P834 Pulmonary arterial hypertension in patients with direct-acting antiviral medications for hepatitis C virus infection - a prospective observational cohort study

Funding Acknowledgements

Kantonsspital Olten, Switzerland

Background

In patients with chronic hepatitis C virus (HCV) infection, a higher risk for pulmonary artery hypertension (PAH) has been described after interferon (IFN) therapy. With the development of direct-acting antiviral (DAA) agents, vast improvements have been made in tolerance and less complications of HCV treatment. However, except of a few case reports, to date no clinical study about the evidence of PAH in patients with DAA medication for HCV infection has been published. We hypothesized that in patients, who receive DAA medication for HCV-infection, the systolic pulmonary artery pressure (sPAP) will not change significantly during and after competition of the therapy and there may be a lower post treatment risk for PAH within the population.

Methods

We prospectively enrolled patients who underwent treatment with DAA for chronic HCV infection. The patients received a transthoracic echocardiography (TTE) for the measurement of the pulmonary artery pressure before, during (8 weeks after starting the medication) and 8 weeks after completion of the HCV medication for evaluation of sPAP (figure 1). The whole treatment period took 8-12 weeks.

Results

Between June 2016 and October 2018, 33 patients completed the study protocol. In mean, the patient’s age was 50.1 ± 1.4 years and 30% of the population were female. Three patients (9 %) were HCV and human immunodeficiency virus (HIV) coinfected. The patients received different treatment regimens, according to hepatitis C genotype and co-medication. The left ventricular systolic and diastolic function were normal in all patients before treatment was started (left ventricular ejection fraction 60.7% [59.7 – 61.7%], E/A 1.18 [1.0 – 1.37]). The following table depicts the right ventricular parameter before the DAA therapy was started, 8 weeks after therapy start, and 8 weeks after therapy was completed. The analysis showed no significant difference between the sPAP in all three groups (25.9 ± 1.2 mmHg vs. 26.0 ± 1.3 mmHg vs.26.9 ± 1.1 mmHg, p-value 0.37, see figure).

Conclusion

DAA-therapy in chronic HCV infected patients is not associated with PAH in a follow-up of 2 months after the treatment was completed.

Echocardiography Data Echocardiography data Before DAA medication was started 8 weeks after DAA-therapy 8 weeks after completion of DAA-therapy p-Value Right ventricular fractional area change (FAC), % 49.1 ± 1.4 51.7 ± 1.0 51.8 ± 1.1 0.09 Tricuspid Annular Plane Systolic Excursion (TAPSE), mm 25.2 ± 1.1 25.8 ± 0.7 24.3 ± 0.5 0.4 Right ventricular/right atrial gradient, mmHg 19.9 ± 1.0 20.5 ± 0.9 21.0 ± 0.8 0.24 Systolic pulmonary artery pressure (sPAP), mmHg 25.9 ± 1.2 26.0 ± 1.3 26.9 ± 1.1 0.37

Abstract P834 Figure. Multiple variable graph of sPAP

Interaction of Np(v) with borate in alkaline, dilute-to-concentrated, NaCl and MgCl2 solutions

The formation of sparingly soluble ternary Na/Mg–Np(v)–borate(s) solid phases in alkaline, dilute-to-concentrated, NaCl and MgCl₂ solutions is confirmed by a multimethod experimental approach.

The surface chemistry of sapphire-c: A literature review and a study on various factors influencing its IEP

A wide range of isoelectric points (IEPs) has been reported in the literature for sapphire-c (α-alumina), also referred to as basal plane, (001) or (0001), single crystals. Interestingly, the available data suggest that the variation of IEPs is comparable to the range of IEPs encountered for particles, although single crystals should be much better defined in terms of surface structure. One explanation for the range of IEPs might be the obvious danger of contaminating the small surface areas of single crystal samples while exposing them to comparatively large solution reservoirs. Literature suggests that factors like origin of the sample, sample treatment or the method of investigation all have an influence on the surfaces and it is difficult to clearly separate the respective, individual effects. In the present study, we investigate cause-effect relationships to better understand the individual effects. The reference IEP of our samples is between 4 and 4.5. High temperature treatment tends to decrease the IEP of sapphire-c as does UV treatment. Increasing the initial miscut (i.e. the divergence from the expected orientation of the crystal) tends to increase the IEP as does plasma cleaning, which can be understood assuming that the surfaces have become less hydrophobic due to the presence of more and/or larger steps with increasing miscut or due to amorphisation of the surface caused by plasma cleaning. Pre-treatment at very high pH caused an increase in the IEP. Surface treatments that led to IEPs different from the stable value of reference samples typically resulted in surfaces that were strongly affected by subsequent exposure to water. The streaming potential data appear to relax to the reference sample behavior after a period of time of water exposure. Combination of the zeta-potential measurements with AFM investigations support the idea that atomically smooth surfaces exhibit lower IEPs, while rougher surfaces (roughness on the order of nanometers) result in higher IEPs compared to reference samples. Two supplementary investigations resulted in either surprising or ambiguous results. On very rough surfaces (roughness on the order of micrometers) the IEP lowered compared to the reference sample with nanometer-scale roughness and transient behavior of the rough surfaces was observed. Furthermore, differences in the IEP as obtained from streaming potential and static colloid adhesion measurements may suggest that hydrodynamics play a role in streaming potential experiments. We finally relate surface diffraction data from previous studies to possible interpretations of our electrokinetic data to corroborate the presence of a water film that can explain the low IEP. Calculations show that the surface diffraction data are in line with the presence of a water film, however, they do not allow to unambiguously resolve critical features of this film which might explain the observed surface chemical characteristics like the dangling OH-bond reported in sum frequency generation studies. A broad literature review on properties of related surfaces shows that the presence of such water films could in many cases affect the interfacial properties. Persistence or not of the water film can be crucial. The presence of the water film can in principle affect important processes like ice-nucleation, wetting behavior, electric charging, etc.

