Excitation-contraction coupling describes the series of events that begins with propagated action potential on the muscle fiber surface membrane and leads to the twitch contraction of the fiber. The generation of an action potential during excitation requires rapid sequential changes in membrane conductances of Na+, Ca2+, and K+ ions that depend upon the opening and closing of the respective channels. Myotonic disorders are inherited diseases whose clinical manifestations include electrophysiological signs such as increased excitability and delayed relaxation of the muscles after voluntary contraction. All these disorders appears to be due to an abnormality of the muscle itself since they persist after section or blocking of the motor nerve after curarization. Most experimental and clinical data suggest that human myotonia arises from genetically-induced structural and functional alterations of the muscle membrane. The purpose of this article is to focus on the more recent developments in the molecular and pharmacological analysis of cation transporting systems such as ionic channels and (Na+, K+) ATPase in myotonic disorders.Special issue dedicated to Dr. Lawrence Austin. 相似文献
The transport activities of two primary ATP-dependent organic-anion transporters in the tonoplast of isolated barley (Hordeum vulgare L. cv. Klaxon) vacuoles have been characterised with N-ethylmaleimide glutathione (NEM-SG) and taurocholate as substrates. The transporters showed different sensitivities to organic anions and a variety of transport inhibitors and drugs. The vacuolar uptake of NEM-SG was inhibited by carbonylcyanide 4-trifluoromethoxyphenylhydrazone, 4,4-diisothiocyanatostilbene-2,2-disulfonic acid (DIDS), S-(2,4-dinitrophenyl)glutathione, alkyl-S-glutathione derivatives and taurocholate but stimulated by probenecid. The uptake of taurocholate was inhibited by vinblastine, DIDS and probenecid. Both transporters were unaffected by verapamil. The kinetic properties of the transporters indicate a general preference for amphiphilic anions with some substrate overlap. These characteristics of the transporters are similar to those displayed by the multidrug resistance protein of mammalian drug-resistant cells. We suggest that these vacuolar transporters be described as plant multispecific organic anion transporters (pMOATs).Abbreviations Bm-S
bimane S-glutathione
- DIDS
4,4-diisothiocyanatostilbene-2,2-disulfonic acid
- DNP-SG
S-(2,4-dinitrophenyl)glutathione
- FCCP
carbonylcyanide 4-trifluoromethoxyphenylhydrazone
- LTC4
cysteinyl leukotriene
- MDR
multidrug transporter
- MRP
multidrug resistance protein
- NEM-SG
N-ethylmaleimide glutathione
We thank Prof E. Martinoia for technical advice on the uptake experiments and Prof J. Palmer for helpful discussions and suggestions. M.B.-K. was partially sponsored by a grant from Stichting VSB Fonds, The Netherlands. IACR receives grant-aided support from the Biotechnology and Biological Science Research Council of the United Kingdom 相似文献
A sequence comparison of the two membrane-associated (MA) domains of the cystic fibrosis transmembrane conductance regulator (CFTR), multidrug resistance transporter (MDR), and -factor pheromone export system (STE6) proteins, each of which are believed to contain a total of 12 transmembrane (TM) segments, reveals significant amino acid homology and length conservation in the loop regions that connect individual TM sequences. Similar structural homology is observed between these proteins, hemolysin B (HLYB) and the major histocompatibility-linked peptide transporter, HAM1, the latter two which contain a single MA domain composed of six TM segments. In addition, there are specific sequences that are conserved within the TM segments of the five different membrane proteins. This observation suggests that the folding topologies of the MA domains of MDR, STE6, and CFTR in the plasma membrane are likely to be very similar. The sequence analysis also reveals that there are three characteristic motifs (a pair of aromatic residues, LTLXXXXXXP and GXXL) that are conserved in MDR, STE6, HLYB, HAM1, but not in CFTR. We propose that although CFTR may be evolutionarily related to these other membrane proteins, it belongs to a separate subclass. 相似文献
