Uncoupling of cardiac cells by doxyl stearic acids specificity and mechanism of action

Determinants of Cx43 Channel Gating and Permeation: The Amino Terminus

Separate connexin domains partake in proposed gating mechanisms of gap junction channels. The amino-terminus (NT) domains, which contribute to voltage sensing, may line the channel's cytoplasmic-facing funnel surface, stabilize the channel's overall structure through interactions with the transmembrane domains and each other, and integrate to form a compound particle to gate the channel closed. Interactions of the carboxyl-terminus (CT) and cytoplasmic loop (CL) domains underlie voltage- and low pH-triggered channel closure. To elucidate potential cooperation of these gating mechanisms, we replaced the Cx43NT with the Cx37NT (chimera Cx43(∗)NT37), leaving the remainder of the Cx43 sequence, including the CT and CL, unchanged. Compared to wild-type Cx43 (Cx43WT), Cx43(∗)NT37 junctions exhibited several functional alterations: extreme resistance to halothane- and acidification-induced uncoupling, absence of voltage-dependent fast inactivation, longer channel open times, larger unitary channel conductances, low junctional dye permeability/permselectivity, and an overall cation selectivity more typical of Cx37WT than Cx43WT junctions. Together, these results suggest a cohesive model of channel function wherein: 1) channel conductance and size selectivity are largely determined by pore diameter, whereas charge selectivity results from the NT domains, and 2) transition between fully open and (multiple) closed states involves global changes in structure of the pore-forming domains transduced by interactions of the pore-forming domains with either the NT, CT, or both, with the NT domains forming the gate of the completely closed channel.

Connexin hemichannel and pannexin channel electrophysiology: how do they differ?

Connexin hemichannels are postulated to form a cell permeabilization pore for the uptake of fluorescent dyes and release of cellular ATP. Connexin hemichannel activity is enhanced by low external [Ca(2+)]o, membrane depolarization, metabolic inhibition, and some disease-causing gain-of-function connexin mutations. This paper briefly reviews the electrophysiological channel conductance, permeability, and pharmacology properties of connexin hemichannels, pannexin 1 channels, and purinergic P2X7 receptor channels as studied in exogenous expression systems including Xenopus oocytes and mammalian cell lines such as HEK293 cells. Overlapping pharmacological inhibitory and channel conductance and permeability profiles makes distinguishing between these channel types sometimes difficult. Selective pharmacology for Cx43 hemichannels (Gap19 peptide), probenecid or FD&C Blue #1 (Brilliant Blue FCF, BB FCF) for Panx1, and A740003, A438079, or oxidized ATP (oATP) for P2X7 channels may be the best way to distinguish between these three cell permeabilizing channel types. Endogenous connexin, pannexin, and P2X7 expression should be considered when performing exogenous cellular expression channel studies. Cell pair electrophysiological assays permit the relative assessment of the connexin hemichannel/gap junction channel ratio not often considered when performing isolated cell hemichannel studies.

Visualization of Ins(1,4,5)P3 dynamics in living cells: two distinct pathways for Ins(1,4,5)P3 generation following mechanical stimulation of HSY-EA1 cells

In the present study, the contribution of inositol (1,4,5)-trisphosphate [Ins(1,4,5)P(3)] generation on the mechanical-stimulation-induced Ca(2+) response was investigated in HSY-EA1 cells. Mechanical stimulation induced a local increase in the cytosolic concentration of Ins(1,4,5)P(3) ([IP(3)](i)), as indicated by the Ins(1,4,5)P(3) biosensor LIBRAvIII. The area of this increase expanded like an intracellular Ins(1,4,5)P(3) wave as [IP(3)](i) increased in the stimulated region. A small transient [IP(3)](i) increase was subsequently seen in neighboring cells. The phospholipase C inhibitor U-73122 abolished these Ins(1,4,5)P(3) responses and resultant Ca(2+) releases. The purinergic receptor blocker suramin completely blocked increases in [IP(3)](1) and the Ca(2+) release in neighboring cells, but failed to attenuate the responses in mechanically stimulated cells. These results indicate that generation of Ins(1,4,5)P(3) in response to mechanical stimulation is primarily independent of extracellular ATP. The speed of the mechanical-stimulation-induced [IP(3)](i) increase was much more rapid than that induced by a supramaximal concentration of ATP (1 mM). The contribution of the Ins(1,4,5)P(3)-induced Ca(2+) release was larger than that of Ca(2+) entry in the Ca(2+) response to mechanical stimulation in HSY-EA1 cells.

