Permeability of a cell membrane junction. Dependence on energy metabolism

Laminaria japonica Extract, an Inhibitor of Clavibater michiganense Subsp. Sepedonicum

Bacterial ring rot of potato is one of the most serious potato plant and tuber diseases. Laminaria japonica extract was investigated for its antimicrobial activity against Clavibater michiganense subsp. sepedonicum (Spieckermann & Kotthoff) Davis et al., the causative agent of bacterial ring rot of potato. The results showed that the optimum extraction conditions of antimicrobial substances from L. japonica were an extraction temperature of 80°C, an extraction time of 12 h, and a solid to liquid ratio of 1∶25. Active compounds of L. japonica were isolated by solvent partition, thin layer chromatography (TLC) and column chromatography. All nineteen fractionations had antimicrobial activities against C. michiganense subsp. sepedonicum, while Fractionation three (Fr.3) had the highest (P<0.05) antimicrobial activity. Chemical composition analysis identified a total of 26 components in Fr.3. The main constituents of Fr.3 were alkanes (80.97%), esters (5.24%), acids (4.87%) and alcohols (2.21%). Antimicrobial activity of Fr.3 against C. michiganense subsp. sepedonicum could be attributed to its ability to damage the cell wall and cell membrane, induce the production of reactive oxygen species (ROS), increase cytosolic Ca2+ concentration, inhibit the glycolytic pathway (EMP) and tricarboxylic acid (TCA) cycle, inhibit protein and nucleic acid synthesis, and disrupt the normal cycle of DNA replication. These findings indicate that L. japonica extracts have potential for inhibiting C. michiganense subsp. sepedonicum.

Electrotonic spread of current in monolayer cultures of neonatal rat heart cells

The passive electrical properties of neonatal rat heart cells grown in monolayer cultures were determined. Hyperpolarizing current pulses were injected through one microelectrode via an active bridge circuit. Membrane voltage displacements caused by the injected current pulses were measured at various distances from the first with a second microelectrode. Using a modified least-squares method the experimental results were fitted to a Bessel function, which is the steady-state solution of the differential equation describing the relation between membrane voltage caused by current injection and interelectrode distance in a very large and very thin plane cell. Best fit was obtained with a space constant of 360 μm and an internal resistivity of 500 Ω cm. From these figures, specific membrane resistance was calculated to be 1,300 Ω cm(2), assuming all current to leave through the upper surface of the monolayer.The time constant of the membrane was measured from the time course of the current-induced membrane voltage displacements. From its value of 1.7 msec a membrane capacity of 1.3 μF/cm(2) was calculated.From these results and some literature data on nexus distribution (A. W. Spira,J. Ultrastruct. Res. 34:409, 1971) specific nexus resistance was calculated to range between 0.25 and 1.25 Ω cm(2), depending on the amount of folding of the intercalated discs. The results suggest that spread of activation in monolayer cultures of heart cells by means of local circuit currents is very likely.

Intercellular communication and some structural aspects of membrane junctions in a simple cell system

AChironomus salivary gland consists of a chain of 30 giant cells (G-cells) and 4 to 6 flat cells (F-cells) spanning a lumen. The surface membranes of these cells are linked by two kinds of organized structures: theseptate junction, taking up nearly the entire surface of cell contact, and thegap junction, occupying a small fraction of this surface. (These junctional structures are examined in the electron microscope after La infiltration.) All cells are joined to their immediate neighbors by septate junctions, the G- to G-cells, the F- to F-cells, and the F- to G-cells; the G-cells, at least, are also joined by gap junctions. All cells are also in communication with each other: small inorganic ions, fluorescein (mol. wt. 330) and Procion Yellow (mol. wt. ∼550) pass from one cell interior to the next.

Freeze-induced shrinkage of individual cells and cell-to-cell propagation of intracellular ice in cell chains from salivary glands

The formation of intracellular ice (IIF), usually a lethal event to be avoided when cryopreserving cells, should, however, be enforced during the cryosurgical destruction of tumour cells. IIF has been investigated so far only in single cells in suspension. Because cells in tissues cannot be successfully cryopreserved, in contrast to single cells in suspension, the mechanism of IIF in tissues may depend on factors that facilitate IIF. We studied IIF in cell strands from salivary glands, which represent a simple form of a tissue. Their cells are connected by channels responsible for intercellular communication. A substantial fraction of cell dehydration during freezing occurs before cells are encapsulated by ice, and the degree of this pre-ice-front shrinkage appears to influence IIF. In strands with coupled cells IIF spread from one cell to adjacent cells in a sequential manner with short delays (200-300 ms), suggesting cell-to-cell propagation via intercellular channels. In strands pretreated with decoupling agents (dinitrophenol, heptanol), sequential IIF was absent. Instead, formation of ice was random, with longer and variable delays between consecutive darkenings indicating IIF. Results suggest that the mechanism of IIF spread, and consequently the degree of cryodamage in tissue, can be influenced by the presence of intercellular channels (gap junctions).

