Gap junction channel modulation--a physiological viewpoint

Mono-Heteromeric Configurations of Gap Junction Channels Formed by Connexin43 and Connexin45 Reduce Unitary Conductance and Determine both Voltage Gating and Metabolic Flux Asymmetry

In cardiac tissues, the expression of multiple connexins (Cx40, Cx43, Cx45, and Cx30.2) is a requirement for proper development and function. Gap junctions formed by these connexins have distinct permeability and gating mechanisms. Since a single cell can express more than one connexin isoform, the formation of hetero-multimeric gap junction channels provides a tissue with an enormous repertoire of combinations to modulate intercellular communication. To study further the perm-selectivity and gating properties of channels containing Cx43 and Cx45, we studied two monoheteromeric combinations in which a HeLa cell co-transfected with Cx43 and Cx45 was paired with a cell expressing only one of these connexins. Macroscopic measurements of total conductance between cell pairs indicated a drastic reduction in total conductance for mono-heteromeric channels. In terms of Vj dependent gating, Cx43 homomeric connexons facing heteromeric connexons only responded weakly to voltage negativity. Cx45 homomeric connexons exhibited no change in Vj gating when facing heteromeric connexons. The distributions of unitary conductances (γj) for both mono-heteromeric channels were smaller than predicted, and both showed low permeability to the fluorescent dyes Lucifer yellow and Rhodamine123. For both mono-heteromeric channels, we observed flux asymmetry regardless of dye charge: flux was higher in the direction of the heteromeric connexon for MhetCx45 and in the direction of the homomeric Cx43 connexon for MhetCx43. Thus, our data suggest that co-expression of Cx45 and Cx43 induces the formation of heteromeric connexons with greatly reduced permeability and unitary conductance. Furthermore, it increases the asymmetry for voltage gating for opposing connexons, and it favors asymmetric flux of molecules across the junction that depends primarily on the size (not the charge) of the crossing molecules.

Hemichannel composition and electrical synaptic transmission: molecular diversity and its implications for electrical rectification

Unapposed hemichannels (HCs) formed by hexamers of gap junction proteins are now known to be involved in various cellular processes under both physiological and pathological conditions. On the other hand, less is known regarding how differences in the molecular composition of HCs impact electrical synaptic transmission between neurons when they form intercellular heterotypic gap junctions (GJs). Here we review data indicating that molecular differences between apposed HCs at electrical synapses are generally associated with rectification of electrical transmission. Furthermore, this association has been observed at both innexin and connexin (Cx) based electrical synapses. We discuss the possible molecular mechanisms underlying electrical rectification, as well as the potential contribution of intracellular soluble factors to this phenomenon. We conclude that asymmetries in molecular composition and sensitivity to cellular factors of each contributing hemichannel can profoundly influence the transmission of electrical signals, endowing electrical synapses with more complex functional properties.

Gating properties of heterotypic gap junction channels formed of connexins 40, 43, and 45

Connexins (Cxs) 40, 43, and 45 are expressed in many different tissues, but most abundantly in the heart, blood vessels, and the nervous system. We examined formation and gating properties of heterotypic gap junction (GJ) channels assembled between cells expressing wild-type Cx40, Cx43, or Cx45 and their fusion forms tagged with color variants of green fluorescent protein. We show that these Cxs, with exception of Cxs 40 and 43, are compatible to form functional heterotypic GJ channels. Cx40 and Cx43 hemichannels are unable or effectively impaired in their ability to dock and/or assemble into junctional plaques. When cells expressing Cx45 contacted those expressing Cx40 or Cx43 they readily formed junctional plaques with cell-cell coupling characterized by asymmetric junctional conductance dependence on transjunctional voltage, V(j). Cx40/Cx45 heterotypic GJ channels preferentially exhibit V(j)-dependent gating transitions between open and residual states with a conductance of approximately 42 pS; transitions between fully open and closed states with conductance of approximately 52 pS in magnitude occur at substantially lower ( approximately 10-fold) frequency. Cx40/Cx45 junctions demonstrate electrical signal transfer asymmetry that can be modulated between unidirectional and bidirectional by small changes in the difference between holding potentials of the coupled cells. Furthermore, both fast and slow gating mechanisms of Cx40 exhibit a negative gating polarity.

