[27] Electrophysiology of giant algal cells. Biomembranes Part U: Cellular and Subcellular Transport: Eukaryotic (Nonepithelial) Cells. Elsevier; 1989. pp. 403-43. [DOI: 10.1016/0076-6879(89)74030-1]

Surface deformation during an action potential in pearled cells

Electric pulses in biological cells (action potentials) have been reported to be accompanied by a propagating cell-surface deformation with a nanoscale amplitude. Typically, this cell surface is covered by external layers of polymer material (extracellular matrix, cell wall material, etc.). It was recently demonstrated in excitable plant cells (Chara braunii) that the rigid external layer (cell wall) hinders the underlying deformation. When the cell membrane was separated from the cell wall by osmosis, a mechanical deformation, in the micrometer range, was observed upon excitation of the cell. The underlying mechanism of this mechanical pulse has, to date, remained elusive. Herein we report that Chara cells can undergo a pearling instability, and when the pearled fragments were excited even larger and more regular cell shape changes were observed (∼10-100μm in amplitude). These transient cellular deformations were captured by a curvature model that is based on three parameters: surface tension, bending rigidity, and pressure difference across the surface. In this paper these parameters are extracted by curve-fitting to the experimental cellular shapes at rest and during excitation. This is a necessary step to identify the mechanical parameters that change during an action potential.

Multi-Scale Characean Experimental System: From Electrophysiology of Membrane Transporters to Cell-to-Cell Connectivity, Cytoplasmic Streaming and Auxin Metabolism

The morphology of characean algae could be mistaken for a higher plant: stem-like axes with leaf-like branchlets anchored in the soil by root-like rhizoids. However, all of these structures are made up of giant multinucleate cells separated by multicellular nodal complexes. The excised internodal cells survive long enough for the nodes to give rise to new thallus. The size of the internodes and their thick cytoplasmic layer minimize impalement injury and allow specific micro-electrode placement. The cell structure can be manipulated by centrifugation, perfusion of cell contents or creation of cytoplasmic droplets, allowing access to both vacuolar and cytoplasmic compartments and both sides of the cell membranes. Thousands of electrical measurements on intact or altered cells and cytoplasmic droplets laid down basis to modern plant electrophysiology. Furthermore, the giant internodal cells and whole thalli facilitate research into many other plant properties. As nutrients have to be transported from rhizoids to growing parts of the thallus and hormonal signals need to pass from cell to cell, Characeae possess very fast cytoplasmic streaming. The mechanism was resolved in the characean model. Plasmodesmata between the internodal cells and nodal complexes facilitate transport of ions, nutrients and photosynthates across the nodes. The internal structure was found to be similar to those of higher plants. Recent experiments suggest a strong circadian influence on metabolic pathways producing indole-3-acetic acid (IAA) and serotonin/melatonin. The review will discuss the impact of the characean models arising from fragments of cells, single cells, cell-to-cell transport or whole thalli on understanding of plant evolution and physiology.

On the excitation of action potentials by protons and its potential implications for cholinergic transmission

One of the most conserved mechanisms for transmission of a nerve pulse across a synapse relies on acetylcholine (ACh). Ever since the Nobel Prize-winning works of Dale and Loewi, it has been assumed that ACh-subsequent to its action on a postsynaptic cell-is split into inactive by-products by acetylcholinesterase (AChE). Herein, the widespread assumption of inactivity of ACh's hydrolysis products is falsified. Excitable cells (Chara braunii internodes), which had previously been unresponsive to ACh, became ACh-sensitive in the presence of AChE. The latter was evidenced by a striking difference in cell membrane depolarization upon exposure to 10 mM intact ACh (∆V = -2 ± 5 mV) and its hydrolysate (∆V = 81 ± 19 mV), respectively, for 60 s. This pronounced depolarization, which also triggered action potentials, was clearly attributed to one of the hydrolysis products: acetic acid (∆V = 87 ± 9 mV at pH 4.0; choline ineffective in the range 1-10 mM). In agreement with our findings, numerous studies in the literature have reported that acids excite gels, lipid membranes, plant cells, erythrocytes, as well as neurons. Whether excitation of the postsynaptic cell in a cholinergic synapse is due to protons or due to intact ACh is a most fundamental question that has not been addressed so far.