Inhibition of astrocyte gap junctional communication by ATP depletion is reversed by calcium sequestration

We have studied the possible role of cellular energy status in the regulation of gap junction permeability in rat astrocytes in primary culture. Incubation with the mitochondrial respiratory chain inhibitor antimycin (5 ng/ml) for 16 h caused a significant decrease in ATP concentrations. This effect was accompanied by a dose-dependent inhibition of gap junction permeability as assessed by the scrape-loading/Lucifer yellow transfer technique. No cell death was observed following this treatment. Restoration of cellular ATP levels by a further 24 h incubation in antimycin-free medium reversed the inhibition of Lucifer yellow transfer caused by antimycin. The inhibition of Lucifer yellow transfer brought about by antimycin treatment was also reversed by a short incubation of the cells with the calcium chelator EGTA plus the calcium ionophore A23187. These results suggest that ATP depiction causes a reversible inhibition of gap junction permeability through a calcium-mediated mechanism.

Temperature dependence of embryonic cardiac gap junction conductance and channel kinetics

We have investigated the effects of temperature on the conductance and voltage-dependent kinetics of cardiac gap junction channels between pairs of seven-day embryonic chick ventricle myocytes over the range of 14-26 degrees C. Records of junctional conductance (Gj) and steady-state unit junctional channel activity were made using the whole-cell double patch-clamp technique while the bath temperature was steadily changed at a rate of about 4 degrees C/min. The decrease in Gj upon cooling was biphasic with a distinct break at 21 degrees C. In 12 cell pairs, Q10 was 2.2 from 26 to 21 degrees C, while between 21 and 14 degrees C it was 6.5. The mean Gj at 22 degrees C (Gj22) was 3.0 +/- 2.1 nS, ranging in different preparations from 0.24 to 6.4 nS. At room temperature, embryonic cardiac gap junctions contain channels with conductance states near 240, 200, 160, 120, 80 and 40 pS. In the present study, we demonstrate that cooling decreases the frequency of channel openings at all conductance levels, and at temperatures below 20 degrees C shifts the prevalence of openings from higher to lower conductance states: all 240 pS openings disappear below 20 degrees C; 200 pS openings are suppressed at 17 degrees C; below 16 degrees C 160 and 120 pS events disappear and only 80 and 40 pS states are seen. Temperature also affected the voltage-dependent kinetics of the channels. Application of a 6 sec, 80 mV voltage step across the junction (Vj80) caused a biexponential decay in junctional conductance. Decay was faster at lower temperatures, whereas the rate of recovery of Gj after returning to Vj0 was slowed. Cooling reduced the fast decay time constant, increased both recovery time constants, and decreased the magnitude of Gj decay, thus leaving a 10-16% larger residual conductance (Gss/Ginit, +/- 80 mV Vj) at 18 than at 22 degrees C. From these results we propose that embryonic chick cardiac gap junctions contain at least two classes of channels with different conductances and temperature sensitivities.

Gap junctions in ovarian follicles of Drosophila melanogaster: inhibition and promotion of dye-coupling between oocyte and follicle cells

The analysis of chimeras has shown that communication between germ-line and soma cells plays an important role during Drosophila oogenesis. We have therefore investigated the intercellular exchange of the fluorescent tracer molecule, Lucifer yellow, pressure-injected into the oocyte of vitellogenic follicles of Drosophila. The dye reached the nurse cells via cytoplasmic bridges and entered, via gap junctions, the somatic follicle cells covering the oocyte. The percentage of follicles showing dye-coupling between oocyte and follicle cells was found to increase with the developmental stage up to stage 11, but depended also on the status of oogenesis, i.e., the stage-spectrum, in the respective ovary. During late stage 10B and stage 11, dye-coupling was restricted to the follicle cells covering the anterior pole of the oocyte. No dye-coupling was observed from stage 12 onwards. During prolonged incubation in vitro, the dye was found to move from the follicle cells back into the oocyte; this process was suppressable with dinitrophenol. Dye-coupling was inhibited when prolonged in vitro incubation preceded the dye-injection. Moreover, dye-coupling was inhibited with acidic pH, low [K+], high intracellular [Ca2+], octanol, dinitrophenol, and NaN3, but not with retinoic acid, basic pH, or high extracellular [Ca2+]. Dye-coupling was stimulated with a juvenile hormone analogue and with 20-hydroxyecdysone. Thus, gap junctions between oocyte and follicle cells may play an important role in intercellular communication during oogenesis. We discuss the significance of our findings with regard to the electrophysiological properties of the follicles, and to the coordinated activities of the different cell types during follicle development and during the establishment of polarity in the follicle.

