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

Anesthetics and Cell-Cell Communication: Potential Ca2+-Calmodulin Role in Gap Junction Channel Gating by Heptanol, Halothane and Isoflurane

Cell–cell communication via gap junction channels is known to be inhibited by the anesthetics heptanol, halothane and isoflurane; however, despite numerous studies, the mechanism of gap junction channel gating by anesthetics is still poorly understood. In the early nineties, we reported that gating by anesthetics is strongly potentiated by caffeine and theophylline and inhibited by 4-Aminopyridine. Neither Ca²⁺ channel blockers nor 3-isobutyl-1-methylxanthine (IBMX), forskolin, CPT-cAMP, 8Br-cGMP, adenosine, phorbol ester or H7 had significant effects on gating by anesthetics. In our publication, we concluded that neither cytosolic Ca²⁺_i nor pH_i were involved, and suggested a direct effect of anesthetics on gap junction channel proteins. However, while a direct effect cannot be excluded, based on the potentiating effect of caffeine and theophylline added to anesthetics and data published over the past three decades, we are now reconsidering our earlier interpretation and propose an alternative hypothesis that uncoupling by heptanol, halothane and isoflurane may actually result from a rise in cytosolic Ca²⁺ concentration ([Ca²⁺]_i) and consequential activation of calmodulin linked to gap junction proteins.

Antagonistic Functions of Connexin 43 during the Development of Primary or Secondary Bone Tumors

Despite research and clinical advances during recent decades, bone cancers remain a leading cause of death worldwide. There is a low survival rate for patients with primary bone tumors such as osteosarcoma and Ewing’s sarcoma or secondary bone tumors such as bone metastases from prostate carcinoma. Gap junctions are specialized plasma membrane structures consisting of transmembrane channels that directly link the cytoplasm of adjacent cells, thereby enabling the direct exchange of small signaling molecules between cells. Discoveries of human genetic disorders due to genetic mutations in gap junction proteins (connexins) and experimental data using connexin knockout mice have provided significant evidence that gap-junctional intercellular communication (Gj) is crucial for tissue function. Thus, the dysfunction of Gj may be responsible for the development of some diseases. Gj is thus a main mechanism for tumor cells to communicate with other tumor cells and their surrounding microenvironment to survive and proliferate. If it is well accepted that a low level of connexin expression favors cancer cell proliferation and therefore primary tumor development, more evidence is suggesting that a high level of connexin expression stimulates various cellular process such as intravasation, extravasation, or migration of metastatic cells. If so, connexin expression would facilitate secondary tumor dissemination. This paper discusses evidence that suggests that connexin 43 plays an antagonistic role in the development of primary bone tumors as a tumor suppressor and secondary bone tumors as a tumor promoter.

Inhibitors of connexin and pannexin channels as potential therapeutics

While gap junctions support the exchange of a number of molecules between neighboring cells, connexin hemichannels provide communication between the cytosol and the extracellular environment of an individual cell. The latter equally holds true for channels composed of pannexin proteins, which display an architecture reminiscent of connexin hemichannels. In physiological conditions, gap junctions are usually open, while connexin hemichannels and, to a lesser extent, pannexin channels are typically closed, yet they can be activated by a number of pathological triggers. Several agents are available to inhibit channels built up by connexin and pannexin proteins, including alcoholic substances, glycyrrhetinic acid, anesthetics and fatty acids. These compounds not always strictly distinguish between gap junctions, connexin hemichannels and pannexin channels, and may have effects on other targets as well. An exception lies with mimetic peptides, which reproduce specific amino acid sequences in connexin or pannexin primary protein structure. In this paper, a state-of-the-art overview is provided on inhibitors of cellular channels consisting of connexins and pannexins with specific focus on their mode-of-action and therapeutic potential.

Conduction abnormalities and ventricular arrhythmogenesis: The roles of sodium channels and gap junctions

Ventricular arrhythmias arise from disruptions in the normal orderly sequence of electrical activation and recovery of the heart. They can be categorized into disorders affecting predominantly cellular depolarization or repolarization, or those involving action potential (AP) conduction. This article briefly discusses the factors causing conduction abnormalities in the form of unidirectional conduction block and reduced conduction velocity (CV). It then examines the roles that sodium channels and gap junctions play in AP conduction. Finally, it synthesizes experimental results to illustrate molecular mechanisms of how abnormalities in these proteins contribute to such conduction abnormalities and hence ventricular arrhythmogenesis, in acquired pathologies such as acute ischaemia and heart failure, as well as inherited arrhythmic syndromes.

Connexin channel modulators and their mechanisms of action

Gap junction channels and hemichannels formed by the connexin family of proteins play important roles in many aspects of tissue homeostasis in the brain and in other organs. In addition, connexin channels have been proposed as pharmacological targets in the treatment of a number of human disorders. In this review, we describe the connexin-subtype selectivity and specificity of pharmacological agents that are commonly used to modulate connexin channels. We also highlight recent progress made toward identifying new agents for connexin channels that act in a selective and specific manner. Finally, we discuss developing insights into possible mechanisms by which these agents modulate connexin channel function. This article is part of the Special Issue Section entitled 'Current Pharmacology of Gap Junction Channels and Hemichannels'.

Evidence of intercellular coupling between co-cultured adult rabbit ventricular myocytes and myofibroblasts

Intercellular coupling between ventricular myocytes and myofibroblasts was studied by co-culturing adult rabbit ventricular myocytes with previously prepared layers of cardiac myofibroblasts. Intercellular coupling was examined by: (i) tracking the movement of the fluorescent dye calcein; (ii) immunostaining for connexin 43 (Cx43); and (iii) measurement of intracellular [Ca2+] ([Ca2+]i). The effects of stimulating ventricular myocytes on the underlying myofibroblasts was examined by confocal measurements of [Ca2+]i using fluo-3. When ventricular myocytes were preloaded with calcein and co-cultured with myofibroblasts for 24 h, calcein fluorescence was detected in 52+/-4% (n=8 co-cultures) of surrounding myofibroblasts. Treatment with the gap junction uncoupler heptanol significantly reduced the movement of calcein (12+/-3%, n=6 co-cultures). Immunostaining showed expression of Cx43 in co-cultured myofibroblasts and myocytes. Field stimulation of ventricular myocytes co-cultured with myofibroblasts increased myofibroblast [Ca2+]i, no response was observed after treatment with heptanol or stimulation of fibroblasts in the absence of ventricular myocytes. Action potential parameters of ventricular myocytes in co-culture were similar to control values. However, application of the hormone sphingosine-1-phosphate (S-1-P) to the co-culture caused a depolarization of ventricular myocytes to approximately -20 mV. Sphingosine-1-phosphate had no effect on ventricular myocytes alone. Voltage-clamp measurements of isolated myofibroblasts indicated that S-1-P activated a significant quasi-linear current with a reversal potential of approximately -40 mV. In conclusion, this study shows that stimulation of the ventricular myocyte influences the intracellular Ca2+ of the linked myofibroblast via connexons. These intercellular links also allow the myofibroblasts to influence the electrical activity of the myocyte. This work indicates the nature of the gap junction-mediated bi-directional interactions that occur between ventricular myocyte and myofibroblast.