Calcium influx into dendrites of the leech Retzius neuron evoked by 5-hydroxytryptamine

5-Hydroxytryptamine (5-HT) is a ubiquitous neurotransmitter and neuromodulator that affects neural circuits and behaviours in vertebrates and invertebrates. In the present study, we have investigated 5-HT-induced Ca(2+) transients in subcellular compartments of Retzius neurons in the leech central nervous system using confocal laser scanning microscopy, and studied the effect of 5-HT on the electrical coupling between the Retzius neurons. Bath application of 5-HT (50mM) induced a Ca(2+) transient in axon, dendrites and cell body of the Retzius neuron. This Ca(2+) transient was significantly faster and larger in dendrites than in axon and cell body, and was half-maximal at a 5-HT concentration of 5-12mM. The Ca(2+) transient was suppressed in the absence of extracellular Ca(2+) and by methysergide (100mM), a non-specific antagonist of metabotropic 5-HT receptors, and was strongly reduced by bath application of the Ca(2+) channel blocker Co(2+) (2mM). Injection of the non-hydrolysable GTP analogue GTPgammaS increased and prolonged the dendritic 5-HT-induced Ca(2+) transient. The non-selective protein kinase inhibitor H7 (100mM) and the adenylate cyclase inhibitor SQ22536 (500 mM) did not affect the Ca(2+) transient, and the membrane-permeable cAMP analogue dibutyryl-cAMP (500 mM) did not mimic the effect of 5-HT application. 5-HT reduced the apparent electrical coupling between the two Retzius neurons, whereas suppression of the Ca(2+) influx by removal of external Ca(2+) improved the transmission of action potentials at the electrical synapses which are located between the dendrites of the adjacent Retzius neurons. The results indicate that 5-HT induces a Ca(2+) influx through calcium channels located primarily in the dendrites, and presumably activated by a G protein-coupled 5-HT receptor. The dendritic Ca(2+) increase appears to modulate the excitability of, and the synchronization between, the two Retzius neurons.

Transmembrane alpha-helices in the gap junction membrane channel: systematic search of packing models based on the pair potential function

Recent progress in the field of electron cryo-microscopy and image analysis has shown that there is an overwhelming need to interpret medium resolution (5 to 10 A) three-dimensional maps. Traditional methods of fitting amino acid residues into electron density using molecular modeling programs must be supplemented with further analysis. We have used a potential of mean force (PMF) method, derived from Boltzmann statistics in protein structure, to generate models for the packing of alpha-helices, using pairwise potentials between amino acid residues. The approach was tested using the three-dimensional map of a recombinant cardiac gap junction membrane channel provided by electron cryo-crystallography (Unger et al., 1997; 1999a, 1999b) which had a resolution of 7.5 A in the membrane plane and 21 A in the vertical direction. The dodecameric channel was formed by the end-to-end docking of two hexamers, each of which displayed 24 rods of density in the membrane interior, which was consistent with an alpha-helical conformation for the four transmembrane domains of each connexin subunit. Based on the three-dimensional map and the amino acid sequence for the 4 transmembrane domains determined by hydropathy analysis, we used the modeling utility SymServ (Macke et al., 1998) to build hexameric connexons with 24 transmembrane alpha-helices. Canonical alpha-helices were aligned to the axes of the rods of density and translated along the density so that the center of masses coincided. The PMF function was used to evaluate 162,000 conformations for each of the 24 possible alpha-helical packing models. Since the different packing models yielded different energy distributions, the pair potential function appears to be a promising tool for evaluating the packing of alpha-helices in membrane proteins. The analysis will be refined by energy calculations based on the expectations that the outer boundary of the channel will be formed by hydrophobic residues in contact with the lipids.

Three-dimensional structure of a recombinant gap junction membrane channel

Gap junction membrane channels mediate electrical and metabolic coupling between adjacent cells. The structure of a recombinant cardiac gap junction channel was determined by electron crystallography at resolutions of 7.5 angstroms in the membrane plane and 21 angstroms in the vertical direction. The dodecameric channel was formed by the end-to-end docking of two hexamers, each of which displayed 24 rods of density in the membrane interior, which is consistent with an alpha-helical conformation for the four transmembrane domains of each connexin subunit. The transmembrane alpha-helical rods contrasted with the double-layered appearance of the extracellular domains. Although not indicative for a particular type of secondary structure, the protein density that formed the extracellular vestibule provided a tight seal to exclude the exchange of substances with the extracellular milieu.