Structure of selenium incorporated in pyrite and mackinawite as determined by XAFS analyses

Selenium has a toxic potential leading to diseases by ingestion and a radiotoxic potential as (79)Se radionuclide if discharged from a high-level nuclear waste repository in deep geological formations into the biosphere. Selenium is often associated with sulfides, such as pyrite, the most important near-surface iron sulfide and constituent of host rocks and bentonite backfills considered for radioactive waste disposal. This study was aimed at investigating the incorporation of Se(2-) and Se(4+) into pyrite and mackinawite to determine the relevance of iron sulfides to Se retention and the type of structural bonding. The syntheses of pyrite and mackinawite occurred via direct precipitation in batches and also produced coatings on natural pyrite in mixed-flow reactor experiments (MFR) under anoxic conditions at Se concentrations in the solutions of up to 10(-3) mol/L. Mineralogical analyses by SEM and XRD reveal the formation of pyrite and mackinawite phases. The average Se(2-) uptake in pyrite in batch experiments amounts to 98.6%. In MFR syntheses, it reaches 99.5%, both suggesting a high potential for retention. XAFS results indicate a substitution of sulfur by selenide during instantaneous precipitation in highly supersaturated solutions only. In selenide-doted mackinawite S(2-) was substituted by Se(2-), resulting in a mackinawite-type compound. S(-) is substituted by Se(-) in selenide-doted pyrite, yielding a FeSSe compound as a slightly distorted pyrite structure. Under slighter supersaturated conditions, XAFS results indicate an incorporation of Se(2-) and Se(4+) predominantly as Se(0). This study shows that a substitution of S by Se in iron sulfides is probable only for highly supersaturated solutions under acidic and anoxic conditions. Under closer equilibrium conditions, Se(0) is expected to be the most stable species.

An attempt to explain bimodal behaviour of the sapphire c-plane electrolyte interface

A tentative picture for the charging of the sapphire basal plane in dilute electrolyte solutions allows reconciliation of the available experimental observations within a dual charging model. It includes the MUltiSIte Complexation (MUSIC) model and auto-protolysis of interfacial water. The semi-empirical MUSIC model predicts protonation and deprotonation constants of individual surface functional groups based on crystal structure and bond-valence principles: on the ideal sapphire c-plane only doubly co-ordinated hydroxyl groups exist which cause quasi zero surface potential (defined as the potential in the plane of the surface hydroxyl groups) from pH 5 to 7 and rather weak charging beyond (compared to typical oxide behaviour). MUSIC predictions concur strikingly with recently published sum frequency data for the pH dependence of the so-called "ice-like" water band (interfacial water) and contact angle titrations. Zeta potential as well as second harmonic generation data reveal a sharp IEP of around 4 and a negative surface charge at the pristine point of zero charge predicted by the MUSIC model. New zeta-potential data corroborate (i) the low IEP and its insensitivity to salt concentration and (ii) the second harmonic results. We thus establish two groups of conflicting results arising from different techniques. A conventional model of the mineral electrolyte interface such as the MUSIC model is at odds with the negative zeta potentials in the pH range 5 to 7. Therefore an additional charging mechanism is invoked to explain all the observations. Enhanced auto-protolysis of interfacial water is the most probable candidate for this additional mechanism, in agreement with net water orientation observed with sum frequency generation and second harmonic generation. Our phenomenological explanation is further corroborated by the similarity of the zeta potential vs. pH curves of the c-plane with those of hydrophobic surfaces. Additional support comes from infrared spectroscopic data on thin water films on sapphire c-plane samples. Most stunningly, theoretical calculations on basal planes of this kind suggest a 2D water bilayer that makes such surfaces hydrophobic towards further adsorption of water. The proposed dual charging mode approach comprises the MUSIC model for protonation/deprotonation of the surface aluminols affecting the surface potential and the currently advocated enhanced auto-protolysis picture for hydrophobic surfaces controlling the zeta-potential and can explain the available information in a qualitative way. The respective contributions from the two components of this dual charging mechanism may be different for different single crystal cuts of alumina. Thus interplay between protonation/deprotonation of surface functional groups and auto-protolysis of interfacial water will cause the observed zeta potentials and isoelectric points. Repercussions of one mechanism on the other will result in the most favourable interfacial water structure, which can be followed by non-linear optic techniques like sum frequency generation.

The redox behaviour of plutonium in humic rich groundwater

Experiments with different oxidation states of Pu in GoHy-532 groundwater under reducing conditions reveal that Pu(VI) and Pu(V) are reduced rapidly to Pu(IV) at pH 7. The half-life of the redox reactions Pu(VI)/(V) and Pu(V)/(IV) are in the range of minutes. The rates of both reduction reactions decrease with decreasing pH values. A portion of the Pu(IV) is not stable in the same groundwater and is reduced slowly to Pu(III) in the range of weeks. Ultrafiltration experiments show Pu to be totally bound to the humic substances in the groundwater. At Pu concentrations of 10-5to 10-4M, most of the Pu occurs as a Pu-colloid and/or Pu-colloid bound to humic substances (Pu-colloid-HS) in solution, which is indicated by EXAFS/XANES and XPS measurements.

Analysis of Th(IV)-humate by XPS

The tetravalent actinide humate complexation was investigated for types of bonding between Th(IV) and humic acid. The Th(IV)-humate was prepared in solution, separated by ultrafiltration and analyzed by X-ray photoelectron spectroscopy (XPS). The stability of humic acid against X-ray induced degradation was verified by high-resolution XPS during long-term irradiation. The Th 4f7/2line of Th(IV)-humate can be fitted by one Gaussian-Lorentzian function, indicating that one type of bonding is mainly involved in the complexation. The Th 4f spectrum of Th(IV)-humate is, as compared to those of other Th(IV)-compounds, similar to the Th(IV)-complex with an ion exchanger containing carboxylic functional groups only. The present XPS study corroborates EXAFS results, according to which Th(IV) is predominantly bound to carboxylic groups of humic acid and demonstrates the applicability of the XPS technique for the characterization of metal-humate complexes.

Effects of hydrogen and bromide on the corrosion of spent nuclear fuel andγ-irradiated UO2(s) in NaCl brine

Radiation induced UO2(s) corrosion is studied at elevated hydrogen pressure in NaCl brine containing traces of bromide. Release of Sr, Cs, Tc and actinides was measured in corrosion experiments with spent nuclear fuel pellets in presence of 10−2 mol H2(kg H2O)−1, and 10−4and 10−3 mol Br−(kg H2O)−1, respectively. For comparison, depleted UO2(s) pellets were γ-irradiated in NaCl brine at 10−3 mol H2(kg H2O)−1and 0−10−4 mol Br−(kg H2O)−1, respectively. In the γ-radiolysis experiments a significant increase in the yield of radiolytic products due to Br−is observed. Both, in the γ-radiolysis experiment with Br−and in that without Br−, the UO2(s) sample was oxidized, and the concentration of dissolved uranium was controlled by precipitation of meta-schoepite and clarkeite. In the spent nuclear fuel corrosion experiment under H2overpressure, aqueous concentrations of Tc and Np were in the range of solubilities of Tc(IV) and Np(IV) hydroxides, whereas measured U concentrations were between solubilities of U(VI) and U(IV) phases. The release rate of Sr was significantly increased in the presence of Br−traces. Results of the complementary spent nuclear fuel corrosion andγ-radiolysis experiments allow the conclusion that Br−traces reduce significantly the protective hydrogen effect with respect to the release of certain radionuclides and the yield of radiolytic products.