ATP directly affects junctional conductance between paired ventricular myocytes isolated from guinea pig heart

Effects of ATP on junctional conductance (gj) were investigated in paired ventricular myocytes isolated from guinea pig hearts. One cell of the pair was voltage-clamped with a single-patch pipette, and gj was measured after the perforation of the nonjunctional membrane of the partner cell. The current-voltage relation of gj was linear between -30 and +30 mV. The control gj at 5.0 mM ATP in 88 pairs of cells ranged from 100 to 1,055 nS (average, 268 nS). ATP within the range from 0.1 to 5.0 mM increased gj in a dose-dependent manner. The Hill coefficient was 2.6, and the half-maximum effective concentration of ATP was 0.68 mM. Adenylylimidodiphosphate (2 mM) caused a transient increase in gj in the presence of 0.5 mM ATP, but forskolin (30 microM), cyclic AMP (50 microM), catalytic subunit of cyclic AMP-dependent protein kinase (1 microM), and ADP (10 mM) had no significant effect on gj. The temperature coefficient of gj in the presence of 5.0 mM ATP was 1.29. These findings suggest that gj in paired ventricular myocytes is directly regulated by ATP probably through a specific ligand-receptor interaction between ATP and gap junctional channel protein.

Carbon tetrachloride at hepatotoxic levels blocks reversibly gap junctions between rat hepatocytes

Electrical coupling and dye coupling between pairs of rat hepatocytes were reversibly reduced by brief exposure to halogenated methanes (CBrCl3, CCl4, and CHCl3). The potency of different halomethanes in uncoupling hepatocytes was comparable to their hepatotoxicity in vivo, and the rank order was the same as that of their tendency to form free radicals. The effect of carbon tetrachloride (CCl4) on hepatocytes was substantially reduced by prior treatment with SKF 525A, an inhibitor of cytochrome P-450, and by exposure to the reducing reagent beta-mercaptoethanol. Halomethane uncoupling occurred with or without extracellular calcium and did not change intracellular concentrations of calcium and hydrogen ions or the phosphorylation state of the main gap-junctional protein. Thus the uncoupling appears to depend on cytochrome P-450 oxidative metabolism in which free radicals are generated and may result from oxidation of the gap-junctional protein or of a regulatory molecule that leads to closure of gap-junctional channels. Decreases in junctional conductance may be a rapid cellular response to injury that protects healthy cells by uncoupling them from unhealthy ones.

Effects of protein reagents on electrotonic coupling in crayfish septate axon

The lateral giant axons of the crayfish nerve cord are composed of segments contributed by each ganglion, which are electrotonically coupled by way of gap junctions. We have investigated the involvement of protein residues in regulating the resistance of crayfish junctional channels by determining effects of group-specific protein reagents. When applied to well-coupled axons, the sulfhydryl group reagents N-ethylmaleimide (NEM) and diamide uncoupled the segments; junctional resistance (Rj) was increased without changing membrane resistance or axoplasmic pH (pHi). The uncoupling produced by NEM could be reversed by alkalinization of the cytoplasm (addition of ammonium chloride to the external medium). Another sulfhydryl reagent (p-chloromercuribenzoic acid) increased Rj to a lesser extent. A disulfide reagent and three amino and three carboxyl group reagents had no effect on the Rj of these axons. The effect of group-specific reagents on partially uncoupled axons was tested by applying the drugs to axons previously exposed to weak acids. N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline recoupled partially uncoupled axons by decreasing Rj and prevented subsequent uncoupling of the junction by low pHi. Another carboxyl group reagent, as well as sulfhydryl and amino group reagents, either had no effect or uncoupled the axons further by increasing Rj. These experimental results suggest that amino acid residues, possibly containing carboxyl and sulfhydryl groups, control the opening and closing of junctional channels and may thus be associated with the channels' active sites.

