The osmotic/calcium stress theory of brain damage: Are free radicals involved?

This overview presents data showing that glucose use increases and that excitatory amino acids (i.e., glutamate, aspartate), taurine and ascorbate increase in the extracellular fluid during seizures. During the cellular hyperactive state taurine appears to serve as an osmoregulator and ascorbate may serve as either an antioxidant or as a pro-oxidant. Finally, a unifying hypothesis is given for seizure-induced brain damage. This unifying hypothesis states that during seizures there is a release of excitatory amino acids which act on glutamatergic receptors, increasing neuronal activity and thereby increasing glucose use. This hyperactivity of cells causes an influx, of calcium (i.e. calcium stress) and water movements (i.e., osmotic stress) into the cells that culminate in brain damage mediated by reactive oxygen species.Special issue dedicated to Dr. Frederick E. Samson     

Energy metabolism and selective neuronal vulnerability following global cerebral ischemia

A short period of global ischemia results in the death of selected subpopulations of neurons. Some advances have been made in understanding events which might contribute to the selectivity of this damage but the cellular changes which culminate in neuronal death remain poorly defined. This overview examines the metabolic state of tissue in the post-ischemic period and the relationship of changes to the development of damage in areas containing ischemia-susceptible neurons. During early recirculation there is substantial recovery of ATP, phosphocreatine and related metabolites in all brain regions. However, this recovery does not signal restitution of normal energy metabolism as reductions of the oxidative metabolism of glucose are seen in many areas and may persist for several days. Furthermore, decreases in pyruvate-supported respiration develop in mitochondria from at least one ischemia-susceptible region at times coincident with the earliest histological evidence of ischemia-induced degeneration. These mitochondrial changes could simply be an early marker of irreversible damage but the available evidence is equally consistent with these contributing to the degenerative process and offering a potential site for therapeutic intervention.Submitted as an Overview article for the volume of Neurochemical Research in honor of Alan N. Davison.     

噪声对豚鼠内耳组织能量代谢的影响——噪声致聋机理

豚鼠暴露于125dB SPL噪声下1小时后即刻、1天内耳组织中葡萄糖(46.0±18.3μg/mg蛋白,102.2±56.3μg/mg蛋白)及丙酮酸量(30.6±8.4μg/g蛋白,47.1±15.7μg/g蛋白)均显著地高于对照组(葡萄糖26.4±8.9μg/mg蛋白,丙酮酸23.7±8.9μg/g蛋白),p<0.05。与此同时豚鼠听功能耳廓反射阈衰减dB值(30.0±12.2dB,44.8±2.5dB)显著地低于暴露前水平(49.4±2.9dB,46.2±1.9dB),P相似文献   

Vanadate as an activator of ATP — sensitive potassium channels in mouse skeletal muscle

The inside-out mode of the patch-clamp technique was used to study adenosine-5-triphosphate (ATP)-sensitive K⁺ channels in mammalian skeletal muscle. Vanadate, applied to the cytoplasmic face of excised patches, was a potent activator of ATP-sensitive K⁺ channels. Divalent cations (Mg²⁺, Ca²⁺) were a prerequisite for the activating process. The maximal effect was achieved using 1 mM vanadate dissolved in Ringer, increasing the open-state probability about ninefold. The active 5 + redox form of vanadate which stimulates ATP-sensitive K⁺ channels is likely to be decavanadate V₁₀O _inf28 ^sup6– . ATP concentration-response curves have Hill coefficients near three in internal Na⁺-rich Ringer and between one and two in internal KCl solutions. Half maximal channel blockage was observed at ATP concentrations of 4 and 8 M in Ringer and KCl solutions, respectively. Internal vanadate shifted the curves towards higher ATP concentrations without affecting their slopes. Thus 50% channel blockage occurred at 65 M ATP in internal Ringer containing 0.5 mM vanadate. The results indicate that the affinity and stoichiometry of ATP binding to ATP-sensitive K⁺ channels are strongly modulated by internal cations and that the ATP sensitivity is weakened by vanadate. Offprint requests to: B. Neumcke     

Phloretin affects the fast potassium channels in frog nerve fibres

The effect of phloretin (20-100 M), a dipolar organic compound, on the voltage clamp currents of the frog node of Ranvier has been investigated. The Na currents are simply reduced in size but not otherwise affected. Phloretin has no effect on the slow 4-aminopyridine-resistant K channels. However, the voltage dependence and time course of the fast K conductance (g _K) is markedly altered. The g _K(E) curve, determined by measuring fast tail currents at different pulse potentials, normally exhibits a bend at –50 mV indicating the existence of two types of fats K channels. Phloretin shifts the g _K (E) curve to more positive potentials, reduces its slope and its maximum and abolishes the distinction between the two tpyes of fast K channels. The effect becomes more pronounced with time. Phloretin also markedly slows the opening of the fast K channels, but has much less effect on the closing. Opening can be accelerated again by a long depolarizing prepulse which presumably removes part of the phloretin block. It is concluded that phloretin selectively affects the fast K channels of the nodal membrane. The results are compared with similar observations on the squid giant axon. Offprint requests to: H. Meves     