Highly specific responses to amine odorants of individual olfactory receptor neurons in situ

The main olfactory system of larval Xenopus laevis is made up of at least two subsystems consisting of subsets of olfactory receptor neurons (ORNs) with different transduction mechanisms. One ORN subset lacks the canonical cAMP transduction pathway and responds to amino acid odorants. The second subset has the cAMP transduction pathway but as yet suitable odorants are unknown. Here we report the identification of amines as proper olfactory stimuli for larval X. laevis using functional Ca(2+) imaging and slice preparations of the olfactory system. The response profiles of individual ORNs to a number of amines were extremely complex and mostly highly specific. The great majority of amine-sensitive ORNs responded also to forskolin, an activator of the olfactory cAMP transduction pathway. Most amine-induced responses could be attenuated by the cyclic nucleotide-gated channel inhibitor LY83583. This confirms that most amine-responsive olfactory receptors (ORs) are coupled to the cAMP-dependent transduction pathway. Furthermore, we show that trace amine-associated receptors (TAARs), which have been shown to act as specific ORs for amines in mammals, are expressed in the olfactory organ of X. laevis. The TAARs expressed in Xenopus cannot, however, explain the complex responses of individual ORNs to amines because there are too few of them. This indicates that, in addition to TAARs, there must be other receptor families involved in the detection of amines.

Autologous osteoblasts enhance osseointegration of porous titanium implants

The goal of this study was to assess the osseointegration of porous titanium implants by means of coating with autologous osteoblasts. Titanium implants (8 x 5 x 4 mm) having drill channels with diameters of 400, 500, and 600 microm were coated with autologous osteoblasts obtained from spongiosa chips. The implants were inserted into the distal femora of 17 adult Chinchilla Bastard rabbits (group I). Uncoated implants were inserted as controls in the contralateral femur (group II). The animals were sacrificed after 5, 11, and 42 days. Intravital fluorochrome labeling and microradiography were used for the assessment of bone ingrowth into the titanium channels. In both groups, no bone tissue was formed in the channels up to day 5. On day 11, group I exhibited significantly more (p<0.05) bone tissue (19.8+/-14.0% vs. 5.8+/-9.1%) with greater bone-implant contact (13.3+/-15.1% vs. 5.7+/-5.3%, p<0.05) at the channel mouths than group II. Bone tissue was formed mainly between day 15 and 30 in group I, in group II between day 25 and 40. Six weeks after implantation, bone tissue filled on an average 68.8+/-15.1% of the mouths of the drill channels in implants in group I, the filling for group II was 49.8+/-18.1% (p<0.05). The average bone-implant contact at the channel mouths after six weeks was 56.5+/-13.5% in group I, 40.2+/-21.9% in group II (p<0.05). 600-microm channels showed at this time point the best osseous integration (p<0.05). Coating with autologous osteoblasts accelerates and enhances the osseointegration of titanium implants and could be a successful biotechnology for future clinical applications.

Mediator function of the human Rad51B-Rad51C complex in Rad51/RPA-catalyzed DNA strand exchange

Five Rad51-like proteins, referred to as Rad51 paralogs, have been described in vertebrates. We show that two of them, Rad51B and Rad51C, are associated in a stable complex. Rad51B-Rad51C complex has ssDNA binding and ssDNA-stimulated ATPase activities. We also examined the functional interaction of Rad51B-Rad51C with Rad51 and RPA. Even though RPA enhances Rad51-catalyzed DNA joint formation via removal of secondary structure in the ssDNA substrate, it can also compete with Rad51 for binding to the substrate, leading to suppressed reaction efficiency. The competition by RPA for substrate binding can be partially alleviated by Rad51B-Rad51C. This recombination mediator function of Rad51B-Rad51C is likely required for the assembly of the Rad51-ssDNA nucleoprotein filament in vivo.

Local photolysis using tapered quartz fibres

We describe a versatile, low-cost photolysis system in which sub-millisecond flashes generated by a xenon flash lamp are conveyed through a silica fibre directly onto a cell. The fibre, which is tapered to a sub-micrometre diameter and sputtered with chromium and aluminium, illuminates an area of about 1 microm2. Site and timing of photolysis can be chosen independently from other experimental parameters. As an application example we show the spatially heterogeneous distribution of Ca2+-dependent Cl- channels in olfactory receptor neurons.

Slice culture of the olfactory bulb of Xenopus laevis tadpoles

We report on the development of a slice culture of amphibian brain tissue. In particular, we cultured slices from Xenopus laevis tadpoles that contain the olfactory mucosae, the olfactory nerves, the olfactory bulb and the telencephalon. During 6 days in roller tubes the slices flattened, starting from 250 microm and decreasing to approximately 40 microm, corresponding to about three cell layers. Dendritic processes could be followed over distances as long as 200 microm. Neurons in the cultured slice could be recorded using the patch clamp technique and simultaneously imaged using an inverted laser scanning microscope. We characterized the main neuron types of the olfactory bulb, i.e. mitral cells and granule cells, by correlating their typical morphological features in the acute slice with the electrophysiological properties in both the acute slice and slice culture. This correlation allowed unambiguous identification of mitral cells and granule cells in the slice culture.

Homologous recombinational repair of DNA ensures mammalian chromosome stability

The process of homologous recombinational repair (HRR) is a major DNA repair pathway that acts on double-strand breaks and interstrand crosslinks, and probably to a lesser extent on other kinds of DNA damage. HRR provides a mechanism for the error-free removal of damage present in DNA that has replicated (S and G2 phases). Thus, HRR acts in a critical way, in coordination with the S and G2 checkpoint machinery, to eliminate chromosomal breaks before the cell division occurs. Many of the human HRR genes, including five Rad51 paralogs, have been identified, and knockout mutants for most of these genes are available in chicken DT40 cells. In the mouse, most of the knockout mutations cause embryonic lethality. The Brca1 and Brca2 breast cancer susceptibility genes appear to be intimately involved in HRR, but the mechanistic basis is unknown. Biochemical studies with purified proteins and cell extracts, combined with cytological studies of nuclear foci, have begun to establish an outline of the steps in mammalian HRR. This pathway is subject to complex regulatory controls from the checkpoint machinery and other processes, and there is increasing evidence that loss of HRR gene function can contribute to tumor development. This review article is meant to be an update of our previous review [Biochimie 81 (1999) 87].

