Specific binding of the calcium channel blocker [3H]verapamil to membrane fractions of Chlamydomonas reinhardtii

Studies on ion channel antagonist-binding sites in sunflower protoplasts

The cytological location of ion channel antagonist-binding sites was studied in sunflower protoplasts using the fluorescent probes DM-Bodipy-PAA and DM-Bodipy-DHP. The binding specificity of the probes was established by competition experiments with Bepridil, phenylalkylamine (Verapamil) and dihydropyridine (Nifedipine) which are known as calcium and potassium channel antagonists. Quantitative image analysis of the fluorescence emitted by the protoplasts showed the existence of interactions between PAA- and DHP-binding sites. Moreover, studies on the cytolocalization of the PAA receptors by confocal imaging showed that in freshly isolated protoplasts, DM-Bodipy-PAA binds exclusively at sites located in the cortical region of the cell.

The photoreceptor current of the green algaChlamydomonas

The unicellular algaChlamydomonaspossesses a visual system which guides it to places that are optimal for photosynthetic growth. The rhodopsin, serving as the photoreceptor, conveys light information into a cellular signal. This signal is transm itted via several electrical steps to the flagella, where it modulates the flagellar beating pattern. The first detectable electrical process is the photoreceptor current, which is confined to the eyespot region. It is carried mainly by calcium under physiological conditions. After a flash the current rises with no detectable delay in less than 2 ms to the m axim um and decays within tens of ms. The peak amplitude increases with the stimulus intensity over a range of two orders of magnitude, and the decay kinetic is accelerated in parallel. The light sensitivity of the cell is reduced by a factor of 8 when the eye is oriented away from the light source instead of facing it. This gives a measure for the ‘contrast’ used by the cell during orientation in light. The rhodopsin action spectrum of the photoreceptor current has a maximum at 494 nm, closely matching the absorption maximum of rhodopsin in purified membranes. When the photoreceptor current exceeds a critical level, flagellar currents appear as all-or-non events. Several lines of evidences suggest that these photocurrents are the link between light absorption and alteration of the flagellar beating during a movement response.

The biochemistry and pharmacology of plasma-membrane calcium channels in plants

A review of plasma-membrane calcium channels in higher plants is presented. Data from pharmacological and biochemical studies are used to assess the current state of our knowledge concerning the occurrence of these structures in higher plants. Recent results demonstrate that after purification and reconstitution a phenylalkylamine binding protein will form calcium permeable channels. This result suggests that plants contain structures with some analogy to animal calcium channels. We also suggest that a degree of caution should be observed in the interpretation of results gained in pharmacological studies employing calcium channel active drugs, as it is now clear that even at low doses these com pounds have non-specific effects.

1,4-Dihydropyridine binding sites in moss plasma membranes. Properties of receptors for a calcium channel antagonist

An increase in cytoplasmic calcium is an early event in hormone (cytokinin)-induced vegetative bud formation in the moss Physcomitrella patens. Whole cell and calcium transport studies have implicated 1,4-dihydropyridine-sensitive calcium channels in this increase in cellular calcium. To understand the molecular nature of the dihydropyridine-sensitive calcium channel, we have established conditions for the binding of the arylazide 1,4-dihydropyridine, [3H]azidopine, to its receptor in moss plasma membranes. [3H]Azidopine bound specifically in a saturable and reversible manner. The KD for [3H]azidopine binding was 5.2 nM and the Bmax was 35.6 pmol/mg of protein. Association and dissociation of the receptor and [3H]azidopine were temperature-dependent, and association varied as a function of pH. Binding was inhibited by dihydropyridine, phenylalkylamine, and benzothiazepine calcium channel blockers, bepridil, lanthanum, and N-ethylmaleimide. [3H]Azidopine binding was stimulated by cations including calcium, strontium, manganese, and barium. [3H]Azidopine binding was also stimulated by cytokinin with a Km value for kinetin of 0.13 nM. These studies utilize a simple plant system to provide a biochemical framework for understanding calcium regulation during development and have implications for understanding mechanisms of signal transduction in plants.

The role of calcium in Chlamydomonas photomovement responses as analysed by calcium channel inhibitors

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.

Effects of nifedipine, verapamil, and diltiazem on tip growth inFunaria hygrometrica

Protonemata ofFunaria hygrometrica Sibth. were treated with nifedipine, verapamil, or diltiazem. Responses to each of the drugs were, on the one hand, reduction of growth rate and tip cell length and, on the other hand, formation of apical swellings in caulonema tip cells and of anomalously oriented separation walls between main filaments and young side branches. The first effect is regarded as a more general expression of inhibition while the second complex of effects is attributed to perturbations in directed vesicle transport. Replacement of drug-containing media by normal Knop agar demonstrated the reversibility of inhibitor action: growth parameters were comparable to those of control protonemata within a few hours. A fast reaction, the formation of subapical vacoules, occurred within minutes of drug application and was only observed with verapamil and diltiazem. In connection with this process, rapid migrations of chloroplasts took place, but examination of the microtubule cytoskeleton in such cells by indirect immunofluorescence with a monoclonal antibody against tubulin showed an intact microtubule network. callose deposits in tip cells treated with verapamil. They were polarly distributed and started to appear in cell apices about 2h after the beginning of verapamil application. Two mechanisms of action for the tested inhibitors are discussed: (i) perturbations of membrane permeability by interference with one or more of the cell's Ca(2+)-transport systems, and (ii) a more indirect mechanism affecting vesicle transport via the microfilament system.

Cytoplasmic calcium affects the gating of potassium channels in the plasma membrane ofChara corallina: a whole-cell study using calcium-channel effectors

The action of a wide range of drugs effective on Ca(2+) channels in animal tissues has been measured on Ca(2+) channels open during the action potential of the giant-celled green alga,Chara corallina. Of the organic effectors used, only the 1,4-dihydropyridines were found to inhibit reversibly Ca(2+) influx, including, unexpectedly, Bay K 8644 and both isomers of 202-791. Methoxyverapamil (D-600), diltiazem, and the diphenylbutylpiperidines, fluspirilene and pimozide were found not to affect the Ca(2+) influx. Conversely, bepridil greatly and irreversibly stimulated Ca(2+) influx, and with time, stopped cytoplasmic streaming (which is sensitive to increases in cytoplasmic Ca(2+)). By apparently altering the cytoplasmic Ca(2+) levels with various drugs, it was found that (with the exception of the inorganic cation, La(3+)) treatments likely to lead to an increase in cytoplasmic Ca(2+) levels caused an increase in the rate of closure of the K(+) channels. Similarly, treatments likely to lead to a decrease in cytoplasmic Ca(2+) decreased the rate of K(+) channel closure. The main effect of bepridil on the K(+) channels was to increase the rate of voltage-dependent channel closure. The same effect was obtained upon increasing the external concentration of Ca(2+), but it is likely that this was due to effects on the external face of the K(+) channel. Addition of any of the 1,4-dihydropyridines had the opposite effect on the K(+) channels, slowing the rate of channel closure. They sometimes also reduced K(+) conductance, but this could well be a direct effect on the K(+) channel; high concentrations (50 to 100 μM) of bepridil also reduced K(+) conductance. No effect of photon irradiance or of abscisic acid could be consistently shown on the K(+) channels. These results indicate a control of the gating of K(+) channels by cytoplasmic Ca(2+), with increased free Ca(2+) levels leading to an increased rate of K(+)-channel closure. As well as inhibiting Ca(2+) channels, it is suggested that La(3+) acts on a Ca(2+)-binding site of the K(+) channel, mimicking the effect of Ca(2+) and increasing the rate of channel closure.