Unchanged Molecular-Weight Distribution of Xyloglucans in Outer Tissue Cell Walls along Intact Growing Hypocotyls of Squash (Cucurbita maxima Duch.) Seedlings

The changes in the mechanical properties and compositions ofcell walls in outer and inner tissues were investigated alongthe hypocotyls of squash (Cucurbita maxima Duch.) seedlings.The endogenous growth capacity decreased and the minimum stress-relaxationtime (T_O) of cell walls in outer tissues increased from theapical to the basal region of hypocotyls. A high correlationwas observed between values of To in outer tissues and endogenousgrowth (r=–0.99). The values of T_O in inner tissues didnot change from the apical to the basal region of hypocotyls. In outer tissues, the levels of neutral sugars in pectin decreasedconsiderably from the apical to the basal region of hypocotyls.However, relative amounts of hemicellulose showed little differencealong the hypocotyls. Levels and molecular weights of hemicellulosicxyloglucans in outer tissues were about 2-3 times greater thanthose in inner tissues. The amount of xyloglucans in outer tissuesincreased in the middle region of hypocotyls, and xyloglucansin upper and basal regions had similar molecular weights. Bycontrast, in inner tissues, amounts of cell-wall material decreasedtoward the basal region. Amounts and molecular weights of hemicellulosicxyloglucans also decreased along the hypocotyls. These results clearly show that cell-wall metabolism duringaging of intact growing stem tissues differs markedly betweenouter and inner tissues, and the absence of a simple relationship between the molecular weights of xyloglucans and the mechanicalproperties of the cell walls in outer tissues indicates thatthe changes in the mechanical properties of the cell walls inintact growing tissues cannot be explained only by the molecularweights of xyloglucans. Thus, the regulation of the mechanicalproperties of cell walls in intact growing stems may be somewhatdifferent from that in auxin-treated stem sections, in whichauxin promotes the depolymerization of xyloglucan molecules. (Received November 28, 1991; Accepted November 16, 1992)     

Cytochemical Studies on Phytochrome-Mediated Changes of Ca2+ Localization in Etiolated Oat Coleoptile Cells

The antimonate-staining procedure and X-ray microanalysis techniquewere used to determine the pattern of Ca₂₊ localization in etiolatedoat (Avena sativa L.) coleoptile parenchyma cells. Precipitatesof calcium antimonate, indicating the presence of Ca₂₊ and confirmedby X-ray microanalysis, were found associated with the outerand inner surfaces of the plasma membrane of cells of dark-grownseedlings. After exposure of seedlings to red light, precipitatesof calcium antimonate were additionally observed in cisternaeof the endoplasmic reticulum. In the cells of oat coleoptilesexposed to red light and then followed immediately by farredlight, Ca₂₊ was observed on the outside of the plasma membrane,in cell walls and in the vacuoles. The results suggest thatphytochrome mediates the regulation of the intracellular Ca₂₊localization. Key words: Antimonate procedure, Avena sativa L., Ca²⁺ (localization), phytochrome, X-ray microanalysis     

Pectic epitopes are differentially distributed in the cell walls of potato (Solanum tuberosum) tubers

Six monoclonal antibodies (mAbs) were used to map the distribution of pectic epitopes in the cell walls of potato ( Solanum tuberosum L. cvs Kardal and Karnico) tuber tissue in both light and electron microscopes. Unesterified (mAb JIM 5 epitope) and methyl-esterified (mAb JIM 7 epitope) pectins were abundant and equally distributed in all parenchymal and vascular cell walls. Homogalacturonans (HGAs) involved in Ca²⁺-cross-linking (mAb 2F4 epitope) were localised to the middle lamella and abundant at cell corners. The tuber cortex was densely labelled, but parenchymal cell walls in the perimedullary region contained few epitopes of calcium pectate except at corners and pit fields. In contrast, pectic side-chains were not detectable in the middle lamella of all parenchymal cell walls, except in the cortex where mAb LM6 (arabinan epitope) labelled the entire wall. The galactan epitope (mAb LM5) was localised to a zone very close to the plasmalemma in cortical cell walls and was also less abundant at pit fields and in vascular cell walls. MAb CCRC-M2 (rhamnogalacturonan I epitope) did not cross-react. Our results show that the cell walls of potato tubers are not homogeneous structures and that the pectic composition of the walls is spatially regulated.     