Characterization of innexin gene expression and functional roles of gap-junctional communication in planarian regeneration

Planaria possess remarkable powers of regeneration. After bisection, one blastema regenerates a head, while the other forms a tail. The ability of previously-adjacent cells to adopt radically different fates could be due to long-range signaling allowing determination of position relative to, and the identity of, remaining tissue. However, this process is not understood at the molecular level. Following the hypothesis that gap-junctional communication (GJC) may underlie this signaling, we cloned and characterized the expression of the Innexin gene family during planarian regeneration. Planarian innexins fall into 3 groups according to both sequence and expression. The concordance between expression-based and phylogenetic grouping suggests diversification of 3 ancestral innexin genes into the large family of planarian innexins. Innexin expression was detected throughout the animal, as well as specifically in regeneration blastemas, consistent with a role in long-range signaling relevant to specification of blastema positional identity. Exposure to a GJC-blocking reagent which does not distinguish among gap junctions composed of different Innexin proteins (is not subject to compensation or redundancy) often resulted in bipolar (2-headed) animals. Taken together, the expression data and the respecification of the posterior blastema to an anteriorized fate by GJC loss-of-function suggest that innexin-based GJC mediates instructive signaling during regeneration.

Short-term effects and teratogenicity of heptanol on embryos of Xenopus laevis

This study examined teratogenic and short-term effects of heptanol on Xenopus embryos. Embryos were exposed for 5h to 2.5mM heptanol at different developmental stages. Teratogenic effects were found in embryos treated from cleavage to early neurula stages. Other heptanol concentrations, shorter exposure time, and the effect of temperature were also assayed. Short-term effects of hepatanol exposure were studied during cleavage and all treated blastulae showed cell separation with delaminated blastomeres inside the blastocel. Disruption of cell adhesion in addition to the uncoupling effect could account for heptanol teratogenicity.

Blockade of brain stem gap junctions increases phrenic burst frequency and reduces phrenic burst synchronization in adult rat

Recent investigations have examined the influence of gap junctional communication on generation and modulation of respiratory rhythm and inspiratory motoneuron synchronization in vitro using transverse medullary slice and en bloc brain stem-spinal cord preparations obtained from neonatal (1-5 days postnatal) mice. Gap junction proteins, however, have been identified in both neurons and glia in brain stem regions implicated in respiratory control in both neonatal and adult rodents. Here, we used an in vitro arterially perfused rat preparation to examine the role of gap junctional communication on generation and modulation of respiratory rhythm and inspiratory motoneuron synchronization in adult rodents. We recorded rhythmic inspiratory motor activity from one or both phrenic nerves before and during pharmacological blockade (i.e., uncoupling) of brain stem gap junctions using carbenoxolone (100 microM), 18alpha-glycyrrhetinic acid (25-100 microM), 18beta-glycyrrhetinic acid (25-100 microM), octanol (200-300 microM), or heptanol (200 microM). During perfusion with a gap junction uncoupling agent, we observed an increase in the frequency of phrenic bursts (~95% above baseline frequency; P < 0.001) and a decrease in peak amplitude of integrated phrenic nerve discharge (P < 0.001). The increase in frequency of phrenic bursts resulted from a decrease in both T(I) (P < 0.01) and T(E) (P < 0.01). In addition, the pattern of phrenic nerve discharge shifted from an augmenting discharge pattern to a "bell-shaped" or square-wave discharge pattern in most experiments. Spectral analyses using a fast Fourier transform (FFT) algorithm revealed a reduction in the peak power of both the 40- to 50-Hz peak (corresponding to the MFO) and 90- to 110-Hz peak (corresponding to the HFO) although spurious higher frequency activity (> or =130 Hz) was observed, suggesting an overall loss or reduction in inspiratory-phase synchronization. Although additional experiments are required to identify the specific brain stem regions and cell types (i.e., neurons, glia) mediating the observed modulations in phrenic motor output, these findings suggest that gap junction communication modulates generation of respiratory rhythm and inspiratory motoneuron synchronization in adult rodents in vitro.

Involvement of gap junctional communication in myogenesis

Cell-to-cell communication plays important roles in development and in tissue morphogenesis. Gap junctional intercellular communication (GJIC) has been implicated in embryonic development of various tissues and provides a pathway to exchange ions, secondary messengers, and metabolites through the intercellular gap junction channels. Although GJIC is absent in adult skeletal muscles, the formation of skeletal muscles involves a sequence of complex events including cell-cell interaction processes where myogenic cells closely adhere to each other. Much experimental evidence has shown that myogenic precursors and developing muscle fibers can directly communicate through junctional channels. This review summarizes current knowledge on the GJIC and developmental events involved in the formation of skeletal muscle fibers and describes recent progress in the investigation of the role of GJIC in myogenesis: evidence of gap junctions in somitic and myotomal tissue as well as in developing muscle fibers in situ, GJIC between perfusion myoblasts in culture, and involvement of GJIC in cytodifferentiation of skeletal muscle cells and in myoblast fusion. A model of intercellular signaling is proposed where GJIC participates to coordinate a multicellular population of interacting myogenic precursors to allow commitment to the skeletal muscle fate.

Increased acetylcholine-induced vasodilation in pregnant rats: A role for gap junctional communication

We have tested the hypothesis that increased gap junctional communication contributes to the augmented endothelium-dependent vasodilation in pregnancy. Contractile force and connexin43 expression were measured in aortic rings from nonpregnant and pregnant rats. Norepinephrine-constricted aortas from pregnant rats were more sensitive to acetylcholine, but not to sodium nitroprusside, compared with those from nonpregnant rats. Vessels from pregnant rats, constricted either with 45 mmol/L KCl or with norepinephrine + 10(-4) mol/L N(G)-monomethyl-L-arginine (L-NMMA), an inhibitor of nitric oxide synthase, also exhibited greater relaxation to acetylcholine. Heptanol, an uncoupler of gap junctional communication, inhibited acetylcholine responses in norepinephrine-constricted aortas from nonpregnant rats but greatly impaired acetylcholine relaxation in aortas from pregnant rats. Heptanol also inhibited in both groups acetylcholine responses in vessels constricted with KCl, only minimally affected acetylcholine relaxation in arteries constricted with norepinephrine + L-NMMA, and did not change sodium nitroprusside-induced relaxation. Tetraethylammonium chloride induced greater contractions in control aortas compared with aortas from pregnant rats. Increased connexin43 mRNA levels were found in the uterus and in the mesenteric, uterine, and thoracic aortic arteries, but not in the heart and brain, from pregnant rats. These results suggest that increased gap junctional communication, possibly due to increased gap junction protein expression, may facilitate the effects of endothelium-derived relaxing factors, contributing to the augmented endothelium-dependent relaxation in arteries from pregnant rats.

