Packing constraints and electrostatic surface potentials determine transmembrane asymmetry of phosphatidylethanol

The energetic determinants of the distribution of anionic phospholipids across a phosphatidylcholine (PtdCho) bilayer with different packing constraints in the two leaflets were studied, using (13)CH2-ethyl-labeled phosphatidylethanol (PtdEth) as a (13)C NMR membrane probe. PtdEth is unique in exhibiting a split (13)CH2-ethyl resonance in sonicated vesicles, the two components originating from the inner and outer leaflets, thus permitting the determination of the PtdEth concentration in each leaflet. Small and large unilamellar PtdEth-PtdCho vesicles were prepared in solutions of different ionic strengths. A quantitative expression for the transbilayer distribution of PtdEth, based on the balance between steric and electrostatic factors, was derived. The transbilayer difference in packing constraints was obtained from the magnitude of the PtdEth signal splitting. The electrostatic contribution could be satisfactorily described by the transmembrane difference in Gouy-Chapman surface potentials. At low (0.1-0.25%) PtdEth levels and high (up to 500 mM) salt concentrations, PtdEth had a marked fivefold preference for the inner leaflet, presumably because of its small headgroup, which favors tighter packing. At higher PtdEth content (4.8-9.1%) and low salt concentrations, where electrostatic repulsion becomes a dominant factor, the asymmetry was markedly reduced and an almost even distribution across the bilayer was obtained. In less curved, large vesicles, where packing constraints in the two leaflets are approximately the same, the PtdEth distribution was almost symmetrical. This study is the first quantitative analysis of the balance between steric and electrostatic factors that determines the equilibrium transbilayer distribution of charged membrane constituents.     

Training of medical practitioners in Nigeria (west Africa) for surgery in the rural areas

Awojobi Clinic in Eruwa, Nigeria, a hospital in a rural setting, is used as a case study. Medical practitioners hoping to practise surgery in a rural setting should be able to carry out certain essential life-saving procedures. The curricula of surgical training in medical institutions are examined to determine their relevance to performing surgery in a rural setting, and some suggestions are proffered.     

Comparison of recommended respirable mass dust sampling devices

The draft, "Revised Guide for Respirable Mass Sampling," prepared by the AIHA-ACGIH Aerosol Hazards Committee, recommends five respirable mass sampling devices that sample in accordance with the "Los Alamos" respirable dust criterion. The spectrum of recommended sampling rates for these devices ranges from 1.8 to 430. liters per minute. A study was conducted to determine if simultaneous measurements with these devices yielded comparable respirable mass concentrations. The penetration characteristics of the first stage separators were also evaluated and compared.     

Ribosomes from Xenopus laevis eggs and embryos in a cell-free protein-synthesizing system: translational regulation

Three types of ribosomal preparations from Xenopus laevis eggs and embryos were tested in a cell-free system to study possible translational regulation of protein synthesis as mediated by the ribosome during early amphibian development: type 1, a crude high-speed sediment, mainly containing monoribosomes completely dissociable by 0.5 M KC1; type II, ribosomes washed with 0.5 M KC1; and type III, ribosomes treated with puromycin - 0.5 M KC1. All three types showed an active response to the addition of poly[U]. Type III was found to be the most active: levels of incorporation of 30 phenylalanine residues/ribosome were reached. In all three cases ribosomes prepared from unfertilized eggs were 30-40% less active in vitro than those from cleavage and gastrula stages.     

Renal sodium reabsorption following induction of recovery from volume expansion

In the rat, infusion of a volume of isotonic saline equal to 2% of body weight resulted in an 82% increase in delivery of filtrate out of the proximal tubule but little or, in some animals, no change in the urinary excretion of sodium. By contrast, further degrees of volume expansion resulted in lesser increases in the distal delivery of filtrate, but were associated with a marked increase in the urinary excretion of sodium. Sixty minutes following completion of volume expansion, while the animals were still in positive sosium balance, the urinary excretion of sodium decreased 52% compared to a decrease of only 24% in the distal delivery of filtrate. During the course of progressive volume expansion and during the recovery phase, there was a dissociation between alterations in sodium reabosrption in the proximal convoluted tubule and in the whole kidney. These studies indicate that although the proximal tubule is more sensitive to changes in the extracellular fluid volume, distal nephron sites are ultimately responsible both for the natriuresis of volume expansion and the relative antinatriuresis of the recovery periods.