Quantitative gap junction alterations in mammalian heart cells quickly frozen or chemically fixed after electrical uncoupling

The gap junction morphology was quantified in freeze-fracture replicas prepared from rat auricles that had been either quickly frozen at 6 K or chemically fixed by glutaraldehyde, in a state of normal cell-to-cell conduction or in a state of electrical uncoupling. The general appearance of the gap junctions was similar after both preparative procedures. A quantitative analysis of three gap junctional dimensions provided the following measurements in the quickly frozen conducting auricles (mean +/- SD): P-face particles' diameter 8.27 +/- 0.74 nm (n = 5709), P-face particles' center-to-center distance 10.78 +/- 2.12 nm (n = 4800), and E-face pits' distance 9.99 +/- 2.19 nm (n = 1600). Corresponding values obtained from chemically fixed tissues were decreased by about 3% for the particle's diameter and about 5% for the particles' and pits' distances. Electrical uncoupling by the action of either 1 mM 2-4-dinitrophenol (DNP), or 3.5 mM n-Heptan-1-ol (heptanol), induced a decrease of the particle's diameter, which amounted to -0.69 +/- 0.01 nm (mean +/- SE) in the quickly frozen preparations and -0.71 +/- 0.01 nm in the chemically fixed ones. The particles' distance was decreased by -0.96 +/- 0.04 nm in the quickly frozen samples and by -0.90 +/- 0.03 nm in the chemically fixed ones and the E-face pits' distance was similarly reduced. All differences were statistically significant (P less than 0.001 for all dimensions). Electrical recoupling after the heptanol effect promoted a return of these gap junctional dimensions towards normal values, which was about 50% complete within 20 min. It is concluded that very similar morphological alterations of the gap junctional structure are induced in the mammalian heart by different treatments promoting electrical uncoupling and that these conformational changes appear independently of the preparative procedure. The suggestion that the observed decrease of the particles' diameter is genuinely related to the closing mechanism of the unit cell-to-cell channel set in their centers is thus confirmed.

Gap junction structures after experimental alteration of junctional channel conductance

Gap junctions are known to present a variety of different morphologies in electron micrographs and x-ray diffraction patterns. This variation in structure is not only seen between gap junctions in different tissues and organisms, but also within a given tissue. In an attempt to understand the physiological meaning of some aspects of this variability, gap junction structure was studied following experimental manipulation of junctional channel conductance. Both physiological and morphological experiments were performed on gap junctions joining stage 20-23 chick embryo lens epithelial cells. Channel conductance was experimentally altered by using five different experimental manipulations, and assayed for conductance changes by observing the intercellular diffusion of Lucifer Yellow CH. All structural measurements were made on electron micrographs of freeze-fracture replicas after quick-freezing of specimens from the living state; for comparison, aldehyde-fixed specimens were measured as well. Analysis of the data generated as a result of this study revealed no common statistically significant changes in the intrajunctional packing of connexons in the membrane plane as a result of experimental alteration of junctional channel conductance, although some of the experimental manipulations used to alter junctional conductance did produce significant structural changes. Aldehyde fixation caused a dramatic condensation of connexon packing, a result not observed with any of the five experimental uncoupling conditions over the 40-min time course of the experiments.

Effect of several uncouplers of cell-to-cell communication on gap junction morphology in mammalian heart

Electrical conduction in sheep Purkinje fibers has been blocked by three different procedures: (I) 1 mM 2-4-dinitrophenol, (II) 3.5 mM n-Heptan-1-ol (heptanol), and (III) treatment by a hypotonic (120 mOsmoles) Ca2+-free solution for half an hour, followed by return to normal conditions. The gap junction morphology was analyzed quantitatively in freeze-fracture replicas and compared in electrically conducting and nonconducting fibers. It is found that the three uncouplers of cell-to-cell conduction induce consistent and statistically significant alterations of the gap junction structure. The investigated morphological criteria: (a) P-face junctional particle diameter, control value 8.18 +/- 0.70 nm (mean +/- SD), (b) P-face junctional particles center-to-center spacing, control value 10.23 +/- 1.57 nm, and (c) E-face pits spacing, control value 9.45 +/- 0.98 nm, are, respectively, decreased to 7.46 +/- 0.62 nm, 9.25 +/- 1.34 nm and 8.67 +/- 1.13 nm in Purkinje fibers with complete conduction blocks. All three gap junctional dimensions are seen to decline progressively with time from the onset of an uncoupling treatment towards stable minima reached in half an hour. The observed morphological transitions appear related to the electrical uncoupling for the following reasons: partial electrical uncoupling results in values of the gap junctional dimensions that are intermediate between those measured in electrically coupled and uncoupled preparations, and the three morphological indices are seen to increase again towards control values very soon after electrical conduction has been re-established. It is concluded that the junctional channels closure on electrical uncoupling correlates with a measurable (-0.72 +/- 0.01 nm, difference of the means +/- SE) decrease of the junctional particle diameters.