Further evidence for the selective disruption of intercellular communication by heptanol

The lack of selective gap junctional uncoupling agents has hampered evaluation of the contribution of intercellular communication to pharmacomechanical coupling and vascular contractility. Thus we further explored the utility and selectivity of heptanol as a gap junctional uncoupling agent in isolated rat aortic rings. Fifty-two aortic rings were obtained from 15 rats and were precontracted to approximately 75% of maximum with phenylephrine (PE). When contraction achieved steady state (approximately 5 min), a single concentration of heptanol (200 microM) was added to each aortic ring at 1- to 3-min intervals for up to 42 min post-PE addition. At early time points (5-10 min after PE), heptanol elicited an approximately 50% loss of tension (i.e., relaxation). At subsequent time points post-PE, a gradual and time-dependent decrease in the magnitude of the heptanol-induced relaxation was observed until, after approximately 40 min, addition of heptanol was associated with little, if any, detectable relaxation. Linear regression analysis of the magnitude of the heptanol-induced relaxation vs. the square root of the elapsed time interval (from addition of PE) revealed a highly significant negative correlation (P < 0.001, R = 0.81). Studies conducted on KCl-precontracted aortic rings revealed no detectable heptanol-induced relaxation after development of the steady-state KCl-induced contraction. These data extend our previous observations to further document the potential utility of heptanol as a "relatively selective" uncoupling agent.

Dispersed and aggregated gap junction channels identified by immunogold labeling of freeze-fractured membranes

An indirect immunogold labeling technique was applied to replicas of freeze-fractured membranes of rapidly frozen unfixed cells. The endogenous gap junction protein Cx43 of BICR/M1Rk rat mammary tumor cells was preferentially identified in quasi-crystalline gap junction plaques as were the transfected connexins Cx40, Cx43, and Cx45 in HeLa (human cervical carcinoma) cells. With this method we also detected contact areas with dispersed gap junction channels which are the only structural correlation for endogenous Cx45 in HeLa wild-type cells where no gap junction plaques exist. In double-transfected HeLa cells a colocalization of Cx40 and Cx43 was occasionally detected in quasi-crystalline gap junction plaques, whereas in contact areas with dispersed particles only one Cx type was present. Our results indicate that functional gap junction channels exist outside the quasi-crystalline plaques.

Transient involvement of gap junctional communication before fusion of newborn rat myoblasts

Heptanol-sensitive gap junction communication was characterized by the gap-FRAP method (fluorescence recovery after photobleaching) in confluent rat myoblasts developing in primary culture. Cell to cell dye diffusion was mainly restricted to a short period of the perfusion lag period and disappeared during fusion promotion except between some myoblasts and myotubes. This short period of occurrence of gap junction communication might be transiently and partially involved during the first steps preparing the subsequent fusion, since treatment with an uncoupler (heptanol) reduced the formation of multinucleated myotubes. During subsequent steps, functional gap junctions are not involved between myoblasts in the process of fusing, but a possible secondary involvement for fusion of remaining myoblasts to newly-formed myotubes is discussed. These data, together with results from other authors, suggest a regulatory role of gap junction communication in development and fusion of skeletal muscle cells, by providing a pathway for exchanging small molecules from one myoblast to another.

Connections with connexins: the molecular basis of direct intercellular signaling

Adjacent cells share ions, second messengers and small metabolites through intercellular channels which are present in gap junctions. This type of intercellular communication permits coordinated cellular activity, a critical feature for organ homeostasis during development and adult life of multicellular organisms. Intercellular channels are structurally more complex than other ion channels, because a complete cell-to-cell channel spans two plasma membranes and results from the association of two half channels, or connexons, contributed separately by each of the two participating cells. Each connexon, in turn, is a multimeric assembly of protein subunits. The structural proteins comprising these channels, collectively called connexins, are members of a highly related multigene family consisting of at least 13 members. Since the cloning of the first connexin in 1986, considerable progress has been made in our understanding of the complex molecular switches that control the formation and permeability of intercellular channels. Analysis of the mechanisms of channel assembly has revealed the selectivity of inter-connexin interactions and uncovered novel characteristics of the channel permeability and gating behavior. Structure/function studies have begun to provide a molecular understanding of the significance of connexin diversity and demonstrated the unique regulation of connexins by tyrosine kinases and oncogenes. Finally, mutations in two connexin genes have been linked to human diseases. The development of more specific approaches (dominant negative mutants, knockouts, transgenes) to study the functional role of connexins in organ homeostasis is providing a new perception about the significance of connexin diversity and the regulation of intercellular communication.

Myocardial gap junction organization in ischemia and infarction

Ischemia causes an increase in myocardial resistivity and a decrease in conduction velocity, thereby enhancing cardiac contractile dysfunction and arrhythmic tendency. Myocardial gap junctions, as principal determinants of conduction velocity, may, therefore, be expected to be deranged in ischemia. Despite a lack of consensus, attempts at correlating gap junction ultrastructural morphology with functional state have revealed the component connexons of gap junctions in freeze-fractured myocardium to be in multiple small hexagonal arrays, tending to become randomly distributed and compacted under uncoupling conditions. Further hypoxic uncoupling causes ultrastructural damage and a reduction in gap-junctional surface area. Immunohistochemical detection of connexin43 gap junctions in chronically ischemic non-infarcted human myocardium demonstrates a reduction in junctional surface area within a normal number of intercalated disks per myocyte, and with a normal distribution of junction sizes. In healed canine infarction there are smaller and fewer gap junctions in the fibrotic myocardium adjacent to infarcts, with reductions in overall gap-junctional content and the proportion of side-to-side vs. end-to-end intercellular connections. Immunohistochemical examination of intact human ventricular myocardium shows the myocytes immediately abutting healed infarcts to have connexin43 gap junctions spread longitudinally over the cell surfaces, and not in discrete transversely orientated intercalated disks as in normal myocardium. Early after canine infarction, and before fibrotic healing, the connexin43 gap junctions in myocytes abutting the infarct show disorganization similar to that described in healed human infarcts, suggesting that this disturbance is an early pathophysiological cellular response, and not simply due to later fibrotic distortion. Such changes in gap-junctional organization in myocardial ischemia and infarction may be implicated in the elusive link between subcellular structure, contractile dysfunction and arrhythmogenesis.

Structure--function relationships in gap junctions

Gap junctions are metabolic and electrotonic pathways between cells and provide direct cooperation within and between cellular nets. They are among the cellular structures most frequently investigated. This chapter primarily addresses aspects of the assembly of the gap junction channel, considering the insertion of the protein into the membrane, the importance of phosphorylation of the gap junction proteins for coupling modulation, and the formation of whole channels from two hemichannels. Interactions of gap junctions with the subplasmalemmal cytoplasm on the one side and with tight junctions on the other side are closely considered. Furthermore, reviewing the significance and alterations of gap junctions during development and oncogenesis, respectively, including the role of adhesion molecules, takes up a major part of the chapter. Finally, the literature on gap junctions in the central nervous system, especially between astrocytes in the brain cortex and horizontal cells in the retina, is summarized and new aspects on their structure-function relationship included.

