EFFECTS OF K+AND Ca2+IONS ON MOTILITY AND PHOTOSENSORY RESPONSES OF STENTOR COERULEUS

Photosensory transduction in ciliates. I. An analysis of light-induced electrical and motile responses in Stentor coeruleus

Light-induced membrane potential changes and motile responses have been studied in Stentor cells with intracellular microelectrodes and video microscopy, respectively. Intracellular microelectrode recordings showed that step-up increase in light intensity induced an electrical membrane response which consisted of an initial membrane depolarization (photoreceptor potential) followed by an action potential and maintaining phase of depolarization (afterdepolarization). The amplitude of the receptor potential is dependent on the intensity of light stimulus and the action potential appears with a lag period (latency) after the onset of light stimulus. The extent of the membrane afterdepolarization is dependent on the intensity and duration of stimulus used. A close time correlation has been established between the latency for the action potential and the onset of ciliary reversal (stop response). A time correlation was also observed between the duration of the membrane afterdepolarization and the duration of backward swimming. The action spectrum for the photoreceptor potential amplitude of Stentor resembled the action spectra for the latency of ciliary reversal and the photoresponsiveness, indicating that the photomovement response and membrane potential changes are coupled through the same photosensor system. A hypothesis on the photosensory transduction chain in Stentor is discussed according to which the photoreceptors and the ciliary apparatus is mediated by the membrane potential changes.

Effects of green light on biological systems

Green light (510-565 nm) constitutes a significant portion of the visible spectrum impinging on biological systems. It plays many different roles in the biochemistry, physiology and structure of plants and animals. In only a relatively small number of responses to green light is the photoreceptor known with certainty or even provisionally and in even fewer systems has the chain of events leading from perception to response been examined experimentally. This review provides a detailed view of those biological systems shown to respond to green light, an evaluation of possible photoreceptors and a review of the known and postulated mechanisms leading to the responses.

Photoreceptor pigments for photomovement of microorganisms: some spectroscopic and related studies

Optical spectroscopy of photoreceptor pigments can substantially contribute to our understanding of the molecular processes which are the basis of photoreception and sensory transduction in photomotile microorganisms. The main spectroscopic techniques are briefly illustrated, together with the most significant types of progress that can be achieved. A few "case examples" are discussed in some detail: Halobacterium, with particular attention to the contribution of flash photolysis studies to the identification and characterization of sensory rhodopsins; Euglena, and the role of in vivo microspectrofluorometry in confirming the flavin nature of its photoreceptor pigment; the first suggestions on the rhodopsin-like nature of the Chlamydomonas photosensing system; Stentor and Blepharisma and the contribution of static and time-resolved fluorescence studies to a molecular model of the primary events in their photoreceptor pigments (stentorin and blepharismin) and systems.

The ratio of extracellular Ca2+ to K+ ions affects the photoresponses in Stentor coeruleus

1. Stentor coeruleus exhibits negative phototaxis (due to phototactic orientation response) and step-up photophobic response (avoiding reaction) to visible light. 2. The effect of Ja-value ([K+]/[Ca2+]1/2) and calcium ion concentration of the surrounding medium on the photoresponses in Stentor were studied. 3. The both types of photoresponses in Stentor are greatly affected by the Ja-value. A higher Ja-value medium suppressed the step-up photophobic response of Stentor, whereas the organism showed a higher degree of phototactic orientation response in higher Ja-value solutions. 4. The effect of the Ja-value on the step-up photophobic response was opposite to that on the phototactic orientation response. 5. With increasing calcium concentration but at a constant Ja-value, the number of Stentor showing the step-up photophobic response increased, whereas the phototactic orientation response of Stentor was suppressed at higher Ca2+ concentrations. 6. The effect of the calcium concentration on the photophobic response was also opposite to that on the phototactic orientation response, as in the case of Ja-value effect.