The electroneutral exchange of chloride and bicarbonate across the human erythrocyte membrane is facilitated by Band 3, a 911 amino acid glycoprotein consisting of a 43 kDa N‐terminal cytosolic domain that binds the cytoskeleton, haemoglobin and glycolytic enzymes and a 52 kDa C‐terminal membrane domain that mediates anion transport. Electron microscopy and three‐dimensional image reconstruction of negatively stained two‐dimensional crystals of the dimeric membrane domain revealed a U‐shaped structure with dimensions of 60 × 110 A, and a thickness of 80 A. The structure is open on the top and at the sides, with the monomers in close contact at the base. The basal domain is 40 A thick and probably spans the lipid bilayer. The upper part of the dimer consists of two elongated protrusions measuring 25 × 80 A in projection, with a thickness of 40 A. The protrusions form the sides of a canyon, enclosing a wide space that narrows down and converges into a depression at the centre of the dimer on the top of the basal domain. This depression may represent the opening to a transport channel located at the dimer interface. Based on the available protein‐chemical data, the two protrusions face the cytosolic side of the membrane and they appear to be dynamic.

Primary human muscle cell cultures were established and the regulation of glucose transport was investigated. Primary cultures were allowed to proceed to the stage of myotubes through fusion of myoblasts or were used for clonal selection based on fusion potential. In clonally selected cultures, hexose (2-deoxyglucose) uptake into myotubes was linear within the time of study and inhibitable by cytochalasin B (IC5o = 400 nM). Cytochalasin B photolabeled a protein(s) of45,000-50,000 D in a D-glucose-protectable manner, suggesting identity with the glucose transporters. In the myotube stage, the cells expressed both the GLUT1 and GLUT4 glucose transporter protein isoforms at an average molar ratio of 7:1. Preincubation in media of increasing glucose concentrations (range 5-25 mM) progressively decreased the rate of 2-deoxyglucose uptake. Insulin elevated 2-deoxyglucose uptake in a dose-dependent manner, with half maximal stimulation achieved at 3.5 nM. Insulin also stimulated the transport of the nonmetabolizable hexose 3-0-methylglucose, as well as the activity of glycogen synthase, responsible for nonoxidative glucose metabolism. The oral antihyperglycemic drug metformin stimulated the cytochalasin B-sensitive component of both 2-deoxyglucose and 3-0-methylglucose uptake. Maximal stimulation was observed at 8 h of exposure to 50 MM metformin, and this effect was not prevented by incubation with the protein-synthesis inhibitor cycloheximide. The relative effect of metformin was higher in cells incubated in 25 mM glucose than in 5 mM glucose, consistent with its selective action in hyperglycemic conditions in vivo. Metformin (50 gM for 24 h) was more effective than insulin (1 ,MM for 1 h) in stimulating hexose uptake and the hormone was effective on top of the stimulation caused by the biguanide, suggesting independent mechanisms of action. (J. Clin. Invest.

The membrane domain of human erythrocyte Band 3 protein (M(r) 52,000) was reconstituted with lipids into two‐dimensional crystals in the form of sheets or tubes. Crystalline sheets were monolayers with six‐fold symmetry (layer group p6, a = b = 170 A, gamma = 60 degrees), whereas the symmetry of the tubular crystals was p2 (a = 104 A, b = 63 A, gamma = 104 degrees). Electron image analysis of negatively stained specimens yielded projection maps of the protein at 20 A resolution. Maps derived from both crystal forms show that the membrane domain is a dimer of two monomers related by two‐fold symmetry, with each monomer consisting of three subdomains. In the dimer, two subdomains of each monomer form a roughly rectangular core (40 × 50 A in projection), surrounding a central depression. The third subdomain of the monomer measures approximately 15 × 25 A in projection and appears to be connected to the other two by a flexible link. We propose that the central depression may represent the channel for anion transport while the third subdomain appears not to be directly involved in channel formation.