Chromosome instability and defective recombinational repair in knockout mutants of the five Rad51 paralogs

The Rad51 protein, a eukaryotic homologue of Escherichia coli RecA, plays a central role in both mitotic and meiotic homologous DNA recombination (HR) in Saccharomyces cerevisiae and is essential for the proliferation of vertebrate cells. Five vertebrate genes, RAD51B, -C, and -D and XRCC2 and -3, are implicated in HR on the basis of their sequence similarity to Rad51 (Rad51 paralogs). We generated mutants deficient in each of these proteins in the chicken B-lymphocyte DT40 cell line and report here the comparison of four new mutants and their complemented derivatives with our previously reported rad51b mutant. The Rad51 paralog mutations all impair HR, as measured by targeted integration and sister chromatid exchange. Remarkably, the mutant cell lines all exhibit very similar phenotypes: spontaneous chromosomal aberrations, high sensitivity to killing by cross-linking agents (mitomycin C and cisplatin), mild sensitivity to gamma rays, and significantly attenuated Rad51 focus formation during recombinational repair after exposure to gamma rays. Moreover, all mutants show partial correction of resistance to DNA damage by overexpression of human Rad51. We conclude that the Rad51 paralogs participate in repair as a functional unit that facilitates the action of Rad51 in HR.

Noradrenergic modulation of calcium currents and synaptic transmission in the olfactory bulb of Xenopus laevis tadpoles

Norepinephrine (NE) has various modulatory roles in both the peripheral and the central nervous systems. Here we investigate the function of the locus coeruleus efferent fibres in the olfactory bulb of Xenopus laevis tadpoles. In order to distinguish unambiguously between mitral cells and granule cells of the main olfactory bulb and the accessory olfactory bulb, we used a slice preparation. The two neuron types were distinguished on the basis of their location in the slice, their typical branching pattern and by electrophysiological criteria. At NE concentrations lower than 5 microM there was only one effect of NE upon voltage-gated conductances; NE blocked a high-voltage-activated Ca(2+)-current in mitral cells of both the main and the accessory olfactory bulbs. No such effect was observed in granule cells. The effect of NE upon mitral cell Ca(2+)-currents was mimicked by the alpha(2)-receptor agonists clonidine and alpha-methyl-NE. As a second effect, NE or clonidine blocked spontaneous synaptic activity in granule cells of both the main and the accessory olfactory bulbs. NE or clonidine also blocked the spontaneous synaptic activity in mitral cells of either olfactory bulb. The amplitude of glutamate-induced currents in granule cells was modulated neither by clonidine nor by alpha-methyl-NE. Taken together, the main effect of the noradrenergic, presynaptic, alpha(2)-receptor-mediated block of Ca(2)+-currents in mitral cells appeared to be a wide-spread disinhibition of mitral cells in the accessory olfactory bulb as well as in the main olfactory bulb.

Fluorescence correlation spectroscopy in small cytosolic compartments depends critically on the diffusion model used

Fluorescence correlation spectroscopy (FCS) is a powerful technique for measuring low concentrations of fluorescent molecules and their diffusion constants. In the standard case, fluorescence fluctuations are measured in an open detection volume defined by the confocal optics. However, if FCS measurements are carried out in cellular processes that confine the detection volume, the standard FCS model leads to erroneous results. In this paper, we derive a modified FCS model that takes into account the confinement of the detection volume. Using this model, we have carried out the first FCS measurements in dendrites of cultured neurons. We further derive, for the case of confined diffusion, the limits within which the standard two- and three-dimensional diffusion models give reliable results.

Structure of the olfactory bulb in tadpoles of Xenopus laevis

The structure of the olfactory bulb in tadpoles of Xenopus laevis (stages 54-56) was studied using axon tracing (with biocytin or low-weight dextran) and immunocytochemical techniques. Filling the olfactory nerve with biocytin made the nerve layer and the glomeruli visible. Dye injections into the glomerular layer labeled the lateral olfactory tract. Vice versa, dye injections into the lateral olfactory tract made mitral cells and their glomerular branching patterns visible. Anti-GABA antiserum stained periglomerular and granule cells, while the olfactory nerve and mitral cells were labeled by antiglutamate antiserum. We describe the layering, the numbers of cells and glomeruli, and their localization in both the main and the accessory olfactory bulb.

Low frequency voltage clamp: recording of voltage transients at constant average command voltage

We implemented a simple feedback system that modifies the conventional current clamp mode of a patch clamp amplifier so that transient potentials, such as action potentials and synaptic potentials, can be measured as in the usual current clamp, while the average membrane potential is kept constant at a value chosen by the user. The circuit thus works like the current clamp for high frequency signals and like a voltage clamp for low frequency signals. We delineate its transfer properties and give application examples.

Evidence for simultaneous protein interactions between human Rad51 paralogs

In yeast, the Rad51-related proteins include Rad55 and Rad57, which form a heterodimer that interacts with Rad51. Five human Rad51 paralogs have been identified (XRCC2, XRCC3, Rad51B/Rad51L1, Rad51C/Rad51L2, and Rad51D/Rad51L3), and each interacts with one or more of the others. Previously we reported that HsRad51 interacts with XRCC3, and Rad51C interacts with XRCC3, Rad51B, and HsRad51. Here we report that in the yeast two-hybrid system, Rad51D interacts with XRCC2 and Rad51C. No other interactions, including self-interactions, were found, indicating that the observed interactions are specific. The yeast Rad51 interacts with human Rad51 and XRCC3, suggesting Rad51 conservation since the human yeast divergence. Data from yeast three-hybrid experiments indicate that a number of the pairs of interactions between human Rad51 paralogs can occur simultaneously. For example, Rad51B expression enhances the binding of Rad51C to XRCC3 and to HsRad51D, and Rad51C expression allows the indirect interaction of Rad51B with Rad51D. Experiments using 6xHis-tagged proteins in the baculovirus system confirm several of our yeast results, including Rad51B interaction with Rad51D only when Rad51C is simultaneously expressed and Rad51C interaction with XRCC2 only when Rad51D is present. These results suggest that these proteins may participate in one complex or multiple smaller ones.