Structure of cardiac gap junction intercellular channels

Gap junction proteins, termed connexins, constitute a multigene family of polytopic membrane channel proteins that have four hydrophobic transmembrane domains with the N- and C-termini located on the cytoplasmic membrane face. The principal gap junction protein in the heart, alpha 1 connexin (also designated Cx43), mediates action potential propagation between cells in order to synchronize cardiac contraction. alpha 1 connexin channels are concentrated in gap junction plaques located in the intercalated disks. The intercellular channel is formed by the docking of two hemi-channels, termed connexons, formed by a ring of six 43-kDa alpha 1 connexin subunits. Each subunit is asymmetric with an axial ratio of 4-5:1 with approximately 20 A extending into the extracellular gap approximately 50 A spanning the lipid bilayer and approximately 50 A extending into the cytoplasmic space. We have recently grown two-dimensional crystals of a recombinant C-terminal truncation mutant of alpha 1 connexin (designated alpha 1Cx263T) that are ordered to better than 7 A resolution. Projection density maps derived by electron cryocrystallography revealed that the intercellular channel is lined by six alpha-helices, and there is a second ring of six alpha-helices at the interface with the membrane lipids. These rings of alpha-helices are staggered by 30 degrees, which predicts that the two connexons in the channel are staggered by 30 degrees such that each connexin subunit in one connexon interacts with two subunits in the apposed connexon. Such a quaternary arrangement may confer stability in the docking of the connexons to form a tight electrical seal for intercellular current flow during cardiac conduction.

Structure of gap junction intercellular channels

Gap junctions are formed by a multigene family of polytopic membrane channel proteins, connexins, that have four hydrophobic transmembrane domains and their N and C termini located on the cytoplasmic membrane face. The C-terminal tail plays important roles in channel regulation by pH and phosphorylation. Conserved cysteine residues stabilize the conformation of the extracellular loops that mediate the 'docking' between connexons in the intercellular channel. Over the past year, electron cryocrystallography of two-dimensional crystals of a truncated recombinant alpha 1 (Cx43) has revealed that the transmembrane boundary of the intercellular channel is lined with alpha helices. Furthermore, a ring of alpha helices resides at the interface with the membrane lipids. A three-dimensional analysis based on images recorded from tilted crystals should reveal the location and secondary structure of additional transmembrane domains, as well as provide important structural details about the interactions between connexins within a hemi-channel and connexon-connexon interactions in the extracellular gap.

Inhibition of calmodulin expression prevents low-pH-induced gap junction uncoupling in Xenopus oocytes

The relationship among intracellular pH (pHi), -log10 intracellular Ca2+ concentration (pCai) and gap junctional conductance, the participation of Ca2+ stores, and the role of calmodulin in channel regulation have been studied in Xenopus oocytes, expressing the native connexin (Cx38), exposed to external solutions bubbled with 100% CO2. The time courses of pHi [measured with 2',7'-bis(2-carboxyethyl)-5,6-carboxyfluorscein (BCECF)], pCai (measured with the membrane-associated fura-C18) and junctional conductance (measured with a double voltage-clamp protocol) were compared. The data obtained confirm previous evidence for a closer relationship of junctional conductance with pCai than with pHi. Evidence for an inhibitory effect of intracellularly injected ruthenium red or 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) on CO2-induced uncoupling, coupled to negative results with Ca2+-free external solutions, point to a low-pHi -induced Ca2+ release from internal stores, likely to be primarily mitochondria. The hypothesis proposing a participation of calmodulin in channel gating was tested by studying the effects of calmodulin expression inhibition by intracellular injection of oligonucleotides antisense to the two calmodulin mRNAs expressed in the oocytes. Calmodulin mRNA was permanently eliminated in 5h. The oocytes injected with the antisense nucleotides progressively lost the capacity to uncouple with CO2 within 72 h. The effect of CO2 on junctional conductance was reduced by approximately 60% in 24 h, by approximately 76% in 48 h and by approximately 93% in 72 h. Oocytes that had lost gating sensitivity to CO2 partially recovered gating competency following calmodulin injection. The data suggest that lowered pHi uncouples gap junctions by a Ca2+- calmodulin-mediated mechanism.

