyrnias et al, 2010). The AP activates and modulates sarcolemmal Ca2+ fluxes, including fluxes through the L-type Ca2+ channels (LCC), Na+/Ca2+ exchangers (NCX), Ca2+ ATPase pumps, and background sarcolemmal Ca2+ leak (Bers, 2001). The entry of Ca2+ via LCC and NCX triggers the sarcoplasmic reticulum (SR) Ca2+ release via ryanodine receptors (RyRs). C (TnC) myofilament protein, thereby activating and regulating myocyte contraction (Bridge et al, 1990; Bers, 2001). To investigate relationships between ion fluxes via LCCs and NCXs at voltages corresponding to the early AP plateau, Sobie et al (2008) recently measured the time-dependent Ca2+ concentration profiles ([Ca2+]i) in isolated rabbit ventricular myocytes. Pharmaceutical disruption of SR activity enabled them to examine the contributions of LCC and NCX to Ca2+ trigger flux, which www.frontiersin.org
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