Numerical solution of a nonlinear advance-delay-differential equation from nerve conduction theory

A functional differential equation which is nonlinear and involves forward and backward deviating arguments is solved numerically. The equation models conduction in a myelinated nerve axon in which the myelin completely insulates the membrane, so that the potential change jumps from node to node. The equation is of first order with boundary values given at t=±. The problem is approximated via a difference scheme which solves the problem on a finite interval by utilizing an asymptotic representation at the endpoints, cubic interpolation and iterative techniques to approximate the delays, and a continuation method to start the procedure. The procedure is tested on a class of problems which are solvable analytically to access the scheme's accuracy and stability, then applied to the problem that models propagation in a myelinated axon. The solution's dependence on various model parameters of physical interest is studied. This is the first numerical study of myelinated nerve conduction in which the advance and delay terms are treated explicitly.Supported in part by NSF Grant MCS8301724 and by a Biomedical Research Support Grant 2SO7RR0706618 from NIH     

Environmental implications of recombinant DNA technology

Applications of recombinant DNA technology are discussed as a backdrop for evaluation of the environmental impacts of this technology. Some of applications include using traditional biological techniques for specific purposes, including nitrogen fixation, microbial pesticides, and waste treatment. In these instances the final product lies along a continuum, beginning with an organism marginally performing its function, and ending with one that is highly specialised and very efficient in what it does. One may move along this continuum toward the ‘perfect’ microorganism by using traditional methodologies of mutagenesis and selection, recombinant DNA technology, or a combination of the two.     

Genotypical differences among graminaceous species in release of phytosiderophores and uptake of iron phytosiderophores

Graminaceous species can enhance iron (Fe) acquisition from sparingly soluble inorganic Fe(III) compounds by release of phytosiderophores (PS) which mobilize Fe(III) by chelation. In most graminaceous species Fe deficiency increases the rate of PS release from roots by a factor of 10–20, but in some species, for example sorghum, this increase is much less. The chemical nature of PS can differ between species and even cultivars.The various PS are similarly effective as the microbial siderophore Desferal (ferrioxamine B methane sulfonate) in mobilizing Fe(III) from a calcareous soil. Under the same conditions the synthetic chelator DTPA (diaethylenetriamine pentaacetic acid) is ineffective.The rate of Fe(III)PS uptake by roots of graminaceous species increases by a factor of about 5 under Fe deficiency. In contrast, uptake of Fe from both synthetic and microbial Fe(III) chelates is much lower and not affected by the Fe nutritional status of the plants. This indicates that in graminaceous species under Fe deficiency a specific uptake system for FePS is activated. In contrast, the specific uptake system for FePS is absent in dicots. In a given graminaceous species the uptake rates of the various FePS are similar, but vary between species by a factor of upto 3. In sorghum, despite the low rate of PS release, the rate of FePS uptake is particularly high.The results indicate that release of PS and subsequent uptake of FePS are under different genetic control. The high susceptibility of sorghum to Fe deficiency (lime-chlorosis) is most probably caused by low rates of PS release in the early seedling stage. Therefore in sorghum, and presumably other graminaceous species also, an increase in resistance to lime chlorosis could be best achieved by breeding for cultivars with high rates of PS release. In corresponding screening procedures attention should be paid to the effects of iron nutritional status and daytime on PS release as well as on rapid microbial degradation of PS.     

An assessment of macroinvertebrate functional feeding groups as water quality indicators in the Buffalo River, eastern Cape Province, South Africa

The aim of this paper was to investigate the potential for using functional feeding groups (FFGs) as indicators of water quality conditions in rivers, using the Buffalo River, South Africa, as a specific example. Multivariate classification and ordination techniques were used to investigate species and FFG distributions in relation to a number of physico-chemical variables at 16 sites from the headwaters to the estuary of the Buffalo River.Two-way indicator species analysis (TWINSPAN) of species composition ranked most of the sites sequentially down the river, irrespective of water quality conditions. Ordination of FFGs from a set of riffle samples collected in mid-late summer showed only weak relationships between FFG distribution and water quality changes, except where variables changed sequentially down the river (e.g. pH and temperature). Individual species responses to water quality gradients were examined for nine riffle-dwelling species representing diverse FFGs. Following correspondence analysis of a matrix of environmental variables and species frequencies, some species showed strong associations with defined ranges of some variables. In particular, Adenophlebia auriculata (Leptophlebiidae, Ephemeroptera) from the headwater sampling site, was associated with low pH and low temperature. Simulium damnosum occurred under conditions of high turbidity, while Afronurus harrisoni was found under high concentrations of potassium, ammonium and nitrite ions.We conclude that although there was a distinct headwaters fauna in the Buffalo River, and sequential downstream changes in species composition, most FFGs (apart from shredders) were represented down the whole length of the river. FFG classifications are therefore unlikely to provide useful indications of water quality conditions in the Buffalo River.Using a categorical approach to classifying water quality variables, and by applying correspondence analysis to the resulting matrix, we recognised nine species that could be used to define water quality. These indicator species can be used to define tolerance ranges of the fauna for water quality conditions in different parts of the Buffalo river.     

