Changes in succinate dehydrogenase (SDH) activity in Paramecium caudatum cells in culture as function of age

SDH activity and its frequency distributions were determined cytophotometrically to investigate the change in respiratory oxidative energy metabolism in Paramecium caudatum (P. caudatum) cell populations during their growth in cell culture. Cells from 3 separate cultures were examined on day 2, 5 and 11 post-inoculation as measures of logarithmic, early and late stationary phases, respectively. SDH activity per individual cell is expressed as the increase in total absorbance (TA) at 590 nm of nitroblue tetrazolium (Nitro BT) formazans per min per cell area in a specimen (deltaTA/min/microm2) to exclude the influence of cell size on the data. On day 5, the mean SDH activity was higher, being approx 140% of that on day 2 and decreased significantly to only approx 11% at day 11. As the mean SDH activity rose, a certain portion of the cells demonstrated a wider range of activity than on day 2 leading to an increase in the width of the SDH activity-frequency distribution. Moreover, on day 11, approx 85% of cells shifted toward the lowest range of activity with further increase in width of the distribution as the mean activity declined. These findings suggest that the respiratory oxidative energy metabolism in P. caudatum cells rise while the cells change from logarithmic to early stationary phases and decays during late stationary phase with increase in its extent of variety within cell populations.