Biochemical assay of inhibitors of metabolic cooperation

A significant and reproducible enhancement of purine nucleotide synthesis from hypoxanthine occurs in HAT medium, when communication-competent hypoxanthine-guanine phosphoribosyltransferase (HGPRT+) cells are co-cultured with communication-competent (HGPRT-) LN cells. This enhancement of purine nucleotide synthesis is dependent upon the hypoxanthine concentration and upon the ratio of (HGPRT-): (HGPRT+) cells. Based upon these results a simple biochemical method for the detection of inhibitors of metabolic cooperation between (HGPRT+) cells and (HGPRT-) LN cells is presented. The biochemical method distinguishes inhibitors of metabolic cooperation from inhibitors of hypoxanthine uptake, of hypoxanthine phosphorylation and of nucleic acid synthesis, as well as from general metabolic inhibitors. This method has the advantage that it can be used on a relatively large number of cells, it is simple and not time-consuming, and distinguishes the inhibition of metabolic cooperation by compounds that have a variety of sites of inhibition.

Experimental depression of junctional membrane permeability in mammalian cell culture. A study with tracer molecules in the 300 to 800 Dalton range

Cell-to-cell junctional permeability in mammalian cell cultures was probed with a series of fluorescent tracers ranging 300 to 800 in molecular weight, during treatment with metabolic inhibitors, Ca-transporting ionophore, and carbon dioxide. Treatment with the combination of cyanide and iodoacetic acid (1--2 mM each), but not with either one alone, caused reversible junctional blockade to all tracer molecular species, large and small. (Electrical coupling, however, persisted in a proportion of the junctions tested.) Treatment with the ionophore A23187 (2--10 micrometers) or with CO2 (an atmosphere of 100% CO2 equilibrated with the medium) produced selective junctional blockade: transmission of a 688 and an 817-dalton tracer was generally blocked, while that of a 376-dalton traced and, in certain conditions, that of a 559-dalton one, persisted. The junctional effect of the ionophore required the presence of Ca in the external medium; and effective junctional blockade by CO2 required pretreatment in medium with high Ca concentration or, interchangeably, pretreatment in medium with high CO2 concentration. In one cell type, prolonged exposure to medium with high Ca concentration alone sufficed to block transmission of the 688-dalton tracer. These effects are discussed in terms of the Ca hypothesis of junctional permeability regulation. In comparison with mammalian (or other vertebrate and invertebrate) organized tissues or with insect cell cultures, the mammalian cell cultures are more resistant to junctional blockade. This difference in transmission stability is discussed in terms of the electron-microscopic finding in the mammalian cultures of fine, bilateral cell processes connected by gap junctions.

Drugs that block calmoduLin activity inhibit cell-to-cell coupling in the epidermis of Tenebrio molitor

In many cell systems, the permeability of membrane junctions is modulated by the cytoplasmic level of free Ca++. To examine whether the calcium-dependent regulatory protein calmodulin is involved in this process, the ability of anticalmodulin drugs to influence the cell-to-cell passage of injected current and an organic tracer was tested using standard intracellular glass microelectrode techniques. Several antipsychotics and local anesthetics were found to block junctional communication in the epidermis of the beetle Tenebrio molitor. Treatment of the epidermis with chlorpromazine (0.25 mM) raised intercellular resistance two- to threefold within 20 to 25 min; cell-to-cell passage of electrical current was abolished within 41 +/- 5 min. Loss of electrotonic coupling was accompanied by a block in the cell-to-cell movement of the organic tracer carboxyfluorescein. The reaction is fully reversible, with normal electrotonic coupling being restored within 2 to 4 hr. Other antipsychotics and local anesthetics had similar effects on cell coupling. The order of potency found was: trifluoperazine greater than thioridazine greater than D-butaclamol greater than chlorprothixine = chlorpromazine greater than L-butaclamol greater than dibucaine greater than tetracaine. The relative uncoupling potencies of these drugs correlate well with their known ability to inhibit calmodulin-dependent phosphodiesterase activity. Other anesthetic compounds, procaine and pentobarbital, did not block cell-to-cell communication. Altering the extracellular Ca++ concentration did not affect the rate of uncoupling by antipsychotics, while chelation of extracellular Ca++ with EGTA raised electrotonic coupling. The effect of three metabolic inhibitors on coupling was also examined. Iodoacetate uncoupled the epidermal cells while DNP and cyanide did not. These results are discussed in terms of possible mechanisms by which calmodulin may control junctional communication in this tissue.

