Ca²⁺ channel nanodomains boost local Ca²⁺ amplitude

Local Ca²⁺ signals through voltage-gated Ca²⁺ channels (CaVs) drive synaptic transmission, neural plasticity, and cardiac contraction. Despite the importance of these events, the fundamental relationship between flux through a single CaV channel and the Ca²⁺ signaling concentration within nanometers of its pore has resisted empirical determination, owing to limitations in the spatial resolution and specificity of fluorescence-based Ca²⁺ measurements. Here, we exploited Ca ²⁺-dependent inactivation of CaV channels as a nanometer- range Ca²⁺ indicator specific to active channels. We observed an unexpected and dramatic boost in nanodomain Ca²⁺ amplitude, ten-fold higher than predicted on theoretical grounds. Our results uncover a striking feature of CaV nanodomains, as diffusion-restricted environments that amplify small Ca²⁺ fluxes into enormous local Ca²⁺ concentrations. This Ca²⁺ tuning by the physical composition of the nanodomain may represent an energy-efficient means of local amplification that maximizes information signaling capacity, while minimizing global Ca²⁺ load.