Rapid onset and calcium independence of the gap junction uncoupling induced by heptanol in cultured heart cells

The kinetics of the reversible interruption of gap junction communication by the aliphatic alcohol heptanol and the possible mediation of an increase of the cytosolic Ca2+ concentration have been investigated in pairs of myocytes dissociated from neonatal rat ventricles and cultured for 2-3 days. Junctional communication was estimated by measuring either the cell-to-cell electrical conductance with a double whole-cell voltage-clamp method, or the rate constant of dye diffusion with the fluorescence recovery after photo-bleaching (gap FRAP) technique. Electrical coupling was seen to be abruptly interrupted (in less than 0.5 s) by heptanol (1-3 mM). The cytosolic Ca2+ concentration was not affected, even at a saturating heptanol concentration. Heptanol removal allowed a gradual re-opening of gap junctional channels, as shown by the recovery curve of the cell-to-cell conductance, which is 90% complete within 90 s. These data are consistent with a direct interaction of heptanol with channel proteins or with their lipid environment.

Effect of an externally applied electric field on excitation propagation in the cardiac muscle

Classical theory of potential distribution in cardiac muscle (cable theory) postulates that all effects of electric field (internally or externally applied) should decay exponentially with a space constant of the order of the tissue space constant ( approximately 1 mm). Classical theory does not take into account the cellular structure of the heart. Here, we formulate a mathematical model of excitation propagation taking into account cellular gap junctions. Investigation of the model has shown that the classical description is correct on the macroscopic scale only. At microscopic scale, electric field is modulated with a spatial period equal to the cell size (Plonsey and Barr), with the zero average. A very important new feature found here is that this effect of electric field does not decay at arbitrary big distances from the electrode. It opens the new way to control the excitation propagation in the cardiac muscle. In particular, we show that electric field can modify the velocity of propagation of an impulse in cardiac tissue at arbitrary big distances from electrode. In 2-dimensions, it can make rotating waves drift. To test these predictions, experiments with cardiac preparations are proposed.

Control of rotating waves in cardiac muscle: analysis of the effect of an electric field

The effect of an electric field on rotating waves in cardiac muscle is considered from a theoretical point of view. A model of excitation propagation taking into account the cellular structure of the heart is presented and studied. The application of a direct current electric field along the cardiac tissue is known to induce changes in membrane potential which decay exponentially with distance. Investigation of the model shows that the electric field induces a gradient of potential inside a cell which does not decay with distance, and results in modification of excitation propagation which extends a considerable distance from the electrodes. In two dimensions, it induces a drift of rotating waves. The effect of the electric field on propagation velocity and on rotating waves cannot be obtained in any arbitrary models of cardiac muscle. For an electric field of about 1 V cm-1 and junctional resistances of about 20 M omega, the change in velocity of propagation can be up to several percent, resulting in a drift velocity of rotating waves of the order of 1 cm s-1. To test these predictions, experiments with cardiac preparations are proposed.

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+.

Heptanol-induced decrease in cardiac gap junctional conductance is mediated by a decrease in the fluidity of membranous cholesterol-rich domains

To assess whether alterations in membrane fluidity of neonatal rat heart cells modulate gap junctional conductance (gj), we compared the effects of 2 mM 1-heptanol and 20 microM 2-(methoxy-ethoxy)ethyl 8-(cis-2-n-octylcyclopropyl)-octanoate (A2C) in a combined fluorescence anisotropy and electrophysiological study. Both substances decreased fluorescence steady-state anisotropy (rss), as assessed with the fluorescent probe 1-(4-trimethylammoniumphenyl)-6-phenyl-1,3,5-hexatriene (TMA-DPH) by 9.6 +/- 1.1% (mean +/- SEM, n = 5) and 9.8 +/- 0.6% (n = 5), respectively, i.e., both substances increased bulk membrane fluidity. Double whole-cell voltage-clamp experiments showed that 2 mM heptanol uncoupled cell pairs completely (n = 6), whereas 20 microM A2C, which increased bulk membrane fluidity to the same extent, did not affect coupling at all (n = 5). Since gap junction channels are embedded in relatively cholesterol-rich domains of the membrane, we specifically assessed the fluidity of the cholesterol-rich domains with dehydroergosterol (DHE). Using DHE, heptanol increased rss by 14.9 +/- 3.0% (n = 5), i.e., decreased cholesterol domain fluidity, whereas A2C had no effect on rss (-0.4 +/- 6.7%, n = 5). Following an increase of cellular "cholesterol" content (by loading the cells with DHE), 2 mM heptanol did not uncouple cell pairs completely: gj decreased by 80 +/- 20% (range 41-95%, n = 5). The decrease in gj was most probably due to a decrease in the open probability of the gap junction channels, because the unitary conductances of the channels were not changed nor was the number of channels comprising the gap junction. The sensitivity of nonjunctional membrane channels to heptanol was unaltered in cholesterol-enriched myocytes. These results indicate that the fluidity of cholesterol-rich domains is of importance to gap junctional coupling, and that heptanol decreases gj by decreasing the fluidity of cholesterol-rich domains, rather than by increasing the bulk membrane fluidity.

Intercellular junctions and the application of microscopical techniques: the cardiac gap junction as a case model

Intercellular junctions are fundamental to the interactions between cells. By means of these junctions, the activities of the individual cells that make up tissues are co-ordinated, enabling each tissue system to function as an integrated whole. In this review, the work of the authors on one specific type of junction--the cardiac gap junction--is presented as a case model to illustrate how the application of a range of microscopical methods, as part of a multidisciplinary approach, can help extend our understanding of cell junctions and their functions. In the heart, gap junctions form the low-resistance pathways for rapid impulse conduction and propagation, enabling synchronous stimulation of myocyte contraction. Gap junctions also form pathways for direct intercellular communication, a function of particular importance for morphogenetic signalling during development. The work discussed demonstrates some of the applications of techniques in electron microscopy, immunocytochemistry and confocal scanning laser microscopy to the understanding of the structural basis of the function of gap junctions in the normal adult heart, the developing heart and the diseased heart. Freeze-fracture electron microscopy of heart tissue prepared by rapid freezing techniques, in which excision-related structural damage to the cells is minimized or avoided, makes it possible to deduce the structure of the functioning gap junction in vivo. Gap junctions in hearts that are beating normally in the living animal until the very instant of freezing consist of connexons (transmembrane channels) organized in a quasi-crystalline arrangement, not a 'random' arrangement as proposed in the original hypothesis on the structural correlates of gap junction function. Alterations in connexon arrangement occur in response to ischaemia and hypoxia, though the relationship of these to gap-junctional permeability is indirect. To obtain probes for mapping the distribution of gap junctions in cardiac tissue, polyclonal antisera to synthetic peptides matching portions of the sequence of connexin43, the major gap-junctional protein reported in the heart, were raised. The specificity of the antisera was confirmed by dot blotting, Western blotting and by immunogold labelling of isolated gap junctions. One antiserum (that raised to residues 131-142) was found to be particularly effective as a cytochemical probe. An immunofluorescence labelling procedure for use with confocal scanning laser microscopy was developed to enable the three-dimensional precision mapping of gap junctions through thick slices of cardiac tissue.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of 2,4-dinitrophenol or low [ATP]i on cell excitability and action potential propagation in guinea pig ventricular myocytes

