Effects of Magnesium on Cardiac Excitation-Contraction Coupling

Objective: Magnesium regulates a large number of cellular processes. Small changes in intracellular free Mg²⁺ ([Mg²⁺]_i) may have important effects on cardiac excitability and contractility. We investigated the effects of [Mg²⁺]_i on cardiac excitation-contraction coupling.

Methods: We used our ionic-metabolic model that incorporates equations for Ca²⁺ and Mg²⁺ buffering and transport by ATP and ADP and equations for MgATP regulation of ion transporters (Na⁺-K⁺ pump, sarcolemmal and sarcoplasmic Ca²⁺ pumps).

Results: Model results indicate that variations in cytosolic Mg²⁺ level might sensitively affect diastolic and systolic Ca²⁺, sarcoplasmic Ca²⁺ content, Ca²⁺ influx through L-type channels, efficiency of the Na⁺/Ca²⁺ exchanger and action potential shape. The analysis suggests that the most important reason for the observed effects is a modified normal function of sarcoplasmic Ca²⁺-ATPase pump by altered diastolic MgATP levels.

Conclusion: The model is able to reproduce qualitatively a sequence of events that correspond well with experimental observations during cardiac excitation-contraction coupling in mammalian ventricular myocytes.     

Acute ethanol treatment decreases intracellular calcium-ion transients in mouse single skeletal muscle fibres in vitro

Alcohol misuse frequently leads to muscle weakness, which may also occur in the setting of acute and chronic alcoholic myopathies. At the cellular level, ethanol has been found to interfere with signalling mechanisms in cardiac myocytes, skeletal myotubes, and smooth muscle cells. In this study, we focused on the effects of ethanol on the intracellular calcium ([Ca(2+)](i)) transients responsible for excitation-contraction (EC) coupling in isolated mouse skeletal fibres loaded with the fluorescent Ca(2+) indicator fura-2. Following electrical stimulation, ethanol caused a significant reversible dose-dependent reduction in [Ca(2+)](i) transient amplitude, already significant at 100 mM ethanol (P = 0.03), without modifying resting [Ca(2+)](i). Evaluating the potential loci for the effects of ethanol, we indirectly measured sarcolemmal Ca(2+) entry by monitoring Mn(2+)-quenching of intracellular fura-2 via the nitrendipine-sensitive Ca(2+) channels during electrical pacing. Ethanol at doses of 20 mM and greater caused a dose-dependent reduction in the rate of fura-2 quenching (all at P<0.05). Moreover, the intracellular pool of Ca(2+) releasable by caffeine was found to be reduced at a minimum of 300 mM ethanol (P = 0.05). We conclude that ethanol reduces the [Ca(2+)](i) transients underlying EC coupling in single mouse skeletal muscle fibres. This acute effect of ethanol was primarily due to an inhibitory effect of ethanol on sarcolemmal Ca(2+) influx via voltage-operated Ca(2+)-channels and, to a lesser extent, to a reduction in the Ca(2+) sarcoplasmic reticulum loading state. This inhibitory effect of ethanol may be implicated in the development of muscle weakness with alcohol consumption.     

Extracellular magnesium regulates nuclear and perinuclear free ionized calcium in cerebral vascular smooth muscle cells: possible relation to alcohol and central nervous system injury.

