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

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 (Ca_i). The apparent node-to-node progression of elevated Ca_i 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.