Host Specificity, Parasite Community Size and the Relation between Abundance and its Variance

We investigated the empirical relationship between mean abundance and its variance, known as Taylor’s power law, in fleas parasitic on small mammals. It has been suggested that the exponent of this function, b, represents a true biological character of a species and, dependent on the level of host specificity, varies among species. Other empirical and theoretical studies suggest that exponent b depends on interspecific competition and varies intraspecifically. We tested these hypotheses using data from central and eastern Slovakia. We demonstrate that the slope of Taylor’s relationship (a) is repeatable within a flea species, i.e. the slope represents a true species character; (b) increases with an increase of the degree of flea host specificity; and (c) decreases with an increase in flea community size. We discuss our results with the idea that the host can mediate interactions among and within flea species. Co-ordinating editor: A. Biere     

Aggregation and species coexistence in fleas parasitic on small mammals

The aggregation model of coexistence states that species coexistence is facilitated if interspecific aggregation is reduced relative to intraspecific aggregation. We investigated the relationship between intraspecific and interspecific aggregation in 17 component communities (the flea assemblage of a host population) of fleas parasitic on small mammals and hypothesized that interspecific interactions should be reduced relative to intraspecific interactions, facilitating species coexistence. We predicted that the reduction of the level of interspecific aggregation in relation to the level of intraspecific aggregation would be positively correlated with total flea abundance and species richness of flea assemblages. We also expected that the higher degree of facilitation of flea coexistence would be affected by host parameters such as body mass, basal metabolic rate (BMR) and depth and complexity of burrows. Results of this study supported the aggregation model of coexistence and demonstrated that, in general, a) conspecific fleas were aggregated across their hosts; b) flea assemblages were not dominated by negative interspecific interactions; and c) the level of interspecific aggregation in flea assemblages was reduced in relation to the level of intraspecific aggregation. Intraspecific aggregation tended to be correlated positively to body mass, burrow complexity and mass-independent BMR of a host. Positive interspecific associations of fleas tended to occur more frequently in species-rich flea assemblages and/or in larger hosts possessing deep complex burrows. Intraspecific aggregation increased relative to interspecific aggregation when species richness of flea infracommunities (the flea assemblage of a host individual) and component communities increased. We conclude that the pattern of flea coexistence is related both to the structure of flea communities and affinities of host species.     

Roles of purinergic P2X receptors as pacemaking channels and modulators of calcium-mobilizing pathway in pituitary gonadotrophs

Anterior pituitary cells release ATP and express several subtypes of purinergic P2 receptors, but their biophysical properties and roles in spontaneous and receptor-controlled electrical activity have not been characterized. Here we focused on extracellular ATP actions in gonadotrophs from embryonic, neonatal, and adult rats. In cells from all three age groups, the Ca2+-mobilizing agonist GnRH induced oscillatory, hyperpolarizing, nondesensitizing, and slow deactivating currents. In contrast, ATP induced nonoscillatory, depolarizing, slowly desensitizing, and rapidly deactivating current, indicating that these cells express cation-conducting P2X channels but not Ca2+-mobilizing P2Y receptors. The amplitudes of P2X current response and the rates of receptor desensitization were dependent on ATP concentration. The biophysical and pharmacological properties of P2X currents were consistent with the expression of P2X2 subtype of channels in these cells. ATP-induced rapid depolarization of gonadotrophs lead to initiation of firing in quiescent cells, an increase in the frequency of action potentials in spontaneously active cells, and a transient stimulation of LH release. ATP also influenced GnRH-induced current and membrane potential oscillations and LH release in an extracellular Ca2+-dependent manner. These inositol 1,4,5-triphosphate-dependent oscillations were facilitated, slowed, or stopped, depending of ATP concentration, the time of its application, and the level of Ca2+ content in intracellular stores. These results indicate that, in gonadotrophs, P2X receptors could operate as pacemaking channels and modulators of GnRH-controlled electrical activity and secretion.     

