Conductance Of Gap Junction Channels Research Articles

Heterotypic Cx43/Cx45 gap junction channel conductance depends on pulse rate and could rescue from arrhythmogenic activity in cardiac tissue

Abstract Introduction Gap junction (GJ) channels, formed of connexin (Cx) proteins, conduit electrical excitation in the heart. It is well established that GJ conductance (gj) depends on transjunctional voltage (Vj), but little is still known about its dynamics during the spread of cardiac excitation. Transitional tissue of atrioventricular (AV) node expresses Cx43 and Cx45, which could form heterotypic GJs, which are highly sensitive to Vj. Purpose Evaluation of heterotypic gj dynamics of Cx43/Cx45 channels during the propagation of excitation in cardiac tissue. Methods The CE-4SM model of cardiac tissue, consisting of discrete cells connected through dynamic GJs, was developed by combining Fenton-Karma equations, which define cardiac excitability (CE), and a 4-state model (4SM) of GJ channel gating, which describes the dependence of gj on Vj and its kinetics. Global optimization methods were applied for 4SM parameter evaluation to fit gj timecourses obtained from electrophysiological recordings. Experiments were performed using dual-whole-cell patch clamp in Novikoff cells, endogenously expressing Cx43, and Hela cells transfected with Cx45. Results We developed a combined cardiac excitability model (CE-4SM), that allowed us to study an interaction between the spread of cardiac excitation and GJ channel voltage. Modeling results shows that during propagation of excitation, the phase difference between each action potential (AP) in adjacent cells, generates two subsequent Vj spikes. First spike causes small decrease of gj, while the second one, a minor increase. These impulses can accumulate into a large overall decay until the impulse rate-dependent steady-state is reached. This phenomenon is caused by variation of delays between APs and consequentially, different durations and amplitudes of Vj spikes developed across GJ channels. Interestingly, such gj decrease was capable of causing drift of spiral wave rotor or termination of fibrillation-like arrhythmia during our modeling experiments. Also, we performed in-vitro experiments to evaluate 4SM model parameters and validate simulation results that showed gj decay at a high pulse rate, which could occur during reentry-like tachycardia. Conclusions Voltage gating of Cx43/Cx45 GJ channels could act as a control circuit, that lowers gj between the transitional cells of AV node at high impulse rate. Presumably, this could help to reduce the conduction or even act as a block of high-rate impulses passing through AV node. Also, such gj decrease could cause a drift of spiral waves and disruption of fibrillation-like processes. Funding Acknowledgement Type of funding sources: Public Institution(s). Main funding source(s): Lithuanian University of Health Sciences, Institute of Cardiology

Molecular basis for potentiation of Cx36 gap junction channel conductance by n-alcohols and general anesthetics

In our recent study, we have demonstrated that short carbon chain n-alcohols (up to octanol) stimulated while long carbon chain n-alcohols inhibited the conductance of connexin (Cx) 36 (Cx36) gap junction (GJ) channels. In contrast, GJ channels composed of other types of Cxs all were inhibited by n-alcohols independent of their carbon chain length. To identify the putative structural domains of Cx36, responsible for the dual effect of n-alcohols, we performed structural modeling of Cx36 protein docking with hexanol and isoflurane that stimulated as well as nonanol and carbenoxolone that inhibited the conductance of Cx36 GJs and revealed their multiple common docking sites and a single pocket accessible only to hexanol and isoflurane. The pocket is located in the vicinity of three unique cysteine residues, namely C264 in the fourth, and C92 and C87 in the second transmembrane domain of the neighboring Cx36 subunits. To examine the hypothesis that disulphide bonding might be involved in the stimulatory effect of hexanol and isoflurane, we generated cysteine substitutions in Cx36 and demonstrated by a dual whole-cell patch-clamp technique that in HeLa (human cervix carcinoma cell line) and N2A (mouse neuroblastoma cell line) cells these mutations reversed the stimulatory effect of hexanol and isoflurane to inhibitory one, typical of other Cxs that lack respective cysteines and a specific docking pocket for these compounds. Our findings suggest that the stimulatory effect of hexanol and isoflurane on Cx36 GJ conductance could be achieved by re-shuffling of the inter-subunit disulphide bond between C264 and C92 to the intra-subunit one between C264 and C87.

