Effects of the calcium channel antagonist nicardipine on renal action of endothelin in dogs

Endothelin is a potent vasoconstrictor peptide isolated from cultured vascular endothelial cells. Interaction between endothelin and calcium channel antagonist on the renal hemodynamics and urine formation was studied in anesthetized dogs. Intrarenal arterial administration of the peptide progressively reduced renal blood flow from 139 +/- 22 to 85 +/- 12 ml/min at 20 min after the start of continuous infusion, with no change in systemic blood pressure. Glomerular filtration rate, urine flow and urinary sodium and calcium excretion decreased significantly by 30-50% from the preinfusion control values. An endothelin-induced reduction in renal blood flow was markedly attenuated by pretreatment with the calcium antagonist nicardipine (100 ng/kg/min intrarenally). Changes in glomerular filtration rate and antinatriuretic and anticalciuretic actions by the peptide was not affected by nicardipine treatment. It is suggested that the renal vasoconstrictor action, but not the tubular action, of endothelin is functionally coupled with the activation of dihydropyridine-sensitive calcium channels.

Clathrin is involved in organization of mitotic spindle and phragmoplast as well as in endocytosis in tobacco cell cultures

We previously identified a 175 kDa polypeptide in Lilium longiflorum germinating pollen using a monoclonal antibody raised against myosin II heavy chain from Physarum polycephalum. In the present study, the equivalent polypeptide was also found in cultured tobacco BY-2 cells. Analysis of the amino acid sequences revealed that the 175 kDa polypeptide is clathrin heavy chain and not myosin heavy chain. After staining of BY-2 cells, punctate clathrin signals were distributed throughout the cytoplasm at interphase. During mitosis and cytokinesis, clathrin began to accumulate in the spindle and the phragmoplast and then was intensely concentrated in the cell plate. Expression of the C-terminal region of clathrin heavy chain, in which light chain binding and trimerization domains reside, induced the suppression of endocytosis and the formation of an aberrant spindle, phragmoplast, and cell plate, the likely cause of the observed multinucleate cells. These data strongly suggest that clathrin is intimately involved in the formation of the spindle and phragmoplast, as well as in endocytosis.

New aspects of membrane dynamics of Amoeba proteus contractile vacuole revealed by vital staining with FM 4-64

The contractile vacuole (CV) cycle of Amoeba proteus has been studied by phase contrast and electron microscopy. However, the understanding of membrane dynamics in this cycle is still poor. In this study, we used live imaging by fluorescence microscopy to obtain new insights. We succeeded in staining the CV with a styryl dye, FM 4-64 (N-(3-triethylammoniumpropyl)-4-(6-(4-(diethylamino)phenyl)hexatrienyl)pyridinium dibromide), and obtained the following results. (1) The CV membrane was directly stained with the dye in the external medium when the CV pore opened upon contraction. This indicates that transfer of plasma membrane to the CV does not occur. (2) The membrane dynamics during the CV cycle were elucidated. In particular, the fluorescent CV membrane was maintained as an aggregate just after contraction and the vacuole re-formed from the aggregate. Staining was maintained during continued contraction cycles. We conclude that the CV membrane is maintained during the CV cycle.

Possible association of actin filaments with chloroplasts of spinach mesophyll cells in vivo and in vitro

In palisade mesophyll cells of spinach (Spinacia oleracea L.) kept under low-intensity white light, chloroplasts were apparently immobile and seemed to be surrounded by fine bundles of actin filaments. High-intensity blue light induced actin-dependent chloroplast movement concomitant with the appearance of a couple of long, straight bundles of actin filaments in each cell, whereas high-intensity red light was essentially ineffective in inducing these responses. The actin organization observed under low-intensity white light has been postulated to function in anchoring chloroplasts at proper intracellular positions through direct interaction with the chloroplasts. Intact chloroplasts, which retained their outer envelopes, were isolated after homogenization of leaves and Percoll centrifugation. No endogenous actin was detected by immunoblotting in the final intact-chloroplast fraction prepared from the leaves kept under low-intensity white light or in darkness. In cosedimentation assays with exogenously added skeletal muscle filamentous actin, however, actin was detected in the intact-chloroplast fraction precipitated after low-speed centrifugation. The association of actin with chloroplasts was apparently dependent on incubation time and chloroplast density. After partial disruption of the outer envelope of isolated chloroplasts by treatment with trypsin, actin was no longer coprecipitated. The results suggest that chloroplasts in spinach leaves can directly interact with actin, and that this interaction may be involved in the regulation of intracellular positioning of chloroplasts.

Possible involvement of energy metabolism in the change of cytoplasm organization induced by a protein phosphatase inhibitor, calyculin A, in root hair cells of Limnobium stoloniferum

In root hair cells of Limnobium stoloniferum, transvacuolar strands disperse and cytoplasmic spherical bodies (CSBs) emerge upon treatment with a protein phosphatase inhibitor, calyculin A (CA), whose effects were previously shown to be canceled by simultaneous treatment of the cells with a nonselective protein kinase inhibitor, K-252a. CSB formation is also suppressed by latrunculin B (LB) or cytochalasin D, actin filament depolymerization drugs, or 2,3-butanedione monoxime, an inhibitor of myosin activity. To confirm the involvement of myosin activity in CSB formation induced by CA, we examined the effect of an inhibitor of energy metabolism, NaN3, on CSB formation in root hair cells pretreated simultaneously with CA and LB. In the presence of CA-LB, CSB formation was suppressed due to the depolymerization of actin filaments. When these drugs were removed, the actin filaments recovered and CSBs emerged even in the presence of K-252a. These results indicated that the phosphorylation level in the cells is elevated during the CA-LB treatment and that a phosphorylation level sufficient for the CSB formation was sustained even after CA removal. On the other hand, CSB formation after simultaneous treatment with CA and LB was significantly suppressed in the presence of NaN3. In such cells, actin filament bundles recovered, although their organization was random. The present and previous results suggested that myosin activity is necessary for CSB formation induced by CA, and that myosin regulated by phosphorylation-dephosphorylation is implicated in the organization of the actin cytoskeleton in root hair cells.

Involvement of microtubules in rhizoid differentiation of Spirogyra species

Some species of Spirogyra form rosette-shaped or rod-shaped rhizoids in the terminal cell of the filaments. In the present study, we analyzed an involvement of microtubules (MTs) in rhizoid differentiation. Before rhizoid differentiation, cortical MTs were arranged transversely to the long axis of cylindrical cells, reflecting the diffuse growth. At the beginning of rhizoid differentiation, MTs were absent from the extreme tip of the terminal cell. In the other area of the cell, however, MTs were arranged transversely to the long axis of the cell. In the fully differentiated rosette-shaped rhizoid, MTs were randomly organized. However, at a younger stage of rosette-shaped rhizoids, MTs were sometimes arranged almost transversely in the lobes of the rosette. In the rod-shaped rhizoid, MTs were arranged almost transversely. MT-destabilizing drugs (oryzalin and propyzamide) induced swelling of rhizoids, and neither rosette-shaped nor rod-shaped rhizoids were formed. The role of MTs in rhizoid differentiation was discussed.

