OXIDANTS ACT AS CHEMOREPELLENTS IN PARAMECIUM BY STIMULATING AN ELECTROGENIC PLASMA-MEMBRANE REDUCTASE-ACTIVITY

Paramecium is a valuable eukaryotic model system for studying chemosen sory transduction, adaptation and cellular sensory integration. While millimolar amounts of many attractants hyperpolarize and cause faster forward swimming, oxidants are repellents that depolarize and cause ba ckward swimming at micromolar concentrations. The non-permeant oxidant s cytochrome c, nitro blue tetrazolium and ferricyanide are repellents with half maximal concentrations of 0.4 mu M, 2.2 mu M and 100 mu M r espectively. In vivo reductase activities follow the same order of pot encies. The concentration dependence of the cytochrome c reductase act ivity is well correlated with cytochrome c-induced depolarizations. Th is suggests that plasma membrane reduction of external cytochrome c is electrogenic, causing membrane depolarization and chemorepulsion. The reductase activity also appears to be voltage dependent. Depolarizati on by either K+, Na+, Ca++ or Mg++ correlates with inhibition of both in vivo reductase activities and cytochrome c-induced membrane potenti al changes. These responses were also seen in deciliated cells, showin g that the body plasma membrane is sufficient for the response. Both c hloroquine and diphenyleneiodonium inhibited reductase activities but only at unusually high concentrations. This activity showed no pH depe ndence in the physiological range. We propose that a plasma membrane b ound NADPH-dependent reductase controls oxidant-induced depolarization s and consequent chemorepulsion.