Quantitative digital imaging microscopy, confocal laser scanning microscopy (CLSM), and multiple molecular fluorescent probes were utilized to test the hypothesis that cerebral vascular muscle cell nuclear ([Ca(2+)](n)), perinuclear ([Ca(2+)](pn)), and cytoplasmic free calcium ([Ca(2+)](i)) levels are regulated by the concentration of extracellular free magnesium ions ([Mg(2+)](o)). Primary cultured canine cerebral vascular smooth muscle cells were loaded with either fura-2/AM, indo-1/AM, or fluo-3/AM, and the subcellular Ca(2+) responses to stepwise reduction in [Mg(2+)](o) (i.e., from 1.36 to 0.17 mM) were analyzed over time. With normal 1.36 mM [Mg(2+)](o)-containing incubation media, basal mean [Ca(2+)](i) was 89.6+/-15 nM. Lowering [Mg(2+)](o) to 1.07, 0.88, 0.48, and 0.17 mM resulted in rapid (<4 min) increments in [Ca(2+)](i) going to 213+/-43, 368+/-67, 471+/-77, and 642+/-98 nM, respectively; the longer the exposure time (up to 30 min) to lowered [Mg(2+)](o), the higher the [Ca(2+)](i). Restoration of [Mg(2+)](o) to normal caused decreases in [Ca(2+)](i) to 215.9+/-42.3 nM, but only complete removal of [Ca(2+)](o) returned [Ca(2+)](i) to basal levels. Results show that basal [Ca(2+)](pn) (282+/-92 nM) exceeds basal cytoplasmic Ca(2+) (61+/-27.8 nM) and [Ca(2+)](n) (20+/-7.6 nM). However, reduction of normal [Mg(2+)](o) to 0.48 mM resulted in dramatic, rapid rises in all subcellular compartments, where [Ca(2+)](pn) (1503+/-102 nM)>cytoplasmic Ca(2+) (688+/-49 nM) approximately equal to [Ca(2+)](n) (674+/-12 nM). Nuclear Ca(2+) rose dramatically (e.g., 35-40 times basal levels). Both verapamil (1 microM) and Ni(2+) (5 mM) prevented, completely, the rises in Ca(2+) in all compartments, suggesting that Mg(2+)-dependent Ca(2+) accumulation may be dependent on nuclear, endoplasmic reticulum-Golgi, and cytoplasmic L-type voltage membrane-regulated Ca(2+) channels. The normally low [Ca(2+)](n) suggests that Ca(2+) does not transport passively across the nuclear membrane in cerebral vascular smooth muscle cells. These results may help to explain much of the impact of hypomagnesemic states on cerebral-central nervous system pathobiology, and, particularly, alcohol-induced strokes.     

锌对红细胞膜ATP酶活性影响的研究

从体内和体外 2方面研究锌对红细胞膜 Na+ ,K+ - ATP酶和 Ca2 + ,Mg2 + - ATP酶活性的影响。健康雄性 Wistar大鼠分别喂饲低锌、常量锌和高锌饲料 (饲料锌含量分别为 2 .2、2 8、12 8mg/kg) ,2 5天后各组随机取 8只动物腹主动脉取血后处死 ,测定血清锌含量和红细胞膜 Na+ ,K+ - ATP酶和 Ca2 + ,Mg2 + - ATP酶活性 ;各组剩余 6只动物改变其锌摄入量后继续喂饲 7天后 ,观察锌对红细胞膜 2种 ATP酶活性影响的敏感性。体外实验用新鲜制备的人红细胞膜 ,在不同浓度锌 (0、5、10、5 0、10 0和 5 0 0 μmol/L)溶液中温浴 ,观察 2种 ATP酶活性的变化趋势。结果表明 ,锌对红细胞膜的 2种 ATP酶活性均有影响 ,锌浓度过低或过高可抑制酶活性。与 Na+ ,K+ - ATP酶相比 ,Ca2 + ,Mg2 + - ATP酶对缺锌更敏感且补锌后不易恢复 ,但其对高锌有较强的耐受性。本次研究结果提示锌对于维持红细胞膜 ATP酶的正常活性有重要意义。     

Influence of calcium, magnesium, sodium, potassium and pH on copper toxicity to barley (Hordeum vulgare)

The extent to which Ca(2+), Mg(2+), Na(+), K(+) ions and pH independently influence copper toxicity to barley (Hordeum vulgare) was assessed by measuring root growth in nutrient solutions. Increased Ca(2+) activity resulted in a sixfold decrease in [EC50(cu2+)] values, while a positive relationship between the cation activity and the EC50 was expected. Increased Mg(2+) activity resulted in a twofold increase in [EC50(cu2+)] values. Na(+), K(+) and H(+) activities did not significantly affect Cu(2+) toxicity. The obtained results indicated that competition for binding sites between Cu(2+) and cations such as Ca(2+), Mg(2+), Na(+), K(+) and H(+) is not an important factor in determining Cu(2+) toxicity to H. vulgare. However, the EC50s could, with one exception, be predicted within a factor three based on the free Cu(2+) activity, indicating that the free Cu(2+) activity cannot only be used to predict metal toxicity to aquatic, but also to terrestrial organisms.     

