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 degrees 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 approximate to 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.