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

The intercalated disc: a unique organelle for electromechanical synchrony in cardiomyocytes

The intercalated disc (ID) is a highly specialized structure that connects cardiomyocytes via mechanical and electrical junctions. Although described in some detail by light microscopy in the 19th century, it was in 1966 that electron microscopy images showed that the ID represented apposing cell borders and provided detailed insight into the complex ID nanostructure. Since then, much has been learned about the ID and its molecular composition, and it has become evident that a large number of proteins, not all of them involved in direct cell-to-cell coupling via mechanical or gap junctions, reside at the ID. Furthermore, an increasing number of functional interactions between ID components are emerging, leading to the concept that the ID is not the sum of isolated molecular silos but an interacting molecular complex, an "organelle" where components work in concert to bring about electrical and mechanical synchrony. The aim of the present review is to give a short historical account of the ID's discovery and an updated overview of its composition and organization, followed by a discussion of the physiological implications of the ID architecture and the local intermolecular interactions. The latter will focus on both the importance of normal conduction of cardiac action potentials as well as the impact on the pathophysiology of arrhythmias.

Effects of Three-Month Feeding High Fat Diets with Different Fatty Acid Composition on Myocardial Proteome in Mice

Westernized diet is characterized by a high content of saturated fatty acids (SFA) and a low level of omega-3 polyunsaturated fatty acids (PUFA), often accompanied by an imbalance in the omega-6/omega-3 PUFA ratio. Since increased intake of SFA and n-6 PUFA is considered as a cardiovascular disease risk factor, this study was conducted to determine whether a three-month dietary supplementation of high-fat diets (HFDs) with saturated fatty acids and a significant proportion of various n-6 and n-3 PUFA ratios would affect the architecture and protein expression patterns of the murine heart. Therefore, three HFD (n = 6) feeding groups: rich in SFA, dominated by PUFA with the n-6/n-3–14:1, and n-6/n-3–5:1, ratios were compared to animals fed standard mouse chow. For this purpose, we performed two-dimensional electrophoresis with MALDI-ToF mass spectrometry-based identification of differentially expressed cardiac proteins, and a histological examination of cardiac morphology. The results indicated that mice fed with all HFDs developed signs of hypertrophy and cardiac fibrosis. Animals fed SFA-rich HFD manifested the most severe cardiac hypertrophy and fibrosis lesions, whereas less pronounced changes were observed in the group of animals that ingested the highest amount of omega-3 FA. In general, all HFDs, regardless of FA composition, evoked a comparable pattern of cardiac protein changes and affected the following biological processes: lipid metabolism and FA β-oxidation, glycolysis, TCA cycle, respiratory chain, myocardium contractility, oxidative stress and PUFA eicosanoid metabolism. However, it should be noted that three proteins, namely IDH3A, LDHB, and AK1, were affected differently by various FA contents. High expression of these myocardial proteins found in the group of animals fed a HFD with the highest n-3 PUFA content could be closely related to the observed development of hypertrophy.

Connexins and Disease

Inherited or acquired alterations in the structure and function of connexin proteins have long been associated with disease. In the present work, we review current knowledge on the role of connexins in diseases associated with the heart, nervous system, cochlea, and skin, as well as cancer and pleiotropic syndromes such as oculodentodigital dysplasia (ODDD). Although incomplete by virtue of space and the extent of the topic, this review emphasizes the fact that connexin function is not only associated with gap junction channel formation. As such, both canonical and noncanonical functions of connexins are fundamental components in the pathophysiology of multiple connexin related disorders, many of them highly debilitating and life threatening. Improved understanding of connexin biology has the potential to advance our understanding of mechanisms, diagnosis, and treatment of disease.

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.

Connexins in Cardiovascular and Neurovascular Health and Disease: Pharmacological Implications

Connexins are ubiquitous channel forming proteins that assemble as plasma membrane hemichannels and as intercellular gap junction channels that directly connect cells. In the heart, gap junction channels electrically connect myocytes and specialized conductive tissues to coordinate the atrial and ventricular contraction/relaxation cycles and pump function. In blood vessels, these channels facilitate long-distance endothelial cell communication, synchronize smooth muscle cell contraction, and support endothelial-smooth muscle cell communication. In the central nervous system they form cellular syncytia and coordinate neural function. Gap junction channels are normally open and hemichannels are normally closed, but pathologic conditions may restrict gap junction communication and promote hemichannel opening, thereby disturbing a delicate cellular communication balance. Until recently, most connexin-targeting agents exhibited little specificity and several off-target effects. Recent work with peptide-based approaches has demonstrated improved specificity and opened avenues for a more rational approach toward independently modulating the function of gap junctions and hemichannels. We here review the role of connexins and their channels in cardiovascular and neurovascular health and disease, focusing on crucial regulatory aspects and identification of potential targets to modify their function. We conclude that peptide-based investigations have raised several new opportunities for interfering with connexins and their channels that may soon allow preservation of gap junction communication, inhibition of hemichannel opening, and mitigation of inflammatory signaling.

Regulation of Connexin-Based Channels by Fatty Acids

In this mini-review, we briefly summarize the current knowledge about the effects of fatty acids (FAs) on connexin-based channels, as well as discuss the limited information about the impact FAs may have on pannexins (Panxs). FAs regulate diverse cellular functions, some of which are explained by changes in the activity of channels constituted by connexins (Cxs) or Panxs, which are known to play critical roles in maintaining the functional integrity of diverse organs and tissues. Cxs are transmembrane proteins that oligomerize into hexamers to form hemichannels (HCs), which in turn can assemble into dodecamers to form gap junction channels (GJCs). While GJCs communicate the cytoplasm of contacting cells, HCs serve as pathways for the exchange of ions and small molecules between the intra and extracellular milieu. Panxs, as well as Cx HCs, form channels at the plasma membrane that enable the interchange of molecules between the intra and extracellular spaces. Both Cx- and Panx-based channels are controlled by several post-translational modifications. However, the mechanism of action of FAs on these channels has not been described in detail. It has been shown however that FAs frequently decrease GJC-mediated cell-cell communication. The opposite effect also has been described for HC or Panx-dependent intercellular communication, where, the acute FA effect can be reversed upon washout. Additionally, changes in GJCs mediated by FAs have been associated with post-translational modifications (e.g., phosphorylation), and seem to be directly related to chemical properties of FAs (e.g., length of carbon chain and/or degree of saturation), but this possible link remains poorly understood.