Inhibition of aerobic metabolism leads to a major disruption of cardiac cell homeostasis. The purpose of the present study was twofold: 1) We determined the relative importance of junctional and nonjunctional membrane resistance (Rj and Rm, respectively) in the development of propagation failure during inhibition of aerobic metabolism in guinea pig ventricular cell pairs. 2) We used the patch-action potential clamp technique in single ventricular myocytes to study some of the properties of the membrane channels that are responsible for shortening of action potential duration and eventual failure of cell excitation after metabolic blockade. In most experiments, whole-cell patch pipettes were filled with a solution containing 1 mM EGTA, 5 mM HEPES, and 5 mM ATP. Our results in cell pairs showed that pharmacological inhibition of aerobic metabolism with the mitochondrial uncoupler 2,4-dinitrophenol (DNP) led to a drop in Rm followed by an increase in Rj. The increase in Rj was not sufficient to cause a measurable delay in cell-to-cell propagation, whereas the drop in Rm consistently led to failure of cell excitation. Similar results were obtained in additional experiments in which the EGTA concentration in the pipette was reduced to 50 microM. Similar results were also obtained by loading the recording patch pipettes with a solution containing only 0.1 mM ATP. Our patch-action potential clamp experiments, on the other hand, revealed that DNP induced the opening of time- and voltage-independent membrane channels, with a unitary conductance of 23 pS. The channels allowed for the passage of outward current in the voltage range of the action potential, and the increase in membrane patch conductance correlated with the observed shortening of action potential duration during DNP superfusion. Our experiments provide the first simultaneous recordings of action potentials and DNP-induced channel currents in guinea pig ventricular myocytes. Overall, the data provide new evidence for the understanding of the cellular and subcellular mechanisms involved in the development of slow conduction velocity and propagation block after metabolic blockade.

Co-ordination of pumping in isolated bovine lymphatic vessels

1. Segments of bovine mesenteric lymphatic of varying diameter taken from different parts of the lymphatic tree were cut to 20 mm in length and set up so that measurements could be made of spontaneous isometric contractions. 2. There was considerable variability in frequency of spontaneous contractions but this was independent of resting tension. There was no significant correlation between lymphatic diameter and inherent frequency of contraction. 3. Isolated segments of bovine mesenteric lymphatic 70-80 mm in length were cannulated and set up in a three-compartment organ bath which allowed independent temperature control in each compartment. Pressure was recorded at inflow and outflow ends and experiments were video recorded. 4. Contractile activity was normally initiated at the end of the lymphatic maintained at the higher temperature and the contractile wave was propagated along the length of the vessel. 5. Propagation could occur either in the direction of valve orientation (orthograde) or retrogradely. The volume of fluid pumped was not significantly affected by the direction of propagation. 6. Perfusion of the central compartment with Krebs solution at 0-2 degrees C disrupted normal propagation and allowed the two parts of the lymphatic to contract at different frequencies although the two parts maintained an approximately 2:1 ratio. 7. Perfusion of the central compartment with 10 mM-heptanol also disrupted normal propagation but the rates on either side of the partition bore no harmonic relationship to one another. 8. These results suggest that relatively short segments of lymph duct have the ability to contract spontaneously and that their inherent frequencies are not determined by their position in the lymphatic tree. The results are consistent with the existence of electrical coupling along the lymphatic's length and they suggest that over distances of at least 80 mm independent pacemakers are capable of mutual entrainment at a frequency representing a compromise between the fastest and slowest components.

Gap junctions in rabbit corneal epithelium: limited permeability and inhibition by cAMP

Rabbit corneas were mounted in Ussing chambers, and the apical membrane of the superficial cells (SCs) was permeabilized by exposure to digitonin in a Ca(2+)-free Ringer solution. This treatment resulted in the generation of large (60.7 +/- 13.2 microA/cm2, n = 25) Na(+)-dependent tear (T)-to-stroma (S) short-circuit currents (Isc). The Isc was abolished by ouabain and by 1.4 mM Ca2+ and was inhibited by heptanol, 18 alpha-glycyrrhetinic acid, and dieldrin, effects consistent with the notion that corneal transepithelial fluxes include translocations through gap junctions (GJs) before basolateral membrane transport. T-to-S Isc were also generated when T-side Na+ was replaced by K+, eliciting a T-to-S K+ flux via basolateral K+ channels and when, with either Na+ or K+ on the T side, channels were introduced at the apical membrane with amphotericin B. The Isc in all four conditions exhibited similar sensitivity to GJ inhibitors and were inhibited by adenosine 3',5'-cyclic monophosphate (cAMP) elevation. Fluorophotometry combined with SC permeabilization with digitonin demonstrated that the half-time for the SC to sub-SC movement of 2',7'-bis(carboxyethyl)-5,6-carboxyfluorescein (mol wt 540) exceeded 3 h. These results indicate that junctional communications along the epithelial stratification axis are highly restricted and modulated by cAMP concentration.

Differential effects of heptanol, potassium, and tetrodotoxin on reentrant ventricular tachycardia around a fixed obstacle in anisotropic myocardium

BACKGROUND

The aim of this study was to test the hypothesis that electrical uncoupling and depression of the fast sodium channels have differential effects on propagation of the electrical impulse relative to the fiber orientation.

METHODS AND RESULTS

In a model of reentrant ventricular tachycardia (VT) (mean cycle length, 144 +/- 13 msec) around a ring of anisotropic myocardium in 10 Langendorff-perfused rabbit hearts, the effects of extracellular K+ concentration [( K+]o) and heptanol were studied. [K+]o and heptanol each had a dose-dependent effect on VT cycle length. However, high [K+]o slowed the VT mainly by depressing longitudinal conduction, whereas heptanol preferentially depressed transverse conduction. The ratio between longitudinal and transverse conduction velocities progressively decreased with high [K+]o and progressively increased with heptanol. Heptanol terminated VT at a mean concentration of 3.5 +/- 0.5 mM. The cycle length before termination was 446 +/- 120 msec (p less than 0.001). In eight of 10 experiments, termination occurred by failure of conduction during transverse propagation. VT terminated at a mean [K+]o of 11.6 +/- 1.8 mM. The cycle length before termination was 493 +/- 341 msec (p less than 0.01). In seven of 10 cases, termination occurred by failure of conduction during longitudinal propagation. In the remaining five episodes (two with heptanol and three with high [K+]o), termination occurred by collision of the reentrant beat with an antidromic impulse being reflected within the ring. In a separate series of six hearts, tetrodotoxin was administered during VT. Like high [K+]o, tetrodotoxin prolonged the cycle length of the VT by preferentially slowing longitudinal conduction, and VT was terminated by longitudinal block.

CONCLUSIONS

During reentrant VT, electrical uncoupling of cells by heptanol or modification of active membrane properties by high [K+]o or tetrodotoxin has a differential depressing effect on propagation of the impulse relative to the fiber orientation.