Mechanosensory ion channels in charophyte cells: the response to touch and salinity stress

Mechanosensitive (MS) ion channels are activated by mechanical stress and then transduce this information into electrical signals. These channels are involved in the growth, development and response to environmental stress in higher plants. Detailed analyses of the electrophysiology in higher plants are difficult because such plants are composed of complex tissues. The large cells of the charophytes facilitate electrophysiological measurements and allow us to study MS ion channels at the level of single cells. We draw parallels between the process of touch-perception in freshwater Chara, and the turgor-regulating response to osmotic shock in salt-tolerant Lamprothamnium. In terms of electrophysiology, these responses can be considered in three stages: (1) stimulus perception, (2) signal transmission and (3) induction of response. In Chara the first stage is due to the receptor potential (RPD), a transient depolarization with a critical threshold that triggers action potentials, which are responsible for stages (2) and (3). Receptor potentials are generated by MS ion channels. Action potentials involve a transient influx of Ca(2+) to the cytoplasm, effluxes of K(+) and Cl(-) and a temporary decrease of turgor pressure. Reducing cell turgor increases sensitivity to mechanical stimulation. In Lamprothamnium, a hypotonic shock produces an extended depolarization that resembles an extended RPD and is responsive to osmotic rather than ionic changes. Like the action potential, a critical threshold depolarization triggers Ca(2+) influx, opening of Ca(2+)-sensitive Cl(-) channels and K(+) channels; effluxes that last over an hour and result in turgor regulation. These processes show us, in primal form and at the level of single cells, how mechanoperception occurs in higher plants. Recent progress in research into the role of MS ion channels in the freshwater and salt-tolerant Characeae is reviewed and the relevance of these findings to plants in general is considered.

Involvement of cortical microtubules in plastic extension regulated by gibberellin in Lemna minor root

We aimed to analyze the rheological characteristics during elongation of the root segments in Lemna minor. The elastic component of segment elongation (EC) increased for the first 6 h, and then almost stopped. However, the plastic component of the segment elongation (PC) began to rapidly increase from 6 h onwards. Uniconazole-P, a gibberellin biosynthesis inhibitor, inhibited the total elongation of root segments (TE), and this inhibition was mainly caused by suppression of the rapid increase in the PC after 6 h. Concomitant with this inhibition, the cortical microtubule (CMT) array within root epidermal cells became disorganized in the presence of uniconazole-P from 6 h onwards. Adding GA3 abolished the inhibition of TE by uniconazole-P treatment, and this recovery was caused not by the increase in the EC but by an increase in the PC. Furthermore, the CMT arrays also recovered their characteristic organization in the presence of GA3. These findings suggest that endogenous gibberellin accelerates TE by activating the PC via control of CMT arrays. This conclusion is also supported by rheological analysis where propyzamide was used to disrupt microtubules. We suggest that endogenous gibberellin controls the PC via its influence over the transverse arrangement of CMTs.

Electrical perception of "death message" in Chara: involvement of turgor pressure

Plants show various defense responses upon wounding. Surviving cells must perceive a "death message" from killed cells in order to start the signal processing that results in defense responses. The initial step in perception of the death message by a surviving cell was studied by taking advantage of the filamentous morphology of characean algae. A specimen comprising two adjoining internodal cells was prepared. One cell (the victim cell) was killed by cutting and any changes in the membrane potential of the neighboring cell (the receptor cell) were analyzed. Upon cutting the victim cell, at least one of three kinds of response were induced in the receptor cell: (1) slow depolarization lasting more than 10 min, (2) action potentials and (3) small spikes. The first of these response types, slow depolarization, was ubiquitous and is the focus of the present study. Two cell properties were essential for generation of this depolarization. (1) Presence of high cell turgor pressure was necessary. (2) The depolarization was generated only at the nodal end of the receptor cell, not at the flank. I concluded that the death message from the killed cell contains the information that turgor pressure has been lost. The mechanism by which this is translated into the slow depolarization of the receptor cell was discussed.

Isolation of cortical MTs from tobacco BY-2 cells

We isolated the cortical microtubules (CMTs) from tobacco BY-2 cells to identify their components. By centrifugation of protoplasts homogenized in the presence of taxol, a MT-stabilizing reagent, in a density gradient of Percoll, we obtained membranous vesicles to which MTs forming a sheet-like bundle were attached. Rhodamine-conjugated Ricinus communis agglutinin I (RCA-I), a lectin that bound to the surface of protoplasts, stained these vesicles, indicating that they were plasma membrane (PM) vesicles that retained CMTs. CMTs were released by solubilization of PM vesicles with Triton X-100. A sheet-like array of CMTs was retained even after solubilization of PM vesicles. Immunoblot analysis of the isolated CMTs demonstrated the presence of tubulin, actin, the 65 kDa microtubule-associated protein (MAP) and a 130 kDa RCA-I binding protein. Purification of the isolated CMTs by the temperature dependent disassembly-reassembly cycling method revealed four polypeptides, 190, 120, 85 and 65 kDa, co-assembling with CMTs.

Possible involvement of 65 kda MAP in elongation growth of azuki bean epicotyls

Although regulation of the dynamics of plant microtubules (MTs) by microtubule-associated proteins (MAPs) has been suggested, the mechanism has not yet been elucidated. As one candidate, a MAP composed of a 65 kDa polypeptide (65 kDa MAP) has been isolated from tobacco cultured cells [Jiang and Sonobe (1993), J. Cell Sci 105: 8911. To investigate the physiological role of the 65 kDa MAP in situ, we analyzed the changes in content and colocalization of this MAP with cortical MTs in relation to elongation growth, using azuki bean epicotyls (Vigna angularis Ohwi et Ohashi). All apical, intermediate, and basal segments prepared from 6 d seedlings showed high growth activity. In 12 d seedlings, growth activity of intermediate and basal segments was low, although that of apical segments was high. The relationship between the growth activity and the orientation of cortical MTs in the epidermal cells was analyzed. Cells could be classified into four types with respect to orientation of cortical MTs: transverse (T), oblique (O), longitudinal (L) to the vertical axis of cells, and random (R). In rapidly growing segments, three types of cells, T, O, L, were observed at similar ratios. In such segments, significant amounts of the 65 kDa MAP were expressed, and it colocalized well with cortical MTs. In segments showing low growth activity, most of the cells showed oblique and longitudinal orientation of cortical MTs. In such segments, the content of the 65 kDa MAP was low. These results suggested involvement of this 65 kDa MAP in regulation of the elongation growth of this epicotyl.

