Patch clamp recordings from membranes which contain gap junction channels

Identification of oscillatory firing neurons associated with locomotion in the earthworm through synapse imaging

We used FM imaging to identify neurons that receive sensory feedback from the body wall in a circuit for octopamine (OA)-evoked rhythmic locomotion in the earthworm, Eisenia fetida. We visualized synapses in which postsynaptic neurons receive the sensory feedback, by using FM1-43 dye to label the synapses of both motor and sensory pathways that are associated with locomotion, then clearing the motor pathway synapse labeling, and finally identifying the target synapses by distinguishing physiologically functional synapses through destaining using a high-K(+) solution. A pair of synaptic regions associated with the sensory feedback was found to be located two or three cell body-widths away from the midline, between the anterior parts of the roots of the second lateral nerves (LNs) at the segmental ganglia (SGs). Using conventional intracellular recording and dye loading of the cell bodies surrounding these synaptic regions, we identified a pair of bilateral neurons with cell bodies larger than those of other cells in these regions, and named them "Oscillatory firing neurons Projecting to Peripheral nerves" (OPPs). These had a bipolar shape and projected neurites to the ipsilateral first and third LNs, fired rhythmically, and had a burst timing synchronized with the motor pattern bursts from the ipsilateral first LNs. Current injection into an OPP caused firing in the ipsilateral first LNs, supporting the hypothesis that OPPs functionally project to the peripheral nerves. OPPs also sent neurites to the adjacent anterior and posterior SGs, suggesting connections with the adjacent segments. We conclude that FM imaging can be used to identify neurons involved in specific functions, and that OPPs are the first neurons to be associated with OA-induced locomotion in the earthworm.

Magnesium gating of cardiac gap junction channels

We aimed to study kinetics of modulation by intracellular Mg(2+) of cardiac gap junction (Mg(2+) gate). Paired myocytes of guinea-pig ventricle were superfused with solutions containing various concentrations of Mg(2+). In order to rapidly apply Mg(2+) to one aspect of the gap junction, the non-junctional membrane of one of the pair was perforated at nearly the connecting site by pulses of nitrogen laser beam. The gap junction conductance (G(j)) was measured by clamping the membrane potential of the other cell using two-electrode voltage clamp method. The laser perforation immediately increased G(j), followed by slow G(j) change with time constant of 3.5 s at 10 mM Mg(2+). Mg(2+) more than 1.0 mM attenuated dose-dependently the gap junction conductance and lower Mg(2+) (0.6 mM) increased G(j) with a Hill coefficient of 3.4 and a half-maximum effective concentration of 0.6 mM. The time course of G(j) changes was fitted by single exponential function, and the relationship between the reciprocal of time constant and Mg(2+) concentration was almost linear. Based on the experimental data, a mathematical model of Mg(2+) gate with one open state and three closed states well reproduced experimental results. One-dimensional cable model of thirty ventricular myocytes connected to the Mg(2+) gate model suggested a pivotal role of the Mg(2+) gate of gap junction under pathological conditions.

Gap junctions in Malpighian tubules ofAedes aegypti

We present electrical, physiological and molecular evidence for substantial electrical coupling of epithelial cells in Malpighian tubules via gap junctions. Current was injected into one principal cell of the isolated Malpighian tubule and membrane voltage deflections were measured in that cell and in two neighboring principal cells. By short-circuiting the transepithelial voltage with the diuretic peptide leucokinin-VIII we largely eliminated electrical coupling of principal cells through the tubule lumen,thereby allowing coupling through gap junctions to be analyzed. The analysis of an equivalent electrical circuit of the tubule yielded an average gap-junction resistance (Rgj) of 431 kΩ between two cells. This resistance would stem from 6190 open gap-junctional channels,assuming the high single gap-junction conductance of 375 pS found in vertebrate tissues. The addition of the calcium ionophore A23187 (2 μmol l–1) to the peritubular Ringer bath containing 1.7 mmol l–1 Ca2+ did not affect the gap-junction resistance, but metabolic inhibition of the tubule with dinitrophenol (0.5 mmol l–1) increased the gap-junction resistance 66-fold,suggesting the regulation of gap junctions by ATP. Lucifer Yellow injected into a principal cell did not appear in neighboring principal cells. Thus, gap junctions allow the passage of current but not Lucifer Yellow. Using RT-PCR we found evidence for the expression of innexins 1, 2, 3 and 7 (named after their homologues in Drosophila) in Malpighian tubules. The physiological demonstration of gap junctions and the molecular evidence for innexin in Malpighian tubules of Aedes aegypti call for the double cable model of the tubule, which will improve the measurement and the interpretation of electrophysiological data collected from Malpighian tubules.

Selective permeability of gap junction channels

Gap junctions mediate the transfer of small cytoplasmic molecules between adjacent cells. A family of gap junction proteins exist that form channels with unique properties, and differ in their ability to mediate the transfer of specific molecules. Mutations in a number of individual gap junction proteins, called connexins, cause specific human diseases. Therefore, it is important to understand how gap junctions selectively move molecules between cells. Rules that dictate the ability of a molecule to travel through gap junction channels are complex. In addition to molecular weight and size, the ability of a solute to transverse these channels depends on its net charge, shape, and interactions with specific connexins that constitute gap junctions in particular cells. This review presents some data and interpretations pertaining to mechanisms that govern the differential transfer of signals through gap junction channels.