Possible involvement of ATP-purinoceptor signalling in the intercellular synchronization of intracellular Ca2+ oscillation in cultured cardiac myocytes

Isolated and cultured neonatal cardiac myocytes contract spontaneously and cyclically. The contraction rhythms of two isolated cardiac myocytes, each of which beats at different frequencies at first, become synchronized after the establishment of mutual contacts, suggesting that mutual entrainment occurs due to electrical and/or mechanical interactions between two myocytes. The intracellular concentration of free Ca(2+) also changes rhythmically in association with the rhythmic contraction of myocytes (Ca(2+) oscillation), and such a Ca(2+) oscillation was also synchronized among cultured cardiac myocytes. In this study, we investigated whether intercellular communication other than via gap junctions was involved in the intercellular synchronization of intracellular Ca(2+) oscillation in spontaneously beating cultured cardiac myocytes. Treatment with either blockers of gap junction channels or an un-coupler of E-C coupling did not affect the intercellular synchronization of Ca(2+) oscillation. In contrast, treatment with a blocker of P2 purinoceptors resulted in the asynchronization of Ca(2+) oscillatory rhythms among cardiac myocytes. The present study suggested that the extracellular ATP-purinoceptor system was responsible for the intercellular synchronization of Ca(2+) oscillation among cardiac myocytes.

Ionic blockade of the rat connexin40 gap junction channel by large tetraalkylammonium ions

The rat connexin40 gap junction channel is permeable to monovalent cations including tetramethylammonium and tetraethylammonium ions. Larger tetraalkyammonium (TAA(+)) ions beginning with tetrabutylammonium (TBA(+)) reduced KCl junctional currents disproportionately. Ionic blockade by tetrapentylammonium (TPeA(+)) and tetrahexylammonium (THxA(+)) ions were concentration- and voltage-dependent and occurred only when TAA(+) ions were on the same side as net K(+) efflux across the junction, indicative of block of the ionic permeation pathway. The voltage-dependent dissociation constants (K(m)(V(j))) were lower for THxA(+) than TPeA(+), consistent with steric effects within the pore. The K(m)-V(j) relationships for TPeA(+) and THxA(+) were fit with different reaction rate models for a symmetrical (homotypic) connexin gap junction channel and were described by either a one- or two-site model that assumed each ion traversed the entire V(j) field. Bilateral addition of TPeA(+) ions confirmed a common site of interaction within the pore that possessed identical K(m)(V(j)) values for cis-trans concentrations of TPeA(+) ions as indicated by the modeled I-V relations and rapid channel block that precluded unitary current measurements. The TAA(+) block of K(+) currents and bilateral TPeA(+) interactions did not alter V(j)-gating of Cx40 gap junctions. N-octyl-tributylammonium and -triethylammonium also blocked rCx40 channels with higher affinity and faster kinetics than TBA(+) or TPeA(+), indicative of a hydrophobic site within the pore near the site of block.

Gap junction blockers decrease defibrillation thresholds without changes in ventricular refractoriness in isolated rabbit hearts

BACKGROUND

The maintenance and termination of reentry arrhythmias are determined by tissue properties such as refractoriness and conduction velocity. Although the effects of Na(+) and K(+) channel block on electrophysiological properties and defibrillation threshold (DFT) have been studied, little is known about the effect of gap junction blockers on defibrillation and tissue electrophysiological properties.