The Residues in the Amino Terminal and First Extracellular Domains and Intracellular Magnesium Influence Cx50 Unitary Gap Junction Channel Conductance

The Residues in the Amino Terminal and First Extracellular Domains and Intracellular Magnesium Influence Cx50 Unitary Gap Junction Channel Conductance

0123 : Effect of the connexin43 downregulation on the electrical gap junction channels in RLE cells

In cells that co-express connexin 43 (Cx43) and Cx40 we have previously shown that the formation and the electrical properties of the gap junction channels (GJCs) is regulated by the increased expression of Cx40 and the ratio of Cx43:Cx40, which demonstrated a relative contribution of each Cx. We aim to further investigate this regulation in the case of a down-regulation of the expression of Cx43. Rat Liver Epithelial (RLE) cells that express Cx43 have been stably transfected with the inducible Ecdysone system to co-express accurate ratios of Cx43:Cx40 (0.9, 1.0 and 1.8). The level of expression of Cx40 is controlled by different concentrations of Ponasterone A, while the level of expression of Cx43 is constant. Cells are transfected with specific small interference RNA (SiRNA) against Cx43 to investigate the specific contribution of Cx43 in the formation and the electrical properties of GJCs by performing dual voltage clamp recordings on cell pairs. Unexpectedly, in native cells the down-regulation of Cx43 increased the electrical coupling, without affecting the voltage-dependence and the single channel conductances characteristic of homotypic Cx43 GJCs. On the contrary, in Cx40-induced cells the down-regulation of Cx43 increased the electrical coupling, the voltage-dependence and the unitary conductance of GJCs in a ratio dependent manner. This suggests a different hetero-assembling of Cxs to non-treated cells, while none homotypic Cx40 channels were observed. These data reveals a specific homo- and hetero-assembling of Cxs and regulation of the formation of GJCs ensured by distinct contribution of Cx43 and Cx40. This leads to determinant specific electrical properties of GJCs for regulating the impulse propagation in the healthy heart, which become pro-arrhythmic in the case of Cxs dysfunction. The biochemical characterization of the down-regulation of Cx43 will be correlated to our findings and to the cardiac pattern of expression. The author hereby declares no conflict of interest

Role of the intercalated disc in cardiac propagation and arrhythmogenesis.

This review article discusses mechanisms underlying impulse propagation in cardiac muscle with specific emphasis on the role of the cardiac cell-to-cell junction, called the “intercalated disc.”The first part of this review deals with the role of gap junction channels, formed by connexin proteins, as a determinant of impulse propagation. It is shown that, depending on the underlying structure of the cellular network, decreasing the conductance of gap junction channels (so-called “electrical uncoupling”) may either only slow, or additionally stabilize propagation and reverse unidirectional propagation block to bidirectional propagation. This is because the safety factor for propagation increases with decreasing intercellular electrical conductance. The role of heterogeneous connexin expression, which may be present in disease states, is also discussed. The hypothesis that so-called ephaptic impulse transmission plays a role in heart and can substitute for electrical coupling has been revived recently. Whereas ephaptic transmission can be demonstrated in theoretical simulations, direct experimental evidence has not yet been presented. The second part of this review deals with the interaction of three protein complexes at the intercalated disc: (1) desmosomal and adherens junction proteins, (2) ion channel proteins, and (3) gap junction channels consisting of connexins. Recent work has revealed multiple interactions between these three protein complexes which occur, at least in part, at the level of protein trafficking. Such interactions are likely to play an important role in the pathogenesis of arrhythmogenic cardiomyopathy, and may reveal new therapeutic concepts and targets.

Atrial fibrillation‐associated Connexin40 mutants make hemichannels and synergistically form gap junction channels with novel properties

Mutations of Cx40 (GJA5) have been identified in people with lone chronic atrial fibrillation including G38D and M163V which were found in the same patient. We used dual whole cell patch clamp procedures to examine the transjunctional voltage (Vj) gating and channel conductance properties of these two rare mutants. Each mutant exhibited slight alterations of Vj gating properties and increased the gap junction channel conductance (γj) by 20-30 pS. While co-expression of the two mutations had similar effects on Vj gating, it synergistically increased γj by 50%. Unlike WTCx40 or M163V, G38D induced activity of a dominant 271 pS hemichannel.