Calcium channels in photosynthetic eukaryotes: implications for evolution of calcium-based signalling

Much of our current knowledge on the mechanisms by which Ca(2+) signals are generated in photosynthetic eukaryotes comes from studies of a relatively small number of model species, particularly green plants and algae, revealing some common features and notable differences between 'plant' and 'animal' systems. Physiological studies from a broad range of algal cell types have revealed the occurrence of animal-like signalling properties, including fast action potentials and fast propagating cytosolic Ca(2+) waves. Genomic studies are beginning to reveal the widespread occurrence of conserved channel types likely to be involved in Ca(2+) signalling. However, certain widespread 'ancient' channel types appear to have been lost by certain groups, such as the embryophytes. More recent channel gene loss is also evident from comparisons of more closely related algal species. The underlying processes that have given rise to the current distributions of Ca(2+) channel types include widespread retention of ancient Ca(2+) channel genes, horizontal gene transfer (including symbiotic gene transfer and acquisition of bacterial genes), gene loss and gene expansion within taxa. The assessment of the roles of Ca(2+) channel genes in diverse physiological, developmental and life history processes represents a major challenge for future studies.

The effect of aluminium on bioelectrical activity of the Nitellopsis obtusa cell membrane after H+-ATPase inhibition

Aluminium induced membrane potential (Em) changes and potential changes during repolarization phase of the action potential (AP) in the internodal cells of Nitellopsis obtusa after blocking H+-ATPase activity by DCCD were investigated. Micromolar concentrations of DCCD are sufficient to give complete and irreversible inhibition of proton pumping. The membrane potential was measured by conventional glass-microelectrode technique. We found that the half-amplitude pulse duration differs significantly between standard conditions, after DCCD application, and after H+-ATPase blocking and subsequent Al3+ treatment: 4.9, 7.7 and 17.2 seconds, respectively. We propose that in the short term (2 hours) treatment of Al3+, the decrease in membrane potential was compensated for by H+-ATPase activity. Blocking H+-ATPase activity by DCCD can enhance the influence of Al3+ on the bioelectrical activity of cell membranes.

Action potential in charophytes

The plant action potential (AP) has been studied for more than half a century. The experimental system was provided mainly by the large charophyte cells, which allowed insertion of early large electrodes, manipulation of cell compartments, and inside and outside media. These early experiments were inspired by the Hodgkin and Huxley (HH) work on the squid axon and its voltage clamp techniques. Later, the patch clamping technique provided information about the ion transporters underlying the excitation transient. The initial models were also influenced by the HH picture of the animal AP. At the turn of the century, the paradigm of the charophyte AP shifted to include several chemical reactions, second messenger-activated channel, and calcium ion liberation from internal stores. Many aspects of this new model await further clarification. The role of the AP in plant movements, wound signaling, and turgor regulation is now well documented. Involvement in invasion by pathogens, chilling injury, light, and gravity sensing are under investigation.

Effects of light and inhibitors of ATP-synthesis on the chloride carrier of the alga Valonia utricularis: is the carrier a chloride pump?

The effect of metabolic inhibitors, such as cyanide, antimycin A and azide was studied on the chloride transport system of the giant marine alga Valonia utricularis by using the charge pulse relaxation method. Two clearly defined voltage relaxations were resolved. The addition of 10-30 microM cyanide to the artificial sea water (ASW) bathing the algal cells increased the time constants of the slow voltage relaxation, tau 2, significantly when the algal cells were kept in the dark. The cyanide-effect reached a plateau value at 100-300 microM and was fully reversible when cyanide was removed from the ASW. Analysis of the charge pulse data in terms of the Läuger-model demonstrated that the translocation rates of the free, kS, and the charged carrier, kAS, decreased. The decrease of kS was more pronounced than that of kAS. 10 microM antimycin A and 3 mM azide had similar effects on the rate constants when the light was switched off. Upon illumination the cyanide- and antimycin A-, but not the azide-mediated effects disappeared. At concentrations higher than 1 mM cyanide caused a further, dramatic decrease of kS and kAS, while the surface concentration of the carrier molecules, N0, was not affected. This cyanide-effect was also reversible, but not light-dependent. Measurements of the ATP level showed that 3 mM cyanide reduced the ATP level by about 70% both under light and dark conditions. In the presence of 30 microM cyanide (or 10 microM antimycin A) the ATP level decreased by about 50%, but only in the dark. These results suggest two different effects of cyanide on the Cl(-)-carrier system: in the micromolar concentration range cyanide (and antimycin A) reduced predominantly the translocation of the free carrier by inhibition of ATP synthesis by oxidative phosphorylation, whereas in the millimolar concentration range cyanide apparently inhibits the translocation rates of both the free and charged carriers by its binding to the carrier. The results provide some evidence that the chloride transport of V. utricularis could be coupled to metabolic energy but it is an open question whether it is a pump or not.