Pharmacology of the SV channel in the vacuolar membrane of Chenopodium rubrum suspension cells

Vacuolar ion channels in the liverwort Marchantia polymorpha: influence of ion channel inhibitors

Potassium-permeable slow activating vacuolar channels (SV) and chloride-permeable channels in the vacuole of the liverwort Marchantia polymorpha were characterized in respect to calcium dependence, selectivity, and pharmacology. The patch-clamp method was used in the study of ion channel activity in the vacuoles from the liverwort Marchantia polymorpha. The whole-vacuole recordings allowed simultaneous observation of two types of currents-predominant slow activated currents recorded at positive voltages and fast activated currents recorded at negative voltages. Single-channel recordings carried out in the gradient of KCl indicated that slow activated currents were carried by potassium-permeable slowly activating vacuolar channels (SV) and fast activated currents-by chloride-permeable channels. Both types of the channels were dependent in an opposite way on calcium, since elimination of this ion from the cytoplasmic side caused inhibition of SV channels, but the open probability of chloride-permeable channels even increased. The dependence of the activity of both channels on different types of ion channel inhibitors was studied. SV channels exhibited different sensitivity to potassium channel inhibitors. These channels were insensitive to 3 mM Ba²⁺, but were blocked by 3 mM tetraethyl ammonium (TEA). Moreover, the activity of the channels was modified in a different way by calcium channel inhibitors. 200 µM Gd³⁺ was an effective blocker, but 50 µM ruthenium red evoked bursts of the channel activity resulting in an increase in the open probability. Different effectiveness of anion channel inhibitors was observed in chloride-permeable channels. After the application of 100 µM Zn²⁺, a decrease in the open probability was recorded but the channels were still active. 50 µM DIDS was more effective, as it completely blocked the channels.

TPC1-SV channels gain shape

The most prominent ion channel localized in plant vacuoles is the slow activating SV type. Slow vacuolar (SV) channels were discovered by patch clamp studies as early as 1986. In the following two decades, numerous studies revealed that these calcium- and voltage-activated, nonselective cation channels are expressed in the vacuoles of all plants and every plant tissue. The voltage-dependent properties of the SV channel are susceptible to modulation by calcium, pH, redox state, as well as regulatory proteins. In Arabidopsis, the SV channel is encoded by the AtTPC1 gene, and even though its gene product represents the by far largest conductance of the vacuolar membrane, tpc1-loss-of-function mutants appeared not to be impaired in major physiological functions such as growth, development, and reproduction. In contrast, the fou2 gain-of-function point mutation D454N within TPC1 leads to a pronounced growth phenotype and increased synthesis of the stress hormone jasmonate. Since the TPC1 gene is present in all land plants, it likely encodes a very general function. In this review, we will discuss major SV channel properties and their impact on plant cell physiology.

Sodium fluxes through nonselective cation channels in the plasma membrane of protoplasts from Arabidopsis roots

The aim of the present work was to characterize Na(+) currents through nonselective cation channels (NSCCs) in protoplasts derived from root cells of Arabidopsis. The procedure of the protoplast isolation was modified to increase the stability of Arabidopsis root protoplasts in low external Ca(2+) by digesting tissue in elevated Ca(2+). Experiments in whole-cell and outside-out modes were carried out. We found that Na(+) currents in Arabidopsis root protoplasts were mediated by cation channels that were insensitive to externally applied tetraethylammonium(+) and verapamil, had no time-dependent activation (permanently opened or completely activated within 1-2 ms), were voltage independent, and were weakly selective for monovalent cations. The selectivity sequence was as follows: K(+) (1.49) > NH(4)(+) (1.24) > Rb(+) (1.15) approximately equal to Cs(+) (1.10) approximately equal to Na(+) (1.00) > Li(+) (0.73) > tetraethylammonium(+) (0.47). Arabidopsis root NSCCs were blocked by H(+) (pK approximately equal to 6.0), Ca(2+) (K(1/2) approximately equal to 0.1 mM), Ba(2+), Zn(2+), La(3+), Gd(3+), quinine, and the His modifier diethylpyrocarbonate. They were insensitive to most organic blockers (nifedipine, verapamil, flufenamate, and amiloride) and to the SH-group modifier p-chloromercuriphenyl sulfonic acid. Voltage-insensitive, Ca(2+)-sensitive single channels were also resolved. Properties of Arabidopsis root NSCCs are discussed and compared with characteristics of similar conductances studied previously in plants and animals. It is suggested that NSCCs present a distinct group of plant ion channels, mediating toxic Na(+) influx to the cell and probably having other important roles in physiological processes of plants.