Properties of extracellular polysaccharides of potato ring rot pathogen and corresponding recognition sites of potato cell walls

The properties of extracellular polysaccharides of the potato ring rot pathogen Clavibacter michiganensis subsp. sepedonicus (Cms) and the corresponding recognition sites isolated from cell walls of potato suspension cultures have been studied. Extracellular polysaccharides of Cms consist of 4-6 components, which differ greatly in molecular mass (from <1 kD to >700 kD), and are capable of formation of associates stabilized by electrostatic interactions in the presence of calcium. Using affinity column chromatography, sites possessing affinity for the total extracellular polysaccharide complex of Cms were isolated from cell walls of suspension cultures of three potato varieties with different resistance to the pathogen. The content of the receptor sites consisting of glycopeptides and sugars for the variety devoid of resistance was 10 times greater than that for the resistant variety. In the receptor fraction for the latter variety, only sugars were found. The molecular masses of the components of the receptor fraction of cell walls were from 39 to 86 kD. Polypeptides in the recognition sites for the resistant variety escaped detection in electrophoretic patterns. Study of the amino acid composition of the receptor sites of cell walls showed that the sites of the resistant variety contained trace amounts of only seven amino acids. In the sites of the receptive variety 14 amino acids were found, the content of polar amino acids being twice as large as that of nonpolar amino acids. Among polar amino acids, glutamine and glycine prevailed, whereas among nonpolar amino acids valine was dominant. We suggest that one of the reasons of variety-specific resistance of potato to Cms is the absence or a low content of the sites revealing the affinity for bacterial extracellular polysaccharides on the plant cell surface.     

Evidence for in vitro binding of pectin side chains to cellulose

Pectins of varying structures were tested for their ability to interact with cellulose in comparison to the well-known adsorption of xyloglucan. Our results reveal that sugar beet (Beta vulgaris) and potato (Solanum tuberosum) pectins, which are rich in neutral sugar side chains, can bind in vitro to cellulose. The extent of binding varies with respect to the nature and structure of the side chains. Additionally, branched arabinans (Br-Arabinans) or debranched arabinans (Deb-Arabinans; isolated from sugar beet) and galactans (isolated from potato) were shown bind to cellulose microfibrils. The adsorption of Br-Arabinan and galactan was lower than that of Deb-Arabinan. The maximum adsorption affinity of Deb-Arabinan to cellulose was comparable to that of xyloglucan. The study of sugar beet and potato alkali-treated cell walls supports the hypothesis of pectin-cellulose interaction. Natural composites enriched in arabinans or galactans and cellulose were recovered. The binding of pectins to cellulose microfibrils may be of considerable significance in the modeling of primary cell walls of plants as well as in the process of cell wall assembly.     

The Effect of pH on the Binding of Calcium to Pea Epicotyl Cell Walls and its Implications for the Control of Cell Extension