Electrotonic coupling in internally perfused crayfish segmented axons

We have developed a technique for cannulation and internal perfusion of crayfish segmented lateral axons. Experiments on perfused and non-perfused axons lead to the following conclusions: 1. Internally perfused segmented axons behave very similarly to non-perfused axons. 2. The axial electrical resistance of the junctional region is almost as low as a comparable segment of axon. 3. Neither intracellular Ca2+ nor H+ is effective in disrupting the intercellular communication pathway in perfused axons. On the basis of these findings we have formulated a hypothesis for cellular control of intercellular coupling based on the existence of a soluble intermediate for Ca2+ or H+-induced uncoupling. This hypothesis is consistent with data from both internally perfused and non-perfused axons.

The permeability of the cell-to-cell membrane channel and its regulation in mammalian cell junctions

Mammalian cell-to-cell channels show polar permselective properties discriminating against negatively charged 14 A-wide molecules and are more restrictive than the channels of insect cell junctions. The channel permeability is modulated by conditions affecting the concentration of intracellular ionic Ca: elevation of the external Ca load (B cells), treatment of cell cultures with Ca-transporting ionophore (in the presence of external Ca, but not in its absence), treatment with a combination of cyanide and iodoacetate, or with high levels of carbon dioxide, all cause depression of channel permeability. Treatment of cell cultures with cyclic AMP or its more permeable derivative, dibutyryl cyclic AMP, produces increase in permeability. A similar channel up regulation is observed upon elevation of the endogenous level of cyclic AMP by serum deprivation or lowering of cell density.

On the electrotonic coupling mechanism of crayfish segmented axons: temperature dependence of junctional conductance

It is generally accepted that the mechanism for electrotonic coupling involves the presence of hydrophilic channels connecting the cytoplasm of neighboring cells. These channels are presumed to be water filled holes. To test this hypothesis, we measured the temperature dependence of coupling parameters and calculated the specific resistance of junctional synapses of crayfish segmented axons. Results demonstrate that: (i) low temperature increases the junctional resistance in a manner that depends on the time course of cooling; (ii) the specific junctional resistance is, at most, 1-20 omega cm2. These results are consistent with a hypothesis of cell communication based on hydrophilic channels and suggest the presence of a temperature-dependent component of these channels.

Inhibition by tryptophan of nucleolar RNA synthesis in salivary gland nuclei of Chironomus thummi

Tryptophan, but not glutamine and lysine, inhibits the incorporation of 3H-uridine into nucleoli of explanted salivary glands by about 60%. In isolated nuclei also, the nucleolar incorporation of 3H-uridine triphosphate is reduced to about 50% by tryptophan. It is concluded that tryptophan acts directly at the nuclear level and the possible mechanisms.

Effect of 2-4-dinitrophenol on intercellular communication in mammalian cardiac fibres

The effect of 2-4-dinitrophenol (DNP) on cell communication, in canine Purkinje fibres, was investigated. It was found that DNP (0.5 MM) suppressed the electrical coupling in about 10 min. This effect of DNP was largely due to an increment in intracellular longitudinal resistance. The longitudinal movement of fluorescein (mol. wt. 320) along Purkinje strands, followed with the cut-end method, was also suppressed by DNP (0.5 mM). The decoupling action of DNP was related to release of Ca from intracellular stores and increase in free (Ca)i. The intracellular injection of EDTA reestablished the electrical coupling of Purkinje cells previously uncoupled by DNP. The results described in this paper indicate that cell communication in heart fibres is greatly dependent on the synthesis of high energy phosphate bonds.

TQ-ST segment mapping: critical review and analysis of current concepts

Controversy and confusion surround many aspects of TQ-ST segment mapping today. Technical standards pertaining to the recording and measurement of the TQ-ST deflection have not been uniformly established nor has the correlative value of the deflection as an indicator of myocardial injury been clearly ascertained. The TQ-ST deflection is believed to originate primarily although not exclusively as a result of extracellular potassium accumulation in the ischemic region and subsequent establishment of a transmembrane potential gradient during diastole and systole at the ischemic boundary. Nonspatial factors (including electrolytes, antiarrhythmic agents, heart rate) influence the TQ-ST deflection by altering this gradient. Spatial factors (including ischemic area and shape, electrode location) alter the relative position of the ischemic boundary to the electrode site and as such can be analyzed with the solid angle theorem. Further study of the complex behavior of the TQ-ST segment deflection, particularly in the presence of pharmacologic intervention, is necessary before mapping techniques can be used reliably in clinical studies designed to quantitate and modify ischemic damage.

