CA-2+ IONS MEDIATE THE PHOTOPHOBIC RESPONSE IN BLEPHARISMA AND STENTOR

The effects of changes in external free Ca2+ ions and presence in exte rnal solution of Ca2+ channel blockers or ionophore on light-dependent motile responses in Blepharisma and Stentor protozoans, were studied using microscope video recording. Cells entering an illuminated field show a step-up photophobic response, these usually occur with some del ay. A decrease in the concentration of external free Ca2+ or the addit ion of the Ca2+ channel blockers, pimozide or diltiazem, to the medium significantly lowers the photosensitivity and prolongs the delay of t he step-up photophobic responses in Stentor and Blepharisma. The Ca2ionophore A23187 added to the medium inhibits photophobic response and markedly increase the response time at lower extracellular Ca2+. The results suggest that Ca2+ ions are involved in the light transduction chains in Blepharisma and Stentor ciliates.