Effect of cadmium on transmembrane Na+ and K+ transport systems in human erythrocytes.

The effects of cadmium (Cd2+) on Na+,K(+)-ATPase in disrupted human erythrocyte membranes and on various transmembrane Na+ and K+ transport systems in intact erythrocyte suspensions were studied. Cadmium2+ inhibited the erythrocyte Na+,K(+)-ATPase enzyme with a 50% inhibition at a Cd2+ concentration of 6.25 microM. The Cd2+ inhibition in the human erythrocyte was non-competitive with respect to Na+,K+, and ATP. Cadmium2+ exerted no acute effect, however, on the Na+,K(+)-ATPase pump activity as measured by the ouabain sensitive 86Rb uptake or Na+ efflux in intact red blood cells. Cadmium2+ also inhibited the Ca2+ dependent K+ channels in human red blood cells, whereas it had no effect on Na+,K+ cotransport, Na+,Li+ countertransport, anion carrier, and the number of active Na+ pump units. The data indicate that in human erythrocytes under acute conditions Cd2+ exerts an inhibitory effect on Na+,K(+)-ATPase enzyme in disrupted erythrocytes and the Ca2+ stimulated K+ efflux in intact red blood cells without affecting the Na+ pump, Na+,K+ cotransport, and Na+,Li+ countertransport activity.     

Effect of cadmium on transmembrane Na+ and K+ transport systems in human erythrocytes.

The effects of cadmium (Cd2+) on Na+,K(+)-ATPase in disrupted human erythrocyte membranes and on various transmembrane Na+ and K+ transport systems in intact erythrocyte suspensions were studied. Cadmium2+ inhibited the erythrocyte Na+,K(+)-ATPase enzyme with a 50% inhibition at a Cd2+ concentration of 6.25 microM. The Cd2+ inhibition in the human erythrocyte was non-competitive with respect to Na+,K+, and ATP. Cadmium2+ exerted no acute effect, however, on the Na+,K(+)-ATPase pump activity as measured by the ouabain sensitive 86Rb uptake or Na+ efflux in intact red blood cells. Cadmium2+ also inhibited the Ca2+ dependent K+ channels in human red blood cells, whereas it had no effect on Na+,K+ cotransport, Na+,Li+ countertransport, anion carrier, and the number of active Na+ pump units. The data indicate that in human erythrocytes under acute conditions Cd2+ exerts an inhibitory effect on Na+,K(+)-ATPase enzyme in disrupted erythrocytes and the Ca2+ stimulated K+ efflux in intact red blood cells without affecting the Na+ pump, Na+,K+ cotransport, and Na+,Li+ countertransport activity.     

Cardiovascular effects of selective serotonin reuptake inhibitor antidepressants

This paper reviews the cardiovascular effects of fluoxetine and other selective serotonin reuptake inhibitors comparing with those of tricyclic antidepressants. The authors survey the electrophysiological mechanisms and the recent data referring on drug's actions on different ionic currents/channels. The paper primarily focuses on preclinical data, showing various effects of fluoxetine and citalopram on cardiac and smooth muscle preparations and on cardiac ionic currents. At concentrations of 0.5-50 microM, fluoxetine and citalopram exhibit depressant effects on Ca(2+)- and Na(+)-dependent electrophysiological parameters of different cardiac preparations and on cardiac Ca2+ current. At concentrations of 0.1-10 microM, fluoxetine and citalopram elicit relaxation of both vascular and intestinal smooth muscles. These results provide evidence for inhibition of cardiac Na+, Ca2+ and more recently K+ channels by fluoxetine and citalopram at concentrations close to therapeutic level. The inhibition of cardiac Ca2+, Na+ and K+ and vascular Ca2+ channels by fluoxetine and citalopram may explain most cardiovascular side effects observed occasionally with the drugs during the chronic treatment. The inhibitory effects on cardiac Ca2+, Na+ and K+ channels of fluoxetine and citalopram may result in antiarrhythmic/proarrhythmic actions. Thus fluoxetine, citalopram and other selective serotonin reuptake inhibitors similarly to tricyclic antidepressants, may exhibit cardiovascular depressant effects. The paper summarizes drug interactions that may lead risk of arrhythmia and vascular side effects. Taking all these into consideration, in depressed patients having also cardiac or liver disorders, these antidepressants should be also more rigorously applied.     