Cell walls were prepared from the epicotyls of dark-grown pea(Pisum sativum L.) seedlings. The walls were found to bind externally-added⁴⁵Ca²⁺, with a binding constant of 4 ? 10^–4 mol dm^–3and a maximum capacity of 1.5 ? 10^–8 g-ions of Ca²⁺ perg fresh weight of epicotyl. The binding capacity decreased asthe pH of the medium was decreased below 6.0, suggesting thatthe calcium was bound by an anionic group with an apparent pKof 4.7. More than half the calcium binding was due to polygalacturonicacid in the wall, since up to 60% of the calcium binding capacitywas removed by pre-incubation of the cell walls with polygalacturonase(E.C.3.2.1.15). Only small decreases in calcium binding wereseen following pre-incubation with protease, nucleases, phospholipaseand hemicellulase. These results indicate that calcium willbe displaced from the cell wall at hydrogen ion concentrationswhich are known to occur in the wall during wall extension.They are consistent with a mechanism by which calcium inhibitswall extension by forming ionic bridges between polygalacturonicacid molecules, and also with the hypothesis that calcium andhydrogen ions exert opposing influences on cell wall extensionby competing for the same binding sites on the polygalacturonicacid. Key words: Pea epicotyl, Cell wall, Calcium, pH     

Characterization of the Sclerotinia sclerotiorum cell wall proteome

We used a proteomic analysis to identify cell wall proteins released from Sclerotinia sclerotiorum hyphal and sclerotial cell walls via a trifluoromethanesulfonic acid (TFMS) digestion. Cell walls from hyphae grown in Vogel's glucose medium (a synthetic medium lacking plant materials), from hyphae grown in potato dextrose broth and from sclerotia produced on potato dextrose agar were used in the analysis. Under the conditions used, TFMS digests the glycosidic linkages in the cell walls to release intact cell wall proteins. The analysis identified 24 glycosylphosphatidylinositol (GPI)‐anchored cell wall proteins and 30 non‐GPI‐anchored cell wall proteins. We found that the cell walls contained an array of cell wall biosynthetic enzymes similar to those found in the cell walls of other fungi. When comparing the proteins in hyphal cell walls grown in potato dextrose broth with those in hyphal cell walls grown in the absence of plant material, it was found that a core group of cell wall biosynthetic proteins and some proteins associated with pathogenicity (secreted cellulases, pectin lyases, glucosidases and proteases) were expressed in both types of hyphae. The hyphae grown in potato dextrose broth contained a number of additional proteins (laccases, oxalate decarboxylase, peroxidase, polysaccharide deacetylase and several proteins unique to Sclerotinia and Botrytis) that might facilitate growth on a plant host. A comparison of the proteins in the sclerotial cell wall with the proteins in the hyphal cell wall demonstrated that sclerotia formation is not marked by a major shift in the composition of cell wall protein. We found that the S. sclerotiorum cell walls contained 11 cell wall proteins that were encoded only in Sclerotinia and Botrytis genomes.     

Autolysis Promotes the Extension Capacity of Zea mays Coleoptile Cell Walls in Response to Acid pH Solutions

The relationship between autolytic degradation of ß(1–3),(1–4)-D-glucanand acid pH-induced extension of isolated Zea mays cell wallshas been investigated using a constant-load extension technique.Acidic buffer (4.5) was able to induce an additional extension(E_a) on cell walls already extended at pH 6.8 buffer under a20 g-mass load, indicating that the additional extension (E_a)was the parameter that better represented the effect of thedifferent treatments on the mechanical properties of maize coleoptilecell walls. The additional extension in response to acidic pHwas higher when cell walls had been previously autolysed for24 h at pH 5.5. Furthermore, the acid-pH effect was dependenton the presence during the constant load extension of some thermo-labilefactors, suggesting the participation of expansins. Acid pHincreased E_a of native cell walls through an increase in theplastic extension (E_p) in agreement with a one step mechanismleading directly to irreversible (plastic) wall extension assuggested by Cosgrove (1977). The autolytic degradation of ß(1–3),(1–4)-D-glucan was also able to modify the mechanicalproperties of maize coleoptile cell walls increasing its elasticextension (E_e) in response to pH 4.5 buffer but that modificationonly leads to an increase in wall extension when expansins areactive, suggesting a cooperation between ß-glucanturnover and expansin action. (Received August 5, 1998; Accepted March 16, 1999)     

Interaction of Pseudomonas solanacearum Lipopolysaccharide and Extracellular Polysaccharide with Agglutinin from Potato Tubers