The Role of Amino Terminus of Mouse Cx50 in Determining Transjunctional Voltage-Dependent Gating and Unitary Conductance

Amino-terminus and carboxyl-terminus of connexins have been proposed to be responsible for the transjunctional voltage-dependent gating ( V j-gating) and the unitary gap junction channel conductance ( γ j). To better understand the molecular structure(s) determining the V j-gating properties and the γ j of Cx50, we have replaced part of the amino-terminus of mCx50 by the corresponding domain of mCx36 to engineer a chimera Cx50-Cx36N, and attached GFP at the carboxyl-terminus of mCx50 to construct Cx50-GFP. The dual whole-cell patch-clamp technique was used to test the resulting gap junction channel properties in N2A cells. The Cx50-Cx36N gap junction channel lowered the sensitivity of steady-state junctional conductance to V j ( G j/ V j relationship), slowed V j-gating kinetics, and reduced γ j as compared to Cx50 channel. Cx50-GFP gap junction channel showed similar V j-gating properties and γ j to Cx50 channel. We further characterized a mutation, Cx50N9R, where the Asn (N) at the ninth position of Cx50 was replaced by the corresponding Arg (R) at Cx36. The G j/ V j relationship of Cx50N9R channel was significantly changed; most strikingly, the macroscopic residual conductance ( G min) was near zero. Moreover, the single Cx50N9R channel only displayed one open state ( γ j = 132 ± 4 pS), and no substate could be detected. Our data suggest that the NT of Cx50 is critical for both the V j-gating and the γ j, and the introduction of a positively charged Arg at the ninth position reduced the G min with a correlated disappearance of the substate at the single channel level.

Short-range functional interaction between connexin35 and neighboring chemical synapses.

Auditory afferents terminating as mixed, electrical, and chemical, synapses on the goldfish Mauthner cells constitute an ideal experimental model to study the properties of gap junctions in the nervous system as well as to explore possible functional interactions with the other major form of interneuronal communication--chemically mediated synapses. By combining confocal microscopy and freeze-fracture replica immunogold labeling (FRIL), we found that gap junctions at these synapses contain connexin35 (Cx35), the fish ortholog of the neuron-specific human and mouse connexin36 (Cx36). Conductance of gap junction channels at these endings is known to be dynamically modulated by the activity of their co-localized chemically mediated glutamatergic synapses. By using simultaneous pre- and postsynaptic recordings at these single terminals, we demonstrate that such functional interaction takes place in the same ending, within a few micrometers. Accordingly, we also found evidence by confocal and FRIL double-immunogold labeling that the NR1 subunit of the NMDA glutamate receptor, proposed to be a key regulatory element, is present at postsynaptic densities closely associated with gap junction plaques containing Cx35. Given the widespread distribution of Cx35- and Cx36-mediated electrical synapses and glutamatergic synapses, our data suggest that the local functional interactions observed at these identifiable junctions may also apply to other electrical synapses, including those in mammalian brain.

Transfection of mammalian cells with connexins and measurement of voltage sensitivity of their gap junctions

Vertebrate gap junction channels are formed by a family of more than 20 connexin proteins. These gap junction proteins are expressed with overlapping cellular and tissue specificity, and coding region mutations can cause human hereditary diseases. Here we present a summary of what has been learned from voltage clamp studies performed on cell pairs either endogenously expressing gap junctions or in which connexins are exogenously expressed. General protocols presented here are currently used to transfect mammalian cells with connexins and to study the biophysical properties of the heterologously expressed connexin channels. Transient transfection is accomplished overnight with maximal expression occurring at about 36 h; stable transfectants normally can be generated within three or four weeks through colony selection. Electrophysiological protocols are presented for analysis of voltage dependence and single-channel conductance of gap junction channels as well as for studies of chemical gating of these channels.

Nontransformed cells can normalize gap junctional communication with transformed cells

We demonstrate that the Src kinase can augment gap junctional communication between cells derived from homozygous null Cx43 knockout mice. The total conductance between Src transformed cells was nearly twice that of nontransformed cells. In addition, the unitary conductance of the majority of single channel events between transformed cells was about 35% greater than that of nontransformed cells. Analysis showed that both nontransformed and transformed cells expressed at least two populations of channels, suggesting that Src increased junctional conductance by up-regulating one population and/or by increasing the unitary conductance of another population of channels. Interestingly, the conductance displayed by heterologous pairs of transformed and nontransformed cells resembled that of nontransformed cells. The majority of single channel events between heterologous pairs shifted back to lower conductances that were exhibited by nontransformed cells. Thus, nontransformed cells can effectively “normalize” the conductance of gap junction channels expressed by adjacent tumor cells.