Regulation of elongation growth by gibberellin in root segments of Lemna minor

Hormonal control of elongation growth was analyzed in segments excised from the elongation zone of Lemna roots. Exogenous GA3 did not promote the segment elongation but rather inhibited it. Uniconazole-P, a gibberellin biosynthesis inhibitor, significantly inhibited the segment elongation, and the inhibitory effect was completely nullified by GA3. In the epidermis, cell elongation was inhibited, but lateral cell expansion was not affected by uniconazole-P. Orientation of cortical microtubules of epidermal cells was disturbed by treatment with uniconazole-P for 12 h, and the disorganization of cortical microtubules was ameliorated by GA3. These findings suggested that disorganization of cortical microtubules induced inhibition of elongation growth of root. However, stabilization of cortical microtubules by taxol, a microtubule-stabilizing agent, did not affect the inhibition of segment elongation by uniconazole-P. These results suggested that endogenous gibberellin controls the elongation growth of root by regulating cell elongation.

Interaction of actin filaments with the plasma membrane in Amoeba proteus: studies using a cell model and isolated plasma membrane

We prepared a cell model of Amoeba proteus by mechanical bursting to study the interaction between actin filaments (AFs) and plasma membrane (PM). The cell model prepared in the absence of Ca2+ showed remarkable contraction upon addition of ATP. When the model was prepared in the presence of Ca2+, the cytoplasmic granules formed an aggregate in the central region, having moved away from PM. Although this model showed contraction upon addition of ATP in the presence of Ca2+, less contraction was noted. Staining with rhodamine-phalloidin revealed association of AFs with PM in the former model, and a lesser amount of association in the latter model. The interaction between AFs and PM was also studied using the isolated PM. AFs were associated with PM isolated in the absence of Ca2+, but were not when Ca2+ was present. These results suggest that the interaction between AFs and PM is regulated by Ca2+.

Isolation of a novel 190 kDa protein from tobacco BY-2 cells: possible involvement in the interaction between actin filaments and microtubules

Interaction between actin filaments (AFs) and microtubules (MTs) has been reported in various plant cells, and the presence of a factor(s) connecting these two cytoskeletal networks has been suggested, but its molecular entity has not been elucidated yet. We obtained a fraction containing MT-binding polypeptides, which induced bundling of AFs and of MTs. A 190 kDa polypeptide which associated with AFs was selectively isolated from the fraction. This polypeptide was thought to have an ability to bind to both AFs and MTs. We raised a monoclonal antibody against the 190 kDa polypeptide. Immunostaining demonstrated the association of the 190 kDa polypeptide with AF bundles and with MT bundles formed in vitro. Immunocytochemical studies throughout the cell cycle revealed that the 190 kDa polypeptide was localized in the nucleus before nuclear envelope breakdown, and in the spindle and the phragmoplast during cell division. After the re-formation of the nuclear envelope, the 190 kDa polypeptide was sequestered to the daughter nuclei. Using the antibody, we succeeded in cloning a cDNA encoding the 190 kDa polypeptide.

Demonstration of Cl- requirement for inhibition of vacuolar acidification by cycloprodigiosin in situ

Applying vacuole-perfusion and plasma membrane permeabilization techniques to internodal cells of Chara, we analyzed the requirement of Cl- for the action of cycloprodigiosin (cPrG) to inhibit vacuole acidification in situ. By combining the two techniques, the Cl- concentration on both sides of the tonoplast could be controlled. In permeabilized cell fragments lacking Cl- in the vacuole, the inhibitory effect of cPrG on vacuole acidification was cancelled. On the other hand, Cl- in the cytoplasm was not needed for the cPrG action. These results supported the function of cPrG as a H+/Cl- symporter. Requirement of Cl- for the cPrG action was also demonstrated in vacuole-perfused living cells. This is the first report on the mechanism of cPrG action in situ.

Regulation of root growth by gibberellin in Lemna minor

Hormonal control of root growth was studied in Lemna minor. Although addition of gibberellic acid (GA3) to the culture medium did not promote the root growth, a gibberellin biosynthesis inhibitor, uniconazole P (Un-P), significantly inhibited root growth. Both length and diameter of roots in Un-P-treated plants were significantly smaller than those in control plants, mainly caused by inhibition of cell division. In epidermal cells, the length was slightly decreased and the width increased by Un-P treatment, indicating inhibition of elongation growth. GA3 completely nullified the inhibition caused by Un-P. Transverse cortical microtubules (CMTs) of epidermal cells in the elongation zone were significantly fragmented by treatment with Un-P, but not by that in the presence of GA3. The cellulose microfibril array in the Un-P-treated cells was more random and more oblique than that in the control cells. These results suggested that root growth in L. minor is regulated by endogenous gibberellin.

Calcium-calmodulin suppresses the filamentous actin-binding activity of a 135-kilodalton actin-bundling protein isolated from lily pollen tubes

We have isolated a 135-kD actin-bundling protein (P-135-ABP) from lily (Lilium longiflorum) pollen tubes and have shown that this protein is responsible for bundling actin filaments in lily pollen tubes (E. Yokota, K. Takahara, T. Shimmen [1998] Plant Physiol 116: 1421-1429). However, only a few thin actin-filament bundles are present in random orientation in the tip region of pollen tubes, where high concentrations of Ca(2+) have also been found. To elucidate the molecular mechanism for the temporal and spatial regulation of actin-filament organization in the tip region of pollen tubes, we explored the possible presence of factors modulating the filamentous actin (F-actin)-binding activity of P-135-ABP. The F-actin-binding activity of P-135-ABP in vitro was appreciably reduced by Ca(2+) and calmodulin (CaM), although neither Ca(2+) alone nor CaM in the presence of low concentrations of Ca(2+) affects the activity of P-135-ABP. A micromolar order of Ca(2+) and CaM were needed to induce the inhibition of the binding activity of P-135-ABP to F-actin. An antagonist for CaM, W-7, cancelled this inhibition. W-5 also alleviated the inhibition effect of Ca(2+)-CaM, however, more weakly than W-7. These results suggest the specific interaction of P-135-ABP with Ca(2+)-CaM. In the presence of both Ca(2+) and CaM, P-135-ABP organized F-actin into thin bundles, instead of the thick bundles observed in the absence of CaM. These results suggest that the inhibition of the P-135-ABP activity by Ca(2+)-CaM is an important regulatory mechanism for organizing actin filaments in the tip region of lily pollen tubes.

