Physics of Selective Conduction and Point Mutation in Biological Ion Channels.

We introduce a statistical and linear response theory of selective conduction in biological ion channels with multiple binding sites and possible point mutation. We derive an effective grand-canonical ensemble and generalized Einstein relations for the selectivity filter, assuming strongly coordinated ionic motion, and allowing for ionic Coulomb blockade. The theory agrees well with data from the KcsA K^{+} channel and a mutant. We show that the Eisenman relations for thermodynamic selectivity follow from the condition for fast conduction and find that maximum conduction requires the binding sites to be nearly identical.

Ionic Coulomb blockade and the determinants of selectivity in the NaChBac bacterial sodium channel.

Mutation-induced transformations of conductivity and selectivity in NaChBac bacterial channels are studied experimentally and interpreted within the framework of ionic Coulomb blockade (ICB), while also taking account of resonant quantised dehydration (QD) and site protonation. Site-directed mutagenesis and whole-cell patch-clamp experiments are used to investigate how the fixed charge Qf at the selectivity filter (SF) affects both valence selectivity and same-charge selectivity. The new ICB/QD model predicts that increasing ∣Qf∣ should lead to a shift in selectivity sequences toward larger ion sizes, in agreement with the present experiments and with earlier work. Comparison of the model with experimental data leads to the introduction of an effective charge Qf∗ at the SF, which was found to differ between Aspartate and Glutamate charged rings, and also to depend on position within the SF. It is suggested that protonation of the residues within the restricted space of the SF is important in significantly reducing the effective charge of the EEEE ring. Values of Qf∗ derived from experiments on divalent blockade agree well with expectations based on the ICB/QD model and have led to the first demonstration of ICB oscillations in Ca2+ conduction as a function of the fixed charge. Preliminary studies of the dependence of Ca2+ conduction on pH are qualitatively consistent with the predictions of the model.

Human in vivo comparative study of zinc and copper transmetallation after administration of magnetic resonance imaging contrast agents.

RATIONALE AND OBJECTIVES: The authors compare in vivo transmetallation of three magnetic resonance contrast agents in humans in terms of their kinetic stability. METHODS: Blood and urine samples were taken before and after the intravenous injection of gadolinium (Gd)-HP-DO3A, Gd-DTPA, or Gd-DTPA-BMA at 0.1 mmol/kg to healthy volunteers. Serum and urine were assayed for zinc, copper, and Gd, using inductively coupled plasma atomic emission spectrometer. RESULTS: Gadolinium-DTPA-BMA caused the highest increase in zinc excretion among the three agents. Gadolinium-HP-DO3A did not cause a significant increase in zinc excretion. In serum, although Gd-DTPA-BMA exhibited a decrease in zinc concentration, the difference between the drugs was not statistically significant. CONCLUSIONS: The difference in observed zinc excretion among the chelates studied reflects in vivo transmetallation of the magnetic resonance contrast media and correlates with the respective kinetic inertia for transmetallation, rather than thermodynamic stability constants. Gadolinium-HP-DO3A was found to be the most kinetically inert among the three drugs tested.

Comparative transmetallation kinetics and thermodynamic stability of gadolinium-DTPA bis-glucosamide and other magnetic resonance imaging contrast media.

RATIONALE AND OBJECTIVES: The relative stability and selectivity of magnetic resonance (MR) imaging contrast agents were compared with in vitro transmetallation and ligand competition reactions. MATERIALS AND METHODS: A gadolinium (Gd)-DTPA-BIGA, Gd-DTPA-BMA, Gd-DTPA, or Gd-HP-DO3A solution was mixed with an equimolar solution of zinc (Zn) citrate in 1:1 volume ratio at pH 7.4 in a transmetallation assay. In a ligand competition reaction, a DTPA or EDTA solution was mixed with an equimolar solution of Gd-DTPA-BIGA, Gd-DTPA-BMA, or Gd-HP-DO3A. The progress of the reactions was followed with high-performance liquid chromatography (HPLC) using fluorescence detection. RESULTS: In the transmetallation study with Zn citrate, the order of kinetic stability is Gd-HP-DO3A > Gd-DTPA > Gd-DTPA-BIGA > Gd-DTPA-BMA. The order of stability in the ligand competition reaction is Gd-HP-DO3A > Gd-DTPA-BIGA > Gd-DTPA-BMA. CONCLUSIONS: Transmetallation kinetics shows major differences among currently used MR imaging contrast media. Equilibrium thermodynamic constants and selectivity constants calculated therefrom do not explain the relative kinetic stability and metal selectivity of MR contrast media in the presence of competing metal ions and ligands.

Pseudotumor cerebri: CT findings and correlation with vision loss.

OBJECTIVE: The purpose of this study was to determine if orbital and cerebral CT can be used to distinguish normal patients from those with pseudotumor cerebri, and to correlate CT findings with the severity of visual impairment. SUBJECTS AND METHODS: Seventeen patients with a clinical diagnosis of pseudotumor cerebri were compared with 20 age- and sex-matched control subjects. Thin-section coronal and axial CT scans of the orbit and whole-brain axial CT scans were available for all subjects. The diameter of the optic nerve sheath, the degree of reversal of the optic nerve head, the presence and degree of empty sellae, and ventricular and sulcal sizes were evaluated without knowledge of whether or not the subject had pseudotumor cerebri. The same parameters were compared for two subgroups of patients with pseudotumor cerebri: those with mild vision loss and those with severe vision loss. RESULTS: Patients with pseudotumor cerebri had significantly larger optic nerve sheaths than did control subjects (6.5 +/- 0.83 mm vs 5.4 +/- 0.69 mm). Radiologic evidence of papilledema with reversal of the optic nerve head was found in 12 of 17 patients compared with one of 20 control subjects. An empty sella was seen more frequently and to a greater degree in patients with pseudotumor cerebri than in control subjects (16 vs seven). Eight of nine patients with severe vision loss and four of eight patients with mild to moderate vision loss had reversal of the optic nerve head; the degree was greater in the group with severe vision loss. No difference in ventricular size or sulcal enlargement was seen between any of the groups. The opening CSF pressures of the two groups with vision loss were not significantly different (348 +/- 80 mm H2O vs 391 +/- 98 mm H2O). CONCLUSION: In addition to the role of CT in excluding intracranial disease as a basis for the clinical syndrome of pseudotumor cerebri, thin-section CT of the orbits reveals a constellation of findings, including enlarged optic nerve sheaths, reversal of the optic nerve head, and empty sellae in patients with pseudotumor cerebri. Furthermore, severe vision loss in these patients correlates with more frequent and more severe reversal of the optic nerve head.

