BACKGROUND AND PURPOSE:Occlusion of the AOP results in a characteristic pattern of ischemia: bilateral paramedian thalamus with or without midbrain involvement. Although the classic imaging findings are often recognized, only a few small case series and isolated cases of AOP infarction have been reported. The purpose of this study was to characterize the complete imaging spectrum of AOP infarction on the basis of a large series of cases obtained from multiple institutions.

Gramicidin A (gA) molecules were covalently linked with a dioxolane ring. Dioxolane-linked gA dimers formed ion channels, selective for monovalent cations, in planar lipid bilayers. The main goal of this study was to compare the functional single ion channel properties of natural gA and its covalently linked dimer in two different lipid bilayers and HCl concentrations (10-8000 mM). Two ion channels with different gating and conductance properties were identified in bilayers from the product of dimerization reaction. The most commonly observed and most stable gramicidin A dimer is the main object of this study. This gramicidin dimer remained in the open state most of the time, with brief closing flickers (tau(closed) approximately 30 micros). The frequency of closing flickers increased with transmembrane potential, making the mean open time moderately voltage dependent (tau(open) changed approximately 1.43-fold/100 mV). Such gating behavior is markedly different from what is seen in natural gA channels. In PEPC (phosphatidylethanolamine-phosphatidylcholine) bilayers, single-channel current-voltage relationships had an ohmic behavior at low voltages, and a marked sublinearity at relatively higher voltages. This behavior contrasts with what was previously described in GMO (glycerylmonooleate) bilayers. In PEPC bilayers, the linear conductance of single-channel proton currents at different proton concentrations was essentially the same for both natural and gA dimers. g(max) and K(D), obtained from fitting experimental points to a Langmuir adsorption isotherm, were approximately 1500 pS and 300 mM, respectively, for both the natural gA and its dimer. In GMO bilayers, however, proton affinities of gA and the dioxolane-dimer were significantly lower (K(D) of approximately 1 and 1.5 M, respectively), and the g(max) higher (approximately 1750 and 2150 pS, respectively) than in PEPC bilayers. Furthermore, the relationship between single-channel conductance and proton concentration was linear at low bulk concentrations of H+ (0.01-2 M) and saturated at concentrations of more than 3 M. It is concluded that 1) The mobility of protons in gramicidin A channels in different lipid bilayers is remarkably similar to proton mobilities in aqueous solutions. In particular, at high concentrations of HCl, proton mobilities in gramicidin A channel and in solution differ by only 25%. 2) Differences between proton conductances in gramicidin A channels in GMO and PEPC cannot be explained by surface charge effects on PEPC membranes. It is proposed that protonated phospholipids adjacent to the mouth of the pore act as an additional source of protons for conduction through gA channels in relation to GMO bilayers. 3) Some experimental results cannot be reconciled with simple alterations in access resistance to proton flow in gA channels. Said differences could be explained if the structure and/or dynamics of water molecules inside gramicidin A channels is modulated by the lipid environment and by modifications in the structure of gA channels. 4...

Two different stereoisomers of the dioxolane-linked gramicidin A (gA) channels were individually synthesized (the SS and RR dimers;. Science. 244:813-817). The structural differences between these dimers arise from different chiralities within the dioxolane linker. The SS dimer mimics the helicity and the inter- and intramolecular hydrogen bonding of the monomer-monomer association of gA's. In contrast, there is a significant disruption of the helicity and hydrogen bonding pattern of the ion channel in the RR dimer. Single ion channels formed by the SS and RR dimers in planar lipid bilayers have different proton transport properties. The lipid environment in which the different dimers are reconstituted also has significant effects on single-channel proton conductance (g(H)). g(H) in the SS dimer is about 2-4 times as large as in the RR. In phospholipid bilayers with 1 M [H(+)](bulk), the current-voltage (I-V) relationship of the SS dimer is sublinear. Under identical experimental conditions, the I-V plot of the RR dimer is supralinear (S-shaped). In glycerylmonooleate bilayers with 1 M [H(+)](bulk), both the SS and RR dimers have a supralinear I-V plot. Consistent with results previously published (. Biophys. J. 73:2489-2502), the SS dimer is stable in lipid bilayers and has fast closures. In contrast, the open state of the RR channel has closed states that can last a few seconds, and the channel eventually inactivates into a closed state in either phospholipid or glycerylmonooleate bilayers. It is concluded that the water dynamics inside the pore as related to proton wire transfer is significantly different in the RR and SS dimers. Different physical mechanisms that could account for this hypothesis are discussed. The gating of the synthetic gA dimers seems to depend on the conformation of the dioxolane link between gA's. The experimental results provide an important framework for a detailed investigation at the atomic level of proton conduction in different and relatively simple ion channel structures.

BACKGROUND AND PURPOSE:While enhanced T1WI is considered the "gold standard" for detection of internal auditory canal pathology, unenhanced fluid-sensitive sequences have shown high sensitivity for lesion identification. Our purpose was to evaluate the diagnostic accuracy of an unenhanced MR imaging protocol using axial CISS and coronal T2WI for detection of small (10 mm or less) internal auditory canal lesions.

