Evidence for a transition in the cortical membranes of Paramecium

Ever since the pioneering studies in the 1960s and 70s, the importance of order transitions for cell membrane functions has remained a matter of debate. Recently, it has been proposed that the nonlinear stimulus-response curve of excitable cells, which manifests in all-or-none pulses (action potentials (AP)), is due to a transition in the cell membrane. Indeed, evidence for transitions has accumulated in plant cells and neurons, but studies with other excitable cells are expedient in order to show if this finding is of a general nature. Herein, we investigated intact, motile specimens of the "swimming neuron" Paramecium. The cellular membranes were labelled with the solvatochromic fluorophores LAURDAN or Di-4-ANEPPDHQ. Subsequently, a cell was trapped in a microfluidic channel and investigated by fluorescence spectroscopy. The generalized polarization (GP) of the fluorescence emission from cell cortical membranes (probably plasma and alveolar membranes) was extracted by an edge-finding algorithm. The thermo-optical state diagram, i.e. the dependence of GP on temperature, exhibited clear indications for a reversible transition. This transition had a width of ~10-15 °C and a midpoint that was located ~4 °C below the growth temperature. The state diagrams with LAURDAN and Di-4-ANEPPDHQ had widely identical characteristics. These results suggested that the cortical membranes of Paramecium reside in an order transition regime under physiological growth conditions. Based on these findings, membrane potential fluctuations, spontaneous depolarizing spikes, and thermal excitation of Paramecium was interpreted.