Purine-induced expression of urate oxidase and enzyme activity in Atlantic salmon (Salmo salar). Cloning of urate oxidase liver cDNA from three teleost species and the African lungfish Protopterus annectens

The peroxisomal enzyme urate oxidase plays a pivotal role in the degradation of purines in both prokaryotes and eukaryotes. However, knowledge about the purine-induced expression of the encoding gene is lacking in vertebrates. These are the first published sequences of fish urate oxidase, which were predicted from PCR amplified liver cDNAs of Atlantic salmon (Salmo salar), Atlantic cod (Gadus morhua), Atlantic halibut (Hippoglossus hippoglossus) and African lungfish (Protopterus annectens). Sequence alignment of different vertebrate urate oxidases revealed amino acid substitutions of putative functional importance in the enzyme of chicken and lungfish. In the adult salmon, expression of urate oxidase mRNA predominated in liver, but was also identified in several nonhepatic organs including brain, but not in skeletal muscle and kidney. Juvenile salmon fed diets containing bacterial protein meal (BPM) rich in nucleic acids showed a significant increase in liver urate oxidase enzyme activity, and urea concentrations in plasma, muscle and liver were elevated. Whereas salmon fed the 18% BPM diet showed a nonsignificant increase in liver mRNA levels of urate oxidase compared with the 0% BPM-fed fish, no further increase in mRNA levels was found in fish receiving 36% BPM. The discrepancy between urate oxidase mRNA and enzyme activity was explained by rapid mRNA degradation or alternatively, post-translational control of the activity. Although variable plasma and liver levels of urate were detected, the substrate increased only slightly in 36% BPM-fed fish, indicating that the uricolytic pathway of Atlantic salmon is intimately regulated to handle high dietary purine levels.     

Intracellular calcium measurements as a method in studies on activity of purinergic P2X receptor channels

Extracellular nucleotide-activated purinergic receptors (P2XRs) are a family of cation-permeable channels that conduct small cations, including Ca²⁺, leading to the depolarization of cells and subsequent stimulation of voltage-gated Ca²⁺ influx in excitable cells. Here, we studied the spatiotemporal characteristics of intracellular Ca²⁺ signaling and its dependence on current signaling in excitable mouse immortalized gonadotropin-releasing hormone-secreting cells (GT1) and nonexcitable human embryonic kidney cells (HEK-293) cells expressing wild-type and chimeric P2XRs. In both cell types, P2XR generated depolarizing currents during the sustained ATP stimulation, which desensitized in order (from rapidly desensitizing to nondesensitizing): P2X₃R > P2X_2b + X₄R > P2X_2bR > P2X_2a + X₄R > P2X₄R > P2X_2aR > P2X₇R. HEK-293 cells were not suitable for studies on P2XR-mediated Ca²⁺ influx because of the coactivation of endogenously expressed Ca²⁺-mobilizing purinergic P2Y receptors. However, when expressed in GT1 cells, all wild-type and chimeric P2XRs responded to agonist binding with global Ca²⁺ signals, which desensitized in the same order as current signals but in a significantly slower manner. The global distribution of Ca²⁺ signals was present independently of the rate of current desensitization. The temporal characteristics of Ca²⁺ signals were not affected by voltage-gated Ca²⁺ influx and removal of extracellular sodium. Ca²⁺ signals reflected well the receptor-specific EC₅₀ values for ATP and the extracellular Zn²⁺ and pH sensitivities of P2XRs. These results indicate that intracellular Ca²⁺ measurements are useful for characterizing the pharmacological properties and messenger functions of P2XRs, as well as the kinetics of channel activity, when the host cells do not express other members of purinergic receptors. ATP-gated receptor channels; inward currents; intracellular calcium signals; desensitization-inactivation; voltage-gated calcium influx; localized and global calcium signals     

Leptin acts at the bovine adenohypophysis to enhance basal and gonadotropin-releasing hormone-mediated release of luteinizing hormone: differential effects are dependent upon nutritional history

