Longitudinal gradient coil optimization in the presence of transient eddy currents.

The switching of magnetic field gradient coils in magnetic resonance imaging (MRI) inevitably induces transient eddy currents in conducting system components, such as the cryostat vessel. These secondary currents degrade the spatial and temporal performance of the gradient coils, and compensation methods are commonly employed to correct for these distortions. This theoretical study shows that by incorporating the eddy currents into the coil optimization process, it is possible to modify a gradient coil design so that the fields created by the coil and the eddy currents combine together to generate a spatially homogeneous gradient that follows the input pulse. Shielded and unshielded longitudinal gradient coils are used to exemplify this novel approach. To assist in the evaluation of transient eddy currents induced within a realistic cryostat vessel, a low-frequency finite-difference time-domain (FDTD) method using the total-field scattered-field (TFSF) scheme was performed. The simulations demonstrate the effectiveness of the proposed method for optimizing longitudinal gradient fields while taking into account the spatial and temporal behavior of the eddy currents.     

Correction of frequency and phase variations induced by eddy currents in localized spectroscopy with multiple echo times.

As a consequence of the time-varying magnetic field induced by eddy currents, frequency drifting occurs when the sampling window of localized spectroscopy continuously shifts. The frequency drifting and the concomitant phase variations can severely affect spectroscopy results when data are acquired with multiple echo times (TEs), such as in the measurement of glutamate (Glu) concentration using the TE-averaged method. Specifically, the averaged spectra are further broadened and distorted in the presence of residual eddy currents, and editing of the coupled spins of Glu C4 protons is affected, resulting in errors in the measured relative intensity ratio. Postacquisition correction using unsuppressed water as reference can effectively minimize this detrimental effect, as manifested by the significantly enhanced signal intensity. Also, it is demonstrated that the methyl signals of creatine (Cr) at 3.0 ppm and choline (Cho) at 3.2 ppm can be used as internal references in finding frequency and phase disparities between different TEs.     

Effects of bile acids on ventricular muscle contraction and electrophysiological properties: studies in rat papillary muscle and isolated ventricular myocytes

Summary The effects of sodium salts of various bile acids on the contractile force and the electrophysiological properties of rat ventricular muscle were studied in vitro. Primary, conjugated, and secondary bile acids were studied in a concentration range of 10^–9–10^–6 mol/l, which corresponds to concentrations found in the plasm of patients with cholestatic jaundice. In general, the bile acid induced a negative inotropic effect which was manifested as a reduction in active tension, maximum rate of tension activation, and maximum rate of tension relaxation. Twitch duration and time to peak tension were unaffected by the bile acids. The negative inotropism was associated with a reduction in ventricular action potential duration. Resting potential, action potential amplitude, and maximum upstroke velocity of phase 0 depolarization were unaffected. Voltage clamp experiments in rat ventricular myocytes demonstrated that sodium taurocholate decreased the slow inward current and slightly increased the outward potassium current. Hence, these effects on the membrane currents are probably responsible for the negative inotropic effect. Send offprint requests to O. Binah at the above address     

