Photolysis of caged Ca2+ induces trichocyst discharge in Paramecium caudatum

Trichocyst discharge, ciliary reversal, and cell body contraction in Parame cium spp. have all been claimed to be regulated by the intracellular Ca2+ c oncentration ([Ca2+](i)) at the cortical region of the cell. We injected ca ged Ca2+ into P. caudatum cells and applied ultraviolet (UV) light to the c ell for 125 ms. This did not induce trichocyst discharge but did induce bot h ciliary reversal and cell body contraction. A re-application of UV for 12 5 ms triggered trichocyst discharge. These results demonstrate that (1) tri chocyst discharge and ciliary reversal and cell body contraction are contro lled by [Ca2+](i) and (2) the threshold of [Ca2+](i) for trichocyst dischar ge is higher than those for the other two functions.