Light induces inositol trisphosphate elevation in Blepharisma japonicum

Photoinduced formation of inositol 1,4,5-trisphosphate (Ins[1,4,5]P-3) was examined using a specific radioimmunoassay to investigate the molecular mec hanisms of light signal transduction mediating photophobic responses in the ciliate Blepharisma japonicum. Application of light stimuli of moderate in tensity to dark-adapted cells induced a rapid and significant increase in t he basal Level of Ins(1,4,5)P-3, with a peak at about 20 s, Thereafter, the level of ins(1,4,5)P-3 declined to the resting value within the subsequent 100 s, Light stimuli of higher intensity raised the cell Ins(1,4,5)P-3 con tent to still higher levels within about 20 s, but the decaying time course was considerably prolonged, hi ciliates incubated under dark conditions wi th agents interfering with the inositol signalling pathway, like neomycin a nd Li+, the basal levels of Ins(1,4,5)P-3 were lower than in control cells, A photoinduced rise of Ins(1,4,5)P-3 content in ciliates treated with neom ycin or Li+ was significantly inhibited in a dose-dependent manner. Depolar izing ionic stimuli in dark-adapted ciliates induced no significant alterat ions of the resting Ins(1,4,5)P-3 Level, indicating a lack of a contributio n of this kind of stimulation to the inositol turnover, These studies are t he first in vivo demonstration of a possible role for inositol trisphosphat e! as a second messenger ill the light signal transduction process in the c iliate B. japonicum.