When Na modulates Ca: nonlinear interplay between Na/Ca-exchanger and IP3-mediated Ca oscillations in astrocytes

We present a composite model of astrocytic Ca2+ dynamics, which includes both IP3-mediated release from intracellular Ca2+ stores and bidirectional exchange through the plasma membrane, mediated by the Na+/Ca2+-exchanger (NCX). The NCX kinetic model is formulated in the Hodgkin {Huxley formalism, taking into account the glutamate transporter-mediated increase in Na+ during synaptic activity and Na+- and Ca2+-dependent regulation. We define main dynamic regimes and transitions in a point model, describing dynamics in an individual astrocyte segment. We demonstrate NCX-based modulation of store-mediated calcium oscillations (amplitude, frequency and amplitude-frequency), analyze and compare calcium dynamics in models with and without allosteric regulation of NCX by intracellular ions.