Permeability of a cell junction and the local cytoplasmic free ionized calcium concentration: a study with aequorin

A technique is devised to determine the spatial distribution of the free ionized cytoplasmic calcium concentration ([Ca2+]i) inside a cell: Chironomus salivary gland cells are loaded with aequorin, and hte Ca2+-dependent light emission of the aequorin is scanned with an image-intensifier/television system. With this technique, the [Ca2+]i is determined simultaneously with junctional electrical coupling when Ca2+ is microinjected into the cells, or when the cells are exposed to metabolic inhibitors, Ca-transporting ionophores, or Ca-free medium. Ca microinjections elevating the [Ca2+]i in the junctional locale produce depression of junctional membrane conductance. When the [Ca2+]i elevation is confined to the vicinity of one cell junction, the conductance of that junction alone is depressed; other junctions of the same cell are not affected. The depression sets in as the [Ca2+]i rises in the junctional locale, and reverses after the [Ca2+]i falls to baseline. When the [Ca2+]i elevation is diffuse throughout the cell, the conductances of all junctions of the cell are depressed. The Ca injections produce no detectable [Ca2+]i elevations in cells adjacent to the injected one; the Ca-induced change in junctional membrane permeability seems fast enough to block appreciable transjunctional flow of Ca2+. Control injections of Cl- or K+ do not affect junctional conductance. The Ca injections that elevate [Ca2+]i sufficiently to depress junctional conductance also produce under the usual conditions an increase in nonjunctional membrane conductance and, hence, depolarization. But injections that elevate [Ca2+]i at the junction while largely avoiding nonjunctional membrane cause depression of junctional conductance with little or no depolarization. Moreover, elevations of [Ca2+]i in cells clamped near resting potential produce the depression, too. On the other hand, complete depolarization in K medium does not produce the depression, unless accompanied by [Ca2+]i elevation. Thus, the depolarization is neither necessary nor sufficient for depression of junctional conductance. Treatment with cyanide, dinitrophenol and ionophores X537A or A23187 produces diffuse elevation of [Ca2+]i associated with depression of junctional conductance. Prolonged exposure to Ca-free medium leads to fluctuation in [Ca2+]i where rise and fall of [Ca2+]i correlate respectively with fall and rise in junctional conductance.

Permeability of a cell junction during intracellular injection of divalent cations

Divalent cations are microinjected into Chironomus salivary gland cells while the cell-to-cell passage of fluorescein (330 dalton) and electrical coupling are monitored. Injections of Ca and Mg that substantially depolarize the cells produce block or marked slowing fluorescein passage, accompanied by electrical uncoupling. Injections of Ca, Mg or Sr that cause little depolarization, and presumably smaller elevation of divalent cation concentration in the cytoplasm, produce block or marked slowing of fluorescein passage with little or no detectable electrical uncoupling. This partial uncoupling may reflect total closure of a fraction of the channels in junctional membrane or partial closure of all channels.

Is the nexus necessary for cell-to-cell coupling of smooth muscle?

Electronmicroscopic study of electrically coupled smooth muscles was undertaken to determine the distribution of nexuses in various types of smooth muscle. The study revealed that while nexal structures were commonplace in some types of smooth muscle, they were very rare or absent in others, even though in some cases these cells were only a few nanometers distant from one another. The persistence in thin section of these structures in the main circular muscle of dog intestine after poor fixation, fixation under strain, cell shrinkage, and metabolic damage of various sorts seems to rule out the thesis that they are labile. The absence of nexuses in longitudinal muscle of dog intestine examined both by thin section and by freeze fracture suggests that in this tissue they are absent or very rare in vivo and cannot account for electrical coupling. Nexuses were discernible in thin sections of main circular muscle after a variety of experimental conditions of fixation. Metabolic inhibition or in vitro permanganate fixation partially destroyed nexal contacts. These procedures induced tissue, membrane apposition and an accompanying increase in the number of structures which resemble nexuses at low magnification (nexus-like structures). "Nexus-like" structures occurred in all smooth muscle fixed by in vitro permanganate associated with apposition of membranes and poor preservation of basement membrane. A technique of in vitro permanganate fixation was developed which prevented tissue swelling; consequently "nexus-like" structures were absent in tissues so treated. The suggestion is made that some structures described in the literature as nexuses, following permanganate fixation, may represent "nexus-like" structures. The balance of evidence suggests that nexuses need not be present for electrical coupling of some smooth muscle cells, in which other types of cell-to-cell contacts must be invoked.