Mercuric chloride-induced inhibition of different ATPases in the intestine of mudskipper, Boleophthalmus dentatus.

This paper deals with the toxicity of mercuric chloride to different ATPases in the intestine of mudskipper (Boleophthalmus dentatus). Mudskippers were exposed to four sublethal concentrations of mercuric chloride for three durations. The specific activities of Na+, K(+)-ATPase, Ca(2+)-ATPase, Mg(2+)-ATPase, Ca2+, HCO3(-)-ATPase, and Mg2+, HCO3(-)-ATPase were estimated. There was linear inhibition of all the enzymes with increasing mercuric chloride concentration as well as exposure duration. The Na+,K(+)-ATPase was found to be the enzyme most affected, followed by other ion-dependent ATPases. Inhibition of all the enzymes indicates severe damage to the intestinal cells, resulting in a blockage of the transport of substances across the membrane.     

The ionic basis of the anti-ischemic and anti-arrhythmic properties of magnesium in the heart

The role of magnesium (Mg) in the prevention of ischemia-induced injury during cardioplegic arrest and in the treatment of cardiac arrhythmias has been considered. Although Mg possesses negative inotropic properties, potassium (K) is more effective than Mg in inducing cardiac arrest. The rationale for the inclusion of Mg in cardioplegic solutions therefore lies not in its cardioplegic properties, but in its ability to influence other cellular events such as the loss of Mg and K and perhaps to counter the detrimental effects of ischemia by antagonizing calcium (Ca) overload. Most of the Mg in the cardiac cell is complexed with high energy phosphate compounds and the loss of Mg during ischemia may restrict the repletion of ATP upon reperfusion and so impair the return of normal contractile function. The ability of Mg to limit K efflux from the cell is of importance not only in the prevention of ischemia-induced K loss but also in the treatment of digitalis-induced arrhythmias. Elevation of extracellular Mg has been shown to reduce the intracellular sodium ion activity ([Na]i) and this decline in [Na]i can be related to the negative inotropic properties of Mg. Mg may therefore exert some of its antiarrhythmic and antiischemic effects by limiting [Na]i-stimulated Ca influx (or facilitating Ca efflux) and hence preventing cellular Ca overload.     

Interrelationship of dietary Mg intake and electrolyte homeostasis in hamsters: I. Severe Mg deficiency, electrolyte homeostasis, and myocardial necrosis

Epidemiological studies indicate a strong relationship between dietary Mg intake and the incidence of sudden cardiac death. The mechanism by which dietary Mg leads to an increased incidence of cardiovascular disease is unknown but may involve alteration of electrolyte balance. In the present study, tissue electrolyte levels and myocardial pathology were investigated in adult hamsters fed a diet containing no added Mg. Control animals were fed the same diet supplemented with Mg or standard laboratory chow. Hamsters were killed after 4, 8, 12, or 18 days on the test diet, and levels of Na, K, Ca, and Mg were measured in the serum, myocardium, bone, and kidney. The earliest change induced by the test diet was a decrease of the serum Mg and an increase in the Na concentration of the myocardium and other tissues. Following the rise in myocardial Na, the myocardial Ca rose, attaining a fourfold increase by 18 days. K fell in heart and kidney, but not significantly. Although there was no significant change in myocardial Mg, foci of myocardial necrosis, considered to be typical of acute severe Mg deficiency, were found. Myocardial necrosis and the increase in myocardial Ca occurred in parallel. Because of the pattern of observed changes in electrolyte levels, and the potential role of Ca in myocardial injury, the occurrence of myocardial necrosis in these Mg-deficient hamsters is attributable to the increased level of myocardial Ca, rather than to any change in intracellular Mg levels. It is postulated that reduced extracellular Mg levels increase [Na]i through reduction of sarcolemmal (Na+ + K+)-ATPase activity. This would lead to an increase in [Ca]i through Na-Ca exchange.     