Inhibitory and excitatory responses of olfactory receptor neurons of xenopus laevis tadpoles to stimulation with amino acids

Recordings were made from olfactory receptor neurons of Xenopus laevis tadpoles using the patch-clamp technique to investigate the responses of these cells to odorants. Four amino acids (glutamate, methionine, arginine and alanine) both individually and as a mixture were used as stimuli. Of the 156 olfactory neurons tested, 43 showed a response to at least one of the stimuli. Of the cells tested, 19 % responded to glutamate, 16 % to methionine, 12 % to arginine and 10 % to alanine. Each amino acid was able to induce both excitatory and inhibitory responses, although these occurred in different cells. Each amino acid produced approximately equal numbers of inhibitory and excitatory responses. Inhibitory responses could best be observed in the perforated-patch configuration using gramicidin as an ionophore and a recording configuration that is a current-clamp for fast signals and a voltage-clamp for slow signals. The diversity of the odorant responses, in particular the existence of excitatory and inhibitory responses, is not consistent with a single transduction pathway in olfactory neurons of Xenopus laevis tadpoles.

Small conductance potassium channels cause an activity-dependent spike frequency adaptation and make the transfer function of neurons logarithmic

We made a computational model of a single neuron to study the effect of the small conductance (SK) Ca2+-dependent K+ channel on spike frequency adaptation. The model neuron comprised a Na+ conductance, a Ca2+ conductance, and two Ca2+-independent K+ conductances, as well as a small and a large (BK) Ca2+-activated K+ conductance, a Ca2+ pump, and mechanisms for Ca2+ buffering and diffusion. Sustained current injection that simulated synaptic input resulted in a train of action potentials (APs) which in the absence of the SK conductance showed very little adaptation with time. The transfer function of the neuron was nearly linear, i.e., both asymptotic spike rate as well as the intracellular free Ca2+ concentration ([Ca2+]i) were approximately linear functions of the input current. Adding an SK conductance with a steep nonlinear dependence on [Ca2+]i (. Pflügers Arch. 422:223-232; Köhler, Hirschberg, Bond, Kinzie, Marrion, Maylie, and Adelman. 1996. Science. 273:1709-1714) caused a marked time-dependent spike frequency adaptation and changed the transfer function of the neuron from linear to logarithmic. Moreover, the input range the neuron responded to with regular spiking increased by a factor of 2.2. These results can be explained by a shunt of the cell resistance caused by the activation of the SK conductance. It might turn out that the logarithmic relationships between the stimuli of some modalities (e.g., sound or light) and the perception of the stimulus intensity (Fechner's law) have a cellular basis in the involvement of SK conductances in the processing of these stimuli.

The contribution of homologous recombination in preserving genome integrity in mammalian cells

Although it is clear that mammalian somatic cells possess the enzymatic machinery to perform homologous recombination of DNA molecules, the importance of this process in mitigating DNA damage has been uncertain. An initial genetic framework for studying homologous recombinational repair (HRR) has come from identifying relevant genes by homology or by their ability to correct mutants whose phenotypes are suggestive of recombinational defects. While yeast has been an invaluable guide, higher eukaryotes diverge in the details and complexity of HRR. For eliminating DSBs, HRR and end-joining pathways share the burden, with HRR contributing critically during S and G2 phases. It is likely that the removal of interstrand cross-links is absolutely dependent on efficient HRR, as suggested by the extraordinary sensitivity of the ercc1, xpf/ercc4, xrcc2, and xrcc3 mutants to cross-linking chemicals. Similarly, chromosome stability in untreated cells requires intact HRR, which may eliminate DSBs arising during DNA replication and thereby prevent chromosome aberrations. Complex regulation of HRR by cell cycle checkpoint and surveillance functions is suggested not only by direct interactions between human Rad51 and p53, c-Abl, and BRCA2, but also by very high recombination rates in p53-deficient cells.

XRCC2 and XRCC3, new human Rad51-family members, promote chromosome stability and protect against DNA cross-links and other damages

The phenotypically similar hamster mutants irs1 and irs1SF exhibit high spontaneous chromosome instability and broad-spectrum mutagen sensitivity, including extreme sensitivity to DNA cross-linking agents. The human XRCC2 and XRCC3 genes, which functionally complement irs1 and irs1SF, respectively, were previously mapped in somatic cell hybrids. Characterization of these genes and sequence alignments reveal that XRCC2 and XRCC3 are members of an emerging family of Rad51-related proteins that likely participate in homologous recombination to maintain chromosome stability and repair DNA damage. XRCC3 is shown to interact directly with HsRad51, and like Rad55 and Rad57 in yeast, may cooperate with HsRad51 during recombinational repair. Analysis of the XRCC2 mutation in irs1 implies that XRCC2's function is not essential for viability in cultured hamster cells.

Transduction mechanisms in vertebrate olfactory receptor cells

Considerable progress has been made in the understanding of transduction mechanisms in olfactory receptor neurons (ORNs) over the last decade. Odorants pass through a mucus interface before binding to odorant receptors (ORs). The molecular structure of many ORs is now known. They belong to the large class of G protein-coupled receptors with seven transmembrane domains. Binding of an odorant to an OR triggers the activation of second messenger cascades. One second messenger pathway in particular has been extensively studied; the receptor activates, via the G protein Golf, an adenylyl cyclase, resulting in an increase in adenosine 3',5'-cyclic monophosphate (cAMP), which elicits opening of cation channels directly gated by cAMP. Under physiological conditions, Ca2+ has the highest permeability through this channel, and the increase in intracellular Ca2+ concentration activates a Cl- current which, owing to an elevated reversal potential for Cl-, depolarizes the olfactory neuron. The receptor potential finally leads to the generation of action potentials conveying the chemosensory information to the olfactory bulb. Although much less studied, other transduction pathways appear to exist, some of which seem to involve the odorant-induced formation of inositol polyphosphates as well as Ca2+ and/or inositol polyphosphate -activated cation channels. In addition, there is evidence for odorant-modulated K+ and Cl- conductances. Finally, in some species, ORNs can be inhibited by certain odorants. This paper presents a comprehensive review of the biophysical and electrophysiological evidence regarding the transduction processes as well as subsequent signal processing and spike generation in ORNs.

Bioelectronic noses: a status report. Part I

The present state of the art to record or mimic electronically the human senses of olfaction and taste is characterized. In this part I, an introduction to our present understanding in the development of electronic and bioelectronic noses is given. Finally the natural olfactory system is described in detail.

