Fluorescent biosensors illuminate calcium levels within defined beta-cell endosome subpopulations

- We report the first measurements of Ca2+ within defined single endosomes.
- Our novel tool estimated Ca2+ levels of ∼0.5 μM for Rab5a-positive endosomes and ∼2.5 μM for Rab7-positive endosomes.
- Endosomes enrich Ca2+ during the glucose stimulation of pancreatic beta-cells.