BackgroundCellular free Zn2+ concentrations ([Zn2+]) are primarily coordinated by Zn2+-transporters, although their roles are not well established in cardiomyocytes. Since we previously showed the important contribution of a Zn2+-transporter ZnT7 to [Zn2+]i regulation in hyperglycemic cardiomyocytes, here, we aimed to examine a possible regulatory role of ZnT7 not only on [Zn2+]i but also both the mitochondrial-free Zn2+ and/or Ca2+ in cardiomyocytes, focusing on the contribution of its overexpression to the mitochondrial function.MethodsWe mimicked either hyperinsulinemia (by 50-μM palmitic acid, PA-cells, for 24-h) or overexpressed ZnT7 (ZnT7OE-cells) in H9c2 cardiomyoblasts.ResultsOpposite to PA-cells, the [Zn2+]i in ZnT7OE-cells was not different from untreated H9c2-cells. An investigation of immunofluorescence imaging by confocal microscopy demonstrated a ZnT7 localization on the mitochondrial matrix. We demonstrated the ZnT7 localization on the mitochondrial matrix by using immunofluorescence imaging. Later, we determined the mitochondrial levels of [Zn2+]Mit and [Ca2+]Mit by using the Zn2+ and Ca2+ sensitive FRET probe and a Ca2+-sensitive dye Fluo4, respectively. The [Zn2+]Mit was found to increase significantly in ZnT7OE-cells, similar to the PA-cells while no significant changes in the [Ca2+]Mit in these cells. To examine the contribution of ZnT7 overexpression on the mitochondria function, we determined the level of reactive oxygen species (ROS) and the mitochondrial membrane potential (MMP) in these cells in comparison to the PA-cells. There were significantly increased production of ROS and depolarization in MMP and increases in marker proteins of mitochondria-associated apoptosis and autophagy in ZnT7-OE cells, similar to the PA-cells, parallel to increases in K-acetylation. Moreover, we determined significant increases in trimethylation of histone H3 lysine27, H3K27me3, and the mono-methylation of histone H3 lysine36, H3K36 in the ZnT7OE-cells, demonstrating the role of [Zn2+]Mit in epigenetic regulation of cardiomyocytes under hyperinsulinemia through histone modification.ConclusionsOverall, our data have shown an important contribution of high expression of ZnT7-OE, through its buffering and muffling capacity in cardiomyocytes, on the regulation of not only [Zn2+]i but also both [Zn2+]Mit and [Ca2+]Mit affecting mitochondria function, in part, via histone modification. 相似文献
Abstract: Fructose-1,6-bisphosphate (FBP), an intermediate of glucose metabolism, is neuroprotective in brain hypoxia or ischemia. Because the mechanisms for this protection are not clear, we examined the effects of FBP on two important events in brain ischemia, i.e., loss of ATP and release of the excitatory neurotransmitter glutamate. Glutamate release from cortical brain slices was measured fluorometrically (glutamate dehydrogenase)-catalyzed conversion of glutamate to α-ketoglutarate) during hypoxia (Po2 15 mm Hg) or hypoxia plus 100 µ M cyanide. FBP (3.5 m M , with glucose 20 m M ) reduced glutamate release during hypoxia by 55% and during hypoxia/cyanide by 46% ( p < 0.005), and prevented a significant fall in [ATP]. [ATP] was maintained in oxygenated glucose-free conditions with 20 but not 3.5 m M FBP, and fell to <20% of normal with hypoxia. Despite the drop in [ATP], 3.5 or 20 m M FBP without glucose decreased hypoxia-evoked glutamate release. We conclude (1) FBP present without glucose preserves normal [ATP] only when oxygen is available, suggesting limited uptake and metabolism; and (2) FBP decreases hypoxia-evoked glutamate release by processes independent of [ATP]. These results suggest protective actions of FBP that are separate from augmentation of anaerobic energy production, as previously proposed. 相似文献
Increasing evidence indicates that the Eph receptors and their ephrin ligands are involved in the regulation of interactions between neurons and astrocytes. Moreover, astrocytic ephrin‐A3 reverse signaling mediated by EphA4 receptors is necessary for controlling the abundance of glial glutamate transporters. However, the role of ephrin‐A3 reverse signaling in astrocytic function and neuronal death under ischemic conditions remains unclear. In the present study, we found that the EphA4 receptor and its ephrin‐A3 ligand, which were distributed in neurons and astrocytes, respectively, in the hippocampus showed a coincident up‐regulation of protein expression in the early stage of ischemia. Application of clustered EphA4 decreased the expressions of astrocytic glutamate transporters together with astrocytic glutamate uptake capacity through activating ephrin‐A3 reverse signaling. In consequence, neuronal loss was aggravated in the CA1 region of the hippocampus accompanied by impaired hippocampus‐dependent spatial memory when clustered EphA4 treatment was administered prior to transient global ischemia. These findings indicate that EphA4‐mediated ephrin‐A3 reverse signaling is a crucial mechanism for astrocytes to control glial glutamate transporters and prevent glutamate excitotoxicity under pathological conditions.