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PubMed1989

NADH substrate inhibition and enhanced thermal stability of higher plant nitrate reductase immobilized via a monoclonal antibody.

The molecular basis of light-induced circadian rhythms of higher plant NADH:nitrate reductase (EC 1.6.6.1) activity is presently not understood. We have investigated whether the regulatory properties of NADH:nitrate reductase would allow oscillatory or related dynamic behavior. We report here the first example of NADH substrate inhibition of higher plant nitrate reductase in solution and for an immobilized enzyme using a novel immobilization technique with a monoclonal antibody. According to current theories on chemical oscillatory reactions, substrate inhibition will allow bistable and oscillatory behavior when the substrate-enzyme reaction is carried out in an open system. We also found a significant enhanced thermal stability of the immobilized enzyme.

Antibodies, Monoclonal↗
PubMed1990

Probing the role of proline in peptide hormones. NMR studies of bradykinin and related peptides.

The use of NMR methods to study conformational and dynamic aspects of the proline residues in the nonapeptide bradykinin is reviewed. NMR analyses involve considerations of bistable equilibria which include the cis/trans conformational heterogeneity of the imide bond, the cis'/trans' regions of conformational stability which characterize rotation about the C alpha-CO bond (dihedral angle psi), and the interconversion of the pyrrolidine ring of proline between puckered C gamma-endo and C gamma-exo conformations. These conformational features are all characterized by different kinetic behavior, are interdependent with peptide bond conformation, and exhibit sensitivity to amino acid substitutions. Thus, the substitution of Gly6 for Ser6 increases the fractional cis probability of the sixth peptide bond from 0.1 to 0.35. Substitutions of alpha-aminoisobutyric acid (AIB) residues for proline introduce conformational constraints analogous to those in cis' proline. Correlations of pyrrolidine ring conformation and dynamics with the cis/trans ratio of the imide bond have also been observed in model systems. Conformational and activity analyses of [AIB7]-bradykinin provided a stimulus for the development of the first bradykinin antagonist by Stewart and Vavrek (Vavrek RJ and Stewart JM, Competitive antagonists of bradykinin. Peptides 6: 161-164, 1985).

Aminoisobutyric Acids↗
PubMed1986

Effects of magnesium on fictive locomotion induced by activation of N-methyl-D-aspartate (NMDA) receptors in the lamprey spinal cord in vitro.

It has previously been demonstrated that an activation of N-methyl-D-aspartate (NMDA) receptors can induce fictive locomotion as well as tetrodotoxin (TTX)-resistant membrane potential oscillations in certain types of neurones in the in vitro preparation of the lamprey spinal cord. These oscillations in individual neurones depend on voltage-sensitive properties of NMDA-activated channels which are only manifested in the presence of Mg2+. To evaluate the role of these pacemaker-like oscillations in the generation of locomotion, the motor patterns induced by N-methyl-D,L-aspartate (NMA) before and after removal of Mg2+ were compared. It was found that the ventral root burst pattern of fictive locomotion was more irregular after removal of Mg2+, particularly at low burst rates. This suggests that the membrane properties underlying the NMDA-induced TTX-resistant membrane potential oscillations are of importance for the generation of a stable and regular locomotor activity in particular at low rates of fictive locomotion. When fictive locomotion was induced instead by an activation of kainate receptors a removal of Mg2+ had no effect on the motor pattern. The effects of the two K+-channel blockers, tetraethylammonium (TEA) and gallamine were also tested on NMA-induced fictive locomotion. Both compounds caused an increase in the burst frequency. The Mg2+-dependent NMDA-induced bistable membrane properties thus appear to be of importance for the operation of the network which generates the locomotor pattern.

Animals↗
PubMed1986

The induction and modification of voltage-sensitive responses in cat neocortical neurons by N-methyl-D-aspartate.