原花青素对饮食所致高脂血症大鼠心血管的保护作用

[目的]研究原花青素对饮食所致高脂血症大鼠心血管的保护作用机制.[方法]对5周龄Wistar大鼠按体重随机分成4组:正常饮食组(对照组,C组)、高脂饮食组(高脂模型对照组,H组)、高脂饮食 20mg/ks原花青素(药物高剂量,HD组)、高脂饮食 10 mg/kg原花青素(低剂量组,LD组).喂饲4周后测定各组大鼠血脂、氧化应激和心肌瘦素、Na -K -ATPase酶和ca2-Mg2 -ATPase酶水平.[结果]与对照组相比,H组血清总胆固醇(CHO)、三酰甘油(TG)、高密度脂蛋白(HDL)、低密度脂蛋白(LDL)及丙二醛(MDA)水平显著升高,血清SOD、GSH-Px、CAT三种抗氧化酶水平明显降低,心肌Na -K -ATPase酶、Ca2 -Mg2 -ATPase酶和MDA水平降低;HD和LD组血清SOD、GSH-Px、CAT以及心肌Na K -ATPase酶、ca2 -Mg2 -ATPase酶和瘦素水平显著升高,MDA含量降低,但对血脂水平无显著影响.[结论]原花青素具有心血管保护作用,其作用机制至少包括抗氧化作用和增加心肌细胞对瘦素的调节敏感性两个方面.     

Reduction and inactivation of the sarcoplasmic Ca(2+)-ATPase by 2-mercaptoethanol--contrast to the (Na+,K+) ATPase.

The sarcoplasmic Ca(2+)-ATPase was reduced with 300 mM 2-mercaptoethanol at elevated temperatures (40-45 degrees C) with a concomitant loss of ATPase activity. The reduction and inactivation of the Ca(2+)-ATPase proceeded rapidly in the absence of Ca2+. The Ca(2+)-ATPase was also inactivated with 2-mercaptoethanol in the presence of diluted SDS (0.4 mg/ml) even at 20 degrees C. In contrast to the (Na+, K+) ATPase, the inactivated Ca(2+)-ATPase in the presence of diluted SDS was sedimented by the centrifugation at 100,000 x g for 30 min.     

Predicting effects of cations on copper toxicity to lettuce (Lactuca sativa) by the biotic ligand model

A biotic ligand model (BLM) was developed to estimate Cu toxicity to lettuce (Lactuca sativa) in terms of root elongation after 4 d of exposure. Effects of Na(+), K(+), Ca(2+), and Mg(2+) on Cu toxicity were examined. The addition of these cations resulted in a 50-fold difference in the copper median effective activity (EC50 cu2+). However, these variations could not be interpreted entirely as a function of the concentrations of these cations alone. In particular, only the relationship between EC50 cu2+ and the activity of protons was found to be significant in the whole range of pH examined from 5.0 to 7.0. The addition of K(+), Na(+), Ca(2+), and Mg(2+) at concentrations up to 20 mM resulted in a 16-fold difference in EC50 cu2+ values. This difference was significant, as indicated by non-overlapping standard deviations of the negative logarithm of EC50 cu2+ pEC50 cu2+) obtained with (7.37 ± 0.22) and without (6.76 ± 0.22) additions of K(+), Na(+), Ca(2+), and Mg(2+). The variations were not statistically significantly related to concentrations of these cations; therefore, only protons can be integrated in the BLM predicting Cu toxicity to lettuce L. sativa with the important parameters: log K(HBL) =6.27, log K(CuBL) =7.40, and [formula in text]. The lack of significant relationships between EC50 cu2+ and concentrations of the cations was not in line with the main assumption of the BLM about the competition between cations for binding sites.     

