Medizin - Open Access LMU - Teil 04/22
Medizin - Open Access LMU - Teil 04/22
Ludwig-Maximilians-Universität München
Die Universitätsbibliothek (UB) verfügt über ein umfangreiches Archiv an elektronischen Medien, das von Volltextsammlungen über Zeitungsarchive, Wörterbücher und Enzyklopädien bis hin zu ausführlichen Bibliographien und mehr als 1000 Datenbanken reicht. Auf iTunes U stellt die UB unter anderem eine Auswahl an elektronischen Publikationen der Wissenschaftlerinnen und Wissenschaftler an der LMU bereit. (Dies ist der 4. von 22 Teilen der Sammlung 'Medizin - Open Access LMU'.)
A long and complex enhancer activates transcription of the gene coding for the highly abundant immediate early mRNA in murine cytomegalovirus
Using the simian virus 40 "enhancer trap" approach, we have identified a transcription enhancer located just upstream of the major immediate early gene of murine cytomegalovirus. This enhancer has several striking properties. (.) Together with the enhancer ofhuman cytomegalovirus, it is the strongest transcription enhancer found to date. (ö) It is an extremely long enhancer, spanning >700 base pairs. (ÜI) It consists of a rather complex pattern of sequence repeats, the longest of which is 181 base pairs. Also, several types of short sequence motifs are scattered throughout the enhancer in monomeric, heterodimeric, or homodimeric (palindromic) form. These motifs have been identified to be components of other enhancers and promoters, and they are presumably binding sites for specific nuclear factors. Our analysis suggests that enhancers are composed of a modular arrangement of short conserved sequence motifs and that enhancer strength is correlated with the redundancy of these motifs.
Jan 1, 1985
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Activity-dependent excitability changes in normal and demyelinated rat spinal root axons
Myelinated nerve fibres with a reduced safety factor for conduction due to demyelination are easily blocked by trains of impulses. To find out why, in vivo recordings from rat ventral root fibres demyelinated with diphtheria toxin have been supplemented with in vivo and in vitro recordings from normal fibres. Despite a small rise in extracellular potassium activity, normal fibres were invariably hyperpolarized by intermittent trains of impulses. This hyperpolarization resulted in an increase in threshold and also in an enhancement of the depolarizing after-potential and the superexcitable period. Replacement of NaCl in the extracellular solution by LiCl completely blocked both the membrane hyperpolarization and the threshold increase which were normally observed during intermittent trains of impulses. At demyelinated nodes which were blocked by trains of impulses (10-50 Hz), conduction block was preceded by a rise in threshold current and in an increase in internodal conduction time, but by no detectable reduction in the outward current generated by the preceding node. It was found possible to prevent the threshold from changing during a train by automatic adjustment of a d.c. polarizing current. This 'threshold clamp' prevented the conduction failure and virtually abolished the changes in internodal conduction time. The threshold changes were attributed to hyperpolarization, as in normal fibres, since (a) the polarizing current required to prevent them was always a depolarizing current, and (b) they were accompanied by an increase in superexcitability. The post-tetanic depression that can follow continuous trains of impulses was attributed to the combination of increased threshold and enhanced superexcitable period due to hyperpolarization. It is concluded that the susceptibility of these demyelinated fibres to impulse trains is not due to a membrane depolarization induced by extracellular potassium accumulation but to a membrane hyperpolarization as a consequence of electrogenic sodium pumping.
Jan 1, 1985
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Minimal requirements for exocytosis
The membrane-permeabilizing effects of streptolysin O, staphylococcal alpha-toxin, and digitonin on cultured rat pheochromocytoma cells were studied. All three agents perturbed the plasma membrane, causing release of intracellular 86Rb+ and uptake of trypan blue. In addition, streptolysin O and digitonin also damaged the membranes of secretory vesicles, including a parallel release of dopamine. In contrast, the effects of alpha-toxin appeared to be strictly confined to the plasma membrane, and no dopamine release was observed with this agent. The exocytotic machinery, however, remained intact and could be triggered by subsequent introduction of micromolar concentrations of Ca2+ into the medium. Dopamine release was entirely Ca2+ specific and occurred independent of the presence or absence of other cations or anions including K+ glutamate, K+ acetate, or Na+ chloride. Ca2+-induced exocytosis did not require the presence of Mg2+-ATP in the medium. The process was insensitive to pH alterations in the range pH 6.6-7.2, and appeared optimal at an osmolarity of 300 mosm/kg. Toxin permeabilization seems to be an excellent method for studying the minimal requirements for exocytosis.
Jan 1, 1985
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An intracellular analysis of gamma-aminobutyric-acid-associated ion movements in rat sympathetic neurones
Double-barrelled ion-sensitive micro-electrodes were used to measure the changes of the intracellular activities of Cl-, K+, and Na+ (aiCl, aiK, aiNa) in neurones of isolated rat sympathetic ganglia during the action of gamma-aminobutyric acid (GABA). The membrane potential of some of the neurones was manually 'voltage clamped' by passing current through the reference barrel of the ion-sensitive micro-electrode. This enabled us to convert the normal depolarizing action of GABA into a hyperpolarization. A GABA-induced membrane depolarization was accompanied by a decrease of aiCl, aiK and no change in aiNa, whereas a GABA-induced membrane hyperpolarization resulted in an increase of aiCl, aiK and also no change in aiNa. GABA did not change the free intracellular Ca2+ concentration, as measured with a Ca2+-sensitive micro-electrode, whereas such an effect was seen during the action of carbachol. pH-sensitive electrodes, on the other hand, revealed a small GABA-induced extracellular acidification. The inward pumping of Cl- following the normal, depolarizing action of GABA required the presence of extracellular K+ as well as Na+, whereas CO2/HCO3--free solutions did not influence the uptake process. Furosemide, but not DIDS, blocked the inward pumping of Cl-. In conclusion, our data show that only changes in intracellular activities of K+ and Cl- are associated with the action of GABA. Furthermore, they indicate that a K+/Cl- co-transport, and not a Cl-/HCO3- counter-transport, may be involved in the homoeostatic mechanism which operates to restore the normal transmembrane Cl- distribution after the action of GABA.
Jan 1, 1985
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