EFFECTS OF BKCa CHANNELS ON THE REDUCTION OF CYTOSOLIC Ca2+ IN cGMP-INDUCED RELAXATION OF GUINEAPIG TRACHEA

1. In order to examine the mechanisms of cGMP-induced relaxation in airway smooth muscle, the effects of atrial natriuretic peptide (ANP) and 8-brom cGMP on muscle tone were studied by measuring isometric tension, while the effects on cytosolic Ca²⁺ concentrations were studied by measuring the spectra of fura-2 loaded in guinea-pig tracheal strips. 2. Atrial natriuretic peptide and 8-brom cGMP caused a concentration-dependent inhibition of spontaneous tone in the guinea-pig trachea. The relaxant effects of these agents on spontaneous tone were markedly suppressed in the presence of iberiotoxin (IbTX), a selective inhibitor of large-conductance Ca²⁺-activated K⁺ (BKca) channels. Iberiotoxin (30 nmol/L) markedly affected the maximal effect induced by ANP and 8-brom cGMP and augmented EC₇₀ values for ANP and EC₅₀values for 8-brom cGMP approximately 27- and 17-fold, respectively. The inhibitory effects of IbTX on relaxation induced by these agents were diminished in the presence of 1 μmol/L nifedipine, an antagonist of voltage-operated Ca²⁺channels (VOCC). 3. The inhibitory action of ANP and 8-brom cGMP on spontaneous tone was not affected by the presence of 10 μmol/L glibenclamide, an inhibitor of ATP-sensitive K⁺ channels, and 100 nmol/L apamin, an inhibitor of small-conductance Ca²+-activated K⁺ channels. When these agents were applied to tissues precontracted by high (40mmol/L) K⁺, the relaxant effects of these agents markedly diminished. 4. The extracellular Ca²⁺-dependent contraction was inhibited in the presence of 0.3 μmoI/L ANP or 0.1 mmol/L 8-brom cGMP. Concentration—response curves to extracellular Ca²⁺ (0.03—2.4 mmol/L) were markedly diminished by exposure to these agents. The maximal effect induced by extracellular Ca²⁺ was affected by these agents. 5. Atrial natriuretic peptide caused an inhibition of spontaneous tone accompanied by a reduction in the intracellular Ca²⁺ concentration. In the presence of IbTX, the elimination of both muscle tone and cytosolic Ca²⁺ by ANP was suppressed. 6. We conclude that ANP and 8-brom cGMP activate BKca channels and that the inhibition of Ca²⁺ influx through VOCC, mediated by BKca channel activation, may be involved in cGMP-dependent bronchodilation.     

Pharmacological properties of Ca2+-activated K+ currents of ramified murine brain macrophages

Using the whole-cell configuration of the patch clamp technique, calcium-activated potassium currents (I_K,Ca) were investigated in ramified murine brain macrophages. In order to induce I_K,Ca the intracellular concentration of nominal free Ca²⁺ was adjusted to 1μM. The Ca²⁺-activated K⁺ current of brain macrophages did not show any voltage dependence at test potentials between –120 and +30mV. A tenfold change in extracellular K⁺ concentration shifted the reversal potential of I_K,Ca by 51mV. The bee venom toxin apamin applied at concentrations of up to 1μM did not affect I_K,Ca. Ca²⁺-activated K⁺ currents of ramified brain macrophages were highly sensitive to extracellularly applied charybdotoxin (CTX). The half-maximal effective concentration of CTX was calculated to be 4.3nM. In contrast to CTX, the scorpion toxin kaliotoxin did not inhibit I_K,Ca at concentrations between 1 and 50nM. Tetraethylammonium (TEA) blocked 8.0% of I_K,Ca at a concentration of 1mM, whereas 31.4% of current was blocked by 10mM TEA. Several inorganic polyvalent cations were tested at a concentration of 2mM for their ability to block I_K,Ca. La³⁺ reduced I_K,Ca by 72.8%, whereas Cd²⁺ decreased I_K,Ca by 17.4%; in contrast, Ni²⁺ did not have any effect on I_K,Ca. Ba²⁺ applied at a concentration of 1mM reduced I_K,Ca voltage-dependently at hyperpolarizing potentials. Received: 17 January / Accepted: 5 May 1997     

福辛普利预处理对缺氧复氧期豚鼠心室肌细胞Na+/Ca2+交换电流的影响

目的:观察福辛普利晚期预处理对缺氧复氧心室肌细胞Na /Ca2 交换电流的影响。方法:应用全细胞膜片钳方法,记录观察酶解的成年豚鼠心室肌细胞Na /Ca2 交换电流在缺氧复氧期的变化。结果:缺氧复氧明显增加心肌Na /Ca2 交换电流,在-100mV和 60mV时较对照组分别增加41.05%及32.75%,福辛普利预处理后可明显抑制缺氧复氧心肌Na /Ca2 交换电流,在-100mV和 60mV时分别减少17.90%及14.20%,有助于减少钙内流,减轻钙超载。结论:福辛普利预处理可抑制缺氧复氧心肌Na /Ca2 交换电流逆向转运。     