Biophysical characteristics of gap junctions in vascular wall cells: implications for vascular biology and disease

The role gap junction channels play in the normal and abnormal functioning of the vascular wall is the subject of much research. The biophysical properties of gap junctions are an essential component in understanding how gap junctions function to allow coordinated relaxation and contraction of vascular smooth muscle. This study reviews the properties thus far elucidated and relates those properties to tissue function. We ask how biophysical and structural properties such as gating, permselectivity, subconductive states and channel type (heteromeric vs homotypic vs heterotypic) might affect vascular smooth muscle tone.

Rapid and direct effects of pH on connexins revealed by the connexin46 hemichannel preparation

pH is a potent modulator of gap junction (GJ) mediated cell-cell communication. Mechanisms proposed for closure of GJ channels by acidification include direct actions of H+ on GJ proteins and indirect actions mediated by soluble intermediates. Here we report on the effects of acidification on connexin (Cx)46 cell-cell channels expressed in Neuro-2a cells and Cx46 hemichannels expressed in Xenopus oocytes. Effects of acidification on hemichannels were examined macroscopically and in excised patches that permitted rapid (<1 ms) and uniform pH changes at the exposed hemichannel face. Both types of Cx46 channel were found to be sensitive to cytoplasmic pH, and two effects were evident. A rapid and reversible closure was reproducibly elicited with short exposures to low pH, and a poorly reversible or irreversible loss occurred with longer exposures. We attribute the former to pH gating and the latter to pH inactivation. Half-maximal reduction of open probability for pH gating in hemichannels occurs at pH 6.4. Hemichannels remained sensitive to cytoplasmic pH when excised and when cytoplasmic [Ca2+] was maintained near resting ( approximately 10(-7) M) levels. Thus, Cx46 hemichannel pH gating does not depend on cytoplasmic intermediates or a rise in [Ca2+]. Rapid application of low pH to the cytoplasmic face of open hemichannels resulted in a minimum latency to closure near zero, indicating that Cx46 hemichannels directly sense pH. Application to closed hemichannels extended their closed time, suggesting that the pH sensor is accessible from the cytoplasmic side of a closed hemichannel. Rapid closure with significantly reduced sensitivity was observed with low pH application to the extracellular face, but could be explained by H+ permeation through the pore to reach an internal site. Closure by pH is voltage dependent and has the same polarity with low pH applied to either side. These data suggest that the pH sensor is located directly on Cx46 near the pore entrance on the cytoplasmic side.

Monovalent cation permeation through the connexin40 gap junction channel. Cs, Rb, K, Na, Li, TEA, TMA, TBA, and effects of anions Br, Cl, F, acetate, aspartate, glutamate, and NO3

The unitary conductances and permeability sequences of the rat connexin40 (rCx40) gap junction channels to seven monovalent cations and anions were studied in rCx40-transfected neuroblastoma 2A (N2A) cell pairs using the dual whole cell recording technique. Chloride salt cation substitutions (115 mM principal salt) resulted in the following junctional maximal single channel current-voltage relationship slope conductances (gamma 1 in pS): CsC1 (153), RbC1 (148), KC1 (142), NaC1 (115), LiC1 (86), TMAC1 (71), TEAC1 (63). Reversible block of the rCx40 channel was observed with TBA. Potassium anion salt gamma j are: Kglutamate (160), Kacetate (160), Kaspartate (158), KNO3 (157), KF (148), KC1 (142), and KBr (132). Ion selectivity was verified by measuring reversal potentials for current in rCx40 gap junction channels with asymmetric salt solutions in the two electrodes and using the Goldman-Hodgkin-Katz equation to calculate relative permeabilities. The permeabilities relative to Li+ are: Cs+ (1.38), Rb+ (1.32), K+ (1.31), Na+ (1.16), TMA+ (0.53), TEA+ (0.45), TBA+ (0.03), Cl- (0.19), glutamate+ (0.04), and NO(3)- (0.14), assuming that the monovalent anions permeate the channel by forming ion pairs with permeant monovalent cations within the pore thereby causing proportionate decreases in the channel conductance. This hypothesis can account for why the predicted increasing conductances with increasing ion mobilities in an essentially aqueous channel were not observed for anions in the rCx40 channel. The rCx40 effective channel radius is estimated to be 6.6 A from a theoretical fit of the relationship of relative permeability and cation radius.

Monovalent ion selectivity sequences of the rat connexin43 gap junction channel

The relative permeability sequences of the rat connexin 43 (rCx43) gap junction channel to seven cations and chloride were examined by double whole cell patch clamp recording of single gap junction channel currents in rCx43 transfected neuroblastoma 2A (N2A) cell pairs. The measured maximal single channel slope conductances (gammaj, in pS) of the junctional current-voltage relationships in 115 mM XCI were RbC1 (103) > or = CsC1 (102) > KC1 (97) > NaC1 (79) > or = LiC1 (78) > TMAC1 (65) > TEAC1 (53) and for 115 mM KY were KBr (105) > KC1 (97) > Kacetate (77) > Kglutamate (61). The single channel conductance- aqueous mobility relationships for the test cations and anions were linear. However, the predicted minimum anionic and cationic conductances of these plots did not accurately predict the rCx43 channel conductance in 115 mM KC1. Instead, the conductance of the rCx43 channel in 115 mM KC1 was accurately predicted from cationic and anionic conductance-mobility plots by applying a mobility scaling factor Dx/Do, which depends upon the relative radii of the permeant ions to an estimated pore radius. Relative permeabilities were determined for all of the monovalent catious and anions tested from asymmetric salt reversal potential measurements and the Goldman-Hodgkin-Katz voltage equation. These experiments estimate the relative chloride to potassium permeability to be 0.13. The relationship between the relative cation permeability and hydrated radius was modeled using the hydrodynamic equation assuming a pore radius of 6.3 +/- 0.4 A. Our data quantitatively demonstrate that the rCx43 gap junction channel is permeable to monovalent atomic and organic cations and anions and the relative permeability sequences are consistent with an Eisenman sequence II or I, respectively. These predictions about the rCx43 channel pore provide a useful basis for future investigations into the structural determinants of the conductance and permeability properties of the connexin channel pore.