In vitro binding assays were used to study the possible role of a cell wall agglutinin in the attachment to plant cell walls of avirulent strains of the wilt pathogen, Pseudomonas solanacearum. In a nitrocellulose filter assay, radioactively labeled lipopolysaccharide (LPS) from the virulent strain, K60, and the avirulent strain, B1, and extracellular polysaccharide (EPS) from K60 were bound quantitatively by the agglutinin extracted from Katahdin potato tubers. The LPS from B1 had significantly greater agglutinin-binding affinity than that from K60 but not after treatment with deoxycholate, which improved solubility. Highly purified chitotetraose did not inhibit binding of K60 LPS to agglutinin, but binding was inhibited by EPS as well as by diverse anionic polymers (DNA, dextran sulfate, xanthan). Binding of agglutinin to EPS and LPS was inhibited at ionic strengths greater than 0.03 and 0.15 M, respectively. It was concluded that electrostatic charge-charge interactions could account for binding of LPS and EPS to potato agglutinin.     

Expression of a Petunia inflata pectin methyl esterase in Solanum tuberosum L. enhances stem elongation and modifies cation distribution

Transgenic potato (Solanum tuberosum L.) plants were constructed with a Petunia inflata-derived cDNA encoding a pectin methyl esterase (PME; EC 3.1.1.11) in sense orientation under the control of the cauliflower mosaic virus 35S promoter. The PME activity was elevated in leaves and tubers of the transgenic lines but slightly reduced in apical segments of stems from mature plants. Stem segments from the base of juvenile PME-overexpressing plants did not differ in PME activity from the control, whereas in apical parts PME was less active than in the wild-type. During the early stages of development stems of these trangenic plants elongated more rapidly than those of the wild-type. Further evidence that overexpression of a plant-derived PME has an impact on plant development is based on modifications of tuber yield, which was reduced in the transgenic lines. Cell walls from transgenic tubers showed significant differences in their cation-binding properties in comparison with the wild-type. In particular, cell walls displayed increased affinity for sodium and calcium, while potassium binding was constant. Furthermore, the total ion content of transgenic potatoes was modified. Indications of PME-mediated differences in the distribution of ions in transgenic plants were also obtained by monitoring relaxations of the membrane potential of roots subsequent to changes in the ionic composition of the bathing solution. However, no effects on the chemical structure of pectin from tuber cell walls could be detected. Received: 24 March 1999 / Accepted: 20 August 1999     

Polygalacturonase isozyme 2 binding and catalysis in cell walls from tomato fruit: pH and β-subunit effects

The catalytic activity of endopolygalacturonase (PG, EC 3.2.1.15) against pectic polymers in vitro is typically not expressed in vivo. In the present study, the binding and catalytic properties of PG isozyme 2 and the influence of the β-subunit protein were investigated in cell walls prepared from tomato fruit expressing an antisense gene to the β-subunit protein. Cell walls prepared from mature-green fruit were employed for binding and assay of PG2. Walls were provided with rate-limiting quantities of purified PG2 and incubated at 100 mM KCl, pH 4.5, or 25 mM KCl, pH 6.0. Cell walls of both β-subunit antisense and wild-type fruit retained comparable quantities of added PG2. The release of pectin from PG2-loaded walls was proportional to the quantity of added enzyme, consistent with a finite catalytic capacity of individual PG proteins. β-Subunit-antisense cell walls released 2- to 3-fold higher levels of pectin in response to PG2 than did wild-type walls. Cell walls incubated at pH 6.0 released lower quantities and showed less extensive depolymerization of pectins than did walls incubated at pH 4.5. Pectins recovered from ripe fruit were similar in size distribution to polymers released by PG2 at pH 6.0, indicating that pH can influence both quantitative and qualitative aspects of pectin metabolism and may be responsible for the restricted hydrolysis of pectins in vivo. Molecular mass differences were not evident in the polymers rendered freely soluble in response to PG2-mediated hydrolysis of β-subunit-antisense compared with wild-type cell walls. The solubilization of pectin from cell walls was not the sole indicator of the extent of PG-mediated cell wall hydrolysis. Hydrolytic modifications were also evident in a pectic fraction extracted from postcatalytic cell walls with 50 mM CDTA (trans-1,2-cyclohexanediamine-N,N,N′,N′-tetraacetic acid), and were more extensive for the β-subunit-antisense cell walls compared with the wild-type walls. Pectic polymers derived from ethanol insoluble-powders showed molecular mass downshifts during ripening but differences between the β-subunit-antisense and wild-type fruits were not observed.     

