Non-redundant role of the a3 isoform of Na,K-ATPase in neuronal excitability and spiking dynamics

The Na,K-ATPase (NKA) plays a fundamental role in neuronal excitability by maintaining ionic gradients and contributing to electrogenic resting currents. Among its isoforms, the neuron-specific a3 subunit exhibits a uniquely low affinity for intracellular Na, weak voltage dependence, and slightly reduced ATP sensitivity compared to the ubiquitous a1. Although mutations in ATP1A3 are known to cause severe neurological disorders, the specific kinetic features of a3 that underlie its functional specialization remain incompletely understood. Here we employed biophysically detailed models of stretch receptor neurons, grounded in patch-clamp recordings of the a3-isoform current and spiking responses, to dissect the contribution of isoform-specific pump kinetics to firing behavior. Substitution of a1 for a3 abolished the ability to sustain long spike trains and reduced high-frequency entrainment, whereas a3-preserved prolonged discharges and faithful responses to vibratory stimuli. Remarkably, even halved pump density a3 50% preserved superior excitability compared to mixed expression (a 50%/a1 50%), indicating that kinetic profile rather than pump quantity determines firing capacity. Hybrid models revealed that Na; affinity is the decisive factor: retaining the low Na; affinity of a3 preserved excitability, while introducing a1-like voltage or ATP dependence produced only minor effect on simulated neuron discharge. These findings establish a mechanistic explanation for the selective expression of a3 in muscle spindle afferents and other neurons with high-frequency demands, and help to explain why a1 cannot compensate in ATP1A3-linked diseases. More broadly, they highlight the principle that isoform specialization of the NKA is not redundant but tuned to the discharge requirements of distinct neuronal populations.