Gap junction channel gating and permselectivity: their roles in co-ordinated tissue function

1. The gating and permselective properties of gap junction channels are important parameters to determine in delineating their role in co-ordinated tissue function. This is of particular relevance in non-excitable tissues. 2. In the present study gating characteristics of rCx43 are demonstrated. The mean open times are of the order of 0.45-1.1 s. These values are extraordinarily large. 3. The permselectivity of a variety of gap junction channels is also illustrated to show the poor selectivity properties of gap junctions and, hence, their potential to allow the permeation of second messenger molecules. 4. Gating and permselectivity, along with diffusion modelling, produce a picture for gap junction channels that is consistent with physiologically relevant modulation or regulation of gap junction channel patency.

Gap junctions in vascular tissues. Evaluating the role of intercellular communication in the modulation of vasomotor tone

Integration and coordination of responses among vascular wall cells are critical to the local modulation of vasomotor tone and to the maintenance of circulatory homeostasis. This article reviews the vast literature concerning the principles that govern the initiation and propagation of vasoactive stimuli among vascular smooth muscle cells, which are nominally the final effectors of vasomotor tone. In light of the abundance of new information concerning the distribution and function of gap junctions between vascular wall cells throughout the vascular tree, particular attention is paid to this integral aspect of vascular physiology. Evidence is provided for the important contribution of intercellular communication to vascular function at all levels of the circulation, from the largest elastic artery to the terminal arterioles. The thesis of this review is that the presence of gap junctions, in concert with the autonomic nervous system, pacemaker cells, myogenic mechanisms, and/or electrotonic current spread (both hyperpolarizing and depolarizing waves through gap junctions), confers a plasticity, adaptability, and flexibility to vasculature that may well account for the observed diversity in regulation and function of vascular tissues throughout the vascular tree. It is hoped that the summary information provided here will serve as a launching pad for a new discourse on the mechanistic basis of the integrative regulation and function of vasculature, which painstakingly accounts for the undoubtedly complex and manifold role of gap junctions in vascular physiology/dysfunction.

Gap junctions in excitable cells

Gap junction channels are an integral part of the conduction or propagation of an action potential from cell to cell. Gap junctions have rather unique gating and permeability properties which permit the movement of molecules from cell to cell. These molecules may not be directly linked to action potentials but can alter nonjunctional processes within cells, which in turn can affect conduction velocity. The data described in this review reveal that, for the majority of excitable cells, there are two limiting factors, with respect to gap junctions, that affect the conduction/propagation of action potentials. These are (1) the total number of channels and (2) the selective permeability of the channels. Interestingly, voltage dependence and the time course of voltage inactivation (kinetics) are not rate limiting steps under normal physiological conditions for any of the connexins studied so far. Only specialized rectifying electrical synapses utilize strong voltage dependence and rapid kinetics to permit or deny the continued propagation of an action potential.

Size and selectivity of gap junction channels formed from different connexins

Gap junction channels have long been viewed as static structures containing a large-diameter, aqueous pore. This pore has a high permeability to hydrophilic molecules of approximately 900 daltons in molecular weight and a weak ionic selectivity. The evidence leading to these conclusions is reviewed in the context of more recent observations primarily coming from unitary channel recordings from transfected connexin channels expressed in communication-deficient cell lines. What is emerging is a more diverse view of connexin-specific gap junction channel structure and function where electrical conductance, ionic selectivity, and dye permeability vary by one full order of magnitude or more. furthermore, the often held contention that channel conductance and ionic or molecular selectivity are inversely proportional is refuted by recent evidence from five distinct connexin channels. The molecular basis for this diversity of channel function remains to be identified for the connexin family of gap junction proteins.

Description of interacting channel gating using a stochastic Markovian model

Single-channel recordings from membrane patches frequently exhibit multiple conductance levels. In some preparations, the steady-state probabilities of observing these levels do not follow a binomial distribution. This behavior has been reported in sodium channels, potassium channels, acetylcholine receptor channels and gap junction channels. A non-binomial distribution suggests interaction of the channels or the presence of channels or the presence of channels with different open probabilities. However, the current trace sometimes exhibits single transitions spanning several levels. Since the probability of simultaneous transitions of independent channels is infinitesimally small, such observations strongly suggest a cooperative gating behavior. We present a Markov model to describe the cooperative gating of channels using only the all-points current amplitude histograms for the probability of observing the various conductance levels. We investigate the steady-state (or equilibrium) properties of a system of N channels and provide a scheme to express all the probabilities in terms of just two parameters. The main feature of our model is that lateral interaction of channels gives rise to cooperative gating. Another useful feature is the introduction of the language of graph theory which can potentially provide a different avenue to study ion channel kinetics. We write down explicit expressions for systems of two, three and four channels and provide a procedure to describe the system of N channels.