Biosynthesis of furano-terpenes by sweet potato cell culture

Callus was induced from sweet potato root tissue on an agarmedium containing Heller's minerals, vitamins, 2,4-D, yeastextract and sucrose. Furano-terpenes were scarcely detectedin the callus. However, when the callus was transferred to aliquid culture medium and incubated with reciprocal shaking,furano-terpenes were rapidly produced mainly in the culturemedium. Furano-terpene production by the cell culture was suppressedby addition of Ceratocystis fimbriata spores or HgCl₂ to theculture medium. Yeast extract and sucrose in the culture mediumwere important for furano-terpene production. 3-Hydroxy-3-methylglutarylcoenzyme A (HMG-CoA) reductase activity increased in the cells,followed by the production of furano-terpenes. The TLC patternof furano-terpenes produced by the cell culture was essentiallythe same as that produced by sweet potato root tissue infectedby C. fimbriata or treated with HgCl₂, but the quantitativeproportion of the individual furano-terpenes in the former differedmarkedly from that in the latter. (Received January 11, 1979; )     

Development of Epidermal Cell Wall Peroxidases along the Mung Bean Hypocotyl: Possible Involvement in the Cell Wall Stiffening Process

Goldberg, R., Liberman, M., Mathieu, C, Pierron, M. and Catesson,A. M. 1987. Development of epidermal cell wall peroxidases alongthe mung bean hypocotyl: possible involvement in the cell wallstiffening process.—J. exp. Bot. 38: 1378–1390. Ultrastructural investigation showed that in the epidermis ofmung bean hypocotyls, cell wall peroxidatic activities couldbe detected mainly below the maximal elongation zone. In theepidermis the peroxidatic activities were preferentially locatedin the radial cell walls. Cell wall peroxidases were then isolatedfrom epidermal strips and further characterized. The possiblepresence of a H₂O₂-generating system in the epidermis of mungbean hypocotyls was also investigated. When whole segments wereprocessed for electron microscopy, H₂O₂ could be detected cytochemicallyin the cell walls with the CeCl₃ technique. A positive reactionwas obtained in the same location when specimens were incubatedin a 3-3'-diaminobenzidine medium for peroxidases in which H₂O₂was replaced by its possible precursors (NADH or NAD + malate).However, isolated epidermal cell walls could not generate H₂O₂at the expense of NADH although they were able to oxidize thereduced nicotinamide-adenine-dinucleotide. The possible relationshipsbetween peroxidase activities, H₂O₂, and Ca²⁺ ions are discussedwith respect to their involvement in the cell wall stiffeningprocess. Key words: Epidermis, cell wall, elongation, peroxidases     

Crystalline Cellulose in Hydrated Primary Cell Walls of Three Monocotyledons and One Dicotyledon