Acute ischemia-induced gap junctional uncoupling and arrhythmogenesis.

Sudden cardiac death forms a major cause of mortality. Myocardial ischemia-induced ventricular fibrillation (VF) is frequently the underlying mechanism. Ventricular arrhythmias arise in two distinct phases during the first hour of ischemia. The first, the 1A phase, has been extensively studied, and few studies relate to the 1B phase. The latter is associated with intercellular electrical uncoupling, mediated by decreased conductance of gap junction channels. Although the relation between gap junctional uncoupling and decreased conduction velocity appears clear under normoxic conditions, additional factors contribute to conduction slowing during ischemia, and VF occurs preferentially at moderate levels of uncoupling. A potential mechanism of arrhythmias depends on temporary electrotonic depression of intrinsically viable tissue by the large bulk of the ischemic zone. This causes conduction slowing and conduction block in the surviving layers, leading to arrhythmias. These arrhythmias then resolve with progression of uncoupling. It is unknown whether either accelerated uncoupling or maintenance of gap junctional communication is antiarrhythmic. Ischemic preconditioning postpones both gap junctional uncoupling and occurrence of VF. Given the burden of sudden death and the large number of casualties in the low-risk population, there is, even in the era of implantable cardiac defibrillators, need for further understanding the mechanism of ischemia-induced VF.

The obligatory link: role of gap junctional communication in endothelium-dependent smooth muscle hyperpolarization

Although an endothelium-derived hyperpolarizing factor (EDHF) has often been hypothesized to underpin vascular relaxations that are independent of nitric oxide (NO) and prostanoids, bioassay techniques have failed to confirm the existence of a freely transferable EDHF in a consistent fashion. Indeed, observations that inhibitors of direct cell–cell coupling such as connexin–mimetic peptides (e.g. Gap 26 and 27) and glycyrrhetinic acid derivatives attenuate “EDHF-type” smooth muscle hyperpolarizations and relaxations suggest that an electrotonic spread of endothelial hyperpolarization via myoendothelial and homocellular smooth muscle gap junctions plays an obligatory role in such responses. The endothelial hyperpolarization that initiates relaxation results from the opening of K Ca channels and is sustained by capacitative Ca 2+ entry triggered by the depletion of intracellular Ca 2+ stores in the endoplasmic reticulum. EDHF-type relaxations are also associated with a prostanoid-independent synthesis of cAMP that increases the conductance of gap junction channels and enhances the transmission of endothelial hyperpolarization through the vascular wall in a permissive fashion. This review will discuss the roles of these interacting signalling pathways in the mediation of the EDHF phenomenon.

Connexin35 Mediates Electrical Transmission at Mixed Synapses on Mauthner Cells

Auditory afferents terminating as "large myelinated club endings" on goldfish Mauthner cells are identifiable "mixed" (electrical and chemical) synaptic terminals that offer the unique opportunity to correlate physiological properties with biochemical composition and specific ultrastructural features of individual synapses. By combining confocal microscopy and freeze-fracture replica immunogold labeling (FRIL), we demonstrate that gap junctions at these synapses contain connexin35 (Cx35). This connexin is the fish ortholog of the neuron-specific human and mouse connexin36 that is reported to be widely distributed in mammalian brain and to be responsible for electrical coupling between many types of neurons. Similarly, connexin35 was found at gap junctions between neurons in other brain regions, suggesting that connexin35-mediated electrical transmission is common in goldfish brain. Conductance of gap junction channels at large myelinated club endings is known to be dynamically modulated by the activity of their colocalized glutamatergic synapses. We show evidence by confocal microscopy for the presence of the NR1 subunit of the NMDA glutamate receptor subtype, proposed to be a key regulatory element, at these large endings. Furthermore, we also show evidence by FRIL double-immunogold labeling that the NR1 subunit of the NMDA glutamate receptor is present at postsynaptic densities closely associated with gap junction plaques containing Cx35 at mixed synapses across the goldfish hindbrain. Given the widespread distribution of electrical synapses and glutamate receptors, our results suggest that the plastic properties observed at these identifiable junctions may apply to other electrical synapses, including those in mammalian brain.