Arachidonic acid closes innexin/pannexin channels and thereby inhibits microglia cell movement to a nerve injury

Pannexons are membrane channels formed by pannexins and are permeable to ATP. They have been implicated in various physiological and pathophysiological processes. Innexins, the invertebrate homologues of the pannexins, form innexons. Nerve injury induces calcium waves in glial cells, releasing ATP through glial pannexon/innexon channels. The ATP then activates microglia. More slowly, injury releases arachidonic acid (ArA). The present experiments show that ArA itself reduced the macroscopic membrane currents of innexin- and of pannexin-injected oocytes; ArA also blocked K(+) -induced release of ATP. In leeches, whose large glial cells have been favorable for studying control of microglia migration, ArA blocked glial dye-release and, evidently, ATP-release. A physiological consequence in the leech was block of microglial migration to nerve injuries. Exogenous ATP (100 µM) reversed the effect, for ATP causes activation and movement of microglia after nerve injury, but nitric oxide directs microglia to the lesion. It was not excluded that metabolites of ArA may also inhibit the channels. But for all these effects, ArA and its non-metabolizable analog eicosatetraynoic acid (ETYA) were indistinguishable. Therefore, ArA itself is an endogenous regulator of pannexons and innexons. ArA thus blocks release of ATP from glia after nerve injury and thereby, at least in leeches, stops microglia at lesions.

Regulation of connexin36 gap junction channels by n-alkanols and arachidonic acid

We examined junctional conductance (gj) and its dependence on transjunctional voltage in gap junction (GJ) channels formed of wild-type connexin36 (Cx36) or its fusion form with green fluorescent protein (Cx36-EGFP) transfected in HeLa cells or endogenously expressed in primary culture of pancreatic β-cells. Only a very small fraction (∼0.8%) of Cx36-EGFP channels assembled into junctional plaques of GJs were open under control conditions. We found that short carbon chain n-alkanols (SCCAs) increased gj, while long carbon chain n-alkanols resulted in full uncoupling; cutoff is between heptanol and octanol. The fraction of functional channels and gj increased several fold under an exposure to SCCAs, or during reduction of endogenous levels of arachidonic acid (AA) by exposure to fatty acid-free BSA or cytosolic phospholipase A2 inhibitors. Moreover, uncoupling caused by exogenously applied AA can be rescued by BSA, which binds AA and other polyunsaturated fatty acids (PUFAs), but not by BSA modified with 1,2-cyclohexanedione, which does not bind AA and other PUFAs. We propose that under control conditions, Cx36 GJ channels in HeLa transfectants and β-cells are inhibited by endogenous AA, which stabilizes a closed conformational state of the channel that leads to extremely low fraction of functional channels. In addition, SCCAs increase gj by interfering with endogenous AA-dependent inhibition, increasing open probability and the fraction of functional channels.

Myocardial impulse propagation is impaired in right ventricular tissue of Zucker diabetic fatty (ZDF) rats

Background

Diabetes increases the risk of cardiovascular complications including arrhythmias, but the underlying mechanisms remain to be established. Decreased conduction velocity (CV), which is an independent risk factor for re-entry arrhythmias, is present in models with streptozotocin (STZ) induced type 1 diabetes. Whether CV is also disturbed in models of type 2 diabetes is currently unknown.

Methods

We used Zucker Diabetic Fatty (ZDF) rats, as a model of type 2 diabetes, and their lean controls Zucker Diabetic Lean (ZDL) rats to investigate CV and its response to the anti-arrhythmic peptide analogue AAP10. Gap junction remodeling was examined by immunofluorescence and western blotting. Cardiac histomorphometry was examined by Masson`s Trichrome staining and intracellular lipid accumulation was analyzed by Bodipy staining.

Results

CV was significantly slower in ZDF rats (56±1.9 cm/s) compared to non-diabetic controls (ZDL, 66±1.6 cm/s), but AAP10 did not affect CV in either group. The total amount of Connexin43 (C×43) was identical between ZDF and ZDL rats, but the amount of lateralized C×43 was significantly increased in ZDF rats (42±12 %) compared to ZDL rats (30±8%), p<0.04. Judged by electrophoretic mobility, C×43 phosphorylation was unchanged between ZDF and ZDL rats. Also, no differences in cardiomyocyte size or histomorphometry including fibrosis were observed between groups, but the volume of intracellular lipid droplets was 4.2 times higher in ZDF compared to ZDL rats (p<0.01).

Conclusion

CV is reduced in type 2 diabetic ZDF rats. The CV disturbance may be partly explained by increased lateralization of C×43, but other factors are likely also involved. Our data indicates that lipotoxicity potentially may play a role in development of conduction disturbances and arrhythmias in type 2 diabetes.

Calcium and connexin-based intercellular communication, a deadly catch?

Ca(2+) is known as a universal messenger mediating a wide variety of cellular processes, including cell death. In fact, this ion has been proposed as the 'cell death master', not only at the intracellular but also at the intercellular level. The most direct form of intercellular spread of cell death is mediated by gap junction channels. These channels have been shown to propagate cell death as well as cell survival signals between the cytoplasm of neighbouring cells, reflecting the dual role of Ca(2+) signals, i.e. cell death versus survival. Its precursor, the unopposed hemichannel (half of a gap junction channel), has recently joined in as a toxic pore connecting the intracellular with the extracellular environment and allowing the passage of a range of substances. The biochemical nature of the so-called intercellular cell death molecule, transferred through gap junctions or released/taken up via hemichannels, remains elusive but several studies pinpoint Ca(2+) itself or its messenger inositol trisphosphate as the responsible masters in crime. Although direct evidence is still lacking, indirect data including Ca(2+) involvement in intercellular communication and cell death, and effects of intercellular communication on intracellular Ca(2+) homeostasis, support this hypothesis. In addition, hemichannels and their molecular building blocks, connexin or pannexin proteins, may exert their effects on Ca(2+)-dependent cell death at the intracellular level, independently from their channel functions. This review provides a cutting edge overview of the current knowledge and underscores the intimate connection between intercellular communication, Ca(2+) signalling and cell death.

Reduced conduction reserve in the diabetic rat heart: role of iPLA2 activation in the response to ischemia

Hearts from streptozotocin (STZ)-induced diabetic rats have previously been shown to have impaired intercellular electrical coupling, due to reorganization (lateralization) of connexin43 proteins. Due to the resulting reduction in conduction reserve, conduction velocity in diabetic hearts is more sensitive to conditions that reduce cellular excitability or intercellular electrical coupling. Diabetes is a known risk factor for cardiac ischemia, a condition associated with both reduced cellular excitability and reduced intercellular coupling. Activation of Ca(2+)-independent phospholipase A(2) (iPLA(2)) is known to be part of the response to acute ischemia and may contribute to the intercellular uncoupling by causing increased levels of arachidonic acid and lysophosphatidyl choline. Normally perfused diabetic hearts are known to exhibit increased iPLA(2) activity and may thus be particularly sensitive to further activation of these enzymes. In this study, we used voltage-sensitive dye mapping to assess changes in conduction velocity in response to acute global ischemia in Langendorff-perfused STZ-induced diabetic hearts. Conduction slowing in response to ischemia was significantly larger in STZ-induced diabetic hearts compared with healthy controls. Similarly, slowing of conduction velocity in response to acidosis was also more pronounced in STZ-induced diabetic hearts. Inhibition of iPLA(2) activity using bromoenol lactone (BEL; 10 μM) had no effect on the response to ischemia in healthy control hearts. However, in STZ-induced diabetic hearts, BEL significantly reduced the amount of conduction slowing observed beginning 5 min after the onset of ischemia. BEL treatment also significantly increased the time to onset of sustained arrhythmias in STZ-induced diabetic hearts but had no effect on the time to arrhythmia in healthy control hearts. Thus, our results suggest that iPLA(2) activation in response to acute ischemia in STZ-induced diabetic hearts is more pronounced than in control hearts and that this response is a significant contributor to arrhythmogenic conduction slowing.