Biodistribution and magnetic resonance imaging of cross-linked DTPA polysaccharides.

Large polysaccharide complexes, cross-linked with DTPA and chelated with gadolinium have been tested for various potential uses for magnetic resonance imaging (MRI) in rats. Biodistribution and pharmacokinetic data for several of the soluble polymers are presented and compared with Gd-DTPA and GdCl3. By varying the initial polysaccharide length and ratio of DTPA to glucose units, polymers of molecular weights (mol wt) from 17,000 to several million were formed, giving soluble material, gels, or particles. The larger polymers (mol wt greater than 100,000) demonstrate prolonged enhancement of the intravascular space, striking renal enhancement, and moderate hepatic uptake. Small particulate material (less than 10 microns) was also successfully used for intravascular enhancement. The material is metabolized and excreted in urine.

Cross-linked DTPA polysaccharides for magnetic resonance imaging. Synthesis and relaxation properties.

An attempt was made to develop a macromolecular paramagnetic contrast agent that would be easily and inexpensively synthesized, low in toxicity, and quickly eliminated from the body after completion of the desired imaging studies. Under more rigorous conditions than attainable with proteins, the bis-anhydride of DTPA was reacted in anhydrous, hot DMSO with dextrans ranging in size from 17,000 molecular weight (MW) to 150,000 MW in various ratios of DTPA to glucose units from 1:1 to 1:40. Up to several hundred DTPA units per saccharide complex could be attached with this technique. Depending on the initial polysaccharide length and ratio of DTPA to glucose units, various degrees of cross-linking occur. Average MW of the polymers ranging from 17,000 to particles several million MW could be synthesized. The ester bond thus formed is stable for prolonged periods in solution. Depending on the polymer size, soluble material, gels, or particles are formed. Relaxation rates of the soluble material are similar or better than Gd-DTPA on a molar basis of Gd3+. The particulate material demonstrates T2 shortening out of proportion to T1, as expected for solid material, from increased susceptibility effects.

MR contrast agents: an overview.

In this article an overview of current and potential MR contrast agents is given. The mechanism of action of contrast agents and their relationship to both T1 and T2 relaxation are explored. Both paramagnetic and superparamagnetic substances are considered. Various physical states of these materials, including small ionic, lipophilic, and macromolecular forms are explored as possible contrast agents. Several clinical examples are given and speculation is made about the future potential of MR contrast agents.

Absence of sympathetic sprouting in the rat olfactory bulb after cholinergic denervation.

Sympathetic axons have been shown to invade the rat hippocampal formation after lesions of the septohippocampal (cholinergic) projection. To test the generality of this type of transmitter-specific sprouting within the central nervous system, lesions were placed within the horizontal limb of the diagonal band in order to denervate the main olfactory bulb (MOB) of its cholinergic innervation. Some animals also received autologous transplants of the superior cervical ganglion to the region of the MOB. There was no evidence of sympathetic axon growth into the MOB under any circumstance as assessed with qualitative fluorescence histochemistry. In addition, retrograde fluorescent dye studies demonstrated that the basal forebrain neurons projecting to the neocortex, where sympathetic sprouting has also been reported, are a separate population from those which project to the MOB. Although it is difficult to unequivocally interpret negative results, these data indicate that cholinergic denervation does not induce sympathetic sprouting throughout the central nervous system. The availability of a brain region where sympathetic sprouting does not occur may provide clues to understanding the conditions which permit such neuronal plasticity in the hippocampal formation.

A quantitative evaluation of suture and tubulization nerve repair techniques.

The radial and sural nerves in cats were transected and either left unrepaired, repaired with 11-0 epineurial sutures, sutured and treated with topical application of Kenalog, or tubulated with hypoantigenic collagen. One year later the nerves were examined electrophysiologically, using single unit recording techniques, and histologically, by light microscopy. The results indicate that regeneration through the neuroma was comparable in all four groups, but that reinnervation of sensory structures in the skin was most successful in the tubulated nerves.

Biological activity of 1,N6-ethenothiamin, a fluorescent analog of thiamin, in rats.

The ability of the recently synthesized fluorescent analog of thiamin, 1,N6-ethenothiamin, to substitute for thiamin in rats in vivo has been studied. 50 micrograms of ethenothiamin/100 g body weight/day injected in rats allowed about one-half the weight gain with 50 micrograms/100g/day of thiamin. The activities of the pyruvate and alpha-keto-glutarate dehydrogenases of liver mitochondria were partially restored by 50 micrograms/100g/day of ethenothiamin. Ethenothiamin at this level did not appear to restore the branched-chain alpha-keto acid dehydrogenase activities to any extent. When 150 micrograms of ethenothiamin/100g body weight/day was injected, growth was optimum, but these enzyme activities still did not achieve normal levels.