Electron microscopic image analysis of cardiac gap junction membrane crystals

Cardiac gap junctions play an important functional role in the myocardium by electrically coupling adjacent cells, thereby providing a low resistance pathway for cell-to-cell propagation of the action potential. Two-dimensional crystallization of biochemically isolated rat ventricular gap junctions has been accomplished by an in situ method in which membrane suspensions are sequentially dialyzed against low concentrations of deoxycholate and dodecyl-beta-D-maltoside. Lipids are partially extracted without solubilizing the protein, and the increased protein concentration facilitates two-dimensional crystallization in the native membrane environment. The two-dimensional crystals have a nominal resolution of 16 A and display plane group symmetry p6 with a = b = 85 A and gamma = 120 degrees. Projection density maps show that the connexons in cardiac gap junctions are formed by a hexameric cluster of alpha 1 connexin subunits. Protease cleavage of alpha 1 connexin from 43 to 30 kDa releases approximately 13kDa from the carboxy-tail, and the projection density maps are not significantly altered. Uranyl acetate stain penetrates the ion channel, whereas phosphotungstic acid is preferentially deposited over the lipid regions. This differential staining can be used to selectively probe the central channel of the connexon and the interface between the connexon and the lipid. The hexameric design of alpha 1 connexons appears to be a recurring quaternary motif for the multigene family of gap junction proteins.

Gap junction gating sensitivity to physiological internal calcium regardless of pH in Novikoff hepatoma cells

Gap junction conductance (Gj) and channel gating sensitivity to voltage, Ca2+, H+, and heptanol were studied by double whole-cell clamp in Novikoff hepatoma cell pairs. Channel gating was observed at transjunctional voltages (Vj) > +/- 50 mV. The cells readily uncoupled with 1 mM 1-heptanol. With heptanol, single (gap junctional) channel events with unitary conductances (gamma j) of 46 and 97 pS were detected. Both Ca(2+)-loading (EGTA.Ca) and acidifying (100% CO2) solutions caused uncoupling. However, CO2 was effective when Ca2+i was buffered with EGTA (a H(+)-sensitive Ca-buffer) but not with BAPTA (1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid) (a H(+)-insensitive Ca-buffer), suggesting a Ca(2+)-mediated H+ effect on gap junctions. This was tested by monitoring the Gj decay at different pCai values (9, 6.9, 6.3, 6, and 5.5; 1 mM BAPTA) and pHi values (7.2 or 6.1, 10 mM 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid and 2-(N-morpholino)ethansulphonic acid, respectively). With pCai > or = 6.9 (pH 7.2 or 6.1), Gj decreased to 10-70% of initial values in approximately 40 min, following single exponential decays (tau = approximately 28 min). With pCai 6-6.3 (pH 7.2 or 6.1), Gj decreased to 10-25% of initial values in 15 min (tau = approximately 5 min); the Student t gave a P = 0.0178. With pCa 5.5 the cells uncoupled in less than 1 min (tau = approximately 20 s). Low pHi affected neither time course nor shape of Gj decay at any pCai tested. The data indicate that these gap junctions are sensitive to [Ca2+]i in the physiological range (< or = 500 nM) and that low pHi, without an increase in [Ca2+]i, neither decreases Gj nor increases channel sensitivity to Ca2+.

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.

Amine modulation of electrical coupling in the pyloric network of the lobster stomatogastric ganglion

1. The neurons of the pyloric network of the lobster (Panulirus interruptus) stomatogastric ganglion organize their rhythmic motor output using both chemical and electrical synapses. The 6 electrical synapses within this network help set the firing phases of the pyloric neurons during each rhythmic cycle. We examined the modulatory effects of the amines dopamine (DA), serotonin (5HT) and octopamine (Oct) on coupling at all the electrical synapses of the pyloric network. 2. Electrical coupling within the pacemaker group [anterior burster (AB) to pyloric dilator (PD), and PD-PD] was non-rectifying, while coupling at the other electrical synapses [AB to ventral dilator (VD), PD-VD, lateral pyloric (LP) to pyloric (PY), and PY-PY] was rectifying. 3. Dopamine decreased or increased the coupling strength of all the pyloric electrical synapses: the sign of the effect depended upon which neuron was the target of current injection. For example, DA decreased AB-->PD coupling (i.e., when current was injected into the AB) but increased coupling in the other direction, PD-->AB. Dopamine decreased AB to VD coupling when current was injected into either neuron. Serotonin also had mixed effects; it enhanced PD-->AB coupling but decreased AB to VD and PD to VD coupling in both directions. Octopamine's only effect was to reduce PD-->VD coupling. 4. Dopamine increased the input resistance of the AB neuron but decreased the input resistance of the PD and VD neurons. Serotonin reduced the input resistance of the VD and PY neurons, while Oct did not significantly change the input resistance of any pyloric neuron. 5. The characteristic modulation of electrical coupling by each amine may contribute to the unique motor pattern that DA, 5HT and Oct each elicit from the pyloric motor network.