Phosphorylation of connexin43 on serine 306 regulates electrical coupling

BACKGROUND

Phosphorylation is a key regulatory event in controlling the function of the cardiac gap junction protein connexin43 (Cx43). Three new phosphorylation sites (S296, S297, S306) have been identified on Cx43; two of these sites (S297 and S306) are dephosphorylated during ischemia. The functional significance of these new sites is currently unknown.

OBJECTIVE

The purpose of this study was to examine the role of S296, S297, and S306 in the regulation of electrical intercellular communication.

METHODS

To mimic constitutive dephosphorylation, serine was mutated to alanine at the three sites and expressed in HeLa cells. Electrical coupling and single channel measurements were performed by double patch clamp. Protein expression levels were assayed by western blotting, localization of Cx43, and phosphorylation of S306 by immunolabeling. Free hemichannels were assessed by biotinylation.

RESULTS

Macroscopic conductance in cells expressing S306A was reduced to 57% compared to wild type (WT), whereas coupling was not significantly changed in cells expressing either S296A or S297A. S306A-expressing cells displayed similar protein and free hemichannel abundance compared to WT Cx43, whereas the fractional area of plaques in cell-to-cell interfaces was increased. However, single channel measurements showed a WT Cx43 main state conductance of 119 pS, whereas the main state conductance of S306A channels was reduced to 95 pS. Furthermore, channel gating was affected in S306A channels.

CONCLUSION

Lack of phosphorylation at serine 306 results in reduced coupling, which can be explained by reduced single channel conductance. We suggest that dephosphorylation of S306 partly explains the electrical uncoupling seen in myocardial ischemia.

Phosphatidylinositol-bisphosphate regulates intercellular coupling in cardiac myocytes

Changes in the lipid composition of cardiac myocytes have been reported during cardiac hypertrophy, cardiomyopathy, and infarction. Because a recent study indicates a relation between low phosphatidylinositol-bisphosphate (PIP(2)) levels and reduced intercellular coupling, we tested the hypothesis that agonist-induced changes in PIP(2) can result in a reduction of the functional coupling of cardiomyocytes and, consequently, in changes in conduction velocity. Intercellular coupling was measured by Lucifer Yellow dye transfer in cultured neonatal rat cardiomyocytes. Conduction velocity was measured in cardiomyocytes grown on microelectrode arrays. Intercellular coupling was reduced by angiotensin II (43.7 +/- 9.3%, N = 11) and noradrenaline (58.0 +/- 10.7%, N = 11). To test if reduced intercellular coupling after agonist stimulation was caused by PIP(2)-depletion, myocytes were stimulated by angiotensin II (57.3 +/- 5.7%, N = 14) and then allowed to recover in medium with or without wortmannin (an inhibitor of PIP(2) synthesis). Intercellular coupling fully recovered in control medium (102.1 +/- 8.9%, N = 10), whereas no recovery occurred in the presence of wortmannin (69.3 +/- 7.8%, N = 12). Inhibition of PKC, calmodulin, or arachidonic acid production did not affect the response to either angiotensin II or noradrenaline. Furthermore, decreasing or increasing PIP(2) also decreased and increased intercellular coupling, respectively. This supports the role of PIP(2) in the regulation of intercellular coupling. In beating myocytes, conduction velocity was reduced by angiotensin II stimulation, and recovery after wash out was prevented by inhibition of PIP(2) production. Reductions in PIP(2) inhibit intercellular coupling in cardiomyocytes, and stimulation by physiologically relevant agonists reduces intercellular coupling by this mechanism. The reduction in intercellular coupling lowered conduction velocity.

The presynaptic effects of arachidonic acid and prostaglandin E2 at the frog neuromuscular junction

Arachidonic acid and prostaglandin E2 decreased the frequency of miniature endplate potentials with producing any changes in the their amplitude-time parameters. Arachidonic acid and prostaglandin E2 decreased the quantum composition of endplate currents and the amplitude of the third phase of the nerve ending response, which reflects currents though potential-dependent K+ channels. A perineural method was used to demonstrate that arachidonic acid and prostaglandin E2 suppressed the nerve ending Ca2+ current. The cyclooxygenase blocker indomethacin increased neurotransmitter secretion and decreased the third phase of the nerve ending response. The effects of arachidonic acid and prostaglandin E2 on evoked neurotransmitter release were not seen in the presence of indomethacin, while the third phase of the response continued to show a reduction. It is suggested that prostaglandin E2 mediates the effects of arachidonic acid on spontaneous and evoked neurotransmitter secretion, Ca2+ currents, and Ca2+ -dependent K+ currents. In addition, arachidonic acid and prostaglandin E2 had their own effects on potential-dependent K+ currents in nerve endings.

Hypoxia/reoxygenation reduces microvascular endothelial cell coupling by a tyrosine and MAP kinase dependent pathway

Communication of electrical signals along the microvascular endothelium plays a key role in integrating microvascular function required for local regulation of blood flow. The aim of the present study was to examine the effect of a short-term hypoxia (0.1% O(2), 1 h) plus reoxygenation (H/R) on electrical coupling in cultured monolayers of microvascular endothelial cells (rat skeletal muscle origin). To assess coupling, we used a current injection technique and a Bessel function model to compute the intercellular resistance (an inverse measure of coupling) and cell membrane resistivity (a measure of resistance to current leakage across the cell membrane). H/R resulted in rapid (within 4 min after reoxygenation) and sustained (up to 100 min) reduction in intercellular coupling, but it did not alter membrane resistivity. H/R did not alter gap junction protein connexin 43 expression nor its tyrosine phosphorylation as determined by immunoblot and immunoprecipitation analyses. Inhibition of mitochondrial respiration (1 mM NaCN) did not mimic the effect of H/R. However, pre-treatment of monolayers with tyrphostin A48 (1.5 microM), PP2 (10 nM) (tyrosine kinase inhibitors), U 0126 (20 microM), and PD 98059 (5 microM) (MEK1/2 inhibitors) inhibited the H/R-induced reduction in coupling. These results indicate that endothelial cell coupling was reduced quickly after reoxygenation, via activation of a tyrosine and MAP kinase dependent pathway. We predict that a short-term H/R can rapidly compromise microvascular function in terms of reduced cellular communication along the vascular wall.