METHODS AND RESULTS

Triplicate DFTs (volts) were obtained before and 15 minutes after 4 micromol/L 16-doxyl-stearic acid (16-DSA, n=8), 1 mmol/L 1-heptanol (n=12) (both gap junction blockers), 3 microg/mL lidocaine (a sodium channel blocker) (n=8), and respective controls (n=27) in isolated perfused rabbit hearts. DFT decreased after 16-DSA (23+/-14%, P<0.01) and 1-heptanol (21+/-16%, P<0.01) but increased after lidocaine (26+/-28%, P<0.05). Ventricular fibrillation cycle length (VFCL) and QRS duration increased after all 3 agents, by 36+/-19% and 44+/-16% (16-DSA), 87+/-42% and 49+/-15% (heptanol), and 57+/-20% and 43+/-26% (lidocaine), respectively (all P<0.01). Spatially averaged temporal VFCL dispersion decreased significantly after all 3 agents, by 47+/-42% (16-DSA, P<0.05), 74+/-19% (1-heptanol, P<0.01), and 82+/-13% (lidocaine, P<0.01), respectively. Ventricular effective refractory period and monophasic action potential duration at 90% repolarization were unchanged after 16-DSA and 1-heptanol (P=NS) but increased after lidocaine (16+/-13%, P<0.01, and 6+/-5%, P=NS, respectively). There were no significant changes in DFT or any other electrophysiological variable in control hearts.

CONCLUSIONS

Electrical uncoupling by 16-DSA and 1-heptanol significantly lowers DFT and dispersion of VFCL without altering refractoriness; lidocaine, at doses resulting in similar slowing of conduction, increases DFT.

Intestinal tachyarrhythmias during small bowel ischemia

The electrical control activity (ECA) of the bowel is the omnipresent slow electrical wave of the intestinal tract. Characterization of small bowel electrical activity during ischemia may be used as a measure of intestinal viability. With the use of an animal model of mesenteric ischemia, serosal electrodes and a digital recording apparatus utilizing autoregressive spectral analysis were used to monitor the ECA of 20 New Zealand White rabbits during various lengths of ischemia. ECA frequency fell from 18.2 +/- 0.5 cycles per minute (cpm) at baseline to 12.2 +/- 0.9 cpm (P < 0.05) after 30 min of ischemia and was undetectable by 90 min of ischemia in all animals. Tachyarrhythmias of the ECA were recorded in 55% of the animals as early as 25 min after ischemia was induced and lasted from 1 to 48 min. Frequencies ranged from 25 to 50 cpm. These tachyarrhythmias were seen only during ischemia, suggesting that they are pathognomonic for intestinal ischemia. The use of the detection of ECA changes during intestinal ischemia may allow earlier diagnosis of mesenteric ischemia.

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.

Intercellular calcium signaling via gap junction in connexin-43-transfected cells

In excitable cells, intracellular Ca2+ is released via the ryanodine receptor from the intracellular Ca2+ storing structure, the sarcoplasmic reticulum. To determine whether this released Ca2+ propagates through gap junctions to neighboring cells and thereby constitutes a long range signaling network, we developed a cell system in which cells expressing both connexin-43 and ryanodine receptor are surrounded by cells expressing only connexin-43. When the ryanodine receptor in cells was activated by caffeine, propagation of Ca2+ from these caffeine-responsive cells to neighboring cells was observed with a Ca2+ imaging system using fura-2/AM. Inhibitors of gap junctional communication rapidly and reversibly abolished this propagation of Ca2+. Together with the electrophysiological analysis of transfected cells, the observed intercellular Ca2+ wave was revealed to be due to the reconstituted gap junction of transfected cells. We next evaluated the functional roles of cysteine residues in the extracellular loops of connexin-43 in gap junctional communication. Mutations of Cys54, Cys187, Cys192, and Cys198 to Ser showed the failure of Ca2+ propagation to neighboring cells in accordance with the electrical uncoupling between transfected cells, whereas mutations of Cys61 and Cys68 to Ser showed the same pattern as the wild type. [14C]Iodoacetamide labeling of free thiols of cysteine residues in mutant connexin-43s showed that two pairs of intramolecular disulfide bonds are formed between Cys54 and Cys192 and between Cys187 and Cys198. These results suggest that intercellular Ca2+ signaling takes place in cultured cells expressing connexin-43, leading to their own synchronization and that the extracellular disulfide bonds of connexin-43 are crucial for this process.

