THE ROLE OF CALCIUM IN Chlamydomonas PHOTOMOVEMENT RESPONSES AS ANALYSED BY CALCIUM CHANNEL INHIBITORS

Abstract

Phototaxis and light-induced stop responses in Chlamydomonas are known to be calcium dependent. We show that phototaxis is stereoselectively inhibited by dihyropyridines, verapamil, diltiazem, omega-conotoxin and pimozide, all inhibitors of slow L-type calcium channels. In contrast, the stop response in Chlamydomonas can be specifically reduced only by omega-conotoxin and pimozide. The light-regulated calcium uptake as detected by 45calcium can be completely suppressed by verapamil and omega-conotoxin but not by diltiazem or any of the dihyropyridine-type calcium channel inhibitors. We conclude that phototaxis and stop response in Chlamydomonas are regulated by three distinguishable drug receptor sites. One of them controls phototaxis and is sensitive to verapamil. The second site controls stop response and phototaxis and shows a high sensitivity to omega-conotoxin and pimozide. These two drug receptors seem to be localized in the plasma membrane and function as ion channels. In addition, calcium influences internal signal transduction from the photoreceptor to the flagella. This internal role of calcium is inhibited by the dihydropyridine binding to a dihydropyridine receptor protein. The arylazide-1,4-dihydropyridine[3H]azidopine binds with a Kd = 35 nM to a 50 kDa protein located in one of the internal cell membranes. Azidopine binding is fully reversible and can be partially inhibited by nimodipine and PN-200110. This protein is the first identified dihyropyridine receptor in an unicellular plant cell. It might serve as an internal calcium regulating channel in Chlamydomonas.

Conversion of beating mode inChlamydomonas flagella induced by electric stimulation

“…Together with the indication about the site of electric excitation, our findings provide experimental support for the presence of voltagesensitive calcium channels in the flagellar membrane, which has been suggested by earlier studies [Harz and Hegemann, 1991;Harz et al, 1992;Beck and Uhl, 1994]. The photophobic response has been shown to be inhibited by some calcium-channel blockers (pimozide and v-conotoxin GVIA) [Hegemann et al, 1990]. The inhibitory effect of these blockers can be attributed to their inhibitory action on the photoreceptor current and the flagellar current.…”

“…To test this possibility, the effect of calcium-channel blockers was examined. We first used two calcium channel blockers, pimozide and v-conotoxin GVIA, both of which have been reported to effectively inhibit the photophobic response [Hegemann et al, 1990]. Pimozide has been reported to inhibit the photophobic response at 10 µM.…”

Two distinct, calcium-mediated, signal transduction pathways can trigger deflagellation in Chlamydomonas reinhardtii

“…We found that several Ca 2* channel blockers known to inhibit other Ca2+-mediated behaviors in Ch/amydomonas (Hegemann et al, 1990;Goodenough et al, 1993) were ineffective at blocking acid-or mastoparan-induced deflagellation. These included omega-conotoxin (up to 5 #M with a 3-h preincubation), diltiazem (up to 100/zM), D-600 (10 /zM), Ni ~* (up to 1 mM), and Co 2+ (up to 1 mM) (data not a lOO 1 shown).…”

Photosensory Adaptation in Aneural Organisms*

“…Even though these channels have not yet been isolated, their existence has subsequently been demonstrated by the use of specific blockers of calcium channels (Dolle and Nultsch, 1988a,b;Hegemann et al, 1990). Several lines of evidence suggest that these channels play a role in the sensory transduction and photosensory adaptation.…”

“…Calcium seems thus to mimic the effect of previous irradiation indicating an adaptive function. For Chlamydomonas, both dark-and light adaptation do indeed depend on Ca2+ (Hegemann and Bruck, 1989;Hegemann et al, 1990).…”