Phytanic acid accumulation is associated with conduction delay and sudden cardiac death in sterol carrier protein-2/sterol carrier protein-x deficient mice

INTRODUCTION

The sterol carrier protein-2 gene encodes two functionally distinct proteins: sterol carrier protein-2 (SCP2, a peroxisomal lipid carrier) and sterol carrier protein-x (SCPx, a peroxisomal thiolase known as peroxisomal thiolase-2), which is involved in peroxisomal metabolism of bile acids and branched-chain fatty acids. We show in this study that mice deficient in SCP2 and SCPx (SCP2null) develop a cardiac phenotype leading to a high sudden cardiac death rate if mice are maintained on diets enriched for phytol (a metabolic precursor of branched-chain fatty acids).

METHODS AND RESULTS

In 210 surface and 305 telemetric ECGs recorded in wild-type (C57BL/6; wt; n = 40) and SCP2 null mice (n = 40), no difference was observed at baseline. However, on diet, cycle lengths were prolonged in SCP2 null mice (262.9 +/- 190 vs 146.3 +/- 43 msec), AV conduction was prolonged (58.3 +/- 17 vs 42.6 +/- 4 ms), and QRS complexes were wider (19.1 +/- 5 vs 14.0 +/- 4 ms). In 11 gene-targeted Langendorff-perfused hearts isolated from SCP2 null mice after dietary challenge, complete AV blocks (n = 5/11) or impaired AV conduction (Wenckebach point 132 +/- 27 vs 92 +/- 10 msec; P < 0.05) could be confirmed. Monophasic action potentials were not different between the two genotypes. Left ventricular function studied by echocardiography was similar in both strains. Phytanic acid but not pristanic acid accumulated in the phospholipid fraction of myocardial membranes isolated from SCP2 null mice.

CONCLUSION

Accumulation of phytanic acid in myocardial phospholipid membranes is associated with bradycardia and impaired AV nodal and intraventricular impulse conduction, which could provide an explanation for sudden cardiac death in this model.

Regulation of astrocyte gap junctions by hypoxia-reoxygenation

Confluent cultures of rat cortical astrocytes were subjected to 12-h hypoxia (<1% O(2)) followed by reoxygenation. Just after hypoxia, the cellular distribution, phosphorylation state and levels of connexin43 (Cx43), as well as the extent of dye coupling were as in control conditions. Nonetheless, 15-30 min after reoxygenation, dye coupling was transiently reduced by approximately 70%. The reduction in dye coupling occurred without changes in the state of phosphorylation or levels of Cx43. Nevertheless, it was correlated with a decrease in Cx43 reactivity found at membrane appositions and the appearance of intracellular Cx43-positive vesicle-like structures of variable size, suggesting internalization of gap junction channels. Reoxygenation-induced cellular uncoupling and redistribution of Cx43 were prevented by melatonin (500 microM), a potent-free radical scavenger, or indomethacin (50 microM), an inhibitor of the cyclooxygenase-dependent arachidonic acid metabolism. In astrocytes cultured under normoxia, the state of phosphorylation of Cx43 was not affected by antimycin A, a blocker of the mitochondrial oxidative metabolism, but phosphorylation was drastically reduced by iodoacetate, a blocker of anaerobic glycolysis. Thus, these results strongly suggest that reoxygenation-induced uncoupling is mediated by arachidonic acid byproducts that induce, at least, disorganization of Cx43 gap junction channels.

Possible mechanism(s) of arachidonic acid-induced intracellular acidosis in rat cardiac myocytes

Arachidonic acid (AA) and other nonesterified fatty acids (FAs) have been shown to exert harmful effects during cardiac ischemia. By continuously measuring intracellular pH (pH(i)) changes in neonatal and adult cardiac myocytes, we have found, for the first time, that 10 micromol/L AA induces a substantial intracellular acidosis (0.3 to 0.4 pH units). We have ruled out the possibilities that the AA-induced acidosis is caused by (1) inhibition or stimulation of the pH(i) regulators, (2) protein kinase C activation or the generation of AA metabolites or free radicals, or (3) activation of NADPH oxidase or an inward H(+) current. The AA-induced acidosis fits to a simple diffusion mechanism, as proposed by Kamp and Hamilton (flip-flop model) for artificial phospholipid bilayers. The important properties found in the cardiac myocyte are that (1) the initial rate of acid flux (J(H)) increases with the AA concentration (2 to 50 micromol/L), (2) FAs with a (-)COOH group (eg, AA, oleic acid, and linoleic acid) induce intracellular acidification, but FAs with a (-)COOCH(3) group (eg, AA methyl ester) have little effect on the pH(i), (3) tetradecylamine (FA amine) induces intracellular alkalosis, and, most importantly, (4) both the AA- and tetradecylamine-induced pH(i) changes can be reversed by 0.3% BSA. Because a low concentration of AA (10 micromol/L) can induce a substantial acidosis, the possible involvement of the FA-evoked acidosis in the negative inotropic effect during cardiac ischemia is discussed. The full text of this article is available at http://www. circresaha.org.

Cardiac ionic currents and acute ischemia: from channels to arrhythmias

The aim of this review is to provide basic information on the electrophysiological changes during acute ischemia and reperfusion from the level of ion channels up to the level of multicellular preparations. After an introduction, section II provides a general description of the ion channels and electrogenic transporters present in the heart, more specifically in the plasma membrane, in intracellular organelles of the sarcoplasmic reticulum and mitochondria, and in the gap junctions. The description is restricted to activation and permeation characterisitics, while modulation is incorporated in section III. This section (ischemic syndromes) describes the biochemical (lipids, radicals, hormones, neurotransmitters, metabolites) and ion concentration changes, the mechanisms involved, and the effect on channels and cells. Section IV (electrical changes and arrhythmias) is subdivided in two parts, with first a description of the electrical changes at the cellular and multicellular level, followed by an analysis of arrhythmias during ischemia and reperfusion. The last short section suggests possible developments in the study of ischemia-related phenomena.

Arachidonic acid-induced dye uncoupling in rat cortical astrocytes is mediated by arachidonic acid byproducts

Arachidonic acid (AA) induced a concentration- and time-dependent reduction in gap junction-mediated dye coupling between cultured astrocytes. The effect was greatly diminished by inhibition of cyclooxygenases and lipoxygenases. The action of a low concentration of AA (5 microM) was also prevented by Ca2+-free extracellular solution or a high concentration of melatonin, a potent free radical scavenger, but not by Nomega-nitro-l-arginine, a nitric oxide (NO) synthase inhibitor. Thus, this effect may depend on Ca2+ influx and oxygen free radicals but not on NO generation. Cellular uncoupling induced by a high (100 microM), but not a low, AA concentration was rapidly reversed by washing with albumin containing solution. After reversal from 5 min but not 2.5 min inhibition with a high AA concentration dye coupling between astrocytes became refractory to a low concentration of AA, suggesting desensitization of the response elicited by a low concentration of the fatty acid. Dye uncoupling occurred without changes in levels and state of phosphorylation (immunoblotting and 32P-incorporation) of connexin43, the main astrocyte gap junctional protein. However, maximal cell uncoupling induced by a low (Slow action) but not by a high (Fast action) AA concentration was paralleled by a reduction in connexin43 (immunofluorescence) at cell-to-cell contacts. It is proposed that the AA-induced dye uncoupling is mediated by byproducts that induce rapid channel closure or slow removal of connexin43 gap junctions.