Cell-cell communication in the embryo of Caenorhabditis elegans

We have investigated the pattern of cell-cell communication in embryos of the free-living soil nematode Caenorhabditis elegans. For this, we have established a method for microinjection of tracer dyes into individual blastomeres. After iontophoresis of fluorescent dyes of different molecular weights (Lucifer yellow, LY, M(r) 457; rhodamine-labeled dextran, RD, M(r) 4000), we can visualize intercellular communication pathways. The dye-spread of LY, indicating communication via gap junctions, becomes first visible in the late 2-cell stage. From the 4-cell stage onward all cells appear to be well coupled by communication channels, which allow the free diffusion of LY. In contrast, RD remains restricted to the injected cell and its descendants. After the primordial germcell P4 has been generated in the 24-cell stage, dye-spread of LY into this cell and its somatic sister D is delayed. However, the restricted dye-coupling of D is only temporary. After a brief period it joins the somatic compartment. With the beginning of the morphogenesis phase the two existing germline cells (the daughters of P4) are completely uncoupled from the soma, while the latter still forms a single dye-coupling compartment. Only during the second half of embryogenesis different separate somatic communication compartments are established. We followed the pattern of intercellular communication in the alimentary tract and found a progressive restriction into smaller dye-coupling units. Our data are compared to those found in other systems and discussed with respect to cellular determination and differentiation.

Effects of acid Ca2+ Ringer on passive electrical properties and intracellular ion activities in leech Retzius neuron

1. A significant drop in effective input resistance of the free membrane and an increase in effective coupling resistance in acid Ca2+ Ringer (complete replacement of Na+ with Ca2+, pH 4) compared to control medium has been obtained in leech Retzius neurons. 2. In neutral Ca2+ Ringer (pH 7.2), effective input resistance increased while effective coupling resistance did not change. In acid sodium, leech Ringer (pH 4) effective input resistance increased while coupling resistance decreased. 3. Ten millimolar manganese and 10 mmol tetraethylammonium did not block conductance changes obtained in acid Ca2+ Ringer. 4. Intracellular activity of Na+ decreased, cellular activity of Cl- increased and intracellular K+ activity was unchanged in both acid and neutral Ca2+ Ringer. 5. The main difference was intracellular acidification in acid Ca2+ Ringer while intracellular pH was unchanged in neutral Ca2+ Ringer. 6. We discuss the possibility that in acid Ca2+ Ringer, intracellular acidification in leech neurons may be responsible for accompanying conductive changes.

Possible role during exocytosis of a Ca(2+)-activated channel in neurohypophysial granules

Ion channels from bovine neurohypophysial granules were incorporated into artificial lipid bilayers. The larger amplitude channel is permeable to cations and exhibits multiple conductances. The channel opens only in the presence of free Ca2+, but is inhibited by relatively high Ca2+ concentrations. Release of vasopressin from permeabilized neurohypophysial terminals also shows a similar biphasic dependence on Ca2+. Release is selectively inhibited by low concentrations of the long-chain alcohol octanol, but not by high concentrations of ethanol, as is the neurosecretory granule Ca(2+)-activated cation channel. Furthermore, Ca(2+)-evoked release and channel activity are both inhibited by the long-chain tetraethylammonium analogs decamethonium and decyl-triethyl ammonium bromide. The close correlation between channel and release properties lead us to conclude that the Ca(2+)-activated channel is involved in peptide secretion.