Calcium channels in higher plants

Calcium channels are involved principally in signal transduction. Their opening results in increased cytoplasmic Ca(2+) concentration, and the spatial and temporal variations in this are thought to elicit specific physiological responses to diverse biotic and abiotic stimuli. Calcium-permeable channels have been recorded in the plasma membrane, tonoplast, endoplasmic reticulum, chloroplast and nuclear membranes of plant cells. This article reviews their electrophysiological properties and discusses their physiological roles. Emphasis is placed on the voltage-dependent and elicitor-activated Ca(2+) channels of the plasma membrane and the depolarisation-activated (SV), hyperpolarisation-activated, IP(3)- and cADPR-dependent Ca(2+) channels of the tonoplast. The closing of stomatal guard cells in the presence of abscisic acid (ABA) is used to illustrate the co-ordination of Ca(2+) channel activities during a physiological response.

Cooperative block of the plant endomembrane ion channel by ruthenium red

Effects of ruthenium red (RR) on the slow Ca(2+)-activated Ca(2+)-permeable vacuolar channel have been studied by patch-clamp technique. Applied to the cytosolic side of isolated membrane patches, RR at concentrations of 0.1-5 microM produced two distinct effects on single channel kinetics, long lasting closures and a flickering block of the open state. The first effect was largely irreversible, whereas the second one could be washed out. The extent of flickering block steeply increased (zdelta = approximately 1.35) with the increase of cytosol-positive voltage, dragging RR into the channel pore. At least two RR ions are involved in the block according to Hill coefficient n = approximately 1.30 for the dose response curves. The on-rate rate of the drug binding linearly depended on the RR concentration, implying that one RR ion already plugged the pore. The blocked state was further stabilized by binding of the second RR. This stabilization was in excess of that predicted by independent binding as the dependence of unblocking rate on RR concentration revealed. A cooperative model was therefore employed to describe the kinetic behavior of RR binding. At zero voltage the half-blocking RR concentration of 36 microM and the bimolecular on-rate constant of 1.8 x 10(8) M(-1) s(-1) were estimated.

Voltage- and Ca(2+)-dependence of the K+ channel in the vacuolar membrane of Chenopodium rubrum L. suspension cells

Voltage- and Ca(2+)-dependence of the slow-activating SV-K+ channel in the vacuolar membrane of Chenopodium rubrum suspension cells has been analyzed using the patch clamp technique in the vacuole-attached, outside-out and whole-vacuolar configuration. Patch-pipette perfusion was applied to measure Ca2+ dependence of single channels in the attached-configuration. Using the PCLAMP-software (Axon Instruments), an algorithm was developed to extract reliable individual channel data from multi-channel activity records, including open probability, mean open and closed times, as well as time constants for open and closed distributions. The channel conductance of the major open state was about 83 pS (seal resistance > 8 G omega) at 30 mV (transmembrane voltage Vm, vacuole negative), and symmetrical 100 mM KCl. the channel exhibited a strong voltage- and a weak Ca(2+)-activation: increasing Vm from 40 to 100 mV is equivalent to a Ca2+ concentration change from 10(-7) to 10(-4) M. Mean open probabilities at Vm = 30 mV were 0.03 with 1 microM and 0.09 with 100 microM Ca2+. Mean open times were approx. 7 ms, and almost independent of both, voltage and Ca2+. Mean closed times, however, varied in a strongly voltage- and Ca(2+)-dependent manner, e.g., at Vm = 30 mV dropped from 205 to 67 ms, if Ca2+ was raised from 10(-6) to 10(-4) M. Open and closed distributions of events within bursts could be fitted by the sum of two exponentials with time constants between 0.3 and 11 ms.