Recombinant ovine leptin (oleptin) stimulates an acute increase in the secretion of LH in fasted, but not in normal-fed, cows through an augmentation of the magnitude of individual pulses of LH. Herein, we tested the hypothesis that this effect could be accounted for by functional changes at the adenohypophyseal (AP) level. Eleven ovariectomized, estradiol-implanted cows were assigned to one of two dietary groups: normal-fed (n = 6) and fasted (fasted for 72 h; n = 5). After the animals were killed, the adenohypophyses were collected and AP explants were perifused with Krebs-Ringer bicarbonate buffer (KRB) for a total of 6.5 h, including a 2-h treatment at 2.5 h with KRB or increasing doses of oleptin and a challenge at 4.5 h with 50 ng of GnRH. To test for effects of leptin at the hypothalamic level, explants encompassing the medial basal hypothalamus-infundibular complex (HYP) were incubated in KRB alone (control) or in KRB containing 1000 ng of oleptin. Basal release of LH from AP explants treated with leptin was greater (P < 0.02) than that from control-treated explants in fasted, but not in normal-fed, cows. To the contrary, leptin-treated explants from normal-fed, but not from fasted, cows released more (P < 0.001) LH in response to GnRH than control-treated tissues. Neither fasting nor leptin affected (P > 0.1) the secretion of GnRH from HYP explants. These observations support the hypothesis that leptin modulates the secretion of LH in mature cows, to a large extent, by its direct actions at the AP. Differential manifestations of these effects are dependent upon nutritional history.     

F(ab)'2-mediated neutralization of C3a and C5a anaphylatoxins: a novel effector function of immunoglobulins

High-dose intravenous immunoglobulin (IVIG) prevents immune damage by scavenging complement fragments C3b and C4b. We tested the hypothesis that exogenous immunoglobulin molecules also bind anaphylatoxins C3a and C5a, thereby neutralizing their pro-inflammatory effects. Single-cell calcium measurements in HMC-1 human mast cells showed that a rise in intracellular calcium caused by C3a and C5a was inhibited in a concentration-dependent manner by IVIG, F(ab)2-IVIG and irrelevant human monoclonal antibody. C3a- and C5a-induced thromboxane (TXB2) generation and histamine release from HMC-1 cells and whole-blood basophils were also suppressed by exogenous immunoglobulins. In a mouse model of asthma, immunoglobulin treatment reduced cellular migration to the lung. Lethal C5a-mediated circulatory collapse in pigs was prevented by pretreatment with F(ab)2-IVIG. Molecular modeling, surface plasmon resonance (SPR) and western blot analyses suggested a physical association between anaphylatoxins and the constant region of F(ab)2. This binding could interfere with the role of C3a and C5a in inflammation.     

Influence de l'intensité et de la longueur d'onde de la lumière sur le rendement des radicaux libres dans les semences de tomate et de carotte

Influence of light intensity and wavelength on the yield of free radicals in seeds of tomato and carrot.
Seeds of tomato ( Lycopersicum esculentum L. cv. Gribovsky-1180) and carrot ( Daucus carota L. cv. Santanet) were irradiated by continuous light of different wavelengths (white, blue, green, red, far-red and He-Ne laser) and different power density. Free radical output was measured using Electron Spin Resonance spectra (ESR). The dependence of the content of free radicals (FR) on the light power density was established. The yield of FR in the seeds irradiated with monochromatic light of the same power density was inversely proportional to the wavelength. The ESR spectra showed maximum absolute number of FR for white light. Successive irradiations gave cumulative effects on the output of free radicals in both directions: increase by the action of white light and decrease by the action of red and far-red light. The contact of irradiated seeds with steam decreased the amount of FR.     

Tracking Down Biotransformation to the Genetic Level: Identification of a Highly Flexible Glycosyltransferase from Saccharothrix espanaensis

Saccharothrix espanaensis is a member of the order Actinomycetales. The genome of the strain has been sequenced recently, revealing 106 glycosyltransferase genes. In this paper, we report the detection of a glycosyltransferase from Saccharothrix espanaensis which is able to rhamnosylate different phenolic compounds targeting different positions of the molecules. The gene encoding the flexible glycosyltransferase is not located close to a natural product biosynthetic gene cluster. Therefore, the native function of this enzyme might be not the biosynthesis of a secondary metabolite but the glycosylation of internal and external natural products as part of a defense mechanism.