The role of plant villin in the organization of the actin cytoskeleton, cytoplasmic streaming and the architecture of the transvacuolar strand in root hair cells of Hydrocharis

In many types of plant cell, bundles of actin filaments (AFs) are generally involved in cytoplasmic streaming and the organization of transvacuolar strands. Actin cross-linking proteins are believed to arrange AFs into the bundles. In root hair cells of Hydrocharis dubia (Blume) Baker, a 135-kDa polypeptide cross-reacted with an antiserum against a 135-kDa actin-bundling protein (135-ABP), a villin homologue, isolated from lily pollen tubes. Immunofluorescence microscopy revealed that the 135-kDa polypeptide co-localized with AF bundles in the transvacuolar strand and in the sub-cortical region of the cells. Microinjection of antiserum against 135-ABP into living root hair cells induced the disappearance of the transvacuolar strand. Concomitantly, thick AF bundles in the transvacuolar strand dispersed into thin bundles. In the root hair cells, AFs showed uniform polarity in the bundles, which is consistent with the in-vitro activity of 135-ABP. These results suggest that villin is a factor responsible for bundling AFs in root hair cells as well as in pollen tubes, and that it plays a key role in determining the direction of cytoplasmic streaming in these cells.

Regulation of cytoplasmic pH under extreme acid conditions in suspension cultured cells of Catharanthus roseus: a possible role of inorganic phosphate

Changes in cytoplasmic pH of suspension-cultured cells of Catharanthus roseus under extreme acid conditions were measured with the pH-dependent fluorescence dye; 2',7'-bis-(2-carboxyethyl)-5 (and-6) carboxyfluorescein (-acetoxymethylester) (BCECF). When cells were treated with 1 mM HCl (pH 3 solution), the cytoplasmic pH first decreased then returned to the original level. Treatment with 10 mM HCl (pH 2 solution) acidified the cytoplasm to a greater extent, and the acidification continued at a constant level throughout the measurement. Treatment with a pH 2 solution resulted in a gradual decrease of the malate content, indicating the operation of biochemical pH regulation mechanism. The pH 2 treatment also caused a sudden decrease of the intracellular level of Pi. The cellular content of total phosphorus did not change during the acidification. The Pi was converted to the organic phosphate form. The ATP level was not increased by the pH 2 treatment, but slightly decreased. The role of Pi, which might be functioning as a regulatory factor of cytoplasmic pH, a non-competitive inhibitor of the H+-pumps of both the plasma membrane and tonoplast is discussed.

The 135-kDa actin-bundling protein from lily pollen tubes arranges F-actin into bundles with uniform polarity

A plant 135-kDa actin-bundling protein (P-135-ABP) isolated from pollen tubes of Lilium longiflorum (Thunb.) binds stoichiometrically to F-actin filaments and bundles them in vitro (E. Yokota et al., 1998, Plant Physiol. 116: 1421-1429). To further understand the mechanism of actin-filament bundle formation by P-135-ABP, the polarity of each F-actin filament in bundles was examined using myosin subfragment 1 (S-1). Dissociation of F-actin filaments from bundles organized by P-135-ABP was induced by S-1. However, F-actin filaments that remained in a bundle and decorated by S-1 showed uniform polarity. These results indicate that P-135-ABP arranges F-actin filaments into bundles with uniform polarity and consequently plays a key role in the orientation of cytoplasmic streaming in pollen tubes.

Effects of aluminum on plasma membrane as revealed by analysis of alkaline band formation in internodal cells of Chara corallina

To study the mechanism of aluminum toxicity in plant cells, the effects of aluminum on alkaline band formation were analyzed in the internodal cells of Chara. After cells were treated with AlCl3, they were examined for their capacity to develop alkaline bands. Treating cells with AlCl3 medium at pH 4.5 completely inhibited alkaline band formation. When either CaCl2 or malic acid was added to the AlCl3 medium (pH 4.5), it did not produce an ameliorative effect, whereas addition of both CaCl2 and malic acid induced a significant ameliorative effect. It was found that treatment at pH 4.5 in the absence of AlCl3 strongly inhibited alkaline band formation. This inhibition by the low pH (4.5) treatment was effectively ameliorated by CaCl2. At higher pH (5.0), malic acid alone produced a significant ameliorative effect on aluminum inhibition of alkaline band formation, but CaCl2 did not. Recovery from aluminum inhibition was also studied. When cells treated with AlCl3 at pH 4.5 were incubated in artificial pond water, they could not recover the capacity to develop alkaline band. When either malic acid or CaCl2 was added to artificial pond water, cells recovered their alkaline band formation. It was concluded that one of the primary targets of aluminum is the plasma membrane and that aluminum affects the plasma membrane from the cell exterior at the beginning of the treatment (within 24 h). It was also suggested that the aluminum treatment impairs the HCO3- influx mechanism but not the OH- efflux mechanism.

Inhibitory regulation of higher-plant myosin by Ca2+ ions

Myosin isolated from the pollen tubes of lily (Lilium longiflorum) is composed of a 170-kD heavy chain (E. Yokota and T. Shimmen [1994] Protoplasma 177: 153-162). Both the motile activity in vitro and the F-actin-stimulated ATPase activity of this myosin were inhibited by Ca2+ at concentrations higher than 10(-6) M. In the Ca2+ range between 10(-6) and 10(-5) M, inhibition of the motile activity was reversible. In contrast, inhibition by more than 10(-5) M Ca2+ was not reversible upon Ca2+ removal. An 18-kD polypeptide that showed the same mobility in sodium dodecyl sulfate-polyacrylamide gel electrophoresis as that of spinach calmodulin (CaM) was present in this myosin fraction. This polypeptide showed a mobility shift in sodium dodecyl sulfate-polyacrylamide gel electrophoresis in a Ca2+-dependent manner. Furthermore, this polypeptide was recognized by antiserum against spinach CaM. By immunoprecipitation using antiserum against the 170-kD heavy chain, the 18-kD polypeptide was coprecipitated with the 170-kD heavy chain, provided that the Ca2+ concentration was low, indicating that this 18-kD polypeptide is bound to the 170-kD myosin heavy chain. However, the 18-kD polypeptide was dissociated from the 170-kD heavy chain at high Ca2+ concentrations, which irreversibly inhibited the motile activity of this myosin. From these results, it is suggested that the 18-kD polypeptide, which is likely to be CaM, is associated with the 170-kD heavy chain as a light chain. It is also suggested that this polypeptide is involved in the regulation of this myosin by Ca2+. This is the first biochemical basis, to our knowledge, for Ca2+ regulation of cytoplasmic streaming in higher plants.