Selectivity of connexin-specific gap junctions does not correlate with channel conductance

Connexins form a variety of gap junction channels that vary in their developmental and tissue-specific levels of expression, modulation of gating by transjunctional voltage and posttranslational modification, and unitary channel conductance (gamma j). Despite a 10-fold variation in gamma j, whether connexin-specific channels possess distinct ionic and molecular permeabilities is presently unknown. A major assumption of the conventional model for a gap junction channel pore is that gamma j is determined primarily by pore diameter. Hence, molecular size permeability limits should increase and ionic selectivity should decrease with increasing channel gamma j (and pore diameter). Equimolar ion substitution of 120 mmol/L KCl for potassium glutamate was used to determine the unitary conductance ratios for rat connexin40 and connexin43, chicken connexin43 and connexin45, and human connexin37 channels functionally expressed in communication-deficient mouse neuroblastoma (N2A) cells. Comparison of experimental and predicted conductance ratios based on the aqueous mobilities of all ions according to the Goldman-Hodgkin-Katz current equation was used to determine relative anion-to-cation permeability ratios. Direct correlation of junctional conductance with dye transfer of two fluorescein-derivatives (2 mmol/L 6-carboxyfluorescein or 2',7'-dichlorofluorescein) was also performed. Both approaches revealed a range of selectivities and permeabilities for all five different connexins that was independent of channel conductance. These results are not consistent with the conventional simple aqueous pore model of a gap junction channel and suggest a new model for connexin channel conductance and permselectivity based on electrostatic interactions. Divergent conductance and permeability properties are features of other classes of ion channels (eg, Na+ and K+ channels), implying similar mechanisms for selectivity.

Differential regulation of distinct types of gap junction channels by similar phosphorylating conditions

Studies on physiological modulation of intercellular communication mediated by protein kinases are often complicated by the fact that cells express multiple gap junction proteins (connexins; Cx). Changes in cell coupling can be masked by simultaneous opposite regulation of the gap junction channel types expressed. We have examined the effects of activators and inhibitors of protein kinase A (PKA), PKC, and PKG on permeability and single channel conductance of gap junction channels composed of Cx45, Cx43, or Cx26 subunits. To allow direct comparison between these Cx, SKHep1 cells, which endogenously express Cx45, were stably transfected with cDNAs coding for Cx43 or Cx26. Under control conditions, the distinct types of gap junction channels could be distinguished on the basis of their permeability and single channel properties. Under various phosphorylating conditions, these channels behaved differently. Whereas agonists/antagonist of PKA did not affect permeability and conductance of all gap junction channels, variable changes were observed under PKC stimulation. Cx45 channels exhibited an additional conductance state, the detection of the smaller conductance states of Cx43 channels was favored, and Cx26 channels were less often observed. In contrast to the other kinases, agonists/antagonist of PKG affected permeability and conductance of Cx43 gap junction channels only. Taken together, these results show that distinct types of gap junction channels are differentially regulated by similar phosphorylating conditions. This differential regulation may be of physiological importance during modulation of cell-to-cell communication of more complex cell systems.

Selective dye and ionic permeability of gap junction channels formed by connexin45

Gap junctions are thought to mediate the direct intercellular coupling of adjacent cells by the gating of an aqueous pore permeable to ions and molecules of up to 1 kD or 8 to 14 A in diameter. We performed ion-substitution and dye-transfer experiments to determine the relative Cl-/K+ conductance and dye permeability of anionic fluorescein derivatives in chick connexin45 (Cx45) channels. We demonstrate that Cx45 forms a 26 +/- 6-picosiemen (pS) channel with a maximum detectable Cl- permeability of 0.2 relative to K+ or Cs+. Although homogeneous channel conductances were observed in multichannel recordings, the open probability estimates were indicative of nonhomogeneous gating behavior and occasional cooperativity. A second conductance state of 19 +/- 4 pS begins to predominate at higher voltages. Cx45 gap junctions are permeable to 2',7'-dichlorofluorescein but are not permeable to the more polar 6-carboxyfluorescein dye. These observations suggest that the Cx45 pore diameter is approximately 10 A and is associated with a fixed negative charge within the junctional channel.

Voltage-dependent gating of single gap junction channels in an insect cell line

De novo formation of cell pairs was used to examine the gating properties of single gap junction channels. Two separate cells of an insect cell line (clone C6/36, derived from the mosquito Aedes albopictus) were pushed against each other to provoke formation of gap junction channels. A dual voltage-clamp method was used to control the voltage gradient between the cells (Vj) and measure the intercellular current (Ij). The first sign of channel activity was apparent 4.7 min after cell contact. Steady-state coupling reached after 30 min revealed a conductance of 8.7 nS. Channel formation involved no leak between the intra- and extracellular space. The first opening of a newly formed channel was slow (25-28 ms). Each preparation passed through a phase with only one operational gap junction channel. This period was exploited to examine the single channel properties. We found that single channels exhibit several conductance states with different conductances gamma j; a fully open state (gamma j(main state)), several substates (gamma j(substates)), a residual state (gamma j(residual)) and a closed state (gamma j(closed)). The gamma j(main state) was 375 pS, and gamma j(residual) ranged from 30 to 90 pS. The transitions between adjacent substates were 1/7-1/4 of gamma j(main state). Vj had no effect on gamma j(main state), but slightly affected gamma j (residual). The lj transitions involving gamma j(closed) were slow (15-60 ms), whereas those not involving gamma j(closed) were fast (< 2 ms). An increase in Vj led to a decrease in open channel probability. Depolarization of the membrane potential (Vm) increased the incidence of slow transitions leading to gamma j(closed). We conclude that insect gap junctions possess two gates, a fast gate controlled by Vj and giving rise to gamma j(substates) and gamma j(residual), and a slow gate sensitive to Vm and able to close the channel completely.

