John Bramhall

Permeability of lipid bilayers to water and ionic solutes

The lipid bilayer moiety of biological membranes is considered to be the primary barrier to free diffusion of water and solutes. This conclusion arises from observations of lipid bilayer model membrane systems, which are generally less permeable than biological membranes. However, the nature of the permeability barrier remains unclear, particularly with respect to ionic solutes. For instance, anion permeability is significantly greater than cation permeability, and permeability to proton-hydroxide is orders of magnitude greater than to other monovalent inorganic ions. In this review, we first consider bilayer permeability to water and discuss proposed permeation mechanisms which involve transient defects arising from thermal fluctuations. We next consider whether such defects can account for ion permeation, including proton-hydroxide flux. We conclude that at least two varieties of transient defects are required to explain permeation of water and ionic solutes.

The origin of a break in arrhenius plots of membrane processes

Abstract The break in Arrhenius plots of membrane permeation rates or protein activities is interpreted as a consequence of the inherent energy-entropy compensation at the lipid phase transition. An experimental result in support of this interpretation is presented.

The use of a fluorescent probe to monitor alterations in trans-membrane potential in single cell suspensions

Summary When single cell suspensions of thymic lymphocytes, splenic lymphocytes and platelets are equilibrated with a carbocyanine dye, 3,3′-dipnetyloxacarbocyanine idoide the fluorescence emission intensity is related to the trans-membrane potential. Addition of valinomycin to these cells provokes a net potassium efflux, and membrane hyperpolarisation, which is reflected as a decrease in fluorescence intensity. Increasing the extra-cellular potassium concentration, and thus decreasing the concentration gradient of that ion across the membrane, reduces the change in fluorescence intensity induced by valinomycin.

Subarachnoid lumbar drains: A case series of fractured catheters and a near miss

PurposeLumbar subarachnoid catheters for cerebrospinal fluid (CSF) drainage (lumbar drains) are indicated for several medical and surgical conditions. A number of complications can occur from the placement of this type of catheter, including catheter breakage from excessive traction or shearing over the Tuohy needle.Clinical featuresFive cases of lumbar subarachnoid catheter breakage/shearing and catheter fragment retention, as well as one near miss, were identified over a one-year period at a single institution. All (n = 6) patients were undergoing neurosurgical procedures. Four patients required surgical retrieval of the catheter fragments. No patient experienced log-term neurological sequelae.DiscussionFrom these experiences, the following risks factors for catheter rupture are identified: 1) intentional or accidental retraction of the catheter through the needle during placement; 2) faulty use of the guidewire; or 3) use of excessive force during removal of the catheter. Methods to prevent such complications are suggested, including minimal use, or complete avoidance of a guidewire.RésuméObjectifLes cathéters lombaires sous-arachnoïdiens pour le drainage (drains lombaires) du liquide céphalorachidien (LCR) sont indiqués pour de nombreuses conditions médicales et chirurgicales. Un nombre de complications peut survenir lors du positionnement de ce type de cathéter, y compris un bris de cathéter dû à une traction excessive ou à un cisaillement de l’aiguille Tuohy.Éléments cliniquesCinq cas de brisldsaillement de cathéter lombaire sous-arachnoïdien et de rétention de fragment de cathéter, ainsi qu’un « near miss», ont été identifiés au cours d’une période d’une année dans une seule institution. Tous les patients (n = 6) subissaient des procédures neurochirurgicales. Quatre patients ont nécessité une récupération chirurgicale des fragments de cathéter. Aucun patient n’a souffert de séquelles neurologiques a long terme.DiscussionNous avons pu identifier les facteurs de risque suiv-ants pour le bris de cathéter suite à ces expériences : 1) rétraction intentionnelle ou accidentelle du cathéter à travers l’aiguille pendant le positionnement; 2) mauvais usage du fil guide; ou 3) utilisation de force excessive pendant l’extraction du cathéter. Certaines méthodes afin de prévenir de telles complications sont suggérées, y compris une utilisation minimale, voire nulle, du fil guide.

Kinetic studies of human erythrocyte membrane resealing.

Following lysis in hypotonic media, human erythrocyte membranes will spontaneously reseal and regain their original low permeability for polar solutes. It is generally accepted that resealing will only occur when the membranes are heated above a critical temperature, and that the membrane lesions are stable under cold conditions. Contrary to these prevailing notions, a detailed investigation of the temperature dependence of resealing kinetics over the temperature range 0-22 degrees C revealed that resealing occurs at measurable rates at temperatures as low as 0 degree C, even in buffers of low ionic strength. At all temperatures studied, initial resealing rates were approximately first-order, and Arrhenius plots of these rates revealed a sharp, singular discontinuity at approx. 7 degrees C.

