Evoked centripetal Ca²⁺ mobilization in cardiac Purkinje cells: insight from a model of three Ca²⁺ release regions

Abnormal oscillations of calcium (Ca2+) concentration in cardiac Purkinje cells (P-cells) have been associated with life-threatening arrhythmias, but the mechanism by which these cells control their Ca2+ level in normal conditions remains unknown. We modelled our previous hypothesis that the principal intracellular Ca2+ compartment (endoplasmic reticulum; ER) which governs intracellular Ca2+ concentration, formed, in P-cells, three concentric and adjacent layers, each including a distinct Ca2+ release channel. We then tested the model against typical Ca2+ variations observed in stimulated P-cells. We found in swine P-cells, as in the rabbit and dog, that stimulation evokes an elevation of Ca2+ concentration first under the membrane, which then propagates to the interior of the cell. Our mathematical model could reproduce accurately this typical ‘centripetal’ Ca2+ spread, hence supporting (1) the existence of the ‘3 layered’ Ca2+ compartment, and (2) its central role in the regulation of Ca2+ concentration in P-cells. To model the ‘centripetal’ Ca2+ spread, local variations of Ca2+ concentration were calculated for a virtual cell environment encompassing three different regions that mimicked the three layers of ER in P-cells. Various tests of the model revealed that the second intermediate layer was essential for ‘forwarding’ the Ca2+ elevation from the periphery to the cell centre. This novel finding suggests that a thin intermediate layer of specific ER Ca2+ channels controls the entire Ca2+ signalling of P-cells. Because Ca2+ plays a role in the electric properties of P-cells, any abnormality affecting this intermediate region is likely to be pro-arrhythmic and could explain the origin of serious cardiac arrhythmias known to start in the Purkinje fibres. Abstract Despite strong suspicion that abnormal Ca2+ handling in Purkinje cells (P-cells) is implicated in life-threatening forms of ventricular tachycardias, the mechanism underlying the Ca2+ cycling of these cells under normal conditions is still unclear. There is mounting evidence that P-cells have a unique Ca2+ handling system. Notably complex spontaneous Ca2+ activity was previously recorded in canine P-cells and was explained by a mechanistic hypothesis involving a triple layered system of Ca2+ release channels. Here we examined the validity of this hypothesis for the electrically evoked Ca2+ transient which was shown, in the dog and rabbit, to occur progressively from the periphery to the interior of the cell. To do so, the hypothesis was incorporated in a model of intracellular Ca2+ dynamics which was then used to reproduce numerically the Ca2+ activity of P-cells under stimulated conditions. The modelling was thus performed through a 2D computational array that encompassed three distinct Ca2+ release nodes arranged, respectively, into three consecutive adjacent regions. A system of partial differential equations (PDEs) expressed numerically the principal cellular functions that modulate the local cytosolic Ca2+ concentration (Cai). The apparent node-to-node progression of elevated Cai was obtained by combining Ca2+ diffusion and ‘Ca2+-induced Ca2+ release’. To provide the modelling with a reliable experimental reference, we first re-examined the Ca2+ mobilization in swine stimulated P-cells by 2D confocal microscopy. As reported earlier for the dog and rabbit, a centripetal Ca2+ transient was readily visible in 22 stimulated P-cells from six adult Yucatan swine hearts (pacing rate: 0.1 Hz; pulse duration: 25 ms, pulse amplitude: 10% above threshold; 1 mm Ca2+; 35°C; pH 7.3). An accurate replication of the observed centripetal Ca2+ propagation was generated by the model for four representative cell examples and confirmed by statistical comparisons of simulations against cell data. Selective inactivation of Ca2+ release regions of the computational array showed that an intermediate layer of Ca2+ release nodes with an ∼30–40% lower Ca2+ activation threshold was required to reproduce the phenomenon. Our computational analysis was therefore fully consistent with the activation of a triple layered system of Ca2+ release channels as a mechanism of centripetal Ca2+ signalling in P-cells. Moreover, the model clearly indicated that the intermediate Ca2+ release layer with increased sensitivity for Ca2+ plays an important role in the specific intracellular Ca2+ mobilization of Purkinje fibres and could therefore be a relevant determinant of cardiac conduction.

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