Calcium in (junctional) intercellular communication and a thought on its behavior in intracellular communication

Electrical synapses between mushroom body neurons are critical for consolidated memory retrieval in Drosophila

Electrical synapses between neurons, also known as gap junctions, are direct cell membrane channels between adjacent neurons. Gap junctions play a role in the synchronization of neuronal network activity; however, their involvement in cognition has not been well characterized. Three-hour olfactory associative memory in Drosophila has two components: consolidated anesthesia-resistant memory (ARM) and labile anesthesia-sensitive memory (ASM). Here, we show that knockdown of the gap junction gene innexin5 (inx5) in mushroom body (MB) neurons disrupted ARM, while leaving ASM intact. Whole-mount brain immunohistochemistry indicated that INX5 protein was preferentially expressed in the somas, calyxes, and lobes regions of the MB neurons. Adult-stage-specific knockdown of inx5 in αβ neurons disrupted ARM, suggesting a specific requirement of INX5 in αβ neurons for ARM formation. Hyperpolarization of αβ neurons during memory retrieval by expressing an engineered halorhodopsin (eNpHR) also disrupted ARM. Administration of the gap junction blocker carbenoxolone (CBX) reduced the proportion of odor responsive αβ neurons to the training odor 3 hours after training. Finally, the α-branch-specific 3-hour ARM-specific memory trace was also diminished with CBX treatment and in inx5 knockdown flies. Altogether, our results suggest INX5 gap junction channels in αβ neurons for ARM retrieval and also provide a more detailed neuronal mechanism for consolidated memory in Drosophila.

A structural and functional comparison of gap junction channels composed of connexins and innexins

Methods such as electron microscopy and electrophysiology led to the understanding that gap junctions were dense arrays of channels connecting the intracellular environments within almost all animal tissues. The characteristics of gap junctions were remarkably similar in preparations from phylogenetically diverse animals such as cnidarians and chordates. Although few studies directly compared them, minor differences were noted between gap junctions of vertebrates and invertebrates. For instance, a slightly wider gap was noted between cells of invertebrates and the spacing between invertebrate channels was generally greater. Connexins were identified as the structural component of vertebrate junctions in the 1980s and innexins as the structural component of pre-chordate junctions in the 1990s. Despite a lack of similarity in gene sequence, connexins and innexins are remarkably similar. Innexins and connexins have the same membrane topology and form intercellular channels that play a variety of tissue- and temporally specific roles. Both protein types oligomerize to form large aqueous channels that allow the passage of ions and small metabolites and are regulated by factors such as pH, calcium, and voltage. Much more is currently known about the structure, function, and structure-function relationships of connexins. However, the innexin field is expanding. Greater knowledge of innexin channels will permit more detailed comparisons with their connexin-based counterparts, and provide insight into the ubiquitous yet specific roles of gap junctions. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 77: 522-547, 2017.

Atomic structure of the innexin-6 gap junction channel determined by cryo-EM

Innexins, a large protein family comprising invertebrate gap junction channels, play an essential role in nervous system development and electrical synapse formation. Here we report the cryo-electron microscopy structures of Caenorhabditis elegans innexin-6 (INX-6) gap junction channels at atomic resolution. We find that the arrangements of the transmembrane helices and extracellular loops of the INX-6 monomeric structure are highly similar to those of connexin-26 (Cx26), despite the lack of significant sequence similarity. The INX-6 gap junction channel comprises hexadecameric subunits but reveals the N-terminal pore funnel, consistent with Cx26. The helix-rich cytoplasmic loop and C-terminus are intercalated one-by-one through an octameric hemichannel, forming a dome-like entrance that interacts with N-terminal loops in the pore. These observations suggest that the INX-6 cytoplasmic domains are cooperatively associated with the N-terminal funnel conformation, and an essential linkage of the N-terminal with channel activity is presumably preserved across gap junction families.

