Gap junction ultrastructure in rat liver parenchymal cells after in vivo ischemia

Gap junctions and non-neoplastic liver disease

Because of their critical role as goalkeepers of hepatic homeostasis, gap junctions are frequent targets in liver disease. This concept has been demonstrated on many occasions in the light of hepatocarcinogenesis. Relatively little focus has been put on the fate of gap junctions in other liver pathologies, including hepatitis, liver fibrosis and cirrhosis, cholestasis and hepatic ischemia and reperfusion injury. The present paper provides an in-depth description of the multiple changes in expression, localization and function of connexins, the molecular constituents of gap junctions. The use of connexins as biomarkers and therapeutic targets in liver disease is also illustrated.

DELAYED UNCOUPLING CONTRIBUTES TO THE PROTECTIVE EFFECT OF HEPTANOL AGAINST ISCHAEMIA IN THE RAT ISOLATED HEART

1. It is known that infusion of the gap junction uncoupler heptanol, before ischaemia or during reperfusion, limits myocardial infarct size. However, whether this cardiac effect is linked to the effect of heptanol on communication across gap junctions has not been elucidated. The aims of the present study were to examine the effect of heptanol on infarct size, arrhythmias and myocardial tissue resistance and to assess whether changes in electrical coupling correlate with cardiac protection. 2. Rat isolated, perfused hearts were subjected to a 24 min infusion of heptanol (0.05, 0.1, 0.5 or 1.0 mmol/L) followed by 20 min regional ischaemia and 60 min reperfusion, or by 70 min global no-flow ischaemia. The effective refractory period, action potential and conduction velocity were measured in papillary muscles from the right ventricle. Heptanol markedly decreased arrhythmia scores during ischaemia and reperfusion, as well as reducing infarct size to a degree similar to that induced by ischaemic preconditioning. In the prolonged ischaemia model, heptanol delayed the onset of uncoupling, increased time to plateau and decreased the maximal rate of uncoupling during ischaemia. Ischaemic preconditioning had similar effects on these parameters. In papillary muscle, heptanol reduced the conduction velocity of the action potential in a dose-dependent manner, but had no significant effect on resting potential, action potential amplitude, action potential duration, maximal upstroke of depolarization or effective refractory period. 3. These results demonstrate that treatment with the gap junction uncoupler heptanol confers cardioprotection against ischaemia and this effect is related to delayed electrical uncoupling during prolonged ischaemia.

Changes in electrical resistivity of swine liver after occlusion and postmortem

The resistivity of swine liver tissue was measured in vivo, during induced ischaemia and post-mortem, so that associated changes in resistivity could be quantified. Plunge electrodes, the four-terminal method and a computer-automated measurement system were used to acquire resistivities between 10Hz and 1 MHz. Liver resistivity was measured in vivo in three animals at 11 locations. At 10 Hz, resistivity was 758 +/- 170 ohm x cm. At 1 MHz, the resistivity was 250 +/- 40 ohm x cm. The resistivity time course was measured during the first 10 min after the liver blood supply in one animal had been occluded. Resistivity increased steadily during occlusion. The change in resistivity of an excised tissue sample was measured during the first 12h after excision in one animal. Resistivity increased during the first 2h by 53% at 10 Hz and by 32% at 1 MHz. After 2h, resistivity decreased, probably owing to membrane breakdown. The resistivity data were fitted to a Cole-Cole circle, from which extracellular resistance Re, intracellular resistance Ri and cell membrane capacitance Cm were estimated. Re increased during the first 2h by 95% and then decreased, suggesting an increase in extracellular volume. Cm increased during the first 4 h by 40%, possibly owing to closure of membrane channels, and then decreased, suggesting membrane breakdown. Ri stayed constant during the initial 6h and then increased.

Endogenous and exogenous modulation of gap junctional intercellular communication: toxicological and pharmacological implications

During the evolution of single-celled organisms to multicellular metazoans, a family of highly conserved genes coding for proteins (connexins), which as hexameric units (connexins), has evolved to form intercellular channels (gap junctions). These gap junctions allow ions and small molecular weight molecules to flow between coupled cells, thereby facilitating synchronization of electrotonic or metabolic cooperation. Control of cell proliferation, cell differentiation and adaptive responses of differentiated cells have been speculated to be biological roles of gap junctions. The regulation of these gap junctions can occur at the transcriptional, translational and posttranslational levels. Transient downregulation by endogenous or exogenous chemicals can bring about adaptive or maladaptive consequences depending on circumstances. Stable abnormal regulation of gap junction function has been associated with the activation of several oncogenes. Several tumor suppressor genes have also been associated with the up-regulation of gap junction function. Since gap junctions exist in all organs of the multi-cellular organisms, the dysfunction of these gap junctions by various toxic chemicals which have cell type/tissue/organ specificity could bring about very distinct clinical consequences, such as embryo lethality or teratogenesis, reproductive dysfunction in the gonads, neurotoxicity of the CNS system, hyperplasia of the skin, and tumor promotion of initiated tissue. Understanding how many non-mutagenic chemicals might alter normal gap junction function should form the basis of "epigenetic" toxicology. On the other hand, restoring normal gap junction function to cells which have dysfunctional intercellular communication could be the basis for a new approach for therapeutic pharmaceuticals.

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

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