Plasma membrane surface potential (psiPM) as a determinant of ion bioavailability: A critical analysis of new and published toxicological studies and a simplified method for the computation of plant psiPM

Plasma membranes (PMs) are negatively charged, and this creates a negative PM surface electrical potential (psiPM) that is also controlled by the ionic composition of the bathing medium. The psiPM controls the distribution of ions between the PM surface and the medium so that negative potentials increase the surface activity of cations and decrease the surface activity of anions. All cations reduce the negativity of psiPM, and these common ions are effective in the following order: Al3+ > H+ > Cu2+ > Ca2+ = Mg2+ > Na+ = K+. These ions, especially H+, Ca2+, and Mg2+, are known to reduce the uptake and biotic effectiveness of cations and to have the opposite effects on anions. Toxicologists commonly interpret the interactions between toxic cations (commonly metals) and ameliorative cations (commonly H+, Ca2+, and Mg2+) as competitions for binding sites at a PM surface ligand. The psiPM is rarely considered in this biotic ligand model, which incorporates the free ion activity model. The thesis of this article is that psiPM effects are likely to be more important to bioavailability than site-specific competition. Furthermore, psiPM effects could give the false appearance of competition even when it does not occur. The electrostatic approach can account for the bioavailability of anions, whereas the biotic ligand model cannot, and it can account for interactions among cations when competition does not occur. Finally, a simplified procedure is presented for the computation of psiPM for plants, and the possible use of psiPM in a general assessment of the bioavailability of ions is considered.     

Synthesis and pharmacological evaluation of 2, 3-dihydro-3-oxo-4H-thieno[3,4-e][1,2,4]thiadiazine 1,1-dioxides as voltage-dependent calcium channel blockers

The synthesis of a novel series of 2,3-dihydro-3-oxo-4H-thieno[3, 4-e][1,2,4]thiadiazine 1,1-dioxides and their pharmacological evaluation as drugs with effects on the rat cardiovascular system are described. The compounds under study were synthesized via Curtius rearrangement of appropriate sulfamoylacylazides which, in turn, were prepared from known starting materials. In isolated rat portal vein, these thienothiadiazines, like verapamil and diazoxide, inhibited the spontaneous motility produced by KCl (20 mM). In addition, the new compounds, like verapamil and unlike diazoxide, also exhibited inhibitory effects in the same preparation when the cell membrane was depolarized by an increased extracellular KCl concentration (80 mM) and, consequently, the membrane potential approached a level close to the K(+) equilibrium potential. Further characterization of this inhibitory activity led to the identification of a selective inhibitory effect of the new compounds on KCl (80 mM)-induced 45Ca(2+) uptake in the same vascular tissue. When tested in vivo (anaesthetized normotensive rats), acute administration of verapamil, diazoxide and some of the most in vitro potent compounds in 45Ca(2+) uptake experiments produced a gradual, dose-dependent and sustained decrease in diastolic arterial blood pressure, devoid of cardiac effects. These results suggest that, like verapamil, the cardiovascular effects produced by the new thienothiadiazines seem to be due, at least in part, to a blockade of transmembrane voltage-dependent calcium channels present in vascular smooth muscle cells and not to an activation of ATP-sensitive K(+) channels. Compounds 5b, 5e and 5i have been selected for further studies as antihypertensive agents.     

雌二醇对溴氰菊酯染毒大鼠皮层突触体神经毒性的影响

目的观察雌激素对溴氰菊酯染毒动物皮层突触体膜兴奋性氨基酸递质释放和ATPase活力的影响.方法用高效液相色谱(HPLC)检测不同水平17β-雌二醇(10-5、10-8、10-111mol/L)对2×10-5mol/L溴氰菊酯染毒的去卵巢大鼠皮层突触体氨基酸释放的影响;用化学比色法检测17β-雌二醇(10-5、10-8、10-11mol/L)对2×10-4mol/L溴氰菊酯染毒的去卵巢大鼠皮层突触体Na+-K+ATPase、Mg2+-ATPase、Ca2+-ATPase、Ca2+-Mg2+-ATPage活力的影响,并观察雌激素受体拮抗剂Tamoxifen对雌二醇作用的影响.结果2×10-5mol/L氰菊酯处理可增加高钾去极化状态下大鼠皮层突触体天冬氨酸(Asp)和谷氨酸(Glu)的释放;10-8、10-11mol/L的17β-雌二醇可抑制溴氰菊酯所致高钾去极化状态下突触体Asp和Glu的释放,对Asp释放抑制率为28.42%、24.36%,对Glu释放抑制率为21.52%、14.57%;Tamoxifen对17β-雌二醇的抑制作用未见拮抗.2×10-4mol/L溴氰菊酯可抑制突触体4种ATPage活力;10-5mol/L雌二醇可拮抗溴氰菊酯对4种ATPase活力的抑制;l0-8、10-11mol/L雌二醇可增加Ca2+-ATPase活力;Tamoxifen对雌二醇的拮抗作用仍未见明显影响.结论 17β-雌二醇对溴氰菊酯染毒所致皮层突触体兴奋性氨基酸递质释放增加和ATPase活力抑制有一定的保护作用,该作用可能是雌二醇非基因学机制的表现.     