Perineurium of sciatic nerve in normal and diabetic rodents: freeze-fracture study of intercellular junctional complexes

A comparative study has been carried out using the freeze-fracture technique on the perineurium of the sciatic nerve from normal and diabetic mice (C57Bl/Ks, BALB/c and CD1 strains) and rats of various ages. The replicas showed that tight junctions connected perineurial cells both within the same cell layer (zonulae occludentes) and between adjacent layers (maculae occludentes). In neonates, a number of zonulae occludentes were characterized by short, incomplete or fragmented ridges at various intervals from each other; in adults, tight junctions appeared as 'mature' networks of interconnected, branching and/or anastomosing strands. Zonulae occludentes of diabetic mice also exhibited frequent interruption of the strands and reduction in the branching of strands. Gap junctions occurred in both zonulae and maculae occludentes of normal and diabetic rats at all ages. In the C57Bl/Ks strain such junctions occurred more frequently in zonulae occludentes of diabetic animals. It is suggested that perineurial cells are coupled by gap junctions to allow fast transfer of ions and small-sized molecules across the layers; under pathological conditions, such as diabetes, the increase in cell-to-cell signalling may be important in controlling the abnormal metabolic situation.

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.

Rapid and reversible secretion changes during uncoupling of rat insulin-producing cells

To determine whether insulin secretion is affected by a blockage of gap junctions between B cells, we have studied the secretion of rat pancreatic islets of Langerhans, primary dispersed islet cells, and cells of the RINm5F line, during short-term exposure to heptanol. Within minutes, this alkanol blocked gap junctions between the B cells of intact islets and abolished their normal secretory response to glucose. These two changes were rapidly and fully reversible after return of the islets to control medium. We further found that heptanol had no significant effect on the glucose-stimulated secretion of single B cells but inhibited that of B cell pairs. In the clone of RINm5F cells, whose junctional coupling and D-glyceraldehyde-induced stimulation of insulin release by aggregated cells were also inhibited by heptanol, this alkanol did not perturb intracellular pH and Ca2+ and the most distal steps of the secretion pathway. In summary, a gap junction blocker affected the secretion of insulin-producing cells by a mechanism which is dependent on cell contact and is not associated with detectable pleiotropic perturbations of the cell secretory machinery. The data provide evidence for the involvement of junctional coupling in the control of insulin secretion.

Large-conductance ion channel measured by whole-cell voltage clamp in single cardiac cells: modulation by beta-adrenergic stimulation and inhibition by octanol

Membrane currents in single cardiac myocytes from adult guinea pigs were studied by means of the patch-clamp technique (whole-cell mode). During spontaneous or caffeine-induced Ca2+ release from the sarcoplasmic reticulum openings of a novel ion channel with large unitary conductance (280 pS) can be recorded. The density of these channels and/or its open-state probability are unusually low. On average in the whole-cell mode simultaneous maximum superposition of only four channels is observed. Opening events of this channel require an intracellular Ca2+ transient. Activation by [Ca2+]i, however, seems to be indirect; maximum opening activity occurs with a delay of several hundred milliseconds after peak [Ca2+]i. Single-channel activity can be enhanced by a cyclic AMP dependent process via beta-adrenergic stimulation of a cell. This can also be mimicked by caffeine, most likely via inhibition of phosphodiesterase. Octanol, an inhibitor of gap-junctional coupling in a variety of tissues, causes a concentration-dependent and reversible decrease in single-channel activity. Unitary conductance is not affected by octanol. The low density of these channels in cardiac membranes and their poor selectivity render any role in normal cardiac electrical activity unlikely. A possible relation of the channel to cardiac gap junctions is discussed.

Structural changes in cardiac gap junctions after hypoxia and reoxygenation: a quantitative freeze-fracture analysis

Isolated rat hearts were subjected to increasing periods of hypoxia with or without subsequent reoxygenation and the gap-junctional particle configuration was followed quantitatively. Irregular contractions were prevented by K(+)-arrest; glucose, counteracting the effects of hypoxia, was omitted. Hyperkalemia alone and a maximum of 20 min of hypoxia do not produce reorganization of the gap-junctional particles normally forming multiple hexagonally packed arrays separated by smooth aisles. After 30 min of hypoxia, the aisles disappear in a proportion of the junctions, thereby increasing the particle density from 9400 +/- 800/microns2 to 10,200 +/- 900/microns2. After 40 min of hypoxia, the normal configuration is no longer found and numerous junctions are arranged as uninterrupted hexagonal lattices. The particles are further condensed to 11,600 +/- 900/microns2. Following reoxygenation after both 30 and 40 min of hypoxia, the proportion of crystalline gap junctions dramatically augments and the mean particle density has further increased significantly. Corresponding thin sections show irreversible cell damage. When reoxygenation is performed with a control solution containing normal levels of K+ and glucose, the particle density does not increase substantially in comparison to the respective 30- and 40-min hypoxic periods. In both groups, the gap junctions display either a normal, a crystalline or an intermediate configuration with crystalline margins and loose centers. The gap-junctional reorganization during hypoxia essentially represents a particle condensation, while the mean center-to-center distances between the particles and pits remain constant. Furthermore, the reappearance of normal gap junctions after reoxygenation appears to depend on glucose availability.

Structure-activity relations of the cardiac gap junction channel

Cardiac gap junction channels play the important roles of synchronizing pacemaker cells and allowing impulse propagation along the conduction system and throughout the ventricular myocardium. These channels, which support current flow in both longitudinal and tranverse directions, are permeable to anions and cations with radii less than approximately 0.5 nm and in rat heart have unitary conductances on the order of 50 pS. This unitary conductance is consistent with channel geometry described by a right cylindrical pore with diameter large enough for the brilliantly fluorescent dye molecule lucifer yellow to pass between cells. These channels, like others in biological systems, are opened and closed by various treatments, a process termed gating. Cytoplasmic acidification reduces junctional conductance (gj), an effect that is apparently potentiated by elevated myoplasmic Ca ions. Reduced gj also occurs in response to a variety of lipophilic molecules, including halothane, heptanol, and unsaturated fatty acids; the mechanism of action may involve disruption of the protein-lipid microenvironment of the gap junction channel. Arachidonic acid uncouples, and this effect is partially, but incompletely, blocked by an inhibitor of the lipoxygenase metabolic pathways. Cyclooxygenase inhibitors have no protective effects. Certain cyclic nucleotides can rapidly increase gj [adenosine 3',5'-cyclic monophosphate (cAMP)] or slightly decrease it [guanosine 3',5'-cyclic monophosphate (cGMP)], and agents that use these cyclic nucleotides as second messengers (isoproterenol and perhaps carbachol, respectively) produce consistent effects. Agents expected to cause protein kinase C activation (tumor-promoting phorbol esters and diacylglycerol) increase gj rapidly. The gap junction protein from rat heart has been cloned and sequenced. From the primary sequence for the protein, plausible sites of action within the putative cytoplasmic domains are proposed for each of these treatments. In response to gating stimuli that close the channel (halothane, CO2, heptanol), unitary channel conductance is unchanged, suggesting that these agents act by reducing open time probability. Together, these properties constitute the beginnings of our endeavor to define pharmacological agents that are potentially useful in therapeutic manipulation of synchronous discharge, conduction velocity, and isochronous wavefront propagation in cardiac tissue.

