Susma Timsina, Zhouruolan Li, Min Zhou

 

Strong electrical coupling through gap junctions equalizes astrocyte membrane potentials (VM) into an isopotential network, termed astrocyte syncytial isopotentiality (SI). In theory, SI renders astrocytic VM effectively constant, preventing VM from tracking the K+ equilibrium potential (EK), which fluctuates with changes in extracellular K+ ([K+]e) concentrations. As a result, SI preserves the electrochemical driving force (VM-EK) required for channel-mediated K+ uptake and release by astrocytes. However, this theoretical framework has not been experimentally demonstrated. Here, we first evaluated astrocyte K+ buffering capacity by sequentially delivering pairs of high- and low-K+ puffs (± Δ 2.5 mM K+ from a baseline 3.5 mM, 150 ms) to voltage-clamped astrocytes in the mouse hippocampal CA1 region. The low-K+ - induced K+ release current was ~1.07-fold greater than the high-K+ - induced K+ uptake current, whereas the K+ uptake current increased with astrocyte VM hyperpolarization, indicating that K+ uptake is an energetically uphill process critically dependent on a hyperpolarized VM. We next examined astrocytic VM responses to high- and low- K+ puffs when astrocytes were either electrically coupled into a syncytium or pharmacologically decoupled with 100 µM Meclofenamic acid (MFA). Consistent with computational model predictions, astrocytic VM deviated minimally from baseline despite rapid, puff-induced shifts in EK. In contrast, under decoupled conditions, astrocytic VM rapidly tracked the reset EK induced by K+ puffs. Thus, astrocytes K+ buffering can be carried out only when astrocytes are electrically coupling into a syncytium. Hence, astrocyte syncytial isopotentiality is an essential glial mechanism critical for maintaining brain K+ homeostasis.