The molecular ordering of cellulose, including its crystallinity,in the unlignified primary cell walls of three monocotyledons(Italian ryegrass, pineapple, and onion) and one dicotyledon(cabbage) was characterized by solid-state ¹³C NMR spectroscopy.These species were chosen because their primary cell walls havedifferent non-cellulosic polysaccharides and this may affectthe molecular ordering of cellulose. Values of the proton rotating-framerelaxation [T_1p(H)] and spin-spin relaxation [T₂(H)] time constantsshowed that the cellulose in the cell walls of all four specieswas in a crystalline rather than an amorphous state. Furthermore,a resolution enhancement procedure showed that the triclinic(I) and the monoclinic (I_rß) crystal forms of cellulosewere present in similar proportions in these cell walls. However,the calculated cross-sectional dimensions of the cellulose crystallitesvaried among the cell walls (in the range 2–3 nm): thelargest were in the Italian ryegrass, the smallest were in theonion and cabbage, and those of intermediate size were in thepineapple. The crystallite dimensions may thus be affected bythe non-cellulosic polysaccha-ride compositions of the cellwalls. ⁴Present address: Food Science Postgraduate Programme, Departmentof Chemistry, The University of Auckland, Private Bag 92019,Auckland, New Zealand.     

Crown-gall tumors possess tumor-specific antigenic sites on their cell walls

Rabbits were injected with cell walls obtained from crown-gall tumor tissue or the corresponding cell walls from normal potato tissue. The serum obtained from rabbits 53 days after they were injected with tumor cell walls contained immunoglobins that reacted with both tumor and normal cell walls as well as with the cells from the inciting strain of Agrobacterium tumefaciens. When this serum was repeatedly absorbed against normal cell walls and the cells of the inciting strain of Agrobacterium tumefaciens, only tumor-specific immunoglobins remained. These immunoglobins did not react with cell walls obtained from meristematic (nontumorous) potato tissue. Yet this same serum reacted with crown-gall tumor cell walls obtained from turnip and carrot discs.     

Direct observation of the structure of mycelial cell walls from the potato blight fungus Phytophthora infestans by solid-state 13C NMR

Mycelial cell walls from the potato blight fungus ( Phytophthora infestans ) (Mont.) de Bary were examined in the solid state by ¹³C nuclear magnetic resonance spectroscopy with cross-polarization and magic-angle spinning. The spectrum was free from interference by spinning sidebands. The main component of the cell walls had the spectral properties of a β -(1,3')-glucan. Protein appeared to be present also. The presence of β -(1,6')-linked glucose residues, cellulose or chitin was not ruled out, but there was no evidence for these as major components of the cell walls.     

Stress relaxation properties of the cell wall of growing intact plants

The cell wall of dark-grown Avena coleoptiles and the epidermisof light-grown mungbean hypocotyls was subjected to stress-relaxationanalysis and the following results were obtained. 1. Actively growing apical regions of the organs, either coleoptilesor hypocotyls, had certain threshold values of minimum stress-relaxationtime, T_O, 0.04 sec for coleoptile cell wall and 0.03 sec forthe epidermal cell wall of hypocotyls. The cell wall of thebasal region of the organs, which were mature and not growing,had a higher value of To. 2. When the apical regions of the organs, either coleoptilesor hypocotyls, ceased to grow, their cell walls showed T_O valuesabove these thresholds. 3. The relaxation rate, b, was small in the cell wall of activelygrowing regions of the organs, compared with that of non-growingregions. 4. The maximum relaxation time, T_m, was variable and no significantrelationship with growth capacity was found. 5. The extensibility, mm/gr, was large not only in activelygrowing regions of the organs but also in fully grown regions,suggesting that the value represents complex properties of thecell wall including the history of cell wall extension. From these results, we concluded that biochemical modificationsoccur in the cell wall matrix of actively growing organs ofeither monocots or dicots, and these are the bases of the capacityof the cell wall to extend and are represented chiefly by T_oand possibly by b. (Received August 12, 1974; )     

