Early changes in membrane potential of Saccharomyces cerevisiae induced by varying extracellular K+, Na+ or H+ concentrations

Recently we introduced a fluorescent probe technique that makes possible to convert changes of equilibrium fluorescence spectra of 3,3’-dipropylthiadicarbocyanine, diS-C 3 (3), measured in yeast cell suspensions under defined conditions into underlying membrane potential differences, scaled in millivolts (Plasek et al. in J Bioenerg Biomembr 44: 559–569, 2012 ). The results presented in this paper disclose measurements of real early changes of plasma membrane potential induced by the increase of extracellular K + , Na + and H + concentration in S. cerevisiae with and without added glucose as energy source. Whereas the wild type and the ∆tok1 mutant cells exhibited similar depolarization curves, mutant cells lacking the two Trk1,2 potassium transporters revealed a significantly decreased membrane depolarization by K + , particularly at lower extracellular potassium concentration [K + ] out . In the absence of external energy source plasma membrane depolarization by K + was almost linear. In the presence of glucose the depolarization curves exhibited an exponential character with increasing [K + ] out . The plasma membrane depolarization by Na + was independent from the presence of Trk1,2 transporters. Contrary to K + , Na + depolarized the plasma membrane stronger in the presence of glucose than in its absence. The pH induced depolarization exhibited a fairly linear relationship between the membrane potential and the pH o of cell suspensions, both in the wild type and the Δtrk1,2 mutant strains, when cells were energized by glucose. In the absence of glucose the depolarization curves showed a biphasic character with enhanced depolarization at lower pH o values.