The sleep-inducing lipid oleamide deconvolutes gap junction communication and calcium wave transmission in glial cells

Oleamide is a sleep-inducing lipid originally isolated from the cerebrospinal fluid of sleep-deprived cats. Oleamide was found to potently and selectively inactivate gap junction-mediated communication between rat glial cells. In contrast, oleamide had no effect on mechanically stimulated calcium wave transmission in this same cell type. Other chemical compounds traditionally used as inhibitors of gap junctional communication, like heptanol and 18beta-glycyrrhetinic acid, blocked not only gap junctional communication but also intercellular calcium signaling. Given the central role for intercellular small molecule and electrical signaling in central nervous system function, oleamide- induced inactivation of glial cell gap junction channels may serve to regulate communication between brain cells, and in doing so, may influence higher order neuronal events like sleep induction.

Conductances and selective permeability of connexin43 gap junction channels examined in neonatal rat heart cells

Myocytes from neonatal rat hearts were used to assess the conductive properties of gap junction channels by means of the dual voltage-clamp method. The experiments were carried out on three types (groups) of preparations: (1) induced cell pairs, (2) preformed cell pairs with few gap junction channels (1 to 3 channels), and (3) preformed cell pairs with many channels (100 to 200 channels) after treatment with uncoupling agents such as SKF-525A (75 micromol/L), heptanol (3 mmol/L), and arachidonic acid (100 micromol/L). In group 1, the first opening of a newly formed channel was slow (20 to 65 ms) and occurred 7 to 25 minutes after physical cell contact. The rate of channel insertion was 1.3 channels/min. Associated with a junctional voltage gradient (Vj), the channels revealed multiple conductances, a main open state [gamma(j)(main state)], several substates [gamma(j)(substates)], and a residual state [gamma(j)(residual state)]. On rare occasions, the channels closed completely. The same phenomena were observed in groups 2 and 3. The existence of gamma(j)(residual state) provides an explanation for the incomplete inactivation of the junctional current (Ij) at large values of Vj in cell pairs with many gap junction channels. The values of gamma(j)(main state) and gamma(j)(residual state) gained from groups 1, 2, and 3 turned out to be comparable and hence were pooled. The fit of the data to a Gaussian distribution revealed a narrow single peak for both conductances. The values of gamma(j) were dependent on the composition of the pipette solution. Solutions were as follows: (1) KCl solution, gamma(j)(main state)=96 pS and gamma(j)(residual state)=23 pS; (2) Cs+ aspartate solution, gamma(j)(main state)=61 pS and gamma(j)(residual state)=12 pS; and (3) tetraethylammonium+ aspartate solution, gamma(j)(main state)=19 pS and gamma(j)(residual state)=3 pS. The respective gamma(j)(main state)-to-gamma(j)(residual state) ratios were 4.2, 5.1, and 6.3. This indicates that the residual state restricts ion permeation more efficiently than does the main state. Transitions of Ij between open states (main open state, substates, and residual state) were fast (<2 ms), and transitions involving the closed state and an open state were slow (15 to 65 ms). This implies the existence of two gating mechanisms. The residual state may be regarded as the ground state of electrical gating controlled by Vj; the closed state, as the ground state of chemical gating.

Intrinsic NMDA-induced oscillations in motoneurons of an adult vertebrate spinal cord are masked by inhibition

Low-frequency membrane potential oscillations were induced in motoneurons (MNs) of isolated hemisected frog spinal cords during N-methyl-D-aspartate (NMDA) application. Oscillations required the presence of physiological Mg2+ and preincubation with strychnine, whereas incubation with bicuculline or phaclofen was not effective. Oscillations were evident in intracellular recordings from single MNs and simultaneous extracellular recordings from lumbar ventral roots. In Mg(2+)-free solution, MNs exhibited irregular transient membrane potential depolarizations that were blocked by D,L-2-amino-5-phosphonopentanoic acid (APV) but not by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). Generation and maintenance of membrane potential oscillations required specific NMDA receptor activation. Oscillations were antagonized by APV but not by CNQX. Strychnine preincubation was required for NMDA to induce oscillations, but was not critical in maintaining them, because oscillations persisted after removal of strychnine. Therefore oscillations are suggested to be an inherent property of the spinal neuronal circuitry. Tetrodotoxin (TTX) blocked spike activity and had a bimodal effect on membrane potential oscillations. Oscillations initially were blocked by TTX, but reappeared spontaneously after 10-40 min. This suggests that maintenance of oscillations, once evoked, does not involve MN firing. Na+ entry through TTX-insensitive Na+ channels and/or NMDA receptor channels, trans-membrane Ca2+ flux, Ca2+ release from intracellular stores, and Ca2+ activated K+ channels were critical in controlling the amplitude and frequency of membrane potential oscillations. It is hypothesized that these unmasked intrinsic oscillations in adult frog spinal cord MNs may represent a premetamorphic spinal oscillator involved in tadpole swimming that becomes suppressed during metamorphosis as strychnine-sensitive inhibition becomes more pronounced.

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.

Photolysis of caged compounds characterized by ratiometric confocal microscopy: a new approach to homogeneously control and measure the calcium concentration in cardiac myocytes

Here we describe the subcellularly uniform control of the intracellular Ca2+ concentration ([Ca2+]i) by flash photolysis of caged Ca2+ or a caged Ca2+ buffer. A mixture of the two Ca2+ indicators Fluo-3 and Fura-red was used together with a laser-scanning confocal microscope to reveal spatial aspects of intracellular Ca2+ signals. The patch clamp technique in the whole-cell variant was applied to load the cells with the indicator mixture together with either DM-nitrophen or diazo-2 and to measure changes in the membrane current. An in vivo calibration was performed to convert the Fluo-3/Fura-red fluorescence ratios to [Ca2+] values. The resulting calibration curve suggested an apparent KD of 1.6 microM, Rmax of 2.15, Rmin of 0.08 and a Hill-coefficient of 0.75 for the indicator mixture. Controlled rupture of the cell membrane revealed a large fraction of immobile intracellular Fura-red fluorescence that may account for the reduced in vivo Rmax value when compared to the in vitro value of 3.1. In cardiac myocytes, flash photolytic release of Ca2+ from DM-nitrophen generated inwardly directed Na+/Ca2+ exchange currents and Ca2+ signals that were graded with the discharged flash-energy. Rapid line-scans revealed subcellularly homogeneous [Ca2+] jumps regardless of the discharged flash energy. Ca2+ signals evoked by L-type Ca2+ currents (ICa) could be terminated rapidly in a spatially homogeneous manner by UV flash photolysis of diazo-2. No side-effects of the photolytic products of DM-nitrophen or diazo-2 with the mixture of Fluo-3/Fura-red were detectable in our experiments. The combination of UV flash photolysis and laser scanning confocal microscopy enabled us to control [Ca2+]i homogeneously on the subcellular level. This approach may improve our understanding of the subcellular properties of cardiac Ca2+ signalling. The technique can also be applied in other cell types and with other signalling systems for which caged compounds are available.