Magnitude and modulation of pancreatic beta-cell gap junction electrical conductance in situ

The parallel gap junction electrical conductance between a beta-cell and its nearest neighbors was measured by using an intracellular microelectrode to clamp the voltage of a beta-cell within a bursting islet of Langerhans. The holding current records consisted of bursts of inward current due to the synchronized oscillations in membrane potential of the surrounding cells. The membrane potential record of the impaled cell, obtained in current clamp mode, was used to estimate the behavior of the surrounding cells during voltage clamp, and the coupling conductance was calculated by dividing the magnitude of the current bursts by that of the voltage bursts. The histogram of coupling conductance magnitude from 26 cells was bimodal with peaks at 2.5 and 3.5 nS, indicating heterogeneity in extent of electrical communication within the islet of Langerhans. Gap junction conductance reversibly decreased when the temperature was lowered from 37 to 30 degrees C and when the extracellular calcium concentration was raised from 2.56 to 7.56 mM. The coupling conductance decreased slightly during the active phase of the burst. Activation of adenylate cyclase with forskolin (10 microM) resulted in an increase in cell-to-cell electrical coupling. We conclude that beta-cell gap junction conductance can be measured in situ under near physiological conditions. Furthermore, the magnitude and physiological regulation of beta-cell gap junction conductance suggest that intercellular electrical communication plays an important role in the function of the endocrine pancreas.

Movement protein of tobacco mosaic virus modifies plasmodesmatal size exclusion limit

The function of the 30-kilodalton movement protein (MP) of tobacco mosaic virus is to facilitate cell-to-cell movement of viral progeny in an infected plant. A novel method for delivering non-plasmalemma-permeable fluorescent probes to the cytosol of spongy mesophyll cells of tobacco leaves was used to study plasmodesmatal size exclusion limits in transgenic plants that express the MP gene. Movement of fluorescein isothiocyanate-labeled dextran (F-dextran) with an average molecular mass of 9400 daltons and an approximate Stokes radius of 2.4 nanometers was detected between cells of the transgenic plants, whereas the size exclusion limit for the control plants was 700 to 800 daltons. No evidence of F-dextran metabolism in the leaves of the transgenic plants was found. Thus, the tobacco mosaic virus movement protein has a direct effect on a plasmodesmatal function.

Diacylglycerol downregulates junctional membrane permeability. TMB-8 blocks this effect

We tested the question whether junctional cell-to-cell communication is regulated by the diacylglycerol branch of the phosphoinositide transmembrane signal pathway. Cultured epithelial rat liver cells were treated with the synthetic diacylglycerol 1-oleoyl-2-acetyl glycerol, while their junctional permeability was probed with the microinjected 443-dalton fluorescent tracer Lucifer Yellow. The treatment reduced junctional permeability (without affecting Lucifer permeability of nonjunctional cell membrane). The effect was dose dependent, with a threshold of about 25 micrograms diacylglycerol/ml in sparse cultures and about 50 micrograms/ml in confluent cultures. The reduction of junctional permeability began within 3 min of diacylglycerol application, peaked within 20 min, and reversed spontaneously within 90 min. The phorbol ester TPA mimicked the diacylglycerol effect, but the (spontaneous) reversal was slower. We propose that cell-to-cell communication is under dual physiological control: an up-regulatory one, as exerted by the cyclic AMP signal route (Loewenstein, W.R., 1985, Biochem. Soc. Symp. London, 50: 43-58), and a downregulatory one, by the diacylglycerol signal route. TMB-8 (54-70 microM)--a blocker of intracellular Ca2+ mobilization--impeded the diacylglycerol action on junctional permeability. It prevented the effect of low diacylglycerol doses completely and it markedly reduced the effect of high doses. (It also counteracted the effect of TPA.) Ca2+ thus emerges as a possible candidate for a role in the junctional downregulation by the diacylglycerol signal route. We tentatively advance two models. In one, leaning closely on the Calcium Hypothesis of cell-to-cell channel regulation (Loewenstein, W.R., 1966, Ann. N.Y. Acad. Sci. 137:441-472), Ca2+ mediates the action of the route on the channel. In the other, Ca2+ acts farther removed from the channel, on protein kinase C. Calmidazolium (5-10 microM)--an inhibitor of calmodulinactivated proteins--did not prevent the diacylglycerol-induced reduction of junctional permeability. Nor did sodium orthovanadate (25 or 50 microM)--an inhibitor of tyrosyl phosphatase--prevent the reversal of diacylglycerol-induced (or TPA-induced) reduction of junctional permeability.