Humoral factors reduce gap junction sensitivity to cytoplasmic pH. I. Organ ablation studies

The sensitivity of gap junctions connecting crayfish lateral axons to uncoupling by axoplasmic acidification was studied after altering the hormonal balance of animals by 1) ablation of eyestalks or sinus glands or 2) inducing long-lasting defensive posturing behavior (stress). Internal pH (pHi) was measured with microelectrodes, and junctional resistance (Rj) was calculated from input and transfer resistances. In isolated nerve cords from intact animals, the maximal Rj (Rjmax) reached after acidification varied diurnally (Rjmax approximately 10 and 0.6 M omega at 0900 and 1800 h, respectively). Basal Rj (20-30 k omega) did not change during the 24-h period. Organ ablation (eyestalks or sinus glands) or stress rendered gap junctions less sensitive to uncoupling by low pHi within 1 h or 2 days; recovery toward control values had different time courses. The reduced pH sensitivity of crayfish junctions seen after eyestalk ablation is attributable to stress in its early phase (lasting 1-2 days) and to ablation of the endocrine organs in its late phase (2-7 days). No striking structural differences accompanied these changes, indicating that the altered properties are not due to major changes in gap junction expression.

Humoral factors reduce gap junction sensitivity to cytoplasmic pH. II. In vitro manipulations

Our previous studies demonstrated a diurnal rhythm in the response of gap junctions between crayfish giant axons to acidification and that the response was reduced after eyestalk ablation, sinus gland removal, or visual stress. In this paper we describe experiments to test whether compounds in the circulating hemolymph were responsible for modulation of the responsiveness gap junction channels to intracellular pH. In axons from destalked animals in which the hemolymph had been replaced with normal saline, the maximal junctional resistance after acidification (Rjmax) reached control values. In contrast, Rjmax reached only 30% of control after acidification in axons from animals that had been destalked but not perfused. Hemolymph drawn after eyestalk ablation was tested on axons from control animals. Treatment with hemolymph drawn 1 day after destalking resulted in control Rjmax values, while treatment with hemolymph drawn 7 days after destalking resulted in Rjmax values of only 5-40%. Similarly, pretreatment for 1 h with 100 microM ecdysterone resulted in low Rjmax values. These experimental results suggest that a circulating compound, most likely ecdysterone or a related molecule, regulates the physiological properties of gap junctions from crayfish lateral axons.

Effects of the anesthetics heptanol, halothane and isoflurane on gap junction conductance in crayfish septate axons: a calcium- and hydrogen-independent phenomenon potentiated by caffeine and theophylline, and inhibited by 4-aminopyridine

This study has monitored junctional and nonjunctional resistance, [Ca2+]i and [H+]i, and the effects of various drugs in crayfish septate axons exposed to neutral anesthetics. The uncoupling efficiency of heptanol and halothane is significantly potentiated by caffeine and theophylline. The modest uncoupling effects of isoflurane, described here for the first time, are also enhanced by caffeine. Heptanol causes a decrease in [Ca2+]i and [H+]i both in the presence and absence of either caffeine or theophylline. A similar but transient effect on [Ca2+]i is observed with halothane. 4-Aminopyridine strongly inhibits the uncoupling effects of heptanol. The observed decrease in [Ca2+]i with heptanol and halothane and negative results obtained with different [Ca2+]o, (Ca2+)-channel blockers (nisoldipine and Cd2+) and ryanodine speak against a Ca2+ participation. Negative results obtained with 3-isobutyl-1-methylxanthine, forskolin, CPT-cAMP, 8Br-cGMP, adenosine, phorbol ester and H7, superfused in the presence and absence of caffeine and/or heptanol, indicate that neither the heptanol effects nor their potentiation by caffeine are mediated by cyclic nucleotides, adenosine receptors and kinase C. The data suggest a direct effect of anesthetics, possibly involving both polar and hydrophobic interactions with channel proteins. Xanthines and 4-aminopyridine may participate by influencing polar interactions. The potentiating effect of xanthines on cell-to-cell uncoupling by anesthetics may provide some clues on the nature of cardiac arrhythmias in patients treated with theophylline during halothane anesthesia.

Effects of caffeine and ryanodine on low pHi-induced changes in gap junction conductance and calcium concentration in crayfish septate axons