Isolation and characterization of RAD51C, a new human member of the RAD51 family of related genes

The yeast and human RAD51 genes encode strand-transfer proteins that are thought to be involved in both recombinational repair of DNA damage and meiotic recombination. In yeast, the Rad51 family of related proteins also includes Rad55, Rad57 and Dmc1. In mammalian cells, five genes in this family have been identified (HsRAD51, XRCC2, XRCC3, RAD51B/hREC2 and HsDMC1), and here we report the isolation of the sixth member, RAD51C. RAD51C was originally identified by a computer screen of the EST database. A full-length approximately 1.3 kb cDNA clone has been isolated that encodes a protein of 376 aa, having a 18-26% aa identity with other human Rad51 family members. RAD51C includes a previously mapped sequenced-tagged site location near the end of chromosome 17q. The RAD51C transcript is expressed in various human tissues, with highest level of expression in testis, followed by heart muscle, spleen and prostate. Yeast two-hybrid experiments indicate that the Rad51C protein binds to two other members of the Rad51 protein family (Xrcc3 and Rad51B) but not to itself. These findings suggest that Rad51C may function similarly to the yeast Rad55 or Rad57 proteins, rather than as a Rad51 functional homolog.

Diurnal and long-term variations of lymph capillary pressure in healthy subjects

The variability of pressure in the cutaneous lymph capillaries on the forefoot was determined in 2 groups of healthy volunteers. In group A, including 12 healthy subjects (8 men, 4 women; mean age 28 years, range 22 to 37 years), measurements were performed in the morning and late afternoon of the same day. In group B (12 healthy subjects, 5 women, 7 men; mean age 53 years, range 23 to 72 years), measurements of lymph capillary pressure were repeated with an interval of 7 weeks. The superficial microlymphatics were visualized by intravital fluorescence microlymphography, cannulated with glass micropipettes, and the lymph capillary pressure was measured using a servonulling pressure system. In group A, lymph capillary pressure measured in the morning (mean 7.5 +/- 4.4 mmHg; range -4 to 16 mmHg) did not differ (p > 0.05) from the pressure in the late afternoon (mean value 5.6 +/- 3.4 mmHg; range-1 to 13 mmHg). In group B, initial lymph capillary pressure (mean 3.9 +/- 2.9 mmHg, range -1.1 to 9.7 mmHg) was not different (p > 0.05) compared with the pressure after 7 weeks (2.9 +/- 2.7 mmHg, range -1.0 to 6.8 mmHg). We conclude that lymph capillary pressure in healthy subjects does not exhibit significant changes during the daytime or over the long term.

Glutamate and N-acetylaspartylglutamate block HVA calcium currents in frog olfactory bulb interneurons via an mGluR2/3-like receptor

1. We investigated the glutamate-mediated modulation of voltage-activated calcium currents in cultured interneurons of the olfactory bulb of Xenopus laevis, with the use of standard patch-clamp whole-cell recordings. 2. Glutamate and N-acetylaspartylglutamate (NAAG) activated (2S,1'R,2'R,3'R)-2-(2,3-dicarboxy-cyclopropyl)glycin-sens iti ve metabotropic glutamate receptors at olfactory bulb interneurons and thereby inhibited calcium influx through presynaptic voltage-gated calcium channels. 3. The glutamate- or NAAG-mediated blockage of calcium channels showed indistinguishable dose-response curves, with K1/2 = 388 and 350 nM for glutamate and NAAG, respectively. At saturating concentrations of glutamate or NAAG, approximately 40% of the maximum calcium currents were blocked. 4. It is concluded that glutamate, which excites interneurons at olfactory bulb reciprocal synapses, also causes a reduction of gamma-aminobutyric acid-mediated feedback inhibition at these synapses by activating a metabotropic-glutamate-receptor-2/3-like receptor.

A transient, RCK4-like K+ current in cultured Xenopus olfactory bulb neurons

A transient K+ current in cultured olfactory bulb neurons of Xenopus tadpoles was studied using the whole-cell patch-clamp technique. The current, which was resistant to 80 mM tetraethylammoniumchloride (TEA) and 10 nM charybdotoxin but blocked by 5 mM 4-aminopyridine (4-AP), activated between -60 and -40 mV and showed time- and voltage-dependent inactivation. Its peak amplitude was nearly independent of the extracellular K+ concentration ([K+]o) in the range of 0.05 to 10 mM, indicating that its conductance increased upon increasing [K+]o. The transient K+ current showed a slow recovery from inactivation with the time for half-maximum recovery from a conditioning pulse to 80 mV for 1 s varying from 100 ms to 500 ms. Complete recovery required as much as 5-10 s at -80 mV, but could be speeded up at hyperpolarized potentials. The current resembles the RCK4 (Kv1.4) current of rat neurons except that its recovery from inactivation was independent of [K+]o. High-frequency stimulation (20-67 Hz) of the neurons with short (5 ms) voltage pulses resulted in a frequency-dependent, progressive inactivation of the transient K+ current. This suggests that, during phasic responses of olfactory bulb neurons, inactivation of the transient K+ current occurs and may lead to lengthening of action potentials and facilitation of synaptic transmission.

Glutamate-mediated release of Ca2+ in mitral cells of the olfactory bulb

1. Effects of glutamate on intracellular Ca2+ concentration ([Ca2+]i) were investigated in cultured olfactory bulb neurons of Xenopus laevis tadpoles. We imaged [Ca2+]i in these cells with the use of a confocal laser scanning microscope and the calcium indicator dyes Fluo3 and FuraRed. 2. In the standard bath solution, application of glutamate through a pipette resulted in an increase of [Ca2+]i in both mitral/ tufted (M/T) cells and interneurons. The increase occurred in all compartments of the cells, although in a nonhomogenous way. 3. In an ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid-buffered bath solution (Ca(2+)-free Ringer solution), glutamate reproducibly induced an increase of [Ca2+]i in M/T cells but not in interneurons. This increase in [Ca2+]i had the following properties: 1) it was delayed with respect to the combined ionotropic and metabotropic response to glutamate, 2) in some cases it stopped before the end of the glutamate application, and 3) it was not affected by D,L-2-amino-5-phosphonopentanoic acid and 6-cyano-7-nitroquinoxaline-2,3-dione added to the bath. It was interpreted as a release of Ca2+ from intracellular calcium stores. 4. Of 47 M/T cells that showed a glutamate-mediated release of [Ca2+]i, 46 also showed Ca2+ influx through ionotropic glutamate receptors, so that intracellular release of [Ca2+]i appeared to be associated with the presence of glutamate-gated ion channels. However, of 110 M/T cells showing an ionotropic response to glutamate, only 46 showed a glutamate-mediated release of [Ca2+]i. 5. The glutamate-mediated release of [Ca2+]i in the dendrites was higher than that in the soma. No indications for calcium waves were found. 6. Quisqualate and (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD) mimicked the effect of glutamate on the release of [Ca2+]i, with potencies quisqualate > glutamate > > ACPD. N-acetyl-aspartyl-glutamate did not induce release of [Ca2+]i. The release was partly blocked by (+)-alpha-methyl-4-carboxyphenylglycine. 7. In conclusion, some but not all M/T cells of the olfactory bulb of X. laevis show a glutamate-mediated and predominantly dendritic increase of [Ca2+]i that is associated with glutamategated channels. The pharmacological properties of the corresponding metabotropic glutamate receptor (mGluR) resemble those of the mGluR1/5 receptors in rat.

