Biophysical models reveal the relative importance of transporter proteins and impermeant anions in chloride homeostasis.

Fast synaptic inhibition in the nervous system depends on the transmembrane flux of Cl ions based on the neuronal Cl driving force. Established theories regarding the determinants of Cl driving force have recently been questioned. Here, we present biophysical models of Cl homeostasis using the pump-leak model. Using numerical and novel analytic solutions, we demonstrate that the Na/K-ATPase, ion conductances, impermeant anions, electrodiffusion, water fluxes and cation-chloride cotransporters (CCCs) play roles in setting the Cl driving force. Our models, together with experimental validation, show that while impermeant anions can contribute to setting [Cl] in neurons, they have a negligible effect on the driving force for Cl locally and cell-wide. In contrast, we demonstrate that CCCs are well-suited for modulating Cl driving force and hence inhibitory signaling in neurons. Our findings reconcile recent experimental findings and provide a framework for understanding the interplay of different chloride regulatory processes in neurons.