The nature of the Ca2+-pump defect in the red blood cells of patients with cystic fibrosis

The reduction in (Ca2+ + Mg2+)-ATPase activity in the cystic fibrosis red blood cells can be attributed to a reduction in the number of active Ca2+ pumps per red blood cell and an altered interaction of calcium ions with the pump. Despite this, the normal free intracellular [Ca2+] is preserved due to a lower rate of passive calcium entry.

The effect of calcium on the lens ultrastructure

The influence of a calcium-enriched solution on pig lens cell structure was studied in vitro. The morphological changes which occurred 20 min or 2 h after bathing in the solution were studied by transmission electron microscopy. After 20-min incubation, small intercellular vacuoles containing membranous and amorphous substance occurred mainly in the epithelial cells. The cytoplasm of the cortical fibers had already been dissolved at this stage. Here the membrane was disrupted and the extracellular spaces widened. After 2-h incubation, the epithelial cells were compressed by adjacent swollen fibers. However, vacuolization in the epithelium had decreased at this stage. At the cortical fiber zone, vacuolar formation had increased extensively. Vacuolization was mainly observed in the intercellular spaces between the intact gap junctions. After 2 h, the main alterations were membrane disruptions and advancing dissolution of the cytoplasm. Deeper cortical fibers were swollen but showed no extracellular vacuoles.

The reduction of calcium current associated with early differentiation of the murine embryo

Membrane currents of intact oocytes and early embryos of the mouse and the hamster were analysed with voltage-clamp techniques. In both mouse and hamster the amplitude of Ca inward currents decreased with time during early development, and they were undetectable by the 8-cell stage, while the threshold potential, alkaline earth cation selectivity, and activation-inactivation kinetics remained unchanged. The reduction of Ca currents was further confirmed in the 2-cell embryo whose cleavage was arrested with use of cytochalasin D, but the process was slightly delayed by comparison with that of the intact embryo. Early differentiation of cytochalasin-D-treated embryos was comparable to that of the intact embryo in terms of intercellular couplings and intercellular fluid accumulation. But these processes were also delayed as in the case of Ca current reduction. The outward current in the hamster embryo which was reflected in the resting membrane conductance began to increase abruptly after the 2-cell stage and seemed to reach the maximum at the end of the 4-cell or 8-cell stage. The increase apparently occurred reciprocally with the decrease in Ca inward current. A similar but much smaller increase in resting membrane conductance also occurred in the cleavage-arrested mouse 2-cell embryo almost at the same development stage at which the abolition of Ca current was found. The possibility is discussed that Ca channels have a role in cell differentiation in early murine embryos.

Characterisation of the Egeria densa Planch. leaf symplast : Inhibition of the intercellular movement of fluorescent probes by group II ions

The hydrophyllic dyes fluorescein glutamic acid, fluorescein glutamylglutamic acid (F(Glu)2), fluorescein hexaglycine, fluorescein leucyldiglutamyl-leucine and 6-carboxyfluorescein are unable to pass the plasmalemma in leaves of E. densa. However, when injected into single cells the dye conjugates of molecular weight 665 dalton or less move freely from cell-to-cell. This intercellular movement presumably occurs via the plant symplast. Movement of F(Glu)2 from the injected cell occurs with greatly reduced frequency when Ca(2+), Mg(2+) or Sr(2+) are injected into the cell immediately prior to the dye. The fraction of dye injections leading to movement declines with increasing group II ion concentration in the electrode tip, up to 10 mM. Sodium and K ions do not affect dye movement. When dye injection is delayed 30 min after Ca(2+) injection, dye movement is no longer inhibited. Thus the cells recover from the Ca(2+) injection, indicating that the ion does not cause major cell damage. Recovery from Mg(2+) injection is not complete within 60 min. Treatment of leaves with chemicals expected to raise the concentration of free intracellular group II ions, notably the mitochondrial uncoupler carbonyl cyanide p-trifluoromethoxyphenyl hydrazone, the inhibitor of mitochondrial Ca(2+) uptake trifluralin, or the ionophore A23187 also inhibits dye movement, while the calmodulin inhibitor trifluoperazine does not. Cytoplasmic streaming is inhibited by Ca(2+) or Mg(2+) injection and by the metabolic inhibitors. However when streaming is stopped by cytochalasin B, dye movement is not inhibited. Hence steaming is not necessary for dye movement. Thus the cytoplasmic concentration of free group II ions may directly regulate the permeability of the plant symplast.