敲减人MCHR2基因抑制CHO细胞的放射性 配体受体结合能力及Ca 2+释放

研究人黑色素浓集激素受体2(MCHR2)基因特异的小发夹RNA(shRNA)真核表达载体pGenesil-1-MCHR2-shRNA对MCHR2表达及特征的影响.将pGenesil-1-MCHR2-shRNA转染到稳定表达人MCHR2基因的CHO细胞中,通过RT-PCR和 Western印迹检测MCHR2表达的变化;放射性配体结合实验（RBA）检测受体最大结合容量B _max及平衡解离常数K_d值的变化;钙流检测实验观察配体MCH刺激后单个细胞Ca ²⁺释放及MCH半数有效浓度EC₅₀的变化.与转染pGenesil-1空载体组比较,pGenesil-1-MCHR2-shRNA 能使MCHR2基因mRNA表达减少45.8％～66.4％;蛋白表达减少44.2％～81.0％; B _max减少39.4％～78.7％,K_d值升高40.9％～81.9％;EC ₅₀升高114.8％～822.4％.MCHR2基因shRNA真核表达载体能有效抑制MCHR2基因的表达,减少B_max、升高K_d值及EC ₅₀,从而对MCHR2生物活性等特征产生影响.     

Growth Delay and Intracellular Changes in Chlorella ellipsoidea C-27 as a Result of Deuteration

The effects of deuterium (D) on Chlorella ellipsoidea C-27 wereinvestigated. Cells grown in a medium prepared with deuteriumoxide (D₂O) showed pronounced delays in cell growth and division;the length of a cell cycle in medium with 100 mol% D₂O was morethan 5 times longer than that in medium prepared in H₂O Thedelay caused by D₂O was not overcome by either indoleaceticacid or kinetin. The biological and ultrastractural characteristicsof deuterated .Chlorella (D-Chlorella) cells were examined.The responses of D-Chlorella to cell wall-digesting enzymesdid not differ from those of normal (H-Chlorella) cells. D-Chlorellacells were enlarged, and cellular components, such as proteins,nucleic acids, lipids and ATP, were present in larger quantitiesthan those in H-cells. The chloroplast of D-Chlorella was enlarged,but the levels of component photosynthetic pigments were significantlyreduced. By contrast, mitochondria of D-Chlorella were smallerthan those of H-cells. These changes in levels of cellular componentsand in the sizes of organelles seem to be unique to deuteration. (Received May 13, 1992; Accepted July 28, 1992)     

Modeling action potential generation and propagation in NRK fibroblasts

Normal rat kidney (NRK) fibroblasts change their excitability properties through the various stages of cell proliferation. The present mathematical model has been developed to explain excitability of quiescent (serum deprived) NRK cells. It includes as cell membrane components, on the basis of patch-clamp experiments, an inwardly rectifying potassium conductance (G_Kir), an L-type calcium conductance (G_CaL), a leak conductance (G_leak), an intracellular calcium-activated chloride conductance [G_Cl(Ca)], and a gap junctional conductance (G_gj), coupling neighboring cells in a hexagonal pattern. This membrane model has been extended with simple intracellular calcium dynamics resulting from calcium entry via G_CaL channels, intracellular buffering, and calcium extrusion. It reproduces excitability of single NRK cells and cell clusters and intercellular action potential (AP) propagation in NRK cell monolayers. Excitation can be evoked by electrical stimulation, external potassium-induced depolarization, or hormone-induced intracellular calcium release. Analysis shows the roles of the various ion channels in the ultralong (30 s) NRK cell AP and reveals the particular role of intracellular calcium dynamics in this AP. We support our earlier conclusion (De Roos A, Willems PH, van Zoelen EJ, and Theuvenet AP. Am J Physiol Cell Physiol 273: C1900–C1907, 1997) that AP generation and propagation may act as a rapid mechanism for the propagation of intracellular calcium waves, thus contributing to fast intercellular calcium signaling. The present model serves as a starting point to further analyze excitability changes during contact inhibition and cell transformation. Hodgkin-Huxley model; intracellular calcium dynamics; L-type calcium conductance; inward rectifier; calcium-activated chloride conductance; gap junctional coupling