Electrophysiologic changes in ischemic ventricular myocardium: I. Influence of ionic, metabolic, and energetic changes

Myocardial ischemia leads to significant changes in the intracellular and extracellular ionic milieu, high-energy phosphate compounds, and accumulation of metabolic by-products. Changes are measured in extracellular pH and K+, and intracellular pH, Ca2+, Na+, Mg2+, ATP, ADP, and inorganic phosphate. Alterations of membrane currents occur as a consequence of these ionic changes, adrenergic receptor stimulation, and accumulation of lactate, amphipathic compounds, and adenosine. Changes in the volume of the extracellular and intracellular spaces contribute further to the ultimate perturbations of active and passive membrane properties that underlie alterations in excitability, abnormal automaticity, refractoriness, and conduction. These characteristic changes of electrophysiologic properties culminate in loss of excitability and failure of impulse propagation and form the substrate for ventricular arrhythmias mediated through abnormal impulse formation and reentry. The ability to detail the changes in ions, metabolites, and high-energy phosphate compounds in both the extracellular and intracellular spaces and to correlate them directly with the simultaneously occurring electrophysiologic changes have greatly enhanced our understanding of the electrical events that characterize the ischemic process and hold promise for permitting studies aimed at developing interventions that may lessen the lethal consequences of ischemia.

Specific inhibition of Na-Ca exchange function by antisense oligodeoxynucleotides

The Na-Ca exchanger is essential for the Ca2+ homeostasis in many cell types. This transporter has been difficult to investigate because no specific inhibitor is available. We have synthesized an antisense oligodeoxynucleotide directed against the rat cardiac Na-Ca exchanger mRNA. To estimate the activity of the Na-Ca exchange in single cultured myocytes, the exchange current (INaCa) was measured with the voltage-clamp technique while the intracellular Ca2+ concentration ([Ca2+]i) was simultaneously recorded. Most cells exposed to antisense oligodeoxynucleotide showed neither an INaCa nor an increase of [Ca2+]i upon extracellular Na+ removal. Liberation of Ca2+ by flashphotolysis of caged Ca2+ was not followed by a decay of [Ca2+]i in cells exposed to the antisense oligonucleotide, whereas in control cells resting [Ca2+]i was reached 6 s after the flash. Control experiments with non-sense and mismatched oligonucleotides were performed to exclude unspecific inhibitory effects. These results demonstrate that the Na-Ca exchange was specifically and completely suppressed and that antisense oligodeoxynucleotides represent a useful tool to investigate the cellular and molecular properties of the Na-Ca exchanger.

Ca-mediated and independent effects of arachidonic acid on gap junctions and Ca-independent effects of oleic acid and halothane

In Novikoff hepatoma cell pairs studied by double perforated patch clamp (DPPC), brief (20 s) exposure to 20 microM arachidonic acid (AA) induced a rapid and reversible uncoupling. In pairs studied by double whole-cell clamp (DWCC), uncoupling was completely prevented by effective buffering of Cai2+ with BAPTA. Similarly, AA (20 s) had no effect on coupling in cells perfused with solutions containing no added Ca2+ (SES-no-Ca) and studied by DPPC, suggesting that Ca2+ influx plays an important role. Parallel experiments monitoring [Ca2+]i with fura-2 showed that [Ca2+]i increases with AA to 0.7-1.5 microM in normal [Ca2+]o, and to approximately 400 nM in SES-no-Ca solutions. The rate of [Ca2+]i increase matched that of Gj decrease, but [Ca2+]i recovery was faster. In cells studied by DWCC with 2 mM BAPTA in the pipette solution and superfused with SES-no-Ca, long exposure (1 min) to 20 microM AA caused a slow and virtually irreversible uncoupling. This result suggests that AA has a dual mechanism of uncoupling: one dominant, fast, reversible, and Ca(2+)-dependent, the other slow, poorly reversible, and Ca(2+)-independent. In contrast, uncoupling by oleic acid (OA) or halothane was insensitive to internal buffering with BAPTA, suggesting a Ca(2+)-independent mechanism only.

Modulation of Ca2+ release in cultured neonatal rat cardiac myocytes. Insight from subcellular release patterns revealed by confocal microscopy

It is well established that in heart muscle the influx of Ca2+ through Ca2+ channels during the action potential is the main trigger for Ca2+ release from the sarcoplasmic reticulum (SR), but intact cardiac tissue and single myocytes are also known to exhibit spontaneous Ca2+ release from the SR under a variety of circumstances. Although conditions favoring spontaneous activity have been examined extensively, mechanisms modulating or regulating spontaneous as well as triggered Ca2+ release are still largely unknown. Using the high spatial and temporal resolution of laser-scanning confocal microscopy, we investigated subcellular aspects of spontaneous and triggered Ca2+ release in isolated rat neonatal myocytes loaded with the Ca(2+)-sensitive fluorescent dye fluo 3. Three distinct patterns of spontaneous Ca2+ release were identified: (1) a homogeneous Ca2+ release, presumably corresponding to Ca2+ release during a spontaneous action potential, (2) a focal or spatially restricted Ca2+ release with no or only limited subcellular propagation, and (3) a Ca2+ release propagating as a wave throughout the entire cell. Pharmacologic tools that interfere with the SR revealed that all release types were critically dependent on the Ca2+ release and uptake function of the SR. From our results we conclude that the probability, extent, and pattern of Ca2+ release are modulated on the subcellular level. The observed spectrum of release patterns can be explained by a space- and time-dependent variability in the positive feedback of the Ca(2+)-induced Ca(2+)-release mechanism within an individual myocyte. Presumably, this variability depends on the existence of subcellular functional elements of the SR. The actual degree of positive feedback may be modulated locally by the Ca(2+)-loading state of each SR element.

Microscopic conduction in cultured strands of neonatal rat heart cells measured with voltage-sensitive dyes

Microscopic discontinuities in electrical activation were assessed in synthetic strands of neonatal rat myocytes cultured on a growth-directing matrix. An optical method using voltage-sensitive dye (RH-237) and a photodiode technique was used for recordings of membrane potential changes with subcellular resolution. Spatial resolution of the method (diameter of measurement area, 5.5 microns; interdiode distance, 30 microns) allowed for simultaneous measurements of cytoplasmic conduction time within a single cell and junctional conduction time across the cell border. In one-dimensional cell chains, where cells were juxtaposed by end-to-end connections but devoid of lateral connections, propagation of the excitation wave was strongly nonuniform: cytoplasmic conduction time was 38 +/- 30 (mean +/- SD) microseconds (n = 37), whereas junctional conduction time was 118 +/- 40 microseconds (n = 27, P < .0001). A mean delay introduced by a single junction was 80 microseconds, or 51% of conduction time. In two-dimensional strands consisting of several cells in width, which exhibited lateral as well as end-to-end connections, inhomogeneity of conduction was smaller: the cytoplasmic and junctional conduction times were 57 +/- 30 (n = 46) and 89 +/- 40 (n = 48) microseconds, respectively (P < .0001); mean junctional conduction delay was 32 microseconds (22% of conduction time). Mathematical modeling suggested that the averaging effect of lateral connections is caused by lateral convergence of local excitatory current beyond and lateral divergence before end-to-end connections. Our results demonstrate that the current flow through lateral cell-to-cell connections smooth the excitation wave front during longitudinal conduction in myocardial tissue.

