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(-)](i) 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.