Axolemmal and septal conduction in the impedance of the earthworm medial giant nerve fiber

Ionic conduction in the axolemmal and septal membranes of the medial giant fiber (MGF) of the earthworm (EW) Lumbricus terrestris was assessed by impedance spectroscopy in the frequency range 2.5-1000 Hz. Impedance loci in the complex plane were described by two semi-circular arcs, one at a lower characteristic frequency (100 Hz) and the other at a higher frequency (500 Hz). The lower frequency arc had a chord resistance of 53 k omega and was not affected by membrane potential changes or ion channel blockers [tetrodotoxin (TTX), 3,4-diaminopyridine (3,4-DAP), 4-aminopyridine (4-AP), and tetraethylammonium (TEA)]. The higher frequency arc had a chord resistance of 274 k omega at resting potential, was voltage-dependent, and was affected by the addition of TTX, 3,4-DAP, 4-AP, and TEA to the physiological EW salines. When all four blockers were added to the bathing solution, the impedance locus was described by two voltage-independent arcs. Considering the effects of these and other (i.e., Cd and Ni) ion channel blockers, we conclude that: 1) the higher frequency locus reflects conduction by voltage-sensitive ion channels in the axolemmal membrane, which contains at least four ion channels selective for sodium, calcium, and potassium (delayed rectifier and calcium-dependent), and 2) the lower frequency locus reflects voltage-insensitive channels in the septal membrane, which separates adjacent MGFs.

Connexin37 forms high conductance gap junction channels with subconductance state activity and selective dye and ionic permeabilities

Gap junctions are thought to mediate the direct intercellular coupling of adjacent cells by the open-closed gating of an aqueous pore permeable to ions and molecules of up to 1 kDa or 10-14 A in diameter. We symmetrically altered the ionic composition or asymmetrically added 6-carboxyfluorescein (6-CF, M(r) = 376), a fluorescent tracer, to pairs of connexin37-transfected mouse neuro2A cells to examine the ionic and dye permeability of human connexin37 channels. We demonstrate that the 300-pS channel formed by connexin37 has an effective relative anion/cation permeability ratio of 0.43, directly converts to at least one intermediate (63 pS) subconductance state, and that 6-CF dye transfer is accompanied by a 24% decrease in unitary channel conductance. These observations favor a new interpretation of the gap junction pore consistent with direct ion-channel interactions or electrostatic charge effects common to more conventional multistate ion channels. These results have distinct implications about the different forms of intercellular signaling (cationic, ionic, and/or biochemical) that can occur depending on the expression and conformation of the connexin channel proteins.

Ion flow in the bath and flux interactions between channels

We present an exact solution to the linearized Nernst-Planck-Poisson equation for spherically symmetric current flow. This solution differs from Levitt's solution (Levitt, D. G. 1992. Biophys. J., Eq. A5) by its dependence on an additional parameter, which is equal to the net ion flux for monovalent ion-selective channels. For ion-selective channels, this solution may provide better boundary conditions to modelling the flow in the channel pore itself, although only at low salt concentrations. We use the solution to estimate the effects of flux interaction between closely packed channels.

Voltage-dependent gap junctional conductance in hepatopancreatic cells of Procambarus clarkii

Properties of gap junction channels present between specific cell types constituting the hepatopancreas of the crayfish (Procambarus clarkii) were investigated using the dual whole cell voltage clamp technique. Four different cell types (E, Fe, R and B) were identified on the basis of their morphology using light and electron microscopy. Although junctional conductance (Gj) could not be measured in B-B cell pairs, junctional currents were resolved in both homologous and heterologous combinations of the other cell types. E-E, Fe-Fe, and E-Fe cell pairs exhibited strong dependence on inside-out voltage (Vi-o), such that Gj increased with hyperpolarization to a maximal plateau reached at approximately -40 mV and was abolished with depolarization > 10 mV. The Gj-Vi-o relationship can be described by a squared Boltzmann relation with A = 0.101 and V0 = 0.135 mV. In this system, sensitivity of the junctions to transjunctional voltage was slight, if present at all. Gating mechanisms were complex, as evidence by the presence of multiple unitary channel conductance states. Single channel recordings showed that large unitary conductances (> 200 pS) were generally found between E-E, Fe-Fe, and E-Fe cell pairs, whereas smaller channel sizes (< 90 pS) were detected between R-R cell pairs.

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.

Multichannel recordings from membranes which contain gap junctions. II. Substates and conductance shifts

Substates which can last up to several seconds are found in the 100-pS channel of the earthworm septum, a putative gap junction channel. The conductance of these substates is highly variable from preparation to preparation, and they are found at almost every fraction of the whole channel conductance. Another phenomenon seen in multichannel recordings is the "conductance shift": here the current passed by several open channels differs from an integral multiple of the current when only one channel is open. These shifts can be modelled by 1) a resistance in series with the channel or 2) long-lived substates. Each of these models fails in particular cases to explain either the magnitude or direction of the shifts. It is possible that both effects are simultaneously present.