Purification of an actin-binding protein composed of 115-kDa polypeptide from pollen tubes of lily

From lily pollen tubes, an actin-binding protein composed of 115-kDa polypeptide was purified sequentially by co-precipitation method with F-actin, hydroxylapatite column, gel filtration column and DE-52 ion exchange column chromatography. This component displayed a tendency to aggregate in solutions of low ionic strength, indicating a hydrophilic characteristic. Under physiological ionic conditions, this component bound to F-actin in an actin-concentration-dependent was saturable manner. Binding of this component to F-actin was independent of ATP and Ca(2+)-concentrations. Fluorescent microscopy revealed that F-actin labeled with rhodamine-phalloidin showed bundling in the presence of this component. Judging from the lack of antibody cross-reactivity, this component does not seem to be related to alpha-actinin of skeletal muscle and plant 135-kDa actin-bundling protein. Therefore, this component is the F-actin binding protein, which has not been identified thus far in plant cells.

Efficient callus initiation from leaf of mangrove plant,Bruguiera sexangula in amino acid medium: Effect of NaCl on callus initiation

Experimental conditions for efficient callus initiation from mangrove plants were investigated. As a source explant, leaf ofBruguiera sexangula was used. Mangrove plant is one of the most famous woody plants which can grow at the salty area. The initiated callus can be a suitable material for the investigation of salt tolerant mechanisms of mangrove plants.Leaf pieces cultured in an Amino Acid medium supplemented with 2 μM 2,4-dichlorophenoxyacetic acid and 2 μMN-(2-chloro-4-pyridyl)-N'-phenylurea at 30 C developed calluses. Microscopic observation suggested that the callus was initiated from the tissue in the vascular bundles in the leaf.We also examined the effect of NaCl on callus initiation and short-term culture of the calluses on the leaves. Callus initiation rate decreased with increasing NaCl concentration higher than 100 mM in the culture media. The medium containing 100 mM NaCl produced the largest callus on the leaf, compared with higher or lower concentrations of NaCl.

Neutralization of human immunodeficiency virus type 1 (HIV-1) with antibody from carriers' plasma against HIV-1 protein p17

It was investigated whether human antibody against HIV-1 protein p17 (anti-p17) in HIV carriers' plasma has the ability to neutralize the infectivity of HIV. By the pretreatment of HIV-1 with anti-p17 from HIV carriers, progeny HIV-1 production from cells infected with virus pretreated with anti-p17 was suppressed and/or delayed. The neutralizing activity of anti-p17 was decreased in the presence of recombinant p17. The latter obviously masked the neutralizing activity of anti-p17. The relevant epitope(s) on p17 is located apparently on the surface of HIV virions and the binding of anti-p17 to p17 impairs the infectivity of HIV. This implies that anti-p17, if stably present in HIV carriers' plasma, may also play an important role in reducing the infectivity of HIV-1 in vivo.

Force-velocity relations of rat cardiac myosin isozymes sliding on algal cell actin cables in vitro

The difference in kinetic properties between two myosin isozymes (V1 and V3) in rat ventricular myocardium was studied by determining the steady-state force-velocity (P-V) relations in the ATP-dependent movement of V1 and V3-coated polystyrene beads on actin cables of giant algal cells mounted on a centrifuge microscope. The maximum unloaded velocity of bead movement was larger for V1 than for V3. The velocity of bead movement decreased with increasing external load applied by the centrifuge microscope, and eventually reached zero when the load was equal to the maximum isometric force (P0) generated by the myosin heads. The maximum isometric force P0 was less than 10 pN, and did not differ significantly between V1 and V3. The P-V curves consisted of a hyperbolic part in the low force range and a non-hyperbolic part in the high force range. The critical force above which the curve deviated from the hyperbola was much smaller for V1 than for V3. An analysis using a model with an extremely small number of myosin heads involved in the bead movement suggested a marked difference in kinetic properties between V1 and V3.

Characean cells as a tool for studying electrophysiological characteristics of plant cells

Characean cells have contributed significantly to various areas of plant cell biology such as cell motility and membrane transport. Since characean cells are very large, various kinds of operations can easily be applied to them. Development of techniques of intracellular perfusion and permeabilization of plasma membrane has facilitated studies on functions of the plasma membrane and the vacuolar membrane (or tonoplast) which is specific to plant cells. The present article is aimed at reviewing the contribution of characean cells to the study of electrophysiological characteristics of plant membranes. Our attention was mainly focused on experiments using plasma membrane-permeabilized cells and intracellularly perfused cells.

The relationship between carbon and water transport in single cells of Chara corallina

The hydraulic resistance of the plasma membrane was measured on single internodal cells of Chara corallina using the method of transcellular osmosis. The hydraulic resistance of the plasma membrane of high CO2-grown cells was significantly higher than the hydraulic resistance of the plasma membrane in low CO2-grown cells. Therefore we tested the possibility that the "bicarbonate transport system", postulated to be present in low CO2-grown cells, serves as a water channel that lowers the hydraulic resistance of the plasma membrane. We were unable to find any correlation between agents that inhibited the "bicarbonate transport system" and agents that increased the hydraulic resistance of low CO2-grown cells. We did, however, find a correlation between the permeability of the cell to water and CO2. We propose that the reduced hydraulic resistance of the plasma membrane of the low CO2-grown cells is a function of a change in either the structural properties of the lipid bilayer or the activity of a CO2 transport protein so that under conditions of reduced inorganic carbon, the plasma membrane becomes more permeable to CO2, and consequently to other small molecules, including H2O, methanol and ethanol.

Depressed sliding velocity of isolated cardiac myosin from cardiomyopathic hamsters: evidence for an alteration in mechanical interaction of actomyosin

We measured the relative sliding velocity of cardiomyopathic hamster cardiac myosin on actin cables by using an in vitro motility assay system. We also investigated the relationship between the velocity and both myosin isozyme content and ATPase activity. Cardiac myosin was obtained from cardiomyopathic hamsters (BIO 14.6; B) aged 3, 6, 9, and 18 months and age-matched controls (F1B; F). Long well-organized actin cables of an alga, Nitellopsis, were used for the motility assay. Small latex beads (2 microns in diameter) were coated with purified cardiac myosin. When myosin-coated beads were introduced into an algal cell in the presence of Mg-ATP, myosin interacted with actin and dragged the beads. Active movement of the beads along the actin cables was observed under a photomicroscope and the velocity was measured. The velocity was significantly lower in B than in F for each age group (0.47 vs. 0.71 microns/s at the age of 3 months, p < 0.05; 0.44 vs. 0.88 microns/s at 6 months, p < 0.01; 0.44 vs. 0.67 microns/s at 9 months, p < 0.01; 0.35 vs. 0.52 microns/s at 18 months, p < 0.05). Both Ca(2+)-activated ATPase activity and the percentage of alpha-myosin heavy chain were also lower in B than in F for each age group. When examined for individual specimens, there was a positive correlation between the velocity and both myosin Ca(2+)-activated ATPase activity (r = 0.84) and percentage of alpha-myosin heavy chain (r = 0.83). These data points of both control and cardiomyopathic hamsters were distributed near the regression line obtained from control and thyroxine-treated rabbits reported previously. The present results indicate that the difference in mechanical properties between control and cardiomyopathic cardiac myosin is attributed to isozyme redistribution and not to a qualitative change in each myosin molecule.