Electrical uncoupling of crayfish septate axons with acidification has been shown to cause a substantial increase in [Ca2+]i which closely matches in percent the increase in junctional resistance. To determine the origin of [Ca2+]i increase, septate axons have been exposed either to drugs that influence Ca2+ release from internal stores, caffeine and ryanodine, or to treatments that affect Ca2+ entry. A large increase in junctional resistance and [Ca2+]i maxima above controls resulted from addition of caffeine (10-30 mM) to acetate solutions, while a substantial decrease in both parameters was observed when exposure to acetate-caffeine was preceded by caffeine pretreatment. In contrast, ryanodine (1-10 microM) always caused a significant decrease in junctional resistance and [Ca2+]i maxima when applied either together with acetate or both before and with acetate. Calcium channel blockers such as La3+, Cd2+ and nisoldipine had no effect, while an increase in the [Ca2+] of acetate solutions either decreased junctional resistance and [Ca2+]i maxima or had no effect. The data suggest that cytoplasmic acidification causes an increase in [Ca2+]i by releasing Ca2+ from caffeine and ryanodine-sensitive Ca2+ stores. The increase in [Ca2+]i results in a decrease in gap junction conductance.

Protein phosphorylation and hydrogen ions modulate calcium-induced closure of gap junction channels

The regulation of the cell-to-cell pathway formed by gap junctions seems to involve the interaction of the junctional channels with either calcium or hydrogen ions, as well as protein phosphorylation and calmodulin. These mechanisms of junctional regulation have been considered to act independently on specific sites of the gap junction protein; however, the possibility that they may be interrelated has not been adequately explored mainly due to the difficulties involved in simultaneous measurement of intracellular cations and protein phosphorylation. To further understanding of mechanisms regulating gap junctions, we have internally perfused coupled lateral axons from crayfish with solutions containing different calcium and hydrogen concentrations under conditions favoring phosphorylation, while monitoring the junctional conductance. We found that calcium ions regulate cell communication probably through a direct interaction with the channel protein. Phosphorylation and low pH do not alter junctional conductance themselves, but appear only to modulate the effects of calcium, possibly by altering the affinity of the channel for calcium. We propose that a combination of free intracellular calcium and protein phosphorylation form an important physiological mechanism regulating intercellular communication.

Increase in gap junction resistance with acidification in crayfish septate axons is closely related to changes in intracellular calcium but not hydrogen ion concentration

Neutral-carrier pH- and Ca-sensitive microelectrodes were used to investigate the relationship between junctional electrical resistance and either pHi or [Ca2+]i in crayfish septate axons uncoupled by acidification. For measuring [Ca2+]i a new neutral carrier sensor sensitive to picomolar [Ca2+] and virtually insensitive to other ions was used. Uncoupling was induced by superfusing the axons with Na-acetate solutions (pH 6.3). With acetate, the time course of changes in junctional resistance differed markedly from that of pHi or [H+]i, and [H+]i peaked 40-90 sec before junctional resistance. The difference in shape and peak time between pHi and junctional resistance curves caused significant hysteresis in the pHi versus junctional resistance relationship. In addition, junctional resistance maxima reached with slow acidification rates were 3-4 times greater than those with fast acidification of similar magnitude. With acetate, [Ca2+]i increased by approximately one order of magnitude from basal values of 0.1-0.3 microM. The curves describing the time course of changes in [Ca2+]i and junctional resistance matched well with each other in shape, peak time and magnitude. Both junctional resistance and [Ca2+]i recovered following a single exponential decay with a time constant of approximately 2 min. Different rates of acidification caused increases in [Ca2+]i and junctional resistance comparable in magnitude. The data indicate that the increase in junctional resistance induced by acidification is more closely related to [Ca2+]i than to [H+]i.

Calmodulin acts as an intermediary for the effects of calcium on gap junctions from crayfish lateral axons

Lateral axons from the abdominal nerve cord of crayfish were internally perfused with the calcium receptor calmodulin (CaM) in solutions with low (pCa greater than 7.0) or high (pCa 5.5) calcium concentrations and studied electrophysiologically and morphologically. Results from these experiments show that when the internal solution contains calcium-activated calmodulin (Ca2+-CaM) the junctional resistance between the axons increases from control values of about 60 to 500-600 k omega in 60 min. In contrast, axons perfused with calmodulin in low calcium solutions maintain their junctional resistance at control levels during the 60-min perfusion. Similar results are obtained when only one or both coupled axons are perfused. The morphological study shows that in the perfused axons the axoplasmic organelles are replaced or grossly perturbed by the perfusion solution up to the region of the synapses. Additionally, in axons perfused with Ca2+-CaM there are regions where the synaptic gap between the membranes decreases from a control 4-6 to 2-3 nm. Both electrophysiological and morphological results can be interpreted as indicating that calcium-activated calmodulin acts directly on the junctional channels to induce their closure.