One of the most striking phenomena in biology is the action potential (AP), a nonlinear pulse with threshold and amplitude saturation (all-or-none-behavior) that propagates along neurons and other cells. In the classical interpretation the AP is considered to be an electrical phenomenon – a regenerating current flowing in a “biological cable”. In contrast, the thermodynamic interpretation has emphasized that conservation laws necessitate pulses and that pulses must manifest as transient changes of all observables of the system (electrical, mechanical, thermal, etc.). It is a key prediction of the latter approach that the cell membrane must undergo thermodynamic state changes during an AP.
In order to characterize the thermodynamic state of an excitable membrane, plant cells (Chara australis) were stained with Di-4-ANEPPDHQ. The location of the dye in the cell membrane was confirmed by confocal microscopy and changes of fluorescence emission were investigated as a function of temperature and extracellular pH. In parallel, emission of the dye was studied in artificial lipid vesicles (DMPC, DMPS) in the vicinity of the main transition temperature. In all these systems, the emission spectrum shifted as a function of membrane state. This shift became nonlinear and was maximal when the membrane underwent a transition . In the excitable cell Di-4-ANEPPDHQ exhibited a transient blueshift by ∼7 nm during an AP. A blueshift also occurred upon cooling and extracellular acidification. These results provided evidence for a sequence of state changes during an AP in which the cellular membrane condenses followed by expansion. This finding is in line with the thermodynamic interpretation of cellular excitability. Future studies should confirm/falsify these findings with other fluorescent dyes or state-sensitive techniques.