Two distinct gating mechanisms in gap junction channels: CO2-sensitive and voltage-sensitive

The chemical gating of single-gap junction channels was studied by the dual whole-cell voltage-clamp method in HeLa cells transfected with connexin43 (HeLa43) and in fibroblasts from sciatic nerves. Junctional current (Ij), single-channel conductance, and Ij kinetics were studied in cell pairs during CO2 uncoupling and recoupling at small transjunctional voltages (Vj < 35 mV: Vj gating absent) and at high Vj (Vj > 40 mV: Vj gating strongly activated). In the absence of Vj gating, CO2 exclusively caused Ij slow transitions from open to closed channel states (mean transition time: approximately 10 ms), corresponding to a single-channel conductance of approximately 120 pS. At Vj > 40 mV, Vj gating induced fast Ij flickering between open, gamma j(main state), and residual, gamma j(residual), states (transition time: approximately 2 ms). The ratio gamma j(main state)/gamma j(residual) was approximately 4-5. No obvious correlation between Ij fast flickering and CO2 treatment was noticed. At high Vj, in addition to slow Ij transitions between open and closed states, CO2 induced slow transitions between residual and closed states. During recoupling, each channel reopened by a slow transition (mean transition time: approximately 10 ms) from closed to open state (rarely from closed to residual state). Fast Ij flickering between open and residual states followed. The data are in agreement with the hypothesis that gap junction channels possess two gating mechanisms, and indicate that CO2 induces channel gating exclusively by the slow gating mechanism.

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.

Evidence that free polyunsaturated fatty acids modify Na+ channels by directly binding to the channel proteins

The effects of free polyunsaturated fatty acids (PUFA) on the binding of ligands to receptors on voltage-sensitive Na+ channels of neonatal rat cardiac myocytes were assessed. The radioligand was [benzoyl-2,5-(3)H] batrachotoxinin A 20alpha-benzoate ([(3)H]BTXB), a toxin that binds to the Na+ channel. The PUFA that have been shown to be antiarrhythmic, including eicosapentaenoic acid (EPA; C20:5n-3), docosahexaenoic acid (DHA; C22:6n-3), eicosatetraynoic acid (ETYA), linolenic acid (C18:3n-3), and linoleic acid (C18:2n-6), inhibited [(3)H]BTXB binding in a dose-dependent fashion with IC50 values of 28-35 microM, whereas those fatty acids that have no antiarrhythmic effects including saturated fatty acid (stearic acid, C18:0), monounsaturated fatty acid (oleic acid; C18:1n-9), and EPA methyl ester did not have a significant effect on [(3)H]BTXB binding. Enrichment of the myocyte membrane with cholesterol neither affected [(3)H]BTXB binding when compared with control cells nor altered the inhibitory effects of PUFA on [(3)H]BTXB binding. Scatchard analysis of [(3)H]BTXB binding showed that EPA reduced the maximal binding without altering the Kd for [(3)H]BTXB binding, indicating allosteric inhibition. The inhibition by EPA of [(3)H]BTXB binding was reversible (within 30 min) when delipidated bovine serum albumin was added. The binding of the PUFA to this site on the Na+ channel is reversible and structure-specific and occurs at concentrations close to those required for apparent antiarrhythmic effects and a blocking effect on the Na+ current, suggesting that binding of the PUFA at this site relates to their antiarrhythmic action.

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.

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.

Inhibition of gap junctional conductance by long-chain acylcarnitines and their preferential accumulation in junctional sarcolemma during hypoxia

Electrophysiological and biochemical sequelae of myocardial ischemia occur within minutes of the onset of myocardial ischemia in vivo. Both conduction delay and conduction block occur rapidly within the same time interval as the accumulation of long-chain acylcarnitines. In the present study, double whole-cell voltage-clamp procedures were used to assess the influence of long-chain acylcarnitines on gap junctional conductance in isolated pairs of canine ventricular myocytes. Long-chain acylcarnitine (5 microM) decreased gap junctional conductance from 153 to 48 nS in a time-dependent and reversible manner. Although the amplitude of junctional current was reduced by 68%, the current continued to demonstrate a linear current-voltage relation. The extent of endogenous accumulation of long-chain acylcarnitines in junctional regions of the sarcolemma was assessed in isolated myocytes in which endogenous free, short-chain, and long-chain acylcarnitine pools had been equilibrated with [3H]carnitine. Under normoxic conditions, long-chain acylcarnitines were not detectable in junctional sarcolemma of myocytes as assessed using electron microscopic autoradiography. Exposure of myocytes to hypoxia (PO2, < 15 mm Hg) for 10 minutes resulted in the preferential accumulation of endogenous long-chain acylcarnitines in junctional sarcolemma (173 +/- 5 x 10(5) molecules/microns 3), a concentration that was sevenfold greater than that found in nonjunctional sarcolemma. Therefore, endogenous long-chain acylcarnitines accumulate preferentially in junctional regions of the sarcolemma during short intervals of hypoxia. Exogenously supplied long-chain acylcarnitines can markedly decrease cellular coupling in a reversible manner, suggesting that this amphiphile may contribute to the marked slowing in conduction velocity in the ischemic heart in vivo, not only by suppressing the rapid Na+ inward current directly, as has been shown previously, but also by decreasing cellular coupling.