Eel ATPase activity as biomarker of thiobencarb exposure

European eels (Anguilla anguilla) were exposed to a sublethal thiobencarb concentration of 0.22 mg/L in a flow-through system for 96 h. Mg(2+) and Na(+)-K(+) adenosine triphosphatase (ATPase) activities were evaluated in gill and muscle tissues at 2, 12, 24, 48, 72, and 96 h of thiobencarb exposure. Gill ATPase activities were rapidly inhibited from 2h of contact onward. Highest inhibition was registered for Na(+), K(+)-ATPase (85%) from 2 to 12h. Both Mg(2+) and total ATPase were inhibited (>73%) during the first hours of toxicant exposure. At the end of the exposure period (96 h) ATPase activities were still different from those of the controls (>50%). Significant inhibition was detected in Na(+), K(+)-ATPase activity (80%) in muscle tissue after 2h and it was maintained over the entire exposure time. However, Mg(2+)-ATPase and total ATPase showed only perturbations after 2 h of exposure. Eels were exposed to 0.22 mg/L of thiobencarb for 96 h and then a recovery period in herbicide-free water was allowed for 192 h. Gill and muscle samples were removed at 8, 24, 72, 96, 144, and 192 h and ATPase activity was evaluated. Following 144 h of recovery, Mg(2+)- and Na(+), K(+)-ATPase activities, as well as total ATPase activity, in gills of those animals previously exposed to 0.22 mg/L of thiobencarb were still significantly different compared to controls. Thiobencarb seems to act to alter the ionic profiles. Since ion-dependent ATPases are known to regulate the influx and efflux of ions across the membrane to maintain the physiological requirements of the cells, the inhibition of Na(+), K(+)-ATPase probably induced osmoregulatory perturbations. On the other hand, thiobencarb exposure causes increases in the muscle water content of A. anguilla. The results indicated that water content increased significantly (>100% higher than the controls) during the first 24 h of exposure.     

Amiloride increases neuronal damage after traumatic brain injury in rats

OBJECTIVE: It is well known that traumatic brain injury (TBI) decreases brain free magnesium (Mg) concentration, and that administration of Mg salts after TBI restores concentration of Mg in brain and improves functional outcome. In the presence of hemorrhage, administration of Mg salts exacerbates the injury process and worsens outcome. An alternative to administration of Mg salts may be to prevent cellular loss of Mg with use of amiloride, which inhibits the Na(+)/Mg(2+) exchange. METHODS: In the present study, male, adult Sprague-Dawley rats were injured using the impact acceleration model of diffuse TBI and administered either 100 mols/kg i.v. amiloride, or an equal volume of 50% DMSO/saline, 30 minutes (min) after injury. RESULTS: Amiloride did not improve functional outcome (motor or cognitive outcome) after TBI relative to vehicle treated controls. Histologically, treatment with amiloride significantly increased hippocampal caspase-3 expression (apoptosis), axonal swellings in the medulla and the degree of dark cell change (cell stress) in the cortex. Phosphorus NMR demonstrated that amiloride did not increase free Mg concentration after injury. CONCLUSIONS: Thus, amiloride is ineffective in preventing Mg loss after TBI when administered 30 min after trauma. Moreover, by administering amiloride after the TBI-related Mg decline has already been initiated, it may exacerbate injury by, in part by inhibiting Na(+)/Mg(2+) antiport and preventing entry of Mg back into the cell, and also by inhibiting other Na(+) linked transporters.