Double whole-cell patch-clamp characterization of gap junctional channels in isolated insect epidermal cell pairs

Double whole-cell patch-clamp methods were used to characterize junctional membrane conductances in epidermal cell pairs isolated from the prepupal integument of the flour beetle, Tenebrio molitor. The mean initial junctional conductance in 267 cell pairs was 9.5 +/- 1.0 nS (range 0-95 nS). Well-coupled cell pairs uncoupled spontaneously with a half-time of 7.6 min. Adding 5 mM ATP to the pipette solution stabilized coupling with less than a 50% drop occurring after 30 min. Nonjunctional membrane potential was the major determinant of junctional conductance with transjunctional potential playing a minor role. Junctional conductance approached 0 pA at nonjunctional membrane potentials greater than 0 mV and increased with hyperpolarization. The voltage at half-maximal conductance was -26 mV. The time course of the reversible changes in junctional conductance were slow (< or = 30 sec) with time-dependent decay occurring faster and recovery occurring slower with increasing depolarization. Single gap junctional channel activity was recorded in uncoupling cell pairs and in poorly coupled ATP-stabilized cell pairs. One main single channel conductance was observed in each cell pair. The mean single channel conductances from all cell pairs in this study ranged from 197-347 pS (mean 248 pS). Single channel conductance was linear over the +/- 60 mV transjunctional voltage range tested. A broad range of subconductance states of the main state representing 5% of the total open time of measurable main state events was observed. Single channel activity was strongly dependent on the nonjunctional membrane potential, increasing with hyperpolarization.

Mechanically induced electrical and intracellular calcium responses in normal and cancerous mammary cells

Mechanically induced channel activities and increase of intracellular calcium ([Ca2+]i) in normal and cancerous murine mammary cells (MMT 060562) were investigated using the patch clamp technique and Fura-2 fluorescence. Both cell types showed similar properties. Upon mechanical stimulation, activation of the Ca(2+)-dependent K+ channel or outward membrane current was recorded in cells which were several cells distant from the stimulated cell. Mechanical stimulation also induced an increase of [Ca2+]i in the touched cell, and this increase of [Ca2+]i spread to the surrounding cells. The [Ca2+]i signal travelled a distance of 100-200 microns within 20-40 s and then diminished. The presence of cell-to-cell communication between adjacent mammary cells through gap junction was indicated by injection of lucifer yellow and measurements of electrical coupling (coupling constant = 0.2-0.3). The mechanically induced increase of the [Ca2+]i signal spread to adjacent cells even when the stimulated cell had no physical contact with them. In the absence of fluid movement, the pattern of the spread of the [Ca2+]i signal was a concentric circle. However, in the presence of fluid movement, the pattern changed to elongate to the direction of the flow. These findings suggested that a certain factor was released from the mechanically stimulated cell to the extracellular space, and this factor induces the increase of [Ca2+]i in surrounding cells.

Multichannel recordings from membranes which contain gap junctions

We have studied multichannel patch-clamp recordings in earthworm axon septal membranes that contain gap junctions. Though all channels have the same conductance and selectivity, the probabilities of the conductance levels in the majority of the recordings could not be fit by assuming independent and identical channels; in these cases, we found that at least two different open probabilities were required to explain the data. The data thus suggest that, within one junctional membrane complex, there exists a heterogenous channel population of similar but not identical channel types. The analysis also revealed cases where cooperativity between individual channels was the only explanation for the amplitude histograms of the observed multichannel activity. The conclusions drawn are based on a theoretical analysis of multichannel current-amplitude histograms. We derive two tests for independent and identical channels. We analyze the effects of mode shifting. These results are based on the ratio of peaks in the histograms; they are independent of the number of channels in the patch and the model of channel gating. In some cases failure to fulfill the criteria of these tests implied an interdependence or cooperativity between channels. Lastly, we have devised statistical tests for stability of the recording in the presence of variance due to finite sample size.

Multiple-channel conductance states and voltage regulation of embryonic chick cardiac gap junctions

We used the double whole-cell voltage-clamp technique on ventricle cell pairs isolated from 7-day chick heart to measure the conductance of their gap junctions (Gj) and junctional channels (gamma j) with a steady-state voltage difference (Vj) applied across the junction. Currents were recorded from single gap junction channels (ij) as symmetrical rectangular signals of equal size and opposite sign in the two cells, and gamma j was measured from ij/Vj. We observed channel openings at six reproducible conductance levels with means of 42.6, 80.7, 119.6, 157.7, 200.4 and 240.3 pS. More than half of all openings were to the 80- and 160-pS conductance levels. The probability that a high conductance event (e.g., 160 or 240 pS) results from the random simultaneous opening of several 40-pS channels is small, based on their frequency of occurrence and on the prevalence of shifts between small and large conductance states with no intervening 40-pS steps. Our results are consistent with three models of embryonic cardiac gap junction channel configuration: a homogeneous population of 40-pS channels that can open cooperatively in groups of up to six; a single population of large channels with a maximal conductance near 240 pS and five smaller substates; or several different channel types, each with its own conductance. Gj was determined from the junctional current (Ij) elicited by rectangular pulses of applied transjunctional voltage as Ij/Vj. It was highest near 0 Vj and was progressively reduced by application of Vj between 20 and 80 mV or -20 and -80 mV. In response to increases in Vj, Gj decayed in a voltage- and time-dependent fashion. After a 6-sec holding period at 0 Vj, the initial conductance (G(init) measured immediately after the onset of an 80-mV step in Vj was nearly the same as that measured by a 10-mV prepulse. However, during 6-sec pulses of Vj greater than +/- 20 mV, Gj declined over several seconds from G(init)to a steady-state value (Gss). At potentials greater than +/- 20 mV the current decay could be fit with biexponential curves with the slow decay time constant (tau 2) 5-20 times longer than tau 1. For the response to a step to 80 mV Vj, for example, tau 1 = 127 msec and tau 2 = 2.6 sec. The rate of current decay in response to smaller positive or negative steps in Vj was slower, the magnitude of the decline was smaller, and the ratio tau 2/tau 1 decreased.(ABSTRACT TRUNCATED AT 400 WORDS)