Human muscle cells were grown in culture from satellite cells present in muscle biopsies and fusion-competent clones were identified. Hexose uptake was studied in fused myotubes of human muscle cells in culture and compared with hexose uptake in myotubes of the rat L6 and mouse C2C12 muscle cell lines. Uptake of 2-deoxyglucose was saturable and showed an apparent Km of about 1.5 mM in myotubes of all three cell types. The Vmax of uptake was about 6000 pmol/(min.mg protein) in human cells, 4000 pmol/(min.mg protein) in mouse C2C12 muscle cells, and 500 pmol/(min.mg protein) in L6 cells. Hexose uptake was inhibited approximately 90% by cytochalasin B in human, rat, and mouse muscle cell cultures. Insulin stimulated 2-deoxyglucose uptake in all three cultures. The hormone also stimulated transport of 3-O-methylglucose. The sensitivity to insulin was higher in human and C2C12 mouse myotubes (half-maximal stimulation observed at 3.5 X 10(-9) M) than in rat L6 myotubes (half-maximal stimulation observed at 2.5 X 10(-8) M). However, insulin (10(-6) M) stimulated hexose uptake to a larger extent (2.37-fold) in L6 than in either human (1.58-fold) or mouse (1.39-fold) myotubes. It is concluded that human muscle cells grown in culture display carrier-mediated glucose uptake, with qualitatively similar characteristics to those of other muscle cells, and that insulin stimulates hexose uptake in human cells. These cultures will be instrumental in the study of human insulin resistance and in investigations on the mechanism of action of antidiabetic drugs.

Although internet use continues to increase and e-commerce sales are expected to exceed US$1 trillion by the end of 2001, there have been few assessments in the literature regarding the implications of this medium for tobacco control eVorts. This commentary explores the challenges that the internet may pose to the key components of a comprehensive tobacco control strategy, and pinpoints potential approaches for addressing these challenges. Four key challenges that the internet presents for tobacco control are identified: unrestricted sales to minors; cheaper cigarettes through tax avoidance and smuggling; unfettered advertising, marketing and promotion; and continued normalisation of the tobacco industry and its products. Potential strategies for addressing these challenges include international tobacco control agreements, national and state regulation, and legal remedies.Globalisation of commerce, investments, and finance is the dominant paradigm among the world's economic and political leaders.1 2 Not only is the internet a major component of globalisation, but it also appears to be a driver of this movement.3 This revolutionary force has changed the mass media landscape in ways that would have been diYcult to predict a mere decade ago. Much debate has transpired over the content and regulation of this new frontier, and the issues are complex and evolving. A tension has developed between accessibility to information and commerce, on one hand, and policies and laws designed to protect the citizenry and uphold social values, on the other.By the end of 2000, internet usage had grown to an estimated 374.9 million users in the top 15 user nations, with 490 million users (79.4 per 1000 people) projected by 2002. 5 Although some tobacco spokespersons deny plans to market their products on the web, 6 the tobacco industry is clearly cognizant of the power of the internet and is working to position itself favourably in this new market. 7Tobacco industry experts predict that within 10 years, at least one fifth of the $40 billion in annual US cigarette sales will occur over the internet. [8][9][10] Numerous issues may have to be addressed if tobacco control eVorts are to keep pace with commerce on the internet. Of critical importance is whether tobacco e-commence threatens the integrity of comprehensive tobacco control programmes, and if so, how this will materialise. Below we describe the challenges the internet may pose to the key components of a comprehensive strategy, and suggest potential solutions for addressing these challenges drawing on developments already occurring. Challenges for tobacco control UNRESTRICTED SALES TO MINORSThe prevention of sales to minors is a key component of a comprehensive tobacco control strategy, 11 but it is clear that there are few barriers to youth buying cigarettes online. A state investigation in Utah found that children aged 10-17 were able to obtain cigarettes with ease via the internet. 8 In an Oregon undercover investigation, four internet based tobacco dealers were found to hav...