Curare binding and the curare-induced subconductance state of the acetylcholine receptor channel

The curare-induced subconductance state of the nicotinic acetylcholine receptor (AChR) of mouse skeletal muscle was examined using the patch-clamp technique. Two mechanisms for the generation of subconductance states were considered. One of these mechanisms entails allosteric induction of a distinct channel conformation through the binding of curare to the agonist binding site. The other mechanism entails the binding of curare to a different site on the protein. Occupation of this site would then limit the flow of ions through the channel. The voltage dependence and concentration dependence of subconductance state kinetics are consistent with curare binding to a site within the channel. The first order rate constant for binding is 1.2 X 10(6) M-1s-1 at 0 mV, and increases e-fold per 118 mV of membrane hyperpolarization. The rate of curare dissociation from this site is 1.9 X 10(2)s-1 at 0 mV, and decreases e-fold per 95 mV hyperpolarization. The equilibrium constant is 1.4 X 10(-4) M at 0 mV, and decreases e-fold per 55 mV hyperpolarization. This voltage dependence suggests that the fraction of the transmembrane potential traversed by curare in binding to this site is 0.46 or 0.23, depending on whether one assumes that one or both charges of curare sense the electric field. Successive reduction and alkylation of the AChR agonist binding sites with dithiothreitol (DTT) and N-ethyl maleimide (NEM), a treatment which results in the loss of responsiveness of the AChR to agonists, produced no change in curare-induced subconductance events, despite the fact that after this treatment most of the channel openings occurred spontaneously. Mixtures of high concentrations of carbamylcholine (CCh) with a low concentration of curare, which produce channel openings gated predominantly by CCH, resulted in subconductance state kinetics similar to those seen in curare alone at the same concentration. Thus displacement by CCh of curare from the agonist binding sites does not prevent curare from inducing subconductances. The results presented here support the hypothesis that curare induces subconductance states by binding to a site on the receptor other than the agonist binding sites, possibly within the channel pore. It is the occupation of this site by curare that limits the flow of ions through an otherwise fully opened channel

Bose-Einstein condensates in a one-dimensional double square well: Analytical solutions of the Nonlinear Schr\"odinger equation and tunneling splittings

We present a representative set of analytic stationary state solutions of the Nonlinear Schr\"odinger equation for a symmetric double square well potential for both attractive and repulsive nonlinearity. In addition to the usual symmetry preserving even and odd states, nonlinearity introduces quite exotic symmetry breaking solutions - among them are trains of solitons with different number and sizes of density lumps in the two wells. We use the symmetry breaking localized solutions to form macroscopic quantum superpositions states and explore a simple model for the exponentially small tunneling splitting.Comment: 11 pages, 11 figures, revised version, typos and references correcte

New Glyptodont from the Codore Formation (Pliocene), Falcón State, Venezuela, its relationship with the Asterostemma problem, and the paleobiogeography of the Glyptodontinae

One of the basal Glyptodontidae groups is represented by the Propalaehoplophorinae (late Oligocene — middle Miocene), whose genera (Propalaehoplophorus, Eucinepeltus, Metopotoxus, Cochlops, andAsterostemma) were initially recognized in Argentinian Patagonia. Among these,Asterostemma was characterized by its wide latitudinal distribution, ranging from southernmost (Patagonia) to northernmost (Colombia, Venezuela) South America. However, the generic assignation of the Miocene species from Colombia and Venezuela (A.? acostae, A. gigantea, andA. venezolensis) was contested by some authors, who explicitly accepted the possibility that these species could correspond to a new genus, different from those recognized in southern areas. A new comparative study of taxa from Argentinian Patagonia, Colombia and Venezuela (together with the recognition of a new genus and species for the Pliocene of the latter country) indicates that the species in northern South America are not Propalaehoplophorinae, but represent the first stages in the cladogenesis of the Glyptodontinae glyptodontids, the history of which was heretofore restricted to the late Miocene — early Holocene of southernmost South America. Accordingly, we propose the recognition of the new genusBoreostemma for the species from northern South America and the restriction ofAsterostemma to the Miocene of Patagonia. Thus, the available data indicate that the Glyptodontinae would in fact have arisen in the northernmost regions of this continent. Their arrival to more southerly areas coincides with the acme of the “Age of Southern Plains”. The Propalaehoplophorinae are geographically restricted to Patagonia

HIV/HBV and HIV/HCV coinfection, and outcomes following highly active antiretroviral therapy

Objectives To assess the prevalence and risk factors for HBV and HCV coinfection in the Australia HIV Observational Database (AHOD), and examine outcomes of HIV disease following initiation of highly active antiretroviral therapy (HAART). Methods Analyses were based on 2086 participants recruited to AHOD by September 2002. Of these, 1605 (77%) had been tested for HBV surface antigen, 1704 (82%) for anti-HCV antibody and 1453 (70%) for both. Demographic and clinical predictors of HBV and HCV coinfection were examined. The impact of HBV and HCV coinfection on HIV disease progression was assessed by Kaplan-Meier survival curves and Cox proportional hazard model of time to AIDS events and death. Results Among those tested, prevalence of HBV surface antigen and HCV antibody were 6.3% and 13.1%, respectively (4.8% and 10.7%, respectively, among the entire cohort). In multivariate analyses, the only independent risk factor for HIV/HBV coinfection was coinfection with HCV. Independent risk factors for HIV/HCV coinfection were HIV exposure category (with people who reported injecting drug use [MSM & IDU, IDU only] or receipt of blood or blood products at markedly increased risk) and HBV coinfection. HIV disease outcomes following first initiation of a HAART regimen were similar for HIV/HBV and HIV/HCV coinfected patients compared with HIV-only patients in terms of AIDS-free survival and detectable HIV virus during the first 12 months. However, patients coinfected with HIV/HCV appeared to have a poorer response to HAART in terms of CD4 count changes, with a CD4 count increase of 32 cells/μL (95% CI 1–67) less than HIV-only patients. Conclusions Coinfection with HBV or HCV is relatively common among HIV-infected participants in AHOD. HIV disease outcomes following HAART do not appear to be adversely affected by HBV/HCV coinfection, except for slightly poorer CD4 count responses in HIV/HCV coinfected patients