Can the light-addressable potentiometric sensor (LAPS) detect extracellular potentials of cardiac myocytes?

The light-addressable potentiometric sensor (LAPS) measures localized photo-induced currents from a silicon wafer, which are dependent on the local surface potential and on the intensity of the light pointer. In this study the ability of the LAPS to record extracellular potentials of adherent cells was investigated. Time dependent LAPS photocurrent signals that correlated in time with contractions were recorded from beating cardiac myocytes cultured on LAPS surfaces. Signals could be recorded both when the LAPS was biased to working points where the photocurrent was maximally sensitive to potential changes and when it was biased to working points where the photocurrent was insensitive to changes in surface potential. Therefore, signals could not be predominantly created by changes in extracellular potential and might be related to mechanical contractions. One possible explanation might be, that the cell-induced modulation of photocurrents arose as a result of cell shape changes. Such alterations in cell shape might have focused and defocused the light pointer and, thus, modulated its intensity. To further test this hypothesis, height changes of beating cardiac myocytes were measured with an atomic force microscope (AFM). They were found to match well with signals derived from LAPS measurements. Therefore, it can be concluded, that LAPS signals were mainly determined by the periodic changes in shape of beating heart cells, and this interference precludes the measurements of extracellular electrophysiological potentials from these cells.