Photogeneration of membrane potential hyperpolarization and depolarization in non-excitable cells

We monitored femtosecond laser induced membrane potential changes in non-excitable cells using patchclamp analysis. Membrane potential hyperpolarization of HeLa cells was evoked by 780 nm, 80 fs laser pulses focused in the cellular cytoplasm at average powers of 30-60 mW. Simultaneous detection of intracellular Ca(2+) concentration and membrane potential revealed coincident photogeneration of Ca(2+) waves and membrane potential hyperpolarization. By using non-excitable cells, the cell dynamics are slow enough that we can calculate the membrane potential using the steady-state approximation for ion gradients and permeabilities, as formulated in the GHK equations. The calculations predict hyperpolarization that matches the experimental measurements and indicates that the cellular response to laser irradiation is biological, and occurs via laser triggered Ca(2+) which acts on Ca(2+) activated K(+) channels, causing hyperpolarization. Furthermore, by irradiating the cellular plasma membrane, we observed membrane potential depolarization in combination with a drop in membrane resistance that was consistent with a transient laser-induced membrane perforation. These results entail the first quantitative analysis of location-dependent laser-induced membrane potential modification and will help to clarify cellular biological responses under exposure to high intensity ultrashort laser pulses.