Relationship between light-induced potential change and internal ATP concentration in tonoplast-free <italic>Chara</italic> cells

Abstract

The effect of the intracellular concentration of ATP ([ATP]1) on the light-induced potential change (LPC) in tonoplast-free Chara cells was studied. The LPC was hardly affected by lowering the [ATP]1 by about 1/10 or by raising it to about 10 times the normal cytoplasmic concentration (0.5–1.3 mM). The insensitivity of LPC to [ATP]1 excludes the possibility that an increase in [ATP]1 due to photosynthesis may induce the LPC. However, extreme lowering of the [ATP]1 to about 1–2 μM completely inhibited LPC, although photosynthetic O2 evolution was not significantly inhibited. This fact supports the hypothesis that light stimulates the putative H+pump fueled by ATP. The uncoupling agents DNP and CCCP greatly depolarized the membrane, and inhibited LPC strongly, but they did not decrease [ATP]1. Photosynthetic O2 evolution was inhibited to some extent by 2 μM CCCP and strongly inhibited by 0.1 mM DNP. Since the membrane resistance increased significantly, these chemicals are believed to act on the membrane as an inhibitor of the electrogenic H+ pump not as an H+conductor. Introduction of 1 mM ATP into cells treated with uncouplers, to a large extent restored their ability to produce LPC although the membrane potential in darkness was maintained at a low level.