Electrostatic forces control the penetration of membranes by charged solutes

Using fluorescent, anionic dyes such as carboxyfluorescein as model solutes, it is shown that the forces allowing such solutes to be retained within sealed lipid vesicles, against a large concentration gradient, can be primarily electrostatic in nature. At temperatures distant from that of the ordered-fluid lipid phase transition a small number of the anionic dye molecules trapped within lipid vesicles are capable of traversing the lipid bilayer and establishing an electrical diffusion potential across the membrane. Further solute movement can then only occur with the concomitant permeation of ions which restore electrical balance. A significant flux of dye can be triggered by (a) increasing the permeability of the membrane to ions (for example by the addition of ionophores such as gramicidin, or by allowing the lipid to approach a phase transition) or by (b) adding lipophilic counterions such as tetraphenylborate or dinitrophenol to the system.

HISTAMINE BLOCKS INTERLEUKIN 2 (IL-2) GENE EXPRESSION AND REGULATES IL-2 RECEPTOR EXPRESSION

Histamine inhibited the proliferative response of human peripheral blood mononuclear cells (PBMC) to the T cell mitogen Phytohemagglutinin-P (PHA-P) in a dose-dependent fashion. This inhibition was mediated via the H2 receptor since cimetidine, a known H2 antagonist, removed the inhibition, whereas the addition of the H1 antagonist Diphenhydramine did not. Inhibition occurred during the inductive phase of the cell cycle, since histamine added 24 hours after PHA-P stimulation had no effect on subsequent T cell proliferation, and was attributable to inhibition of interleukin-2 (IL-2) gene expression. Both secreted IL-2 and messenger RNA coding for IL-2 were inhibited by histamine. In contrast, histamine exerted no inhibitory effect on the expression of cell surface receptors for IL-2 as determined by flow cytometry. Furthermore, histamine-treated cells retained full responsiveness to exogenously administered IL-2, which completely reversed the anti-proliferative effect of histamine. In some donors, histamine enhanced the percentage of IL-2 receptor positive cells. Stimulated PBMC from AIDS KS patients as a group, displayed a lower percentage of IL-2 receptor bearing cells, which was significantly increased by the addition of histamine even at concentrations as low as 10(-6) M and peaking at 10(-3) M. These findings indicate that histamine exerts its anti-proliferative effects on T cells by inhibiting IL-2 production, via blockade of IL-2 gene expression. In addition, histamine seems to exert immunomodulating effects on IL-2 receptor expression, particularly in those individuals with AIDS-KS.

Labeling of the active subunit of cholera toxin from within the membrane bilayer

Abstract Using the photoreactive glycolipid probe 12-(4-azido-2-nitrophenoxy)-stearoylglucosamine-[1-14C], we have effected the radiolabeling of the active A1 subunit of cholera toxin from within the membrane bilayer. The membrane employed as a target was the envelope of Newcastle disease virus which contained the photoreactive probe. Radiolabeling of the A1 subunit occurred after cholera toxin and virus were incubated together for 15 min at 37° and then irradiated at 366 nm for 1 min. Labeling of A1 did not occur when cholera toxin was irradiated in a solution of probe without virus or when the 15 min incubation with virus was performed at 0° instead of at 37°.

Calcium and oestrogen interactions upon the rat thymic lymphocyte plasma membrane.

Summary When isolated thymic lymphocytes are equilibrated with a carbocyanine dye the degree of fluorescence is indicative of the potential difference across the plasma membrane. Increments in extracellular calcium concentrations trigger a decrease in fluorescence intensity which represents a membrane hyperpolarisation. This hyperpolarisation cannot be reversed by subsequent chelation of the calcium. Oestradiol and natural oestrogens reduced this calcium-induced potential alterations. These same steroids also blocked the calcium-induced mitotic stimulation apparent in this same cell type.

Amphotericin B-induced changes in renal membrane permeation: A model of nephrotoxicity

As part of an investigation into the nephrotoxic effects of the polyene antibiotic Amphotericin B we have studied its effects on the ion permeability of purified renal brush border membrane vesicles. Membrane potentials were measured using a potential sensitive carbocyanine dye, and ion permeabilities were calculated from the constant field equation. Amphotericin B significantly altered the ionic permeability sequence of isolated membranes and caused a selectivity for increasing the permeation of anions. Permeability changes induced by 2.0 micrograms/ml Amphotericin B resulted in an estimated hyperpolarization of the membrane from -50 mV to -72 mV. In addition, the kinetic parameters of Na+ dependent transport of organic metabolites were examined. The maximum change in fluorescence was decreased significantly in the presence of Amphotericin B. These results suggest that the ionic state of the renal cell membrane is significantly altered by the presence of Amphotericin B.