Model invariant method for extracting single-channel mean open and closed times from heterogeneous multichannel records

We present a proof that the mean open (and closed) times of the individual channels in a multichannel record can be found in a model-independent fashion. As the results are model independent, they can be derived by assuming the simplest model for all the channels, namely that they all have the basic CLOSED in equilibrium with OPEN scheme. In particular, the method can be applied to patches where the channel population is heterogenous with respect to open probability. Multichannel simulations are performed to test the limits of applicability of this method to restricted amounts of data. One conclusion is that increasing the number of channels does not substantially reduce the errors in estimating the mean times, in spite of the 'increased information' present. We also prove the general applicability of the algorithm of Fenwick et al. (1982) in estimating the mean times without knowledge of the number of channels present, and discuss its limitations. An illustration using experimental data is also given.

Electrical coupling between cells of the insect Aedes albopictus

1. Cell pairs of an insect cell line (Aedes albopictus, clone C6/36) were used to study the electrical properties of intercellular junctions. A double voltage-clamp approach was adopted to control the voltage gradient between the cells and measure the intracellular current flow. 2. Determinations of junctional conductance (gj) revealed two types of intercellular contacts, gap junctions and cytoplasmic bridges. Identification occurred by means of functional criteria, i.e. the dependency of gj on (i) junctional membrane potential, (ii) non-junctional membrane potential, and (iii) heptanol. 3. In cell pairs with putative gap junctions, gj was dependent on the junctional membrane potential (Vj). When determined at the beginning of voltage pulses, gj was insensitive to Vj; when determined at the end of 15 s pulses, it depended on Vj in a bell-shaped manner (70% decrease for a change in Vj of +/- 75 mV). 4. These cell pairs also showed a dependency of gj on the non-junctional membrane potential (Vm). When determined immediately after changing the non-junctional membrane potential in both cells, gj was not affected by Vm; when determined 30 s later, gj was modified by Vm in a S-shaped fashion (100% decrease when Vm was depolarized to +50 mV). 5. Exposure to 3 mM-heptanol gave rise to complete and reversible block of gj in cell pairs with putative gap junctions. 6. Cell pairs susceptible to uncoupling by heptanol revealed junctional currents indicative of the operation of gap junction channels. The single-channel conductance, determined at a Vm of -50 to -70 mV, was 133 pS. 7. In the case of putative cytoplasmic bridges, gj was insensitive to the junctional and non-junctional membrane potential. In addition, it was not affected by 3 mM-heptanol. 8. While most cell pairs showed functional properties characteristic of gap junctions or cytoplasmic bridges, few cell pairs exhibited junctional currents compatible with the co-existence of both junctional structures.

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.

Biophysics of gap junctions

Gap junction channels, now known to be formed of connexins, connect the interiors of apposed cells. These channels can be opened and closed by various physiological stimuli and experimental treatments. They are permeable to ions and neutral molecules up to a size of about 1 kDa or 1.5 nm diameter, including second messengers and metabolites. The processes of gating and of permeation are the subject of this review. Voltage is a readily applied stimulus, and transjunctional voltages, or those between cytoplasm and exterior, affect most junctions. Single channel transitions between open and closed states are rapid and presumably involve a charge movement as occurs with channels of electrically excitable channels of nerve and muscle. Identification of gating domains and charges by domain replacement and site-directed mutagenesis is being pursued. Raising cytoplasmic H+ or Ca2+ concentrations rapidly reduces junctional conductance, and this action is generally reversible, at least in part. A number of lipophilic alcohols, fatty acids and volatile anesthetics have similar actions. Phosphorylation also modulates junctional conductance, and in several cases, sites of phosphorylation are known. These gating processes appear similar to those induced by voltage. Permeability measurement indicates that the channel is aqueous and that permeation is by diffusion with only minor interactions with the channel wall. Differences among junctions are known, but further characterization of connexin and cell specificity is required.

