The combination of fluorescence measurements of membrane potential and intracellular Ca2+ concentration allows correlating the electrical and calcium activity of a cell with spatial precision. The technical advances allowing this type of measurement were achieved only recently and represent an important step in the progress of the voltage imaging approach pioneered over 40 years ago by Lawrence B. Cohen. Here, we show how this approach can be used to investigate the function of Ca2+ channels using the foreseen possibility to extract Ca2+ currents from imaging experiments. The kinetics of the Ca2+ current, mediated by voltage-gated Ca2+ channels, can be accurately derived from the Ca2+ fluorescence measurement using Ca2+ indicators with KD>10 μM that equilibrate in <1 ms. In this respect, the imaging apparatus dedicated to this application is described in detail. Next, we illustrate the mathematical procedure to extract the current from the Ca2+ fluorescence change, including a method to calibrate the signal to charge flux density. Finally, we show an example of simultaneous membrane potential and Ca2+ optical measurement associated with an action potential at a CA1 hippocampal pyramidal neuron from a mouse brain slice. The advantages and limitations of this approach are discussed.
"Using simultaneous voltage and calcium imaging to study fast Ca2+ channels," Neurophotonics 2(2), 021010 (11 February 2015). https://doi.org/10.1117/1.NPh.2.2.021010