Cell-to-cell channels with two independently regulated gates in series: analysis of junctional conductance modulation by membrane potential, calcium, and pH

We study cell-to-cell channels, in cell pairs isolated from Chironomus salivary gland, by investigating the dependence of junctional conductance (gj) on membrane potentials (E1, E2), on Ca2+, and on H+, and we explore the interrelations among these dependencies; we use two separate voltage clamps to set the membrane potentials and to measure gj. We find gj to depend on membrane potentials whether or not a transjunctional potential is present. The pattern of gj dependence on membrane potentials suggests that each channel has two closure mechanisms (gates) in series. These gates pertain, respectively, to the two cell faces of the junction. By treating the steady-state gj as the resultant of two simultaneous but independent voltage-sensitive open/closed equilibria, one within each population of gates (i.e., one on either face of the junction), we develop a model to account for the steady-state gj vs. E relationship. Elevation of cytosolic Ca2+ or H+ at fixed E lowers gj, but at moderate concentrations of these ions this effect can be completely reversed by clamping to more negative E. Overall, the effect of a change in pCai or pHi takes the form of a parallel shift of the gj vs. E curve along the E axis, without change in slope. We conclude (1) that the patency of a cell-to-cell channel is determined by the states of patency of its two gates; (2) that the patency of the gates depends on membrane potentials (not on transjunctional potential), on pCai, and on pHi; (3) that pCai and pHi determine the position of the gj vs. E curve on the E axis; and (4) that neither Ca2+ nor H+ at moderate concentrations alters the voltage sensitivity of gj.

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.

Active modulation of electrical coupling between cardiac cells of the dog. A mechanism for transient and steady state variations in conduction velocity

Propagation velocities of action potentials were measured simultaneously along the longitudinal and transverse axes of cardiac fibers in ventricular muscle. The anisotropic distribution of propagation velocities was found to be altered transiently and in the steady state by the rate and pattern of stimulation and by ouabain. The relative amount of velocity change varied with the direction of propagation and was greatest in the direction perpendicular to the long fiber axis. None of the variables usually associated with the membrane ionic mechanism of depolarization--resting potential, Vmax, and taufoot--showed enough variation to account for the observed changes in velocity. A simplified anisotropic propagation model representing the internal current pathway as an alternating sequence of cytoplasmic and junctional resistance is presented, taking into account the larger contribution to the internal resistance made by the cell couplings in the transverse direction than in the longitudinal direction. On the basis of this model, it was concluded that the observed changes in velocity were due to changes in cell coupling. Both transient and steady state velocity changes were found to correspond to changes in the action potential duration, suggesting that there is a common factor, such as the internal calcium and/or sodium concentrations, linking the control of the action potential duration and the coupling resistance between cardiac cells.

Ammonium sulfate induced uncouplings of crayfish septate axons with and without increased junctional resistance

Cytoplasmic pH (pH1) of crayfish lateral giant axons was monitored using antimony microelectrodes placed near septate junctions and variations of internal pH was induced by short applications of ammonium sulfate in the perfusing bath of the preparation. This treatment produced a rapid cell alkalinization followed, after wash, by acidification rebound. Simultaneously, two successive phases of uncoupling of the septate junction occurred; they had the same time course as those of their associated pH1 movements. Calculation of the electronic coupling parameters indicated that, during alkalinization, coupling was accompanied by an increased axonal membrane conductance (the intimate origin of which was beyond the scope of this study) and resulted from a shunt of the gap junctions; the resistance proper of the latter was unaffected; thus involvement of Ca2+ was ruled out and uncoupling was only an indirect consequence of the electrotonic junction's network configuration. In contrast, and as expected from previous investigations, the junctional membrane resistance was increased during the second phase of cytoplasmic acidification. Evidence that uncoupling can be brought about by a non-junctional membrane increased permeability raises questions about some of the criteria commonly used during investigations of electrotonic transmission.