Gap junctional conductance in ventricular myocyte pairs isolated from postischemic rabbit myocardium

Abnormalities of myocardial gap junction-mediated cell coupling have been implicated in cardiac arrhythmogenesis. The potential role of gap junctional dysfunction in the generation of reperfusion-induced arrhythmias is uncertain. The purpose of this study was to measure the effects of myocardial ischemia and reperfusion on gap junctional conductance (gj) between isolated ventricular myocytes. By using a new experimental model, myocyte pairs were isolated from Langendorff-perfused rabbit hearts 1) after 30 minutes of global normothermic ischemia followed by 30 minutes of reperfusion, 2) after 75 minutes of control perfusion, or 3) immediately after removal of the heart. Myocytes and myocyte pairs were studied using whole-cell recording techniques. Action potential characteristics of cells in all three groups were normal. Despite similar mean gj in all three groups (0.88 +/- 0.27, 1.15 +/- 0.18, and 1.24 +/- 0.25 microS, respectively; p greater than 0.05), the postischemic group was more widely distributed and had a significantly greater proportion of poorly communicating cell pairs than either control group (gj less than 25% of mean in eight of 15 myocyte pairs versus zero of 15 and one of 13, respectively; p less than 0.02). Thus, postischemic myocyte pairs represent a heterogeneous population of electrically coupled cells in which individual deficits in coupling are masked by a normal mean value. In the reperfused intact heart, local disturbances of cell coupling, similarly undetected by gross measures of conduction, could disrupt myocardial conduction and activation on a microscopic scale and thus enhance arrhythmogenicity.

Signal propagation via gap junctions, a key step in the regulation of liver metabolism by the sympathetic hepatic nerves

Cell-to-cell communication via gap junctions has been proposed to be involved in the metabolic actions of sympathetic liver nerves in the rat. The effects of hepatic nerve stimulation and noradrenaline-, PGF2 alpha- and glucagon infusion on glucose metabolism and perfusion flow were studied in perfused rat liver in the absence and presence of the gap junctional inhibitors, heptanol, carbenoxolone and (4 beta)phorbol 12-myristate 13-acetate (4 beta PMA). (i) Stimulation of the hepatic nerve plexus increased glucose output, decreased flow and caused an overflow of noradrenaline into the hepatic vein. (ii) Heptanol completely inhibited not only the nerve stimulation-dependent metabolic and hemodynamic alterations but also the noradrenaline overflow. Thus the heptanol-dependent inhibitions were caused primarily by a strong impairment of transmitter release. (iii) Carbenoxolone inhibited the effects of neurostimulation on glucose metabolism partially by about 50%, whereas it left perfusion flow and noradrenaline overflow essentially unaltered. (iv) 4 beta PMA reduced the nerve stimulation-dependent enhancement of glucose release by about 80% but the noradrenaline-dependent increase in glucose output only by about 30%; the increase in glucose release by PGF2 alpha and by glucagon remained essentially unaltered. 4 beta PMA reduced the nerve stimulation-dependent decrease in portal flow by about 35% but did not affect the noradrenaline-and PGF2 alpha-elicited alterations, nor did it alter noradrenaline overflow. The results allow the conclusion that gap junctional communication plays a major role in the regulation of hepatic carbohydrate metabolism by sympathetic liver nerves, but not by circulating noradrenaline, PGF2 alpha or glucagon.