Recent insights pertaining to sarcolemmal phospholipid alterations underlying arrhythmogenesis in the ischemic heart

Myocardial ischemia in vivo is associated with dramatic electrophysiologic alterations that occur within minutes of cessation of coronary flow and are rapidly reversible with reperfusion. This suggests that subtle and reversible biochemical alterations within or near the sarcolemma may contribute to the electrophysiologic derangements. Our studies have concentrated on two amphipathic metabolites, long-chain acylcarnitines and lysophosphatidylcholine (LPC), which have been shown to increase rapidly in ischemic tissue in vivo and to elicit electrophysiologic derangements in normoxic tissue in vitro. Incorporation of these amphiphiles into the sarcolemma at concentrations of 1 to 2 mole%, elicits profound electrophysiologic derangements analogous to those observed in ischemic myocardium in vivo. The pathophysiological effects of the accumulation of these amphiphiles are thought to be mediated by alterations in the biophysical properties of the sarcolemmal membrane, although there is a possibility of a direct effect upon ion channels. Inhibition of carnitine acyltransferase I (CAT-I) in the ischemic cat heart was found to prevent the increase in long-chain acylcarnitines and LPC and to significantly reduce the incidence of malignant arrhythmias including ventricular tachycardia and fibrillation. This review focuses on the electrophysiologic derangements that are observed during early ischemia and presents data supporting the concept that accumulation of these amphiphiles within the sarcolemma contributes to these changes. The potential contribution of these amphiphiles to the increases in extracellular potassium and intracellular calcium are examined. Finally, recent data pertaining to the accumulation of long-chain acylcarnitines on cell-to-cell uncoupling are presented. In addition to the events reviewed here, there are many other alterations that occur during early myocardial ischemia, but the results from multiple studies over the past two decades indicate that the accumulation of these amphiphiles contributes importantly to arrhythmogenesis and that development of specific inhibitors of CAT-I or phospholipase A2 may be a promising therapeutic strategy to attenuate the incidence of lethal arrhythmias associated with ischemic heart disease in man.

A structural basis for the unequal sensitivity of the major cardiac and liver gap junctions to intracellular acidification: the carboxyl tail length

The regulation of junctional conductance (Gi) of the major cardiac (connexin43; Cx43) and liver (connexin32; Cx32) gap junction proteins by intracellular hydrogen ion concentration (pH; pHi), as well as well as that of a truncation mutant of Cx43 (M257) with 125 amino acids deleted from the COOH terminus, was characterized in pairs of Xenopus laevis oocytes expressing homologous channels. Oocytes were injected with 40 nl mRNAs (2 micrograms/microliters) encoding the respective proteins; subsequently, cells were stripped, paired, and incubated for 20-24 h. Gj was measured in oocyte pairs using the dual electrode voltage-clamp technique, while pHi was recorded simultaneously in the unstimulated cell by means of a proton-selective microelectrode. Because initial experiments showed that the pH-sensitive microelectrode responded more appropriately to acetate than to CO2 acidification, oocytes expressing Cx32 and wild type and mutant Cx43 were exposed to a sodium acetate saline, which was balanced to various levels of pH using NaOH and HCl. pH was changed in a stepwise manner, and quasi-steady-state Gj -pHi relationships were constructed from data collected at each step after both Gj and pHi had reached their respective asymptotic values. A moderate but significant increase of Gj was observed in Cx43 pairs as pHi decreased from 7.2 to 6.8. In both Cx32 and M257 pairs, Gj increased significantly over a wider pH range (i.e., between 7.2 and 6.3). Further acidification reversibly reduced Gj to zero in all oocyte pairs. Pooled data for the individual connexins obtained during uncoupling were fitted by the Hill equation; apparent 50%-maximum (pK;pKa) values were 6.6 and 6.1 for Cx43 and Cx32, respectively, and Hill coefficients were 4.2 for Cx43 and 6.2 for Cx32. Like Cx32, M257 had a more acidic pKa (6.1) and steeper Hill coefficient (6.0) than wild type Cx43. The pKa and Hill coefficient of M257 were very similar to those of Cx32. These experiments provide the first direct comparison of the effects of acidification on Gj in oocyte pairs expressing Cx43 or Cx32. The results indicate that structural differences in the connexins are the basis for their unequal sensitivity to intracellular acidification in vivo. The data further suggest that a common pH gating mechanism may exist between amino acid residues 1 and 256 in both Cx32 and Cx43. However, the longer carboxyl tail of Cx43 relative to Cx32 or M257 provides additional means to facilitate acidification-induced gating; its presence shifts the pKa from 6.1 (Cx32 and M257) to 6.6 (Cx43) in the conductance of these channels.

Effects of phorbol ester on gap junctions of neonatal rat heart cells

Myocytes were isolated from the ventricles of neonatal rat hearts and cultured for 1-3 days. Newly formed cell pairs were used to examine the conductance of gap junctions, gj. Measurements were performed using a dual voltage-clamp method in conjunction with a whole-cell, tight-seal recording. Exposure to the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA, 100-160 nM) led to a decrease in gj. Single-channel events recorded immediately before complete uncoupling yielded a single-channel conductance, gamma j, of 40.5 pS, implying that TPA affects the channel kinetics rather than gamma j. TPA-induced uncoupling was observed at subphysiological levels of cytosolic Ca2+ (pipette solution = 18 nM), not at physiological levels (pipette solution = 170 nM). The effects of TPA could not be mimicked by 250 microM 1-oleoyl-2-acetyl-glycerol (OAG). Preincubation with TPA (up to 24 h) revealed no changes in gj attributable to down-regulation of protein kinase C, PKC. Pretreatment with PKC inhibitors, staurosporine or PKCI, prevented the TPA-dependent decrease in gj. TPA-dependent uncoupling was not impaired by 4-bromophenacyl bromide, an inhibitor of phospholipase A2, PLA2; conversely, an arachidonic acid-dependent decrease in gj was not prevented by PKCI. This suggests that gj regulation does not involve an interaction between PLA2 and PKC.