The effects of sodium, potassium and ATP on a developmental puff sequence in Drosophila salivary glands in vitro

Salivary glands of late third instar larvae of Drosophila melanogaster were isolated at a developmental stage when the release of ecdysone had already taken place. They were then incubated in a chemically defined medium. An ecdysone-dependent developmental puff sequence was measured in vitro and influenced by adding various substances or by changing iso-osmotically the sodium or potassium content of the medium. Trinactin, valinomycin N-ethylmaleimide and KCN blocked the puff sequence, i.e. the regression of the early and the induction of the late ecdysone-dependent puffs probably by increasing the Na+ influx and depleting the ATP content of the cell. A medium that contained Na+ as the only monovalent cation decreased the size of the late ecdysone-dependent puffs and increased the size of other puffs, such as 50 CD. Addition of tetrodotoxin to the normal medium had the opposite effect, i.e., it increased 63 E and inhibited 50 CD. Na+ free medium, inhibition of K+ flux by tetraethylammonium chloride, and application of ouabain did not considerably influence the size of the puffs measured. It is concluded from these results that Na+ in particular has an inhibitory effect on the induction of late ecdysone-dependent puffs. Na+ (and perhaps also K+) may act by modulating the effect of proteins that are involved in gene control mechanisms.

Effect of intracellular injection of calcium and strontium on cell communication in heart

1. The influence of Ca and Sr on the electrical coupling of canine Purkinje cells was investigated by injecting the ions electrophoretically into the cytoplasm. 2. It was found that the intracellular injection produced electrical uncoupling which was spontaneously reversed. 3. No change in resting potential of the cell adjacent to the injection site was found except in those fibres not completely healed. 4. The input resistance of the injected cell increased concomitantly with the establishment of the electrical uncoupling. 5. Caffeine (6mM), added to the extracellular fluid, reduced the rate of spontaneous recoupling. Reduction of temperature of the Tyrode solution had the same effect. 6. The abolition of cell communication produced by Ca injection seems to indicate that the ion plays an important role in the control of junctional conductance in heart fibres.

Passive electrical properties of toad urinary bladder epithelium. Intercellular electrical coupling and transepithelial cellular and shunt conductances

The electrical resistances of the transcellular and paracellular pathways across the toad urinary bladder epithelium (a typical "tight" sodium-transporting epithelium) were determined by two independent sets of electrophysiological measurements: (a) the measurement of the total transepithelial resistance, the ratio of resistance of the apical to the basal cell membrane, and cable analysis of the voltage spread into the epithelium; (b) the measurement of the total transepithelial resistance and the ratio of resistances of both cell membranes before and after replacing all mucosal sodium with potassium (thus, increasing selectively the resistance of the apical membrane). The results obtained with both methods indicate the presence of a finite transepithelial shunt pathway, whose resistance is about 1.8 times the resistance of the transcellular pathway. Appropriate calculations show that the resistance of the shunt pathway is almost exclusively determined by the zonula occludens section of the limiting junctions. The mean resistance of the apical cell membrane is 1.7 times that of the basal cell membrane. The use of nonconducting materials on the mucosal side allowed us to demonstrate that apparently all epithelial cells are electrically coupled, with a mean space constant of 460 microm, and a voltage spread consistent with a thin sheet model.

Low resistance junctions in crayfish. II. Structural details and further evidence for intercellular channels by freeze-fracture and negative staining

The ultrastructure of low resistance junctions between segments of lateral giant fibers in crayfish is studied in sections from specimens fixed either by conventional methods or by glutaraldehyde-H(2)O(2) or by glutaraldehyde-lanthanum. Cross sections through junctions fixed by conventional glutaraldehyde display the usual trilaminar profile of two parallel membranes separated by a narrow gap. Most of the junctional regions appear covered by 500-800 A vesicles which lie on both sides of the junction in rows adjacent to the membranes. Gross sections through junctions fixed by glutaraldehyde-H(2)O(2) display, in regions containing vesicles, membranes with a beaded profile. The beads correspond to globules approximately 125 A in width and approximately 170 A in height arranged in a hexagonal pattern with a unit cell of approximately 200 A. The globules of one membrane match precisely with those of the adjacent membrane, and opposite globules seem to come in contact with each other at the center of the junction. The membrane of the vesicles also contains globules. Occasionally the globules of the vesicles seem to join with those of the junctional membranes, apparently forming intracellular junctions. Injunctions negatively stained by lanthanum the globules are seen organized into two arrangements. Areas containing globules in a hexagonal array with a unit cell of approximately 200 A (swollen pattern) are seen adjacent to areas in which the globules are more closely and disorderly packed (close packing), the minimum center-to-center distance between adjacent globules being approximately 125 A. At higher magnification each globule appears composed of six subunits arranged in a circle around a central region occupied by lanthanum (possibly a pit).