Kinetic properties of the ATP-dependent actin-myosin sliding as revealed by the force-movement assay system with a centrifuge microscope

To study the kinetic properties of the ATP-dependent actin-myosin sliding responsible for muscle contraction, we developed an in vitro force-movement assay system, in which centrifugal forces were applied to myosin-coated polystyrene beads sliding along actin cables of giant algal cells in the presence of ATP. Under constant centrifugal forces directed opposite to the bead movement ("positive" loads), the beads moved with constant velocities. The steady-state force-velocity (P-V) curve thus obtained was double-hyperbolic in shape, being analogous to the P-V curve of single muscle fibers. Under constant centrifugal forces in the direction of the bead movement ("negative" loads), on the other hand, the beads also moved with constant velocities. Unexpectedly, the velocity of bead movement did not increase with increasing negative loads, but decreased markedly (by 20-60%). We also studied the effect of centrifugal forces at right angles with actin cables on the bead movement.

Sliding velocity of isolated rabbit cardiac myosin correlates with isozyme distribution

To investigate the relationship between the mechanical and biochemical properties of cardiac myosin, the sliding velocity of isolated cardiac myosin obtained from both euthyroid and hyperthyroid rabbits on actin cables was measured with an in vitro motility assay system. Ten rabbits (T) were treated with L-thyroxine to induce hyperthyroidism, and eight nontreated animals (N) were used as controls. Myosin was purified from the left ventricles of anesthetized animals. Myosin isozyme content was analyzed by the pyrophosphate gel electrophoresis method, and myosin adenosinetriphosphatase (ATPase) activity was determined on the same sample. Long well-organized actin cables of green algae, Nitellopsis, were used in the in vitro motility assay. Small latex beads were coated with purified cardiac myosin and introduced onto the Nitellopsis actin cables. Active unidirectional movement of the beads on the actin cables was observed under a photomicroscope, and the velocity was measured. The velocity was dependent on ATP concentrations, and the optimal pH for bead movement was approximately 7.0-7.5. The mean velocity was higher in T than in N (0.66 +/- 0.12 vs. 0.32 +/- 0.09 micron/s, P less than 0.01). Both Ca(2+)-activated ATPase activity and the percentage of alpha-myosin heavy chain were also higher in T than in N (0.691 +/- 0.072 vs. 0.335 +/- 0.072 microM Pi.mg-1.min-1, P less than 0.01, and 79 +/- 12 vs. 26 +/- 7%, P less than 0.01, respectively). The velocity of myosin closely correlated with both Ca(+2)-activated myosin ATPase activity (r = 0.87, P less than 0.01) and the percentage of alpha-myosin heavy chain (r = 0.87, P less than 0.01).

A novel regulatory effect of myosin light chain kinase from smooth muscle on the ATP-dependent interaction between actin and myosin

The actin-binding activity of myosin light chain kinase (MLCK) from smooth muscle was studied with special reference to the ATP-dependent interaction between actin and myosin. MLCK in the presence of calmodulin endowed sensitivity to Ca2+ on the movement of actin filaments on phosphorylated myosin from smooth muscle that was fixed on a coverslip. This regulatory effect was not attributable to the kinase activity of MLCK but could be explained by its actin-binding activity. The importance of the actin-binding activity was further substantiated by results of an experiment with Nitellopsis actin-cables in which MLCK regulated the interaction under conditions where MLCK was exclusively associated with the actin-cables.

Active movement of cardiac myosin on Characeae actin cables

The active sliding of cardiac myosin on actin cables was studied using an in vitro movement assay. Cardiac myosin prepared from either adult rabbit or rat hearts was mixed with small latex beads to coat them. Actin cables were obtained from the internodal cells of green algae, Characeae. When the myosin-coated beads suspended in physiological buffer were introduced into the internodal cells, the myosin started to interact with the actin causing the beads to move. The sliding movement of the beads was observed under microscopy and the sliding velocity measured. The observed movement was smooth and the velocity was constant over a long distance. The movement was physiological in nature: a) it was ATP-dependent, but above a certain level of ATP, the velocity was constant; b) the velocity was maximum at pH 7.0, and decreased in both acidic and alkaline conditions. The average sliding velocity of cardiac myosin obtained from rabbit ventricles (0.31 +/- 0.11 micron/s) was slower than that from rat ventricles (1.04 +/- 0.26 micron/s) reflecting the lower ATPase activity of rabbit cardiac myosin. This assay system is considered to be a useful tool linking biochemistry and physiology at the molecular level.

Role of actin in the myosin-linked Ca(2+)-regulation of ATP-dependent interaction between actin and myosin of a lower eukaryote, Physarum polycephalum

Actin-activated ATPase activity of myosin from Physarum polycephalum decreases when it binds Ca2+ and increases when it loses Ca2+. This Ca-inhibition is observed with phosphorylated myosin [Kohama, K. (1990) Trend, Pharmacol. Sci. 11, 433-435]. The activity of dephosphorylated myosin remained at a low level both in the presence and absence of Ca2+, although Ca(2+)-binding ability was much the same as that of the phosphorylated myosin. The effect of phosphorylation has been studied at a conventional actin concentration, which is comparable with that of myosin by weight. When the concentration of actin was increased by 10 times, the dephosphorylated myosin became actin-activatable in the absence of Ca2+, and Ca-inhibition was recovered. As actin exists quite abundantly in non-muscle cells of Physarum, myosin phosphorylation plays virtually no role in regulating actin-myosin-ATP interaction in vivo. Physiologically the interaction may be regulated by Ca2+ by binding to and subsequent release from myosin. Latex beads coated by either phosphorylated or dephosphorylated myosin moved ATP-dependently on the actin cables of Characeae cells to the same extent in the absence of Ca2+, but the movement was abolished by increasing Ca2+. When the interaction was examined by monitoring the movement of actin filaments on myosin fixed on a coverslip, the movement and Ca-inhibition of the movement were detected with phosphorylated, not dephosphorylated, myosin [Okagaki, T., Higashi-Fujime, S., & Kohama, K. (1989) J. Biochem. 106, 955-957].(ABSTRACT TRUNCATED AT 250 WORDS)

Renal and femoral vascular responses to endothelin-1 in dogs: role of prostaglandins