川芎嗪拮抗链霉素耳毒性作用及其离子通道机制

目的 研究川芎嗪（tetraethylplyrazine，TMP）拮抗链霉素耳毒性作用及其对耳蜗外毛细胞外向K^＋通道的影响，寻求两者的相关性，旨在探讨川芎嗪拮抗耳中毒作用的离子通道机制。方法 选取豚鼠60只，随机分为6组，即对照组、链霉素组、川芎嗪低浓度组、川芎嗪高浓度组、川芎嗪低浓度＋链霉素组和川芎嗪高浓度＋链霉素组，分别注射生理盐水（2．5ml/kg）、链霉素（450mg／kg）、川芎嗪（12mg／kg）、川芎嗪（60ms／ks）、川芎嗪（12mg／kg）＋链霉素（450mg／kg）、川芎嗪（60mg／kg）＋链霉素（450mg／kg），用药10天后检测各组豚鼠ABR反应阈，并采用全细胞膜片钳技术观察川芎嗪对耳蜗外毛细胞Ca^2＋敏感K^＋电流和延迟外向K^＋电流的影响。结果 结果表明川芎嗪明显降低链霉素所致的豚鼠ABR反应阈升高，提示川芎嗪具有明显的拮抗链霉素耳毒性作用；川芎嗪能明显增大豚鼠耳蜗外毛细胞Ca^2＋敏感K^＋电流和延迟外向K^＋电流，并呈浓度依赖关系。结论 川芎嗪可能通过增大K^＋通道电流而发挥其降低链霉素耳毒性作用，推测这是其抗耳毒性作用机制之一。     

二氮嗪对大鼠脑缺血再灌注细胞凋亡的影响

目的研究二氮嗪开放线粒体ATP敏感性钾通道对大鼠脑缺血再灌注细胞凋亡的影响。方法采用线栓法建立大鼠局灶性脑缺血再灌注损伤模型,将20只大鼠随机分成4组,假手术组、缺血组、缺血＋二氮嗪治疗组和缺血＋二氮嚷＋MitoK（ATP）通道特异性抑制剂5-HD组。观察各组凋亡细胞数和凋亡相关蛋白Bcl-2、Bax的变化。结果与缺血组比较,二氮嗪使凋亡细胞数明显减少（83．2±9．04 vs 123．96±13．45）,Bcl-2表达增高（0．17±0．01 vs 0．13±0．01）,Bax表达下降（0．15±0．02 vs 0．20±0．03）,差异具有显著性（P＜0．05）。5-HD能取消这些作用（P＜0．05）。结论局灶性脑缺血再灌注损伤时,二氮嗪能通过上调半暗带区Bcl-2蛋白表达,下调Bax蛋白表达,减少神经元凋亡,对脑缺血损伤起保护作用。     

Voltage-gated Currents in Rabbit Retinal Astrocytes

The voltage-gated currents of the astrocytes associated with the retinal capillaries of the rabbit retina were studied using whole-cell patch clamp recording. The resting potential of these cells was −70 ± 4.8 mV (mean ± SEM; n = 54), and the input resistance and cell capacitance were 558 ± 3.6 MΩ and 19.5 ± 1.8 pF respectively. Depolarization to potentials positive to −50 mV evoked rapidly activating inward and outward currents. The inward current was transient, eliminated by substitution of choline for Na⁺ in the bathing solution, and reduced by 50% in the presence of 1 μM tetrodotoxin. The time-to-peak of the Na⁺ current was more than twice that for the Na⁺ current found in retinal neurons. The glial Na⁺ current was half-inactivated at −55 mV. A transient component of the outward K⁺ current was blocked by external 4-aminopyridine while a more sustained component was blocked by external tetraethylammonium. At potentials between −150 and −50 mV the membrane behaved Ohmically. Voltage-gated currents in retinal astrocytes recorded in situ appear qualitatively similar to those described for some glial cells in vitro.     

View ordering for magnetization prepared steady state free precession acquisition: application in contrast-enhanced MR angiography.

Magnetization prepared segmented acquisition requires a view order that maximizes signal contrast during the acquisition of the central portion of k-space. Steady state free precession (SSFP) acquisition further requires a view order that minimizes changes in phase-encoding gradients from one repetition to the next in order to minimize eddy current artifacts. In this article, optimal view ordering schemes satisfying these two requirements are formulated and applied to inversion prepared 3D SSFP contrast-enhanced MR angiography (MRA). Experiments on phantoms and pigs demonstrated improved background suppression and reduced image artifacts.