Loading Fluorescent Ca2+ Indicators into Living Cells
- 1Babraham Institute, Babraham, Cambridge, CB22 3AT, United Kingdom;
- 2Department of Life, Health and Chemical Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA, United Kingdom;
- 3Department of Microbiology and Physiology Systems, University of Massachusetts Medical School, Worcester, Massachusetts 01655;
- 4McMaster Stem Cell and Cancer Research Institute, Faculty of Health Sciences, McMaster University, MDCL 5029, Hamilton, Ontario L8S4L8, Canada
Abstract
Small-molecule fluorescent Ca2+ reporters are the most widely used tools in the field of Ca2+ signaling. The excellent spatial and temporal resolution afforded by fluorescent reporters has driven the understanding of Ca2+ as a messenger in many different cell types. In many situations, the cellular loading and monitoring of fluorescent Ca2+ indicators is quite trivial. However, there are numerous pitfalls that require consideration to ensure that optimal data are recorded. Fluorescent Ca2+ indicators have carboxylic acid groups for binding of Ca2+. Because these “free-acid” forms of the indicators are hydrophilic they cannot readily cross cell membranes and need to be introduced into cells using techniques such as microinjection, pinocytosis, or diffusion from a patch pipette. However, the most convenient and widely used method for loading indicators into cells is as hydrophobic compounds in which the carboxylic acid groups are esterified (commonly as acetoxymethyl [AM] or acetate esters). The ester versions of the indicators permeate the plasma membrane. The Ca2+-sensitive, free-acid form of the indicator is liberated following hydrolysis of the ester groups by intracellular esterases.
Footnotes
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↵5 Correspondence: martin.bootman{at}babraham.ac.uk
- © 2013 Cold Spring Harbor Laboratory Press