Microelectrode measurements of low frequency electric field effects in cells and tissues

The average intensities of electric fields induced into tissue can be calculated if the morphology and conductivities of the tissue are known, and such values provide one estimate of dosage for a given field exposure level. However, the microanatomical structures of living tissue, which include gap junctions, tight junctions, highly charged cell coats, and extracellular matrices, as well as complex cell shapes, precludes a detailed characterization of the field and current distribution near the cells which are actually responding to the electric fields. This suggests that a more useful electric field dose metric may be one based on an induced physical effect on the cells. Electric fields have at least three distinct physical effects on cells: the normal plasma membrane potential will be altered; the ionic currents and ion distributions at the extracellular surface will be modified; and mechanical forces will be imposed at the cell surface. Each of these effects can, in principle, be measured through the application of specific microelectrode techniques. Here, the feasibility of using various intracellular and extracellular recording methods to obtain dosimetric values, as well as the contribution these measurements could make to our understanding of electric field interactions with biological tissue, are discussed.

Connexin46, a novel lens gap junction protein, induces voltage-gated currents in nonjunctional plasma membrane of Xenopus oocytes

Gap junctions are composed of a family of structural proteins called connexins, which oligomerize into intercellular channels and function to exchange low molecular weight metabolites and ions between adjacent cells. We have cloned a new member of the connexin family from lens cDNA, with a predicted molecular mass of 46 kD, called rat connexin46 (Cx46). Since a full-length cDNA corresponding to the 2.8-kb mRNA was not obtained, the stop codon and surrounding sequences were confirmed from rat genomic DNA. The RNA coding for this protein is abundant in lens fibers and detectable in both myocardium and kidney. Western analysis of both rat and bovine lens membrane proteins, using the anti-MP70 monoclonal antibody 6-4-B2-C6 and three anti-peptide antibodies against Cx46 demonstrates that Cx46 and MP70 are different proteins. Immunocytochemistry demonstrates that both proteins are localized in the same lens fiber junctional maculae. Synthesis of Cx46 in either reticulocyte lysate or Xenopus oocytes yields a 46-kD polypeptide; all anti-Cx46 antisera recognize a protein in rat lens membranes 5-10 kD larger, suggesting substantive lenticular posttranslational processing of the native translation product. Oocytes that have synthesized Cx46 depolarize and lyse within 24 h, a phenomenon never observed after expression of rat connexins 32 or 43 (Cx32 and Cx43). Lysis is prevented by osmotically buffering the oocytes with 5% Ficoll. Ficoll-buffered oocytes expressing Cx46 are permeable to Lucifer Yellow but not FITC-labeled BSA, indicating the presence of selective membrane permeabilities. Cx43-expressing oocytes are impermeable to Lucifer Yellow. Voltage-gated whole cell currents are measured in oocytes injected with dilute concentrations of Cx46 but not Cx43 mRNA. These currents are activated at potentials positive to -10 mV. Unlike other connexins expressed in Xenopus oocytes, these results suggest that unprocessed Cx46 induces nonselective channels in the oolemma that are voltage dependent and opened by large depolarizations.

The propagation potential. An axonal response with implications for scalp-recorded EEG

An electrophysiological response of axons, referred to as the "propagation potential," was investigated. The propagation potential is a sustained voltage that lasts as long as an action potential propagates between two widely spaced electrodes. The sign of the potential depends on the direction of action potential propagation. The electrode towards which the action potential is propagating is positive with respect to the electrode from which it is receding. For normal frog sciatic nerves the magnitude of the propagation potential was 17% of the peak of the extracellular action potential; TEA increased it to 32%. For normal earthworm median or lateral giant fibers it was 30%. A ripple pattern on the propagation potential was attributed to variation in resistance along the length of the worm. Cooling increased the duration of the propagation potential and attenuated the higher frequency components of the ripple pattern. Differential records from two widely spaced intracellular microelectrodes in the same axon differed from the propagation potential. The amplitude of the plateau relative to the peak was smaller, it decreased as the action potential propagated from one electrode site to the other, and the potential did not return to zero as rapidly as for extracellular records. When propagation was blocked by heat, the propagation potential slowly decayed. There was no ripple pattern during the decay. In a volume conductor, electrodes contacting the worm did not show the typical propagation potential, but electrodes located a few centimeters away from the worm did. Simple core-conductor models based on classical action potential theory did not reproduce the propagation potential. More complex, modified core-conductor models were needed to accurately simulate it. The results suggest that long, slowly conducting fibers can contribute to the scalp-recorded EEG.

Opioids at low concentration decrease openings of K+ channels in sensory ganglion neurons

Previous studies showed that low concentrations of opioids prolong the calcium-dependent component of the action potential duration (APD) of dorsal root ganglion (DRG) neurons, whereas higher concentrations shorten the APD. In the present study whole-cell voltage-clamp, as well as cell-attached membrane-patch voltage-clamp, recordings demonstrate that application of picomolar to nanomolar concentrations of mu, delta or kappa opioid agonists (DAGO, DPDPE or dynorphin) to DRG neurons in dissociated cell cultures reversibly decreased the activities of voltage-sensitive K+ channels. Pretreatment of DRG neurons with the opioid receptor antagonists, naloxone (30 nM) or diprenorphine (1 nM) prevented mu/delta or kappa opioid-induced decreases in K+ channel activities, respectively. Since opioids added to the bath solution decreased the activities of K+ channels in the membrane patch sealed off by the pipette tip, our results provide strong evidence that some modes of excitatory modulation of the action potential of DRG neurons are mediated by diffusible second messengers. The data are consonant with our previous studies indicating that opioids can elicit excitatory effects on sensory neurons via cholera toxin-sensitive Gs-linked excitatory opioid receptors coupled to cyclic AMP-dependent ionic channels.