Effect of cellular uncoupling by heptanol on conduction in infarcted myocardium

Experiments were performed in vitro on six normal thin ventricular epicardial tissue strips and 10 strips removed from the infarcted regions of dogs 21-60 days after experimental myocardial infarction. Conduction was evaluated by mapping activation sequences at 40-45 sites over an area of 1 x 2 cm during pacing at a basic cycle length of 2,000 msec. The amplitude and length of recorded electrograms were also determined at each site. After control recordings, heptanol, which increases gap junctional resistance, was added to the tissue bath at concentrations ranging between 0.2 and 1.0 mM. In contrast to its effect on normal tissues, heptanol caused 75 of 260 previously active sites in the infarcted tissues to become inactive. The affected sites were located in areas of very slow conduction and/or adjacent to areas of preexisting conduction block. In addition, heptanol decreased the length and degree of fractionation of electrograms recorded in slowly conducting regions of the infarcted tissues. The magnitude of the decrease in electrogram length following heptanol was related to the degree of electrogram abnormality during control as reflected in the ratio of electrogram length to amplitude. Heptanol shortened electrograms by causing local conduction block, which eliminated some components of the fractionated electrograms. In an additional eight epicardial strips removed from the infarcted region, 0.5 mM heptanol had only a slight effect (10.7% decrease) on the maximum rate of membrane depolarization. Thus, heptanol does not act primarily by way of depressing the fast inward current. We conclude from heptanol's effects on conduction and electrogram characteristics that slow and dissociated conduction in the infarcted region is due to an abnormality in gap junctional distribution between surviving cells and/or an abnormality in individual gap junctional function.

Electrical properties of gap junction channels in guinea-pig ventricular cell pairs revealed by exposure to heptanol

Cell pairs were isolated from adult guinea pig ventricles to study the electrical properties of gap junction channels. The experiments involved a double voltage-clamp approach and whole-cell, tight-seal recording. Heptanol decreased the intracellular current, In, in a dose-dependent fashion. Before complete uncoupling, In showed fluctuations suggesting the operation of gated channels. In the presence of 3 mM heptanol, In showed quantal steps arising from spontaneous opening and closing of single channels. The IV-relationship of the channels was linear (range: +/- 95 mV). Analysis of current records revealed the following single-channel conductances, gamma n: Mean value = 37 pS; median value = 33 pS. gamma n was insensitive to the non-junctional membrane potential (range: -90 to +10 mV). 3 mM ATP4- in the pipette solution had no effects on gamma n, 6 mM ATP4- produced a small decrease, and 6 mM ATP + 0.1 mM cAMP- an increase in gamma n. Channel transitions from closed to open state were variable (range of apparent time constants: 2.5-32 ms; mean: 11 ms).

Segmental electrical uncoupling and conduction blocks after calcium removal and replacement in a mammalian auricle

The cell-to-cell electrical conduction has been investigated in control conditions, during calcium depletion and after calcium repletion. When rat auricular strips are bathed in a Ca2+-free, EGTA-containing (5 mM) solution, the resting membrane potential slowly decreases to about -35 mV within 20 min. The electrotonic spread of intracellular current pulses remains similar to that observed in control conditions, with length constants of about 215 microns in the fibre direction and 52 microns perpendicular to it. Restoration of calcium ions to the bathing fluid at 37 degrees C induces an irreversible loss of the all-or-none electrical conduction of the action potential, and the auricular fibres become split up into aggregates of electrically coupled cells delimited by border zones where the electrical coupling and the conduction of action potentials are interrupted. Inside each of those islets the resting membrane potential is uniform, but it may vary abruptly (between about -10 and -80 mV) across the border of two islets. Islets with sufficient levels of membrane potential (less than -60 mV) can generate action potentials that do not propagate to adjacent islets. This fragmentation of the cardiac tissue into electrically independent subunits explains the irreversible loss of the propagated electro-mechanical activity (calcium paradox) that is observed after calcium repletion.

Effects of cellular uncoupling on conduction in anisotropic canine ventricular myocardium

Experiments were performed on canine superfused ventricular epicardial tissue slices to determine the effects of 1.0-2.0 mM heptanol, an uncoupling agent, on conduction longitudinal and transverse to myocardial fiber orientation. Conduction velocities were measured between proximal and distal pairs of epicardial electrodes oriented transverse and longitudinal to the direction of a conducted wavefront evoked by pacing at a basic cycle length of 2,000 msec from one margin of the tissue before and after the addition of heptanol. In a separate group of tissues, the dual bipolar orthogonal electrode was used to sequentially map epicardial activation at 40 to 45 sites in a 1 cm x 2 cm area before and 30 minutes after the introduction of heptanol. In a third group of tissues, transmembrane potentials were recorded with standard microelectrode techniques to determine the effects of heptanol on action potential characteristics. Heptanol did not significantly effect action potential amplitude or maximum rate of depolarization. After 1.0 mM heptanol, conduction velocity began to decrease in 1-2 minutes and reached a steady state in 15-20 minutes. Conduction velocity in the longitudinal direction decreased from a control value of 0.56 +/- 0.13 to 0.46 +/- 0.10 M/sec (+/- SD) at 30 minutes after heptanol (p = 0.005). In the transverse direction, it decreased from 0.24 +/- 0.09 to 0.17 +/- 0.05 M/sec (p = 0.002). The ratio of longitudinal to transverse conduction velocities increased from 2.54 +/- 1.00 to 2.94 +/- 0.82 (p = 0.042). Thus, heptanol preferentially slowed conduction in the transverse direction. Because heptanol did not greatly influence active membrane properties, we used cable equations to calculate the time course of the change in effective junctional resistivity, which rose from 133.2 omega.cm before heptanol to 312.2 omega.cm 30 minutes after heptanol administration. We conclude that heptanol slows conduction velocity by selectively increasing junctional resistivity. The preferential slowing of conduction in the transverse direction is most likely due to the fact that more junctional resistances are encountered per unit distance in the transverse than in the longitudinal direction.

Morphometrical analysis of the gap-junctional area in parenchymal cells of the rat liver after administration of dibutyryl cAMP and aminophylline

In view of the presumed involvement of gap junctions in the coordination of metabolic activities, the influence of cAMP as a regulatory signal of cell metabolism on gap junctions of hepatocytes has been examined. Male rats received two intraperitoneal doses of 10 mg dibutyryl cAMP/100 g body weight with a time interval of 2.5 h and were decapitated 2.5 h later. After this 5-h interval, analysis of freeze-fracture replicas of fixed liver tissue revealed an increase in the mean (+/- SEM) gap-junctional membrane portion on the lateral hepatocyte membranes from 0.049 +/- 0.003 (n = 66) in controls to 0.061 +/- 0.003 (n = 70) in treated rats, while the configuration of the connexons appeared unaltered. This effect could not be reinforced by prior administration of aminophylline: the relative gap-junctional area is similarly extended from 0.054 +/- 0.003 (n = 126) in the control group to 0.065 +/- 0.004 (n = 105) in the experimental animals. Probing for the time course of the junctional response, a group of rats was sacrificed 3 h after the onset of treatment. Already within this time, the gap-junctional area is augmented from 0.042 +/- 0.004 (n = 63) in the concurrent controls to 0.069 +/- 0.006 (n = 42) in the treated rats. These statistically significant increases in area may suggest a stimulating effect of cAMP on gap junctions of hepatocytes in vivo.