Temperature dependence of gap junction properties in neonatal rat heart cells

Cell pairs of neonatal rat hearts were used to study the influence of temperature on the electrical properties of gap junctions. A dual voltage-clamp method was adopted, which allowed the voltage gradient between the cells to be controlled and the intercellular current flow to be measured. Cell pairs with normal coupling revealed a positive correlation between the conductance of the junctional membranes, gj, and temperature. Cooling from 37 degrees C to 14 degrees C led to a steeper decrease in gj, cooling from 14 degrees C to -2 degrees C to a shallower decrease (37 degrees C: gj = 48.3 nS; 14 degrees C: gj = 21.4 nS; -2 degrees C: gj = 17.5 nS), corresponding to a temperature coefficient, Q10, of 1.43 and 1.14 respectively. The existence of two Q10 values implies that gj may be controlled by enzymatic reactions. When gj was low, i.e. below 5 nS (conditions: low temperature; treatment with 3 mM heptanol), it showed voltage-dependent gating. This property was not visible when gj was large, i.e. 20-70 nS (conditions: high temperature; normal saline), presumably because of series resistances (pipette resistance). Cell pairs with weak intrinsic coupling and normally coupled cell pairs treated with 3 mM heptanol revealed a positive correlation between the conductance of single gap-junction channels, gamma j, and temperature (37 degrees C: 75.6 pS; -2 degrees C: 19.6 pS), corresponding to a Q10 of 1.41.

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.

Arachidonic acid-induced Ca2+ release from isolated sarcoplasmic reticulum

Arachidonic acid has been shown to release Ca2+ from isolated skeletal and cardiac sarcoplasmic reticulum (SR) vesicles. The release took place nearly equally well from all fractions of the SR and was only partially inhibited by ruthenium red, suggesting that some other pathway is involved in addition to the SR Ca2+ release channel. Arachidonic acid increased SR Ca2+ efflux even in the presence of several different SR Ca2+ pump inhibitors. It also had considerably less effect on uptake measured in the presence of oxalate and did not appear to inhibit Ca(2+)-dependent ATPase activity. Thus, the SR Ca2+ pump also appears to be minimally perturbed by arachidonic acid. Arachidonyl CoA was more effective at releasing Ca2+ than the parent compound. Arachidonic acid effects were not inhibited by lipoxygenase or cyclooxygenase inhibitors, suggesting that no eicosanoids are involved in the effects under study here. Flunarizine, cinnarizine and propyl-methylenedioxyindene inhibited the Ca2+ release induced by arachidonic acid. The effects of arachidonic acid appear to depend on the ratio of arachidonic acid to membrane vesicles.

Retrograde modulation at developing neuromuscular synapses: involvement of G protein and arachidonic acid cascade

Intracellular loading of nonhydrolyzable GTP analogs into innervated muscle cells in Xenopus cultures led to a marked increase in the frequency of spontaneous synaptic currents (SSCs), while extracellular application of the drugs at the same concentration was without effect. The increase in SSC frequency appeared to be unrelated to changes in the muscle membrane sensitivity toward acetylcholine (ACh), but resulted from an elevated spontaneous ACh secretion from the presynaptic nerve terminal. Postsynaptic loading of arachidonic acid (AA) produced a similar effect as the GTP analogs, and the potentiation effect of both GTP analogs and AA was reversed by an inhibitor of AA metabolism, AA861. Further studies indicate that a lipoxygenase metabolite, 5-HPETE, appears to be a likely candidate for the retrograde factor involved in modulating ACh secretion. These results suggest that G protein activation of the AA cascade in the postsynaptic cell could produce a retrograde signal to modulate transmitter secretion from the presynaptic nerve terminal at developing synapses.

Arachidonic acid and lipoxygenase metabolites uncouple neonatal rat cardiac myocyte pairs

The effects of arachidonic acid (AA) and its metabolites on the conductance (gj) of the gap junctions between neonatal rat myocardial cells was investigated. AA reduced gj in a dose- (2, 5, and 20 microM) and time-dependent fashion. Pretreatment of the cells with an inhibitor of the 5-lipoxygenase pathway, U-70344A, shifted the dose-response curve to the right; pretreatment with indomethacin, an inhibitor of the cyclooxygenase pathway, had no effect. The mean time to uncoupling was 3.7 +/- 0.3, 3.8 +/- 0.9, and 4.6 +/- 0.6 min (means +/- SE, P less than 0.05) for 5 microM AA, 5 microM AA + indomethacin, and 5 microM AA + U-70344A, respectively. Incorporation of AA into membrane phospholipids was not affected by the inhibitor. These studies suggest that complete uncoupling of the cells occurred at membrane concentrations of 3-4 mol%. The data indicate that AA and a 5-lipoxygenase metabolite uncouple neonatal rat heart cells. The data are discussed with respect to the possible underlying mechanism of uncoupling and the potential role of gap junctions in arrhythmia formation in ischemic heart disease.

The potential role of Ca2+ for electrical cell-to-cell uncoupling and conduction block in myocardial tissue

Ca2+ ions are often invoked as potential initiators of cardiac arrhythmias in pathophysiological situations which are associated with an increase of free [Ca2+]i. It is well documented that elevated [Ca2+]i may produce SR release of Ca2+ and oscillations of membrane potential, thereby leading to triggered or spontaneous ectopic activity. The relation among elevated free [Ca2+]i, electrical cell-to-cell coupling, conduction slowing, and reentrant arrhythmias is more speculative. If Ca2+ (e.g. in mechanically injured cells) has direct access to the cellular interconnections (gap junctions), rapid uncoupling occurs at [Ca2+]i which is even within the range of a normal contractile cycle. If cellular integrity is preserved and changes of [Ca2+]i are imposed by extracellular interventions, the effect of [Ca2+]i is critically dependent on pHi. At normal pHi, transcellular conductance remains normal even if [Ca2+]i is increased to bring the cells into a hypercontractile state (> 1-2 microM). At decreased pHi, rapid uncoupling develops at low [Ca2+]i. Comparison of the conduction delay between two cells (or conduction velocity in a simulated conducting medium) with the [Ca2+]i-mediated increase in coupling resistance suggests that the transition from normal conduction velocity to conduction block (a key event in re-entrant arrhythmias) occurs within a relatively narrow range of [Ca2+]i or pHi, almost like a threshold phenomenon. Major efforts have been made in recent years to assess the changes of electrical cell-to-cell coupling and [Ca2+]i in myocardial ischemia. Therefore, the discussion of the role of [Ca2+]i as a modulator of electrical coupling is made in this pathophysiological setting. Comparison of several studies indicate that cell-to-cell resistance and [Ca2+]i in ischemia increase at the same time (10-15 min after perfusional arrest). Since other potential uncoupling processes (delta ATP, delta Mg2+, amphiphilic metabolites, delta pHi) show a similar time-course, it is difficult to attribute cell-to-cell uncoupling in ischemia solely to an increase in [Ca2+]i. Both an initial decrease of membrane excitability and subsequent electrical cell-to-cell uncoupling characterize the early phase of ischemia. The first mechanism is assumed to be more important for the generation of conduction block and re-entry. However, Ca(2+)-induced cell-to-cell uncoupling may partially contribute to the second phase of the early ischemic arrhythmias and mark the transition from reversible to irreversible ischemic damage.