The mobility of protons in a dioxolane-linked gramicidin A channel (D1) is comparable to the mobility of protons in aqueous solutions (Cukierman, S., E. P. Quigley, and D. S. Crumrine. 1997. Biophys. J. 73:2489-2502). Aliphatic alcohols decrease the mobility of H+ in aqueous solutions. In this study, the effects of methanol on proton conduction through D1 channels were investigated in different lipid bilayers and at different HCl concentrations. Methanol attenuated H+ currents in a voltage-independent manner. Attenuation of proton currents was also independent of H+ concentrations in solution. In phospholipid bilayers, methanol decreased the single channel conductance to protons without affecting the binding affinity of protons to bilayers. In glycerylmonooleate membranes, the attenuation of single channel proton conductances qualitatively resembled the decrease of conductivities of HCl solutions by methanol. However, in both types of lipid bilayers, single channel proton conductances through D1 channels were considerably more attenuated than the conductivities of different HCl solutions. This suggests that methanol modulates single proton currents through D1 channels. It is proposed that, on average, one methanol molecule binds to a D1 channel, and attenuates H+ conductance. The Gibbs free energy of this process (DeltaG0) is approximately 1.2 kcal/mol, which is comparable to the free energy of decrease of HCl conductivity in methanol solutions (1.6 kcal/mol). Apolar substances like urea and glucose that do not transport protons in HCl solutions and do not permeate D1 channels decreased solution conductivity and single channel conductance by a considerably larger proportion than methanol. Cs+ currents through D1 channels were considerably less (fivefold) attenuated by methanol than proton currents. It is proposed that methanol partitions inside the pore of gramicidin channels and delays the transfer of protons between water and methanol molecules, causing a significant attenuation of the single channel proton conductance. Gramicidin channels offer an interesting experimental model to study proton hopping along a single chain of water molecules interrupted by a single methanol molecule.

The direct role of the dioxolane group on the gating and single-channel conductance of different stereoisomers of the dioxolane-linked gramicidin A (gA) channels reconstituted in planar lipid bilayers was investigated. Four different covalently linked gA dimers were synthesized. In two of them, the linker was the conventional dioxolane described previously (SS and RR channels). Two gAs were covalently linked with a novel modified dioxolane group containing a retinal attachment (ret-SS and ret-RR gA dimers). These proteins also formed ion channels in lipid bilayers and were selective for monovalent cations. The presence of the bulky and hydrophobic retinal group immobilizes the dioxolane linker in the bilayer core preventing its rotation into the hydrophilic lumen of the pore. In 1 M HCl the gating kinetics of the SS or RR dimers were indistinguishable from their retinal counterparts; the dwell-time distributions of the open and closed states in the SS and ret-SS were basically the same. In particular, the inactivation of the RR was not prevented by the presence of the retinal group. It is concluded that neither the fast closing events in the SS or RR dimers nor the inactivation of the RR are likely to be a functional consequence of the flipping of the dioxolane inside the pore of the channel. On the other hand, the inactivation of the RR dimer was entirely eliminated when alkaline metals (Cs(+) or K(+)) were the permeating cations in the channel. In fact, the open state of the RR channel became extremely stable, and the gating characteristics of both the SS and RR channels were different from what was seen before with permeating protons. As in HCl, the presence of a retinal in the dioxolane linker did not affect the gating behavior of the SS and RR in Cs(+)- or K(+)-containing solutions. Alternative hypotheses concerning the gating of linked gA dimers are discussed.

Background and Purpose: Cervical artery dissection is a major cause of ischemic stroke in the young and presents with various imaging findings including stenosis and intramural hematoma (IMH). Our goal was to determine the relative contribution of lumen findings and IMH to acute ischemic stroke, and if a heavily T1-weighted sequence could more reliably detect IMH. Methods: IRB approval was obtained for this retrospective study of 254 patients undergoing MRI/MRA for suspected dissection. Imaging included standard turbo spin echo (TSE) T1-fat saturation (T1FS) and heavily T1-weighted flow-suppressed Magnetization Prepared Rapid Acquisition Gradient-recalled Echo (MPRAGE) sequences. Subjects with stents (1) or atherosclerotic disease (26) were excluded, leaving 227 subjects. Kappa analysis was used to determine IMH interrater reliability on MPRAGE and T1FS in 4 vessels per subject. Lumen findings, cardiovascular risk factors, medications, and non-dissection stroke sources were recorded. Mixed effects multivariate Poisson regression was used to determine the prevalence ratio (PR) of each factor with acute ischemic stroke, accounting for 4 vessels per patient with backwards elimination to a threshold p-value of 0.10. Results: Patients were 41.9% male, mean age of 47.3±16.6 years, with 114 dissections and 107 strokes. IMH interrater reliability was significantly higher for MPRAGE (kappa=0.83, 95%CI: 0.78–0.86) versus T1FS (0.58, 95%CI: 0.57–0.68). The final acute stroke prediction model included MPRAGE-detected IMH (PR=2.0, 95%CI: 1.1–3.9, p=0.034), stenosis, pseudoaneurysm, male gender, current smoking, and non-dissection stroke sources. The final model had high discrimination for acute stroke (AUC=0.902, 95%CI: 0.872–0.932), compared to models without stenosis (0.861, 95%CI: 0.821–0.902), and without stenosis and IMH (0.831, 95%CI: 0.783–0.879). All three models were significantly different at p<0.05. Conclusions: Along with stenosis, IMH detection significantly contributed to acute ischemic stroke etiology in patients with suspected cervical artery dissection. In addition, IMH detection can be made more reliable with heavily T1-weighted sequences.

Absolute Hounsfield unit values, Hounsfield unit to hematocrit ratios, and venoarterial Hounsfield unit value differences are a useful adjunct in noncontrast head computed tomographic evaluation of cerebral venous thrombosis.