Synchrony and mutual stimulation of yeast cells during fast glycolytic oscillations

Cell synchrony was investigated during glycolytic oscillations in starved yeast cell suspensions at cell densities ranging from 2 x 10⁶-5 x 10⁷ cells ml^-1. Oscillations in NAD(P)H were triggered by inhibition of mitochondrial respiration when intracellular NAD(P)H had reached a steady state after glucose addition. Before macroscopic damping of the oscillations, individual yeast cells oscillated in phase with the cell population. After oscillations had damped out macroscopically, a significant fraction of the cells still exhibited oscillatory dynamics, slightly out-of-phase. At cell concentrations higher than 10⁷ cells ml^-1 the dependence upon cell-density of (i) the damping of glycolytic oscillations and (ii) the amplitude per cell suggested that cell-to-cell interaction occurred. Most importantly, at cell densities exceeding 10⁷ cells ml^-1 the damping was much weaker. A combination of modelling studies and experimental analysis of the kinetics of damping of oscillations and their amplitude, with and without added ethanol, pyruvate or acetaldehyde, suggested that the autonomous glycolytic oscillations of the yeast cells depend upon the balance between oxidative and reductive (ethanol catabolism) fluxes of NADH, which is affected by the extracellular concentration of ethanol. Based on the facts that cell (i) excrete ethanol, (ii) are able to catabolize external ethanol, and (iii) that this catabolism affects their tendency to oscillate, we suggest that the dependence of the oscillations on cell density is mediated through the concentration of ethanol in the medium.