Incorporation of n-3 fatty acids into WB-F344 cell phospholipids inhibits gap junctional intercellular communication

In this investigation, we demonstrate that rat liver epithelial (WB-F344) cells grown in medium supplemented with n-3 fatty acids (FA) results in the inhibition of gap junctional intercellular communication (GJIC). Cells incubated for 48 hr in medium containing 50 microM alpha-linolenate (18:3n-3) resulted in a 60% inhibition of GJIC, compared to control cells, while treatment with gamma-linolenate (18:3n-6) had no effect. Supplementation with octadecatetraenoate (18:4n-3), eicosapentaenoate (20:5n-3), and docosahexaenoate (22:6n-3), inhibited GJIC by 42%, 28%, and 18%, respectively. Incubation with each of the n-3 FA markedly increased the total n-3 FA content of cellular phospholipids (PL). Growing cells in medium containing 50 microM arachidonate (20:4n-6) plus 50 microM 18:3n-3 partially attenuated the inhibition of GJIC induced by 18:3n-3. The mechanism by which n-3 FA inhibit GJIC remains to be determined.

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.

Direct regulation of ion channels by fatty acids

A variety of fatty acids regulate the activity of specific ion channels by mechanisms not involving the enzymatic pathways that convert arachidonic acid to oxygenated metabolites. Furthermore, these actions of fatty acids occur in patches of membrane excised from the cell and are not mediated by cellular signal transduction pathways that require soluble factors such as nucleotides and calcium. Thus, fatty acids themselves appear to regulate the action of channels directly, much as they regulate the action of several purified enzymes, and might constitute a new class of first or second messengers acting on ion channels.

Effects of arachidonic acid on the gap junctions of neonatal rat heart cells

Myocytes were isolated from neonatal rat hearts and grown in tissue-culture dishes for 1-2 days. Spontaneously formed cell pairs were used to study the conductance of gap junctions. The experiments involved a double voltage-clamp approach and whole-cell, tight-seal recording. Exposure to arachidonic acid (AA) produced a quasi dose-dependent decrease in junctional conductance, gi (binding constant, Kd = 4 microM; Hill coefficient, n = 0.75). AA-dependent uncoupling was reversible. Addition of 1 mg/ml albumin to the bath solution accelerated the recovery. During control, cell pairs exhibited a gradual decrease in gi (16.4% in 6 min). Exposure to 20 microM 4-bromophenacyl bromide, a phospholipase inhibitor, suppressed the decay in gi (1.8% in 6 min), suggesting that endogenous AA may be involved in spontaneous uncoupling. The effect of AA on gi was specific. Arachidic acid (100 microM) and arachidonamide (10 microM), structural analogues of AA, had no effect on gi. Currents recorded shortly before complete uncoupling caused by AA, or early during recovery from uncoupling, revealed random opening and closing of single channels. The single channel conductance, gamma i, was not affected by the concentration of AA (1 microM - 100 microM). The mean gamma i turned out to be 33.5 pS. The results suggest that AA-dependent uncoupling was caused via decrease in open channel probability, presumably mediated by a direct action on channel proteins.

Volatile anesthetics block intercellular communication between neonatal rat myocardial cells

The effects of halothane and ethrane on gap junction-mediated intercellular communication and on membrane excitability were examined in cultured neonatal rat cardiac myocytes using whole-cell voltage-clamp and current-clamp techniques. Excitability was maintained at doses of both anesthetics that reversibly abolished current flow through junctional membranes. The degree of reduction of junctional conductance was a steep function of the dose of anesthetic; complete block occurred at lower aqueous concentrations of halothane than ethrane. The time course for loss of communication was rapid; 90% reduction of initial junctional conductance occurred in less than 15 seconds after exposure to 2 mM halothane or 4 mM ethrane. Recovery of junctional conductance and junctional permeability to intracellularly injected Lucifer yellow was rapid and complete on washout of the anesthetics. As junctional conductance was reduced by halothane or ethrane exposure, unitary conductance of the gap junctional channels remained constant at about 50 pS. Uncoupling by these anesthetics is thus attributable to a decrease in the number of conducting channels rather than to reduction of the channel's unitary conductance. The data are discussed with regard to the possible role of this intercellular communication pathway in the arrhythmias and alterations of conduction velocity and contractility produced by volatile anesthetics.