The originally published paper has been updated to include the following new reference, added as ref. 18: Albrecht, T., Zhao, Y., Nguyen, T. H., Campbell, R. E. & Johnson, J. D. Fluorescent biosensors illuminate calcium levels within defined beta-cell endosome subpopulations. Cell Calcium 57, 263-274 (2015). Subsequent references have been renumbered in the reference list and throughout the text. Minor text changes were made in the sentence in which this new reference is first cited: "Previous attempts used endocytic tracers bearing either pH- or Ca -sensitive dyes to serially measure population-averaged pH and apparent Ca in different batches of cells, thus scrambling information from individual endosomes " in the original introduction was changed to "Previous attempts used endocytic tracers bearing either pH- or Ca -sensitive dyes or fluorescent-protein-based sensors to serially measure population-averaged pH and apparent Ca in different batches of cells, thus scrambling information from individual endosomes." These changes have been made in the HTML and PDF versions of the article.