Vascular responses to endothelin were examined with special reference to prostaglandins (PGs). Intrarenal infusion of endothelin (ET)-1 (1-5 ng/kg/min) to dogs caused a transient increase followed by a sustained decrease in renal blood flow, with no change in blood pressure or heart rate. Renal secretion rates of PGE2 and I2 (determined as 6-keto PGF1 alpha) were increased with ET, dose-dependently, and the intrarenal infusion of ET (5 ng/kg/min) elevated the systemic arterial concentration of 6-keto PGF1 alpha from 26 +/- 5 to 83 +/- 14 pg/ml. Because this increase in PG secretion was not affected by the platelet activating factor antagonist, CV 6209, it is unlikely that ET-induced renal PG production was platelet activating factor-mediated. Pretreatment with aspirin abolished completely the increased PG secretion elicited by ET and potentiated the ET-induced reduction renal blood flow. Intrafemoral infusion of ET (5 ng/kg/min) also induced an initial increase followed by a gradual decrease in femoral blood flow, without any increase in PG secretion from the hindlimb. Aspirin had no effects on the femoral hemodynamic action of ET. In addition, initial transient increases in either renal or femoral blood flow by endothelin were not affected by aspirin. Thus, the ET-induced production of renal PGs counteracts the renal vasoconstrictor action of ET, an event in marked contrast to the lack of any apparent involvement of PGs in the femoral hemodynamic action of ET. The ET-induced transient vasodilation shows no apparent relation to the cyclooxygenase products.

Steady-state force-velocity relation in the ATP-dependent sliding movement of myosin-coated beads on actin cables in vitro studied with a centrifuge microscope

To eliminate the gap between the biochemistry of actomyosin in solution and the physiology of contracting muscle, we developed an in vitro force-movement assay system in which the steady-state force-velocity relation in the actin-myosin interaction can be studied. The assay system consists of the internodal cells of an alga, Nitellopsis obtusa, containing well-organized actin filament arrays (actin cables); tosyl-activated polystyrene beads (diameter, 2.8 microns; specific gravity, 1.3) coated with skeletal muscle myosin; and a centrifuge microscope equipped with a stroboscopic light source and a video system. The internodal cell preparation was mounted on the rotor of the centrifuge microscope, so that centrifugal forces were applied to the myosin-coated beads moving along the actin cables in the presence of ATP. Under constant centrifugal forces directed opposite to the bead movement ("positive" loads), the beads continued to move with constant velocities, which decreased with increasing centrifugal forces. The steady-state force-velocity curve thus obtained was analogous to the double-hyperbolic force-velocity curve of single muscle fibers. The unloaded velocity of bead movement was 1.6-3.6 microns/s (20-23 degrees C), while the maximum "isometric" force generated by the myosin molecules on the bead was 1.9-39 pN. If, on the other hand, the beads were subjected to constant centrifugal forces in the direction of bead movement ("negative" loads), the bead also moved with constant velocities. Unexpectedly, the velocity of bead movement did not increase with increasing negative loads but first decreased by 20-60% and then increased towards the initial unloaded velocity until the beads were eventually detached from the actin cables.

Effects of endothelin on renal hemodynamics and renal function in anesthetized dogs

Porcine endothelin was infused directly into the renal artery of anesthetized dogs to evaluate the renal action of endothelin. Endothelin (0.2 to 5.0 ng/kg/min) elicited a dose-dependent reduction in the renal blood blow with no changes in systemic blood pressure. Endothelin infusion (0.2 ng/kg/min) decreased the renal blood flow by 30% and urinary sodium excretion by 50% without any changes in the glomerular filtration rate. However, higher doses did reduce glomerular filtration rate. These data suggest that endothelin preferentially constricts efferent arterioles and that endothelin may enhance the renal reabsorption of sodium.

Endothelin stimulates the renal production of prostaglandin E2 and I2 in anesthetized dogs

The infusion of endothelin into the renal artery of anesthetized dogs (5 ng/kg per min) decreased the renal blood flow without changing the blood pressure, indicating that endothelin caused renal vasoconstriction. The renal secretion rate of prostaglandin E2 and I2 markedly increased and these increases were abolished by pretreatment with aspirin. Furthermore, the renal vasoconstrictor effect of endothelin was potentiated by aspirin, suggesting a role of prostaglandins in the renal action of endothelin.

Mg-ATPase activity and motility of native thick filaments isolated from the anterior byssus retractor muscle of Mytilus edulis

A method for isolating native thick filaments from the anterior byssus retractor muscle (ABRM) of Mytilus edulis is described. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis showed that the isolated thick filament preparation contained mainly paramyosin and myosin but almost no actin. Electron microscopy of negatively stained preparations showed that the isolated thick filaments were tapered at both ends and of various sizes, in the range 5-31 microns in length and 51-94nm in width in the central region. Central bare zones were observed in the smaller filaments, but were not clearly seen in the larger filaments. Mg-ATPase activity of the isolated thick filaments was activated by skeletal muscle F-actin in a Ca2+-dependent manner. The maximal activity was about 20 nmol min-1 mg-1 thick filaments (20 degrees C, pH7.0). Motility of the thick filaments attached to latex beads (diameter, 2 microns) was also studied using the native actin cables of the freshwater alga, Chara. In the presence of Mg-ATP and Ca2+, the beads moved along the actin cables at a maximal velocity of about 1 micron s-1. In the absence of Ca2+, almost no movement was observed. These results show that the isolated thick filaments are structurally intact and retain the essential mechanochemical characteristics of the ABRM myosin.

Simultaneous recordings of force and sliding movement between a myosin-coated glass microneedle and actin cables in vitro

To elucidate the molecular mechanism of muscle contraction resulting from the ATP-dependent actin-myosin interaction, we constructed an assay system with which both the force and the movement produced by the actin-myosin interaction in vitro can be simultaneously recorded and analyzed. The assay system consisted of the giant internodal cells of an alga, Nitellopsis obtusa, which contain well-organized arrays of actin filaments (actin cables) running along the cell long axis, and a glass microneedle (tip diameter, approximately 7 microns; elastic coefficient, approximately 40 pN/microns), which was coated with skeletal muscle myosin at the tip and extended from a micromanipulator at right angles with the actin cables. When the myosin-coated tip of the microneedle was brought into contact with the exposed surface of the actin cables, it exhibited ATP-dependent movement along the actin cables over a distance of 20-150 microns in 20-200 s (20-23 degrees C) and eventually stopped due to a balance between forces generated by the actin-myosin interaction (800-6000 pN) and by the bent microneedle. Since the load on the force-generating myosin molecules increased with the bending displacement of the microneedle (auxotonic condition), the relation between the load and the sliding velocity of the myosin heads past the actin cables was determined from the time course of the microneedle movement recorded with a video system. The shape of the force-velocity curve thus obtained was convex upwards, similar to that of the force-velocity curve of intact frog muscle fibers obtained under the auxotonic condition.