Direct intercellular communication in health and disease: an overview

Direct intercellular communication (cell to cell coupling) is a mechanism for the local transit of information between cells and supplements the endocrine and nervous systems. Electrophysiological, biochemical, histological and cell culture techniques have established the widespread existence of coupling in mammalian tissues, and the importance of the gap junction has been recognised. Information is carried in the form of ions and small molecules between cells, and sensitive apparatus exists within each cell for controlling the permeability of the junctional membrane. The system may be important in the control and co-ordination of cellular metabolism and growth in the embryo and in adult tissues. Disorders of direct intercellular communication may be important in the pathogenesis of some diseases, in particular cancer.

Diameter of the cell-to-cell junctional membrane channels as probed with neutral molecules

The cell-to-cell channels in the junctions of an insect salivary gland and of insect and mammalian cells in culture were probed with fluorescent molecules-neutral linear oligosaccharides, neutral branched glycopeptides, and charged linear peptides. From the molecular dimensions of the largest permeants and smallest impermeants the permeation-limiting channel diameter was obtained: 16 to 20 angstroms for the mammalian cells and 20 to 30 angstroms for the insect cells.

The influence of calcium ions on the synthesis of collagen and glycosaminoglycans in human diploid cells in culture

Synthesis of collagen and glycosaminoglycans of skin fibroblasts derived from an adult normal male in culture under different Ca2+ concentrations were investigated. Collagen synthesis was increased in hypocalcemic culture and decreased in hypercalcemic culture. Ca ionophore A23187 added to the medium caused reduction of collagen synthesis. From these results, it was suggested that intracellular Ca2+ controlled the collagen synthesis of fibroblasts. Uronic acid measurements of cell surface glycosaminoglycans (csGAGS) showed the reduction of uronic acid contents along with an increase of Ca2+ concentration in the medium. 3H-glucosamine and 35S-SO42-incorporation into csGAGS showed that glycosaminoglycans synthesis was reduced along with the increase of Ca2+ concentration in the medium. The results from the uronic acid measurement and the radio-isotope incorporation suggested that sulfated glycosaminoglycans, which contained isuronic acid in high proportions such as dermatan sulfate and heparan sulfate were increased in csGAGS in hypocalcemic culture.

Gap junction formation and regulation in myometrium

Myometrial tissues from pregnant rats were examined by electron microscopy for the presence of gap junctions after incubation in vitro with a variety of agents. Gap junctions were present in low frequency or absent prior to incubation in vitro. The junctions were present in control tissues in high frequency after 48 h incubation. The addition of cycloheximide or actinomycin D inhibited the incorporation of [3H]leucine into TCA-precipitable proteins and prevented gap junction formation. A prostacyclin analog (carbacyclin), a thromboxane synthesis inhibitor, and indomethacin also prevented gap junction formation. Oxytocin had no effect on gap junction formation but isoxsuprine decreased their number and increased their size. Isoxsuprine and isoproterenol also produced electron opaque crystals associated with the gap junctions. Dibutyryl cAMP treatment but not monobutyryl cGMP also increased the size of gap junctions. Based upon this and previous studies, we propose at least four sites for regulation of gap junctions and possible control of labor.

Localization of Ca++-containing antimonate precipitates during mitosis

Intracellular bound Ca++ has been localized throughout mitosis and cytokinesis in two plant species by means of in situ precipitation with potassium antimonate and electron microscope visualization. Identification of Ca++ as the major cation precipitated was made by comparing solubility properties in water, EDTA, and EGTA of the intracellular deposits with respect to those of K+-, Mg++-, and Ca++-antimonate standards. In spermatogenous cells of the water fern, Marsilea vestita, and stomatal complex cells of barley, Hordeum vulgare, antimonate deposits have been found associated with the endoplasmic reticulum (ER), vacuoles, euchromatin/nucleoplasm, and mitochondria. The last contain a much higher density of precipitates in Marsilea than in Hordeum. Dictyosomes and the nuclear envelope of Marsilea also contain antimonate deposits, as do the plasmalemma, cell wall, and phragmoplast vesicles of Hordeum. Microtubule-organizing centers such as kinetochores and the blepharoplast of Marsilea do not stain. In spite of differences in associated antimonate between certain organelles of the two species, the presence of antimonate aong the ER throughout the cell cycle is common to both. Of particular interest are those precipitates seen along the tubules and cisternae of the extensive smooth ER that surrounds and invades the mitotic spindle in both species. The ability to bind divalent cations makes the mitotic apparatus (MA)-associated ER a likely candidate for regulation of free Ca++ levels in the immediate vicinity of structural components and processes that are Ca++-sensitive and proposed to be Ca++-regulated.