Second messenger signaling in olfactory transduction

Olfactory receptor neurons respond to odorants with G-protein mediated increases in the concentration of cyclic adenosine 3',5'-monophosphate (cAMP) and/or inositol 1,4,5-trisphospahte (InsP3). These two second messengers directly regulate opening of cAMP- and InsP3-regulated conductances localized to the apical transduction compartments of the cell (cilia and olfactory knob). In the presence of physiological concentrations of extracellular Ca2+, these second messenger regulated conductances mediate influx of Ca2+ into the olfactory neuron resulting in large, localized increases in intracellular Ca2+ ([Ca2+]i). A significant advance in our understanding of the molecular mechanisms of olfaction is the recent realization that this increase in [Ca2+]i plays an important role as a "third messenger" in olfactory transduction. Second messenger dependent increases in [Ca2+]i cause opening of ciliary Ca(2+)-activated Cl-, cation and/ or K+ channels that can carry a large percentage of the generator current, thus amplifying the signal substantially. As a result of this sequence of events, the generator potential in olfactory neurons can be depolarizing, leading to excitation of the neuron, or hyperpolarizing, leading to suppression of basal action potential firing rate. This dual effect of odorants on olfactory neurons may play an important role in quality coding and in the ability to detect low concentrations of odorants, particularly in complex mixtures.

Different spatial patterns of [Ca2+] increase caused by N- and L-type Ca2+ channel activation in frog olfactory bulb neurones

1. The intracellular calcium concentration ([Ca2+]i) in cultured olfactory bulb neurones of Xenopus laevis tadpoles was imaged using the calcium indicator dyes fluo-3 and Fura Red as well as a laser scanning microscope. 2. Upon extracellular application of brief pulses of a solution with high potassium concentration (high [K+]o), an increase in [Ca2+]i occurred in all neurones observed. During the first 2 days in culture this increase was highest. At later stages (more than 2 days in culture) the increase in [Ca2+]i was non-homogeneous and highest in the dendritic processes. 3. Nifedipine (10 microM) reduced the high [K+]o-induced increase in [Ca2+]i. The reduction was greatest in somata and proximal dendrites. 4. With nifedipine in the bath, the high [K+]o-induced increase of [Ca2+]i was further reduced by the application of omega-conotoxin GVIA (1 microM). The omega-conotoxin-sensitive Ca2+ influx occurred predominantly on dendritic processes. 5. Noradrenaline (NA), as well as the alpha 2-adrenergic receptor agonist clonidine, reduced the high [K+]o-induced increase of [Ca2+]i. This reduction occurred mainly on dendritic processes. 6. Our results suggest a highly non-homogeneous spatial distribution of voltage-gated Ca2+ channels in cultured olfactory bulb neurones. L-type channels were found mainly on somata and their density seemed to decrease on the dendrites with increasing distance from the soma. In contrast, nifedipine-insensitive N-type channels were mainly observed on dendrites and were blocked by omega-conotoxin. NA, as well as clonidine, markedly blocked Ca2+ influx through dendritic N-type Ca2+ channels.

Imaging of L-type Ca2+ channels in olfactory bulb neurones using fluorescent dihydropyridine and a styryl dye

We have imaged the fluorescence of dihydropyridine-Bodipy (fDHP) in cultured olfactory bulb neurones in order to investigate the subcellular distribution of L-type calcium channels in these neurones. The neurones were stained with both fDHP and the voltage-sensitive styryl dye RH414. The fluorescence emission maxima of these dyes were in the green and red ranges of the spectrum, respectively, and were recorded by the 2 photomultiplier channels of a laser scanning microscope. The fDHP images were ratioed with the RH414 images taken simultaneously. The resulting ratio images revealed the spatial distribution of the surface density of L-type calcium channels. This density was highest on somata, in particular at the base of dendrites, and decreased with the distance from this maximum. Two classes of dendritic L-type channel distributions were observed: one with a homogeneously low density and another one with a characteristic gradient of the L-type channel density along proximal dendrites. This subcellular localization of L-type calcium channels is discussed in the light of specific functional roles.

Suppression of a new allele of the yeast RAD52 gene by overexpression of RAD51, mutations in srs2 and ccr4, or mating-type heterozygosity

The RAD52 gene of Saccharomyces cerevisiae is involved both in the recombinational repair of DNA damage and in mitotic and meiotic recombination. A new allele of rad52 has been isolated that has unusual properties. Unlike other alleles of rad52, this allele (rad52-20) is partially suppressed by an srs2 deletion; srs2 mutations normally act to suppress only rad6 and rad18 mutations. In addition, although haploid rad52-20 strains are very X-ray sensitive, diploids homozygous for this allele are only slightly X-ray sensitive and undergo normal meiosis and meiotic recombination. Because rad52-20 diploids homozygous for mating type are very X-ray sensitive, mating-type heterozygosity is acting to suppress rad52-20. Mating-type heterozygosity suppresses this allele even in haploids, because sir mutations, which result in expression of the normally silent mating-type cassettes, were identified among the extragenic revertants of rad52-20. A new allele of srs2 and alleles of the transcriptional regulatory genes ccr4 and caf1 were among the other extragenic revertants of rad52-20. Because other researchers have shown that the RAD51 and RAD52 proteins interact, RAD51 on a high copy number plasmid was tested and found to suppress the rad52-20 allele, but RAD54, 55 and 57 did not suppress. The RAD51 plasmid did not suppress rad52-1. The rad52-20 allele may encode a protein that has low affinity binding to the RAD51 protein. To test whether the selected revertants suppressed rad52-20 by elevating the expression of RAD51, an integrated RAD51-lacZ fusion was genetically crossed into each revertant. Because none of the revertants increased the level of RAD51-lacZ, the revertants must exert their effect by one or more mechanisms that are not mediated by RAD51.