Accelerated sliding of pollen tube organelles along Characeae actin bundles regulated by Ca2+

Pollen tubes show active cytoplasmic streaming. We isolated organelles from pollen tubes and tested their ability to slide along actin bundles in characean cell models. Here, we show that sliding of organelles was ATP-dependent and that motility was lost after N-ethylmaleimide or heat treatment of organelles. On the other hand, cytoplasmic streaming in pollen tube was inhibited by either N-ethylmaleimide or heat treatment. These results strongly indicate that cytoplasmic streaming in pollen tubes is supported by the "actomyosin"-ATP system. The velocity of organelle movement along characean actin bundles was much higher than that of the native streaming in pollen tubes. We suggested that pollen tube "myosin" has a capacity to move at a velocity of the same order of magnitude as that of characean myosin. Moreover, the motility was high at Ca2+ concentrations lower than 0.18 microM (pCa 6.8) but was inhibited at concentration higher than 4.5 microM (pCa 5.4). In conclusion, cytoplasmic streaming in pollen tubes is suggested to be regulated by Ca2+ through "myosin" inactivation.

Adenine-nucleotide levels and metabolism-dependent membrane potential in cells of Nitellopsis obtusa Groves

The relationship between adenine-nucleotide levels and metabolism-dependent membrane potential was studied in cells of Nitellopsis obtusa. Effects of ADP and AMP in the presence of ATP on electrogenic pump activity were measured in the dark, using the continuous perfusion method. Both ADP and AMP acte as competitive inhibitors for ATP, the Ki value for either compound being about 0.4 mM. The role of ADP and AMP as regulating factors for the electrogenic pump was investigated under various metabolic conditions. Application of N2 gas in the dark caused a significant membrane depolarization amounting to 90 mV, but cytoplasmic streaming and membrane excitability were not affected. Under anoxia, the ATP level decreased from 1.6 to 0.5 mM; ADP increased but only slightly, and AMP increased greatly. However, the time course of changes in the adenine nucleotides was not concurrent with that of the membrane-potential changes, thus, the adenine-nucleotide level changes cannot fully account for the N2-elicited depolarization. Under light, although the membrane hyperpolarized, no significant changes in the adenine-nucleotide levels were observed. Therefore, the light-induced membrane hyperpolarization cannot be explained solely by changes in adenine-nucleotide levels.

Dependence of the membrane potential on intracellular ATP concentration in tonoplast-free cells of Nitellopsis obtusa

The membrane potential of tonoplast-free cells of Nitellopsis obtusa Graves in relation to the intracellulcar concentration of ATP ([ATP])i was measured using either the ordinary microelectrode method or the open-vacuole method (M. Tazawa, M. Kikuyama and S. Nakagawa, 1975, Plant Cell Physiol. 16, 611). The intracellular ATP concentration was modified in the microelectrode method by introducing into the cell ATP-regenerating media composed of phosphoenolpyruvate and pyruvate kinase, and in the open-vacuole method by continuously perfusing the cell interior with media of known ATP concentrations. Plots of the membrane potential against the [ATP]i follow a rectangular hyperbola. Using the microelectrode method, the maximum ATP-dependent potential was about-120-130 mV and the apparent K m about 10-30 μM. When the openvacuole method was used, the maximum ATP-dependent potential was about 100 mV and the apparent K m about 100 μM. The membrane was still excitable when the [ATP]i was 10 μM but not at 1.7 μM [ATP]i. The membrane resistance increased in parallel with a decrease in [ATP]i or membrane depolarization, but decreased again at a very low [ATP]i (1.7 μM).

Direct demonstration of the involvement of chloroplasts in the rapid light-induced potential change in tonoplast-free cells of Chara australis. Replacement of Chara chloroplasts with spinach chloroplasts

To directly demonstrate the involvement of chloroplasts in the rapid light-induced potential change we removed chloroplasts by centrifuging tonoplast-free cells of Chara australis. Chloroplast-free cells showed no signs of a potential change on illumination. When a chloroplast-free Chara cell was perfused with a suspension of chloroplasts isolated from other cells of Chara australis, it regained its ability to generate the rapid potential change induced by light. Chloroplasts were isolated from spinach leaves by a method which preserves the chloroplast envelope; thus ensuring the ability to assimilate CO2. When these chloroplasts were introduced into a transparent Chara cell containing no chloroplasts, normal rapid hyperpolarization was induced on illumination. When spinach chloroplasts were treated with a medium of very low osmotic potential, no O2 evolution was detected, but even with these chloroplasts rapid light-induced hyperpolarization was observed. We concluded that chloroplasts are essential for the rapid light-induced potential change and that chloroplasts can be replaced with those of another species, so far as the light-induced potential change is concerned.

Demonstration of the K+ channel in the plasmalemma of tonoplast-free cells of Chara australis

Tonoplast-free and ATP-less internodal cells of Chara australis were made by perfusing cells internally with a medium containing ethyleneglycol-bis-(β-aminoethylether) N, N'-tetraacetic acid, hexokinase and glucose. Such a cell showed rapid light-induced potential change when it was stained with neutral red (5). The rapid light-induced potential change was inhibited by tetraethylammonium, a known inhibitor of the K(+) channel in nerve cells. Tetraethylammonium acted both from the inside of the membrane and from the outside with similar effectiveness. Nonyltriethylammonium inhibited the light response far more effectively than did tetraethylammonium. The results suggest the existence of a K(+) channel which is activated by light in the Chara membrane.

Dependence of the membrane potential of Chara cells on external pH in the presence or absence of internal adenosinetriphosphate

The dependence of the membrane potential (Em) and the membrane resistance (Rm) of Chara australis R. Brown on the pH of the external medium (pH0) was studied by controlling the activity of the plasmamembrane H(+) pump under both light and dark conditions. The activity of the pump was controlled by regulating the internal ATP or Mg(2+) concentration in tonoplast-free cells prepared by vacuolar perfusion. In these cells, which contained Mg · ATP (mgATP cells), Em and Rm were very sensitive to pH0, as in normal cells. Em was more negative in light than in the dark at all pH0 values tested. Tonoplast-free cells with very low [ATP]i (-ATP cells) or [Mg(2+)]i (-Mg cells) showed very weak dependence of Em and Rm on pH0. Thus, the active and not the passive component of Em was sensitive to pH0. At the same time, the high permeability of the plasma membrane to H(+) was questioned. In both-ATP cells and-Mg cells, Em was scarcely affected and Rm markedly decreased on illumination.