Data‐based theoretical identification of subcellular calcium compartments and estimation of calcium dynamics in cardiac myocytes

Key points•  Ca 2+ release from intracellular stores affects the cardiac action potential via currents through L‐type Ca 2+ channels ( I Ca ) and the sodium/calcium exchanger ( I NCX ). •  Dynamic interactions between released calcium, I Ca and I NCX occur in a restricted subcellular compartment, close to Ca 2+ released sites, where calcium concentration (Ca t ) cannot be measured. •  We used a computational model and experimental data to define this compartment and to provide a theoretical basis for estimating Ca t . •  We estimated Ca t from recordings of I Ca and I NCX and optical recordings of whole‐cell calcium concentration (Ca m ). •  Estimated peak Ca t ranged from 6 μ m to 25 μ m , depending on calcium load. Time to equilibrium between Ca t and Ca m was ∼350 ms. The Ca t values are in the range of I Ca and I NCX sensitivity to calcium, implying that there is significant effect of Ca 2+ in this restricted domain on their kinetics and on the action potential during cell excitation.Abstract  In cardiac cells, Ca 2+ release flux ( J rel ) via ryanodine receptors (RyRs) from the sarcoplasmic reticulum (SR) has a complex effect on the action potential (AP). Coupling between J rel and the AP occurs via L‐type Ca 2+ channels ( I Ca ) and the Na + /Ca 2+ exchanger ( I NCX ). We used a combined experimental and modelling approach to study interactions between J rel , I Ca and I NCX in porcine ventricular myocytes. We tested the hypothesis that during normal uniform J rel , the interaction between these fluxes can be represented as occurring in two myoplasmic subcompartments for Ca 2+ distribution, one (T‐space) associated with RyR and enclosed by the junctional portion of the SR membrane and corresponding T‐tubular portion of the sarcolemma, the other (M‐space) encompassing the rest of the myoplasm. I Ca and I NCX were partitioned into subpopulations in the T‐space and M‐space sarcolemma. We denoted free Ca 2+ concentrations in T‐space and M‐space Ca t and Ca m , respectively. Experiments were designed to allow separate measurements of I Ca and I NCX as a function of J rel . Inclusion of T‐space in the model allowed us to reproduce in silico the following important experimental results: (1) hysteresis of I NCX dependence on Ca m ; (2) delay between peak I NCX and peak Ca m during caffeine application protocol; (3) delay between I NCX and Ca m during Ca 2+ ‐induced‐Ca 2+ ‐release; (4) rapid I Ca inactivation (within 2 ms) due to J rel , with magnitude graded as a function of the SR Ca 2+ content; (5) time delay between I Ca inactivation due to J rel and Ca m . Partition of 25% NCX in T‐space and 75% in M‐space provided the best fit to the experimental data. Measured Ca m and I Ca or I NCX were used as input to the model for estimating Ca t . The actual model‐computed Ca t , obtained by simulating specific experimental protocols, was used as a gold standard for comparison. The model predicted peak Ca t in the range of 6–25 μ m , with time to equilibrium of Ca t with Ca m of ∼350 ms. These Ca t values are in the range of LCC and RyR sensitivity to Ca 2+ . An increase of the SR Ca 2+ load increased the time to equilibrium. The I Ca ‐based estimation method was most accurate during the ascending phase of Ca t . The I NCX ‐based method provided a good estimate for the descending phase of Ca t . Thus, application of both methods in combination provides the best estimate of the entire Ca t time course.