InsP3 causes an increase in apical [Ca2+]i by activating two distinct current components in vertebrate olfactory receptor cells

1. Effects of inositol-1,4,5-trisphosphate (InsP3) applied through a patch pipette to Xenopus laevis olfactory receptor cells (ORCs) were studied using the patch-clamp technique in conjunction with calcium imaging with fura-2. 2. InsP3 activated, first, a novel voltage-independent Ca2+ current (ICa) and, second, a nonselective cation current (Icat). 3. The activation of these currents occurred at different intracellular calcium concentrations, and the activation of either current led to a marked increase of [Ca2+]i in the dendritic knob. 4. The results suggest that InsP3 might act as a second messenger in vertebrate olfactory receptor cells by activating, through different mechanisms, a plasma membrane Ca2+ conductance (ICa) and a nonselective cation conductance (Icat).

Chemosensitivity of the osphradium of the pond snail Lymnaea stagnalis

The osphradium of the pond snail Lymnaea stagnalis was studied to determine the stimuli to which this organ responds. The following stimuli were tested: hypoxia, hypercapnia, a mixture of amino acids, a mixture of citralva and amyl acetate and a mixture of lyral, lilial and ethylvanillin. The mean nerve activity consistently increased with elevated PCO2, whereas hypoxia produced variable effects. The nerve activity became rhythmic upon application of citralva and amyl acetate, but it increased in a non-rhythmic way upon application of the other two odorant mixtures tested. Whole-cell patch-clamp recordings were made from a group of 15 neurones that lay next to the issuing osphradial nerve, to determine whether ganglion cells were involved in olfactory signal processing. All neurones tested responded to at least one of the three mixtures of odorants. Both excitatory and inhibitory responses occurred. Our results indicate that the osphradium of the pond snail Lymnaea stagnalis is sensitive to elevated PCO2 as well as to three different classes of odorants. In addition, at least some neurones within the osphradium are involved in the processing of olfactory information.

Sodium/calcium exchanger in olfactory receptor neurones of Xenopus laevis

Ca2+ ions enter neurones through various types of calcium and cation channels. The mechanisms by which Ca2+ ions are spatially buffered and expelled from neurones have been studied considerable less. Using calcium imaging in conjunction with the patch clamp technique, we investigated the Na/Ca exchanger in olfactory neurones and found evidence for its localization on the dendrite. It is suggested that this tends to decouple increases in [Ca2+]i occurring in the transduction compartments of the cell from processes in the soma.

Localization of calcium entry through calcium channels in olfactory receptor neurones using a laser scanning microscope and the calcium indicator dyes Fluo-3 and Fura-Red

The intracellular calcium concentration [Ca2+]i in olfactory receptor neurones of Xenopus laevis was imaged with high spatial and temporal resolution. A new method using a mixture of the calcium indicator dyes Fluo-3 and Fura-Red was employed. The fluorescence patterns in two wavelength bands were measured on the emission side of a confocal laser scanning microscope, and the ratio R of the fluorescence intensities was taken as an estimate of [Ca2+]i. When the neurones were depolarized by elevating the extracellular potassium concentration [K+]o they showed one of three types of responses: a fast increase in [Ca2+]i, a slow increase in [Ca2+]i, or no change in [Ca2+]i. The fast increase in [Ca2+]i took place in the soma compartment. For at least 4 s after the onset of depolarization the calcium distribution in the dendrite remained essentially unchanged. To study the fast increase with high time resolution, line scan images were taken. The neurones were depolarized for brief periods applying a solution containing high [K+] onto the soma from an application pipette. The fast increase in [Ca2+]i began with a delay of about 200 ms and went from the resting concentration to about 110 nM above resting concentration. Following the depolarization, recovery from elevated [Ca2+]i to resting levels had a time constant of about 15 s. The slow response seemed to depend on the removal of [Na+] from the bath rather than on the elevated [K+] in the bath. The response was also observed with Cd2+, Ni2+, and Co2+ (1.5 mM each) in the bath.(ABSTRACT TRUNCATED AT 250 WORDS)

Amiloride-insensitive cation conductance in Xenopus laevis olfactory neurons: a combined patch clamp and calcium imaging analysis

We used digital calcium imaging with Fura-2 in conjunction with the tight-seal whole-cell patch clamp technique to describe a novel cation conductance in olfactory neurons of the clawed toad Xenopus laevis. Substitution of extracellular Ca2+ and Na+ was used as a tool to change [Ca2+]i. When [Ca2+]i was increased to about 450 nM, a conductance gcat activated that was permeable for cations. Upon gcat activation, an increase in [Ca2+]i occurred in the dendritic knob. Once activated, gcat showed no further dependence upon [Ca2+]i. Icat is shown to be different from the current activated by a mixture of the odorants citralva and amyl acetate. We conclude that there are two different cation conductances in the peripheral compartments of olfactory neurons in X. laevis.

Standing calcium gradients in olfactory receptor neurons can be abolished by amiloride or ruthenium red

Digital imaging and the patch clamp technique were used to investigate the intracellular calcium concentration in olfactory receptor neurons using the Ca2+ indicator dyes fura-2 and fura-2/AM. The spatial distribution of Cai2+ as well as its modification by the drugs Amiloride and Ruthenium Red were studied. Resting calcium concentrations in cells loaded with fura-2/AM were between 10 and 200 nM. In cells that were loaded with the pentapotassium salt of fura-2 through the patch pipette, calcium concentrations were in the same range if ATP was added to the pipette solution. Otherwise, Ca2+ reached concentrations of approximately 500 nM. Most of the observed cells showed a standing gradient of calcium, the calcium concentrations in the distal dendritic end of the cell being higher than in the soma. In some cells, the gradient was markedly reduced or abolished by adding either Amiloride or Ruthenium Red to the bath solution. In a few cells, neither drug had any effect upon the gradient. It is suggested that the inhomogenous spatial distribution of intracellular calcium in olfactory cells of Xenopus laevis is brought about by an influx of calcium ions through two different calcium permeable conductances in the peripheral compartments of the cells. The fact that only either Ruthenium Red or Amiloride abolished the standing calcium gradient further suggested that the two conductances blocked were presumably not coexpressed in the same cells.