Cell-to-cell communication in the heart: structure-function correlations

The communicating cell junctions that ensure the electrical and diffusional continuity of the intracellular space in the heart fibres can be switched from their normal conducting, or opened state, to an exceptional non-conducting, or closed state. This electrical uncoupling is observed after cell injury in the presence of Ca2+ ions in the extracellular fluid, after metabolic inhibition and in the presence of aliphatic alcohols (C6 to C9). The correlations between electrical uncoupling and gap junction morphology in the heart are briefly reviewed. A decrease of the distance between P-face particles and between the E-face pits has been found in all investigations, but the functional significance of this observation is not understood at present. A quantitatively very similar decrease of the average particle diameter (about -0.7 nm) has been measured in glutaraldehyde-fixed sheep Purkinje fibres and in unfixed, quickly frozen rat auricles that had been electrically uncoupled by three different procedures. About half of this decrease was reversible on short-term electrical recoupling (within 20 min). It is concluded that a measurable decrease of the connexon diameter correlates with electrical uncoupling.

Cell-to-cell coupling studied by diffusional methods in myocardial cells

The diffusion of large molecular substances from cell to cell in multicellular and enzymatically isolated cell pairs is described. Permeability of the gap junctional membrane to these molecules and the critical diffusing diameter of the myocardial gap junctional channel are discussed.

Cell-to-cell coupling studied in isolated ventricular cell pairs

Cell pairs isolated from adult rat and guinea pig ventricles were used to study the electrical properties of the nexal membrane. Each cell of a pair was connected to a voltage-clamp system so as to enable whole-cell, tight-seal recording. The current-voltage relationship of the nexal membrane was found to be linear, revealing a resistance rn of 2-4 M omega. rn was insensitive to the sarcolemmal membrane potential (range: -90 to +30 mV), and exerted no time-dependent gating behavior (range: 0.1 to 10 s). Lowering pHi yielded a small increase in rn. Vigorous elevations in [Ca2+]i gave rise to an increase in rn which was associated with a cell shortening. Uncoupling caused by aliphatic alcohols or halothane did not produce cell shortening. Cell pairs were also used to study action potential transfer.

Effects of pCai and pHi on cell-to-cell coupling

Internal longitudinal resistance (ri), a determinant of cardiac conduction, is affected by changes in intracellular calcium and protons. However, the role and mechanism by which H+ and Ca2+ may modulate ri is uncertain. Cable analysis was performed in cardiac Purkinje fibers to measure ri during various interventions. In some experiments, intracellular pH (pHi) was recorded simultaneously to study the pHi-ri relation. Both intracellular Ca2+ and H+ independently modified ri. However, internal resistance of cardiac fibers was insensitive to pHi changes compared to other tissues. A latent period preceded the pHi-related changes in ri and the amount of change depended upon methodology. The results suggest that direct action of protons or ri may be subordinate to other regulatory processes. Ionic regulation of internal longitudinal resistance may occur by more than one mechanism: i) direct cationic binding to sites on junctional membrane proteins; and ii) H+- or Ca2+-dependent phosphorylation of junctional proteins.

Complementarity of particles and pits in freeze-fractured hepatic and cardiac gap junctions

Particles and pits of freeze-fractured gap junctions are considered as complementary structures despite the frequent observations of more regular and closer spacings of pits, ascribed to plastic deformation of particle arrays. Recently, however, the noncomplementarity of pits and particles in Purkinje fibers has been reported. To ascertain the relationship between both structures, gap junctions from fixed, cryoprotected liver and myocardium were investigated using spacing and density measurements and complementary replicas. In hepatocyte gap junctions, the center-to-center distances (mean +/- SD) among pits, 9.57 +/- 1.49 nm, and particles, 9.70 +/- 1.77 nm, are not significantly different. Density determinations yielded a slightly higher value for the pits, (11,510 +/- 830)/microns 2, than for the particles, (11,230 +/- 950)/microns 2. In the myocardium, the spacing of the regularly arrayed pits, 9.55 +/- 1.33 nm, barely exceeds the value of 9.44 +/- 1.62 nm for the particles, which show some clustering. However, the packing density for the pits, (10,090 +/- 740)/microns 2, appears a little higher than that of the particles, (9,890 +/- 920)/microns 2. As density and spacing measurements provided no decisive answers, the positions of individual pits and particles of complementary junctional faces were recorded on transparent sheets and compared. In this fashion, a one-to-one correspondence between particles and pits could be established, while small discrepancies may be attributed to plastic deformation. Moreover, the co-linearity of pits and particles may be suggested by the observation of a platinum grain in the center of many pits.

Dynamic interactions and mutual synchronization of sinoatrial node pacemaker cells. A mathematical model

Dynamic interactions and mutual entrainment of coupled sinoatrial pacemaker cells with different intrinsic frequencies were investigated using a computerized mathematical model. Transmembrane potentials were simulated using equations of individual membrane currents based on voltage clamp data for the sinoatrial node. The intrinsic frequency of a given cell was altered by applying bias hyperpolarizing current, or by changing the amount of slow inward current. Cells were coupled through simple ohmic resistances to form linear arrays of two or more cells. Simulations closely reproduced previous experimental work showing that the mutual interactions between pacemakers are mediated electrotonically and show phase dependence. Results from the present simulations provide an explanation for the ionic basis of these phase-dependent interactions. In addition, it is demonstrated that the mutual entrainment of coupled pacemakers can lead to their coordinated behavior (synchronization). Two pacemaker cells can synchronize at simple harmonic (i.e., 1:1, 2:1, etc.) or more complex ratios (3:2, 5:3, etc.), depending on the differences in intrinsic frequencies and the degree of electrical coupling between cells. Simulations using larger numbers of linearly connected cells yielded various patterns of pacemaker activity including 2:1 sinoatrial block and complex dysrhythmic activity. The overall results may be used to predict higher order interactions of thousands of cells comprising the sinus node. Under such a scheme, synchronization occurs not by the conducted influence of a dominant pacemaker cell, but by the mutual "democratic" interaction of individual pacemaker cells.

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

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

Electrical properties of the nexal membrane studied in rat ventricular cell pairs

Cell pairs isolated from adult rat ventricles were used to characterize the electrical properties of the nexal membrane located between the cells. Each cell of a cell pair was connected to a suction pipette so as to enable whole-cell recordings. A double voltage-clamp method was employed which allowed the voltage gradient across the nexal membrane to be controlled. The current-voltage relationship of the nexal membrane was found to be linear over a broad range of transnexal voltages ( +/- 50 mV). The measurements revealed a mean value for the apparent nexal membrane resistance, rn(app), of 3.4 M omega. Taking into account the contribution of an uncompensated series resistance (access resistance), the effective nexal resistance, rn(eff), amounts to 1.7 M omega, approximately. The nexal membrane resistance was found to be insensitive to the sarcolemmal membrane potential, Vm (voltage range tested: -90 mV to +30 mV). The nexal membrane showed no rectifying property, i.e. it allows impulse transmission in both directions equally well. The connexons of the nexal membrane exhibited no time-dependent gating behaviour (time range investigated: 0.1-10 s).