Short-Range Functional Interaction Between Connexin35 and Neighboring Chemical Synapses

Auditory afferents terminating as mixed, electrical, and chemical, synapses on the goldfish Mauthner cells constitute an ideal experimental model to study the properties of gap junctions in the nervous system as well as to explore possible functional interactions with the other major form of interneuronal communication--chemically mediated synapses. By combining confocal microscopy and freeze-fracture replica immunogold labeling (FRIL), we found that gap junctions at these synapses contain connexin35 (Cx35), the fish ortholog of the neuron-specific human and mouse connexin36 (Cx36). Conductance of gap junction channels at these endings is known to be dynamically modulated by the activity of their co-localized chemically mediated glutamatergic synapses. By using simultaneous pre- and postsynaptic recordings at these single terminals, we demonstrate that such functional interaction takes place in the same ending, within a few micrometers. Accordingly, we also found evidence by confocal and FRIL double-immunogold labeling that the NR1 subunit of the NMDA glutamate receptor, proposed to be a key regulatory element, is present at postsynaptic densities closely associated with gap junction plaques containing Cx35. Given the widespread distribution of Cx35- and Cx36-mediated electrical synapses and glutamatergic synapses, our data suggest that the local functional interactions observed at these identifiable junctions may also apply to other electrical synapses, including those in mammalian brain.

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.

Differences in gap junction channels between cardiac myocytes, fibroblasts, and heterologous pairs.

Cultures of neonatal rat heart cells contain predominantly myocytes and fibroblastic cells. Most abundant are groups of synchronously contracting myocytes, which are electrically well coupled through large gap junctions. Cardiac fibroblasts may be electrically coupled to each other and to adjacent myocytes, be it with low intercellular conductances. Nevertheless, synchronously beating myocytes interconnected via a fibroblast were present, demonstrating that nonexcitable cardiac cells are capable of passive impulse conduction. In fibroblast pairs as well as in myocyte-fibroblast cell pairs, no sensitivity to junctional voltage could be detected when transjunctional conductance was > 1-2 nS. However, in pairs coupled by a conductance of < 1 nS, complex voltage-dependent gating was evident; gap junction channel open probability decreased with increasing junctional voltage but a nongated residual conductance remained at all voltages tested. Single gap junction channel conductance between fibroblasts was approximately 21 pS, very similar to an approximately 18-pS channel conductance that was found between myocytes next to the major conductance of 43 pS. Single-channel conductance in heterologous myocyte-fibroblast gap junctions was approximately 32 pS, which matches the theoretical value of 29 pS for gap junction channels composed of a fibroblast connexon and the major myocyte connexon. A site-directed antibody against rat heart gap junction protein connexin43 recognized gap junctions between neonatal cardiomyocytes, as demonstrated by immunocytochemical labeling. In contrast, junctions between fibroblasts showed no labeling, while in myocyte-fibroblast junctions labeling occasionally was present. Our results suggest the existence of two gap junction proteins between neonatal rat cardiocytes, connexin43 and another yet unidentified connexin. An alternative explanation (cell-specific regulation of the conductance of connexin43 channels) is discussed.

Connexin32 gap junction channels in stably transfected cells: unitary conductance

Pairs of SKHep1 cells, which are derived from a highly metastatic human hepatoma, were studied using the whole cell voltage clamp technique with patch-type electrodes containing CsCl as the major ionic species. In 12 of 81 cell pairs, current flow through junctional membranes was detectable; in the remaining 69 cell pairs, junctional conductance was less than the noise limit of our recording apparatus (worst case: 10 pS). Macroscopic junctional conductance (gj) in the small percentage of pairs where it was detectable ranged from 100 to 600 pS. Unitary junctional conductance (gamma j) determined in the lowest conductance pairs or after reducing conductance with a short exposure to the uncoupling agent halothane was 25-35 pS. To study properties of gap junction channels formed of connexin32, the parental SKHep1 cell line was stably transfected with a plasmid containing cDNA that encodes connexin32, the major gap junction protein of rat liver cells. In 85 of 98 pairs of voltage clamped connexin32-transfected SKHep1 cells, macroscopic gj was greater than 1 nS; gj increased with time after dissociation (from 1.8 +/- 0.6 [mean +/- SE; n = 7] nS at 2 h after plating to 9.3 +/- 2.2 [n = 9] nS, the maximal value, at 24 h). Unitary conductance of gap junction channels between pairs of transfected SKHep1 cells was measured in low conductance pairs and after reducing gj by exposure to halothane or heptanol. Histograms of gamma j values in transfected cells, in 10 experiments where greater than 100 transitions were measurable, displayed two peaks; 120-130 pS and 25-35 pS. The smaller size corresponded to channels that were occasionally detected in the parental cells. We therefore conclude that connexin32 forms gap junctions channels of the 120-130 pS size class.