In vivo activation of cirral movement in Stylonychia by calcium

Summary Motor responses of cirri (= organelles consisting of bundles of cilia) in the protozoan Stylonychia are elicited by positive or negative shifts of the membrane voltage from its resting state. The same responses are evoked at voltages near the Ca²⁺ equilibrium potential (E_Ca) applying extremely positive steps under voltage clamp. Motor responses recorded at large positive voltages approaching E_Ca from the negative side corresponded to cirral activation following physiological depolarization from the resting potential (DCA). The hyperpolarization-induced activation of the cirri (HCA) was documented during step potentials positive to E_Ca, suggesting that the observed HCA of the cirri resulted from an efflux of Ca²⁺ from the ciliary space as compared with DCA, which is related to Ca²⁺ influx. The ciliary responses were graded functions of the rising outward or inward driving force for Ca²⁺. Slopes of reciprocal plots of response latencies near E_Ca as a function of membrane potential indicate a removal of Ca²⁺ during HCA which exceeds the free intraciliary Ca²⁺ content at rest. It is suggested that this excess Ca²⁺ is released from axonemal binding sites.     

Use of phlorizin binding to demonstrate induction of intestinal glucose transporters

Summary We used specific binding of phlorizin to the intact intestinal mucosa in order to measure glucose transport site density in intestines of mice fed a high-carbohydrate or no-carbohydrate diet. Nonspecific binding varied with intestinal position but showed only modest dependence on diet. Specific binding to glucose transporters was 1.9 times greater in jejunum of high-carbohydrate mice than of no-carbohydrate mice; this ratio was the same as the ratio for V_max values of actived-glucose uptake between the two diet groups. The gradient in specific binding of phlorizin along the intestine paralleled the gradient in V_max of glucose transport. These results directly demonstrate that the increase in intestinal glucose transport caused by a high-carbohydrate diet is due to induction of glucose transporter. They also indicate that the normal positional graident in glucose transport along the intestine arises from a gradient in transporters, induced by the normal gradient in luminal glucose concentration.     

Calcium-dependent anion channel in the water mold,Blastocladiella emersonii

Summary Injection of depolarizing current into vegetative cells of the water moldBlastocladiella emersonii elicits a regenerative response that has the electrical characteristics of an action potential. Once they have been taken past a threshold of about –40 mV, cells abruptly depolarize to +20 mV or above; after an interval ranging from several hundred milliseconds to a few seconds, the cells spontaneously return to their resting potential near –100 mV. When the action potential was analyzed with voltage-clamp recording, it proved to be biphasic. The initial phase reflects an influx of calcium ions through voltage-sensitive channels that also carry Sr²⁺ ions. The delayed, and more extended, phase of inward current results from the efflux of chloride and other anions. The anion channels are broadly selective, passing chloride, nitrate, phosphate, acetate, succinate and even PIPES. The anion channels open in response to the entry of calcium ions, but do not recognize Sr²⁺. Calcium channels, anion channels and calcium-specific receptors that link the two channels appear to form an ensemble whose physiological function is not known. Action potentials rarely occur spontaneously but can be elicited by osmotic downshock, suggesting that the ion channels may be involved in the regulation of turgor.     

Characterization of a Ca-dependent maxi K channel in the apical membrane of a cultured renal epithelium (JTC-12.P3)

Summary A Ca and potential-dependent K channel of large unit conductance was detected in the apical membrane of JTC-12.P3 cells, a continuous epithelial cell line of renal origin. The open probability of the channel is dependent on membrane potential and cytoplasmic-free Ca concentration. At cell-free configuration of the membrane patch, the open probability shows a bell-shaped behavior as function of membrane potential, which decreases at larger depolarization. With increasing Ca concentration, the width of the bell-shaped curve increases and the maximum shifts into the hyperpolarizing direction. For the first time the kinetics of this channel was analyzed under cell-attached conditions. In this case the kinetics could sufficiently be described by a simple open-closed behavior. The channel has an extremely small open probability at resting potential, which increases exponentially with depolarization. The low probability induces an uncertainty about the actual number of channels in the membrane patch. The number of channels is estimated by kinetic analysis. It is discussed that this K channel is essential for the repolarization of the membrane potential during electrogenic sodium-solute cotransport across the apical membrane.     

Pathophysiologic alterations in Biomphalaria glabrata infected with Angiostrongylus costaricensis

Hemolymph glucose, alkaline phosphatase, lactic dehydrogenase, and creatine phosphokinase in Biomphalaria glabrata infected with Angiostrongylus costaricensis were significantly higher on day 27 postinfection (PI) than in uninfected snails. Hemolymph total calcium from infected snails was less on days 6, 12, and 27 PI than that from controls. Total hemolymph protein was similar for controls and infected animals during the entire study. Throughout the study the mean number of amoebocytes/mm³ hemolymph from infected snails was significantly less than that for controls. Mean total wet weights of digestive gland and foot muscle from infected and uninfected snails was similar throughout the study. Mean μg glycogen/mg wet weight of digestive gland from infected snails was significantly greater on days 24, 27, and 28 PI than that from controls. Mean μg glycogen/mg wet weight of foot muscle from infected snails was significantly reduced between days 12 and 28 PI from that of uninfected snails. It is suggested that hemolymph glucose and digestive gland glycogen in infected snails are augmented by glycogen breakdown in the foot muscle of parasitized animals. Elevations in hemolymph enzymes are due to tissue destruction by larvae emerging from the foot muscle of infected snails. Parasite-induced derangements in shell metabolism underlie observed changes in hemolymph calcium in infected snails.