Junctional resistance and action potential delay between embryonic heart cell aggregates

Spheroidal aggregates of embryonic chick ventricle cells were brought into contact and allowed to synchronize their spontaneous beats. Action potentials were recorded with both intracellular and extracellular electrodes. The degree of electrical interaction between the newly apposed aggregates was assessed by measuring the delay or latency (L) between the entrained action potentials, and by determining directly interaggregate coupling resistance (Rc) with injected current pulses. Aggregate size, contact area between the aggregates, and extracellular potassium concentration (Ko+) were important variables regulating the time-course of coupling. When these variables were controlled, L and Rc were found to be linearly related after beat synchrony was achieved. In 4.8 mM Ko+ L/Rc = 3.7 ms/M omega; in 1.3 mM Ko+ L/Rc = 10.1 ms/M omega. We conclude that action potential delay between heart cell aggregates can be related quantitatively to Rc.

Free calcium in Xenopus embryos measured with ion-selective microelectrodes

A regulatory role for intracellular free calcium has been suggested in cell division and intercellular communication via gap junctions. Previous measurements of cytoplasmic free Ca2+ during cell division and uncoupling have been made with the Ca2+-sensitive photoprotein aequorin. We now report experiments using Ca2+-sensitive microelectrodes to monitor free Ca2+ in the cells and intercellular fluid of early embryos of the amphibian Xenopus laevis. In addition to measuring basal levels, we have looked for changes in free Ca2+ during cell division and during electrical uncoupling of the normally coupled embryonic cells, induced by acidification of the intracellular medium.

Structure and evolution of calcium-modulated proteins

This review suggests that the intracellular functions of calcium are best understood in terms of calcium's functioning as a second messenger. Further, when functioning as a second messenger, calcium completes its mission not by transferring charge nor by binding to lipid but by binding to specific targets, calcium-modulated proteins. This concept is broadly interpreted to include proteins involved in calcium transport. There is strong evidence that many, if not all, of these calcium-modulated proteins are homologs. Their structures and properties are contrasted to those of extracellular calcium-binding proteins which are not homologous to one another or to the intracellular calcium-modulated proteins. Finally, this line of thought leads to a suggestion of the evolutionary reason for the choice of calcium as the sole inorganic second messenger.

Cyclic variation of K+ conductance in pancreatic beta-cells: Ca2+ and voltage dependence

Pulses of hyperpolarizing current were injected through the microelectrode recording the electrical activity of beta-cells in order to measure input resistance. Increase in resistance during depolarization of the slow oscillation ("burst") indicates inactivation of an outward current, probably K+. Decrease in resistance as the plateau commences suggests that the previous depolarization causes activation of an inward current, probably calcium. The postburst hyperpolarization, caused by a late activation of potassium permeability (PK), would result from the increase of intracellular free calcium. An intracellular buffering system may control this intracellular free calcium level. By restoring the silent phases, in the presence of ouabain or high potassium, injection of hyperpolarizing current shows also a voltage dependency of the PK involved in the postburst hyperpolarization. Glucose, by stimulating intracellular binding of calcium, would cause membrane depolarization at glucose levels below threshold and elongation of the plateau phase at higher concentrations.

Direct visualization of cell to cell coupling: transfer of fluorescent probes in living mammalian pancreatic acini

A technique whereby it is possible directly to observe the movement of organic molecules from cell to cell in living mammalian exocrine glands is described. Thin translucent segments of mouse pancreas are mounted in a superfusion bath. Fluorescent probes are injected intracellularly via fine micropipettes and fluorescence observed. Both fluorescein (mol. wt. 332) and procion yellow (mol. wt. 697) are shown to be transferred from the injection cell to neighbouring acinar cells. This shows directly the existence of intercellular communicating pathways.