PHOTOTRANSDUCTION IN BLEPHARISMA AND STENTOR

The ciliates Blepharisma and Stentor exhibit distinct light-avoiding b ehavior (photophobic response). This cell behavior is attributed to ph otoexcitation of receptor chromoproteins located in cell organelles (p igment granules). The photochemical process in the receptors of both t hese ciliates, considered as a primary reaction to light, initiates th e sensory transduction chain leading to delayed generation of membrane photoreceptor potential. The photoreceptor potential is coupled to th e photophobic response (ciliary reversal) through an action potential which is elicited by receptor potential when its amplitude exceeds the membrane excitation threshold. The direct relationship between photoe lectrical transduction and electromotor events is evidenced by a close temporal correlation observed in Blepharisma and Stenter between the latency periods of the action potential and of the cell stop response (receptor/action potentials appear to be instrumental in initiation of ciliary reversal). The photoreceptor potentials in Blepharisma are ge nerated by transient changes in membrane conductance for Ca2+ ions. In Stenter the ionic nature of light-dependent membrane depolarization h as not been fully elucidated; The action potentials in both Blepharism a and Stenter ciliates are produced by any suprathreshold depolarizati on, such as a photoreceptor potential, in the consequence of the Ca2channel and, after a delay, the K+ channel activations.