Biophysical properties of gap junctions between freshly dispersed pairs of mouse pancreatic beta cells

Coupling between beta cells through gap junctions has been postulated as a principal mechanism of electrical synchronization of glucose-induced activity throughout the islet of Langerhans. We characterized junctional conductance between isolated pairs of mouse pancreatic beta cells by whole-cell recording with two independent patch-clamp circuits. Most pairs were coupled (67%, n = 155), although the mean junctional conductance (gj) (215 +/- 110 pS) was lower than reported in other tissues. Coupling could be recorded for long periods, up to 40 min. Voltage imposed across the junctional or nonjunctional membranes had no effect on gj. Up to several hours of treatment to increase intracellular cAMP levels did not affect gj. Electrically coupled pairs did not show transfer of the dye Lucifer yellow. Octanol (2 mM) reversibly decreased gj. Lower concentrations of octanol (0.5 mM) and heptanol (0.5 mM) than required to uncouple beta cells decreased voltage-dependent K+ and Ca2+ currents in nonjunctional membranes. Although gj recorded in these experiments would be expected to be provided by current flowing through only a few channels of the unitary conductance previously reported for other gap junctions, no unitary junctional currents were observed even during reversible suppression of gj by octanol. This result suggests either that the single channel conductance of gap junction channels between beta cells is smaller than in other tissues (less than 20 pS) or that the small mean conductance is due to transitions between open and closed states that are too rapid or too slow to be resolved.

Cloning of a gap junctional protein from vascular smooth muscle and expression in two-cell mouse embryos.

Gap junctional proteins (connexins) form aqueous channels that enable direct cell-cell transfer of ions and small molecules. The distribution and conductance of gap junction channels in cardiac muscle determine the pattern and synchrony of cellular activation. However, the capacity for smooth muscle to restrict contractile events temporally and spatially suggests that cell-cell coupling or its regulation may be decidedly different in this tissue. We isolated a cDNA from vascular smooth muscle which encodes a connexin (Mr 43,187) structurally homologous to cardiac connexin43. Vascular smooth muscle connexin43 mRNA was expressed prominently in smooth muscle tissues, cultured vascular myocytes, and arterial endothelial cells. A model for functional expression of connexins was developed in two-cell B6D2 mouse embryos. Microinjection of in vitro transcribed vascular smooth muscle connexin43 mRNA was shown to be sufficient to induce intercellular coupling in previously uncoupled blastomeres. Through the construction of two deletion mutants of connexin43, we also show that the formation of cell-to-cell connections does not depend upon a predicted cytoplasmic region within 98 residues of the carboxyl terminus. Finally, the identification of connexin43 in smooth muscle and endothelial cells provides supporting evidence for the existence of heterocellular coupling between cells of the vascular intima.

Single channel currents of homo- and heterologous gap junctions between cardiac fibroblasts and myocytes

Recently, the use of the double whole-cell patch-clamp technique enabled conductance measurements of single gap junctional channels. Different values have been measured in pairs of rat lacrimal cells (6), murine acinar cells and chinese hamster ovary cells (9), embryonic chick heart- (10) and neonatal rat heart myocytes (7). We here present evidence that the conductance of gap junction channels between two different cell types originating from the same tissue, neonatal rat heart, is different. In mixed cultures of cardiac fibroblasts and myocytes, gap junction channels between fibroblasts have a single channel conductance of only 22 pS, while those between myocytes have a conductance of 43 pS. Fibroblasts can be electrically coupled to myocytes through channels having an intermediate conductance of 29 pS, a value which matches very well with the theoretically expected conductance of a gap junction channel composed of a fibroblast- and a myoblast connexon (hemichannel). These data provide direct evidence on the single channel level that in heterologous gap junction channels the composing connexons retain their cell-specific properties.