Development of a high-throughput fluorescent no-wash sodium influx assay

Voltage-gated sodium channels (Na_Vs) are key therapeutic targets for pain, epilepsy and cardiac arrhythmias. Here we describe the development of a no-wash fluorescent sodium
influx assay suitable for high-throughput screening and characterization of novel drug leads. Addition of red-violet food dyes (peak absorbance range 495–575 nm) to assays in HEK293 cells heterologously expressing hNa_V1.1–1.8 effectively quenched background fluorescence of the sodium indicator dye Asante NaTRIUM Green-2 (ANG-2; peak emission 540 nm), negating the need for a wash step. Ponceau 4R (1 mM) was identified as a suitable quencher, which had no direct effect on NaV channels as assessed by patch-clamp experiments, and did not alter the pharmacology of the Na_V1.1–1.7 activator veratridine (EC50 10–29 μM) or the Na_V1.1–1.8 inhibitor tetracaine (IC50’s 6–66 μM). In addition, we also identified that the food dyes Ponceau 4R, Brilliant Black BN, Allura Red and Amaranth are effective at quenching the background fluorescence of the calcium indicator dyes fluo-4, fura-2 and fura-5F, identifying them as potential inexpensive alternatives to no-wash calcium ion indicator kits. In summary, we have developed a no-wash fluorescent sodium influx assay suitable for high-throughput screening based on the sodium indicator dye ANG-2 and the quencher Ponceau 4R.