Cloning by functional complementation, and inactivation, of theSchizosaccharomyces pombe homologue of theSaccharomyces cerevisiae geneABC1

TheSaccharomyces cerevisiae geneABC1 is required for the correct functioning of thebc ₁ complex of the mitochondrial respiratory chain. By functional complementation of aS. cerevisiae abc1 ^– mutant, we have cloned aSchizosaccharomyces pombe cDNA, whose predicted product is 50% identical to the Abc1 protein. Significant homology is also observed with bacterial, nematode, and even human amino acid sequences of unknown function, suggesting that the Abc1 protein is conserved through evolution. The cloned cDNA corresponds to a singleS. pombe geneabc1Sp, located on chromosome II, expression of which is not regulated by the carbon source. Inactivation of theabc1Sp gene by homologous gene replacement causes a respiratory deficiency which is efficiently rescued by the expression of theS. cerevisiae ABC1 gene. The inactivated strain shows a drastic decrease in thebc ₁ complex activity, a decrease in cytochromeaa3 and a slow growth phenotype. To our knowledge, this is the first example of the inactivation of a respiratory gene inS. pombe. Our results highlight the fact thatS. pombe growth is highly dependent upon respiration, and thatS. pombe could represent a valuable model for studying nucleo-mitochondrial interactions in higher eukaryotes.     

Functional response of Neoseiulus barkeri Hughes on two-spotted spider mite (Acari: Tetranychidae)

Generalist predatory mites are the common phytoseiid fauna in many agroecosystems, but little attention has been paid to their potential as biological control agents. In this study, we determined the functional responses of adult females of the generalist predator Neoseiulus barkeri Hughes on eggs, larvae, and adults of the two-spotted spider mite, Tetranychus urticae Koch, in the laboratory. Predation experiments were conducted on pepper leaf discs over a 24 h period at 25±1°C, 70–80% RH and 16L:8D photoperiod. Prey densities ranged 5 to 80 eggs, or 5 to 40 larvae, or 1 to 8 female adults of T. urticae per disc. The predation rate of N. barkeri adult females on T. urticae eggs was the same as on its larvae, but the predation rate on adult females was much lower. The role of generalist predatory mites in integrated and biological control of greenhouse pests was discussed.     

Specific monitoring by PCR amplification and bioluminescence of firefly luciferase gene-tagged bacteria added to environmental samples

Abstract The firefly luciferase gene, luc , was demonstrated to hold promise as a specific marker for monitoring of genetically modified bacteria in the environment. PCR amplification and bioluminescence procedures were modified and compared for environmental monitoring of luc -tagged bacteria, using Escherichia coli as a model. The methods were used to track luc -tagged bacterial cells added to intact sediment core microcosms. Detection limits for the luc -tagged cells were the following, expressed as cells per 0.5 g of sediment: 10², by PCR amplification; 10³, by whole cell luminescence; and 10³−10⁴, by measurement of luminescence in cell extracts.     

Metabolic compartmentation and substrate channelling in muscle cells

The published experimental data and existing concepts of cellular regulation of respiration are analyzed. Conventional, simplified considerations of regulatory mechanism by cytoplasmic ADP according to Michaelis-Menten kinetics or by derived parameters such as phosphate potential etc. do not explain relationships between oxygen consumption, workload and metabolic state of the cell. On the other hand, there are abundant data in literature showing microheterogeneity of cytoplasmic space in muscle cells, in particular with respect to ATP (and ADP) due to the structural organization of cell interior, existence of multienzyme complexes and structured water phase. Also very recent experimental data show that the intracellular diffusion of ADP is retarded in cardiomyocytes because of very low permeability of the mitochondrial outer membrane for adenine nucleotidesin vivo. Most probably, permeability of the outer mitochondrial membrane porin channels is controlled in the cellsin vivo by some intracellular factors which may be connected to cytoskeleton and lost during mitochondrial isolation. All these numerous data show convincingly that cellular metabolism cannot be understood if cell interior is considered as homogenous solution, and it is necessary to use the theories of organized metabolic systems and substrate-product channelling in multienzyme systems to understand metabolic regulation of respiration. One of these systems is the creatine kinase system, which channels high energy phosphates from mitochondria to sites of energy utilization. It is proposed that in muscle cells feed-back signal between contraction and mitochondrial respiration may be conducted by metabolic wave (propagation of oscillations of local concentration of ADP and creatine) through cytoplasmic equilibrium creatine and adenylate kinases and is amplified by coupled creatine kinase reaction in mitochondria. Mitochondrial creatine kinase has experimentally been shown to be a powerful amplifier of regulatory action of weak ADP fluxes due to its coupling to adenine nucleotide translocase. This phenomenon is also carefully analyzed.It is easier to explain biochemistry in terms of transport than it is to explain transport in terms of biochemistry. P. Mitchell The Ninth Sir Hans Krebs Lecture, Dresden, July 2, 1978.