The actions of the excitatory amino acid, N-methyl-D-aspartate (NMDA), on layer V neurons of cat sensorimotor cortex were examined in an in vitro slice preparation using current clamp, single electrode voltage clamp (SEVC), and ionic substitution techniques. Low doses of NMDA evoked a slow depolarization with a net decrease of input conductance. Larger doses additionally evoked repetitive firing, rhythmic depolarization shifts (DSs), low-threshold calcium spikes (in the presence of TEA+) and bistable membrane potential behavior. Ionic substitution experiments suggested that entry of both Ca2+ and Na+ ions contributed to the NMDA responses. Attention was focused on the NMDA response with Ca2+ entry blocked. Examination by SEVC revealed that, in both normal cells and in the presence of several blocking agents, NMDA induced a highly voltage-dependent inward ionic current which could result in a region of negative slope conductance on the cell's current-voltage relation. The development of this current seems capable of accounting for all aspects of the observed response, including the DSs and low-threshold Ca2+ spikes. Substitution of TEA+ for most external Na+ (with Ca2+ entry blocked) largely eliminated the NMDA responses and corresponding ionic current. Our results in neocortical neurons are compared to those recently obtained in cultured murine neurons.

Animals↗
PubMed1993

Sustained activation of the triceps surae muscles produced by mechanical stimulation of the genital tract of the female cat.

In decerebrate cats, controlled mechanical stimulation of the perivulvar skin, the vaginal wall or the cervix uteri induced visible hind limb extension. Pressing on the cervix uteri produced the greater response. To quantify these responses, the EMG activity and the tension developed by the normally inserted triceps surae muscles were recorded. The activity induced in these muscles by stimulation of the genital canal outlasted the stimulus by many seconds or a few minutes. These effects disappeared after spinalization at the T12 level. We propose that stimulation of the vaginal canal in the female cat may induce bistability of triceps surae motoneurones.

Animals↗
PubMed1984

Domain structure of flagellin.

The chemotaxis of bacteria such as Salmonella and Escherichia coli involves smooth swimming punctuated by periods of tumbling. In smooth swimming the flagellar filaments are left-handed, in tumbling they are right-handed with a different wavelength. The filaments are constructed from a globular protein, flagellin, by a process of self-assembly. The existing models assume that the flagellin molecule is bistable and longitudinal rows of subunits take one of the two possible conformations. Such a model explains the observed different morphology of the flagellum. We have studied Salmonella and E. coli flagellins in polymeric and monomeric forms by scanning microcalorimetry and circular dichroism. We have inferred that a flagellin molecule consists of several domains, two of which are structured at physiological temperatures and are in the monomeric form, while the others acquire a regular form only in the process of polymerization. This phenomenon may be the basis of a process during which the flagellin molecule, fitting into the flagellum, acquires a conformation analogous to that of the neighbouring molecule in the longitudinal row.

Bacterial Proteins↗
PubMed1996

Spatially explicit models for the population dynamics of a species colonizing an island.

We construct reaction-diffusion models for the population dynamics of a species colonizing an island from a source population on a continent. We view the source population as inducing a density or flux of immigrants onto the island and interpret colonization as succeeding if the population on the island is predicted to persist even when immigration from the continent is stopped. To capture the observation that a sufficiently large population or density must be attained for colonization to succeed, we assume Allee (i.e., bistable) dynamics rather than logistic dynamics for the colonizing population. We consider the cases of colonization in both the absence and presence of a competitor. We use reaction-diffusion theory, especially comparison methods and sub- and supersolutions, to determine how parameters such as the distance from the continent to the island and the dispersal, birth and mortality rates, carrying capacity, and minimum viable population density of the colonizing species affect the outcome of the attempted colonization. In the case of colonization in the presence of a competitor we consider a number of scenarios involving different types and strengths of competition. Our analysis permits us to draw conclusions about the characteristics of a species that make it a good colonizer.

Animals↗
PubMed1983

Failure to find an absolute retinal limit of a putative short-range process in apparent motion.

Previous studies of apparent movement have concluded that the short-range process does not operate when stimulus displacements exceed 15-20' arc. In the present experiments, we studied a bistable apparent-movement display, one of whose perceptual organizations is mediated by short-range process. In both experiments it was found that the perceptual organization mediated by the short-range process could be made dominant at stimulus displacements well in excess of the proposed spatial limit, provided the stimulus elements were made larger. It is concluded that the spatial limit of the short-range process is a relative, not absolute, one. Current knowledge regarding the short-range process is reviewed, and an hypothesis regarding the functional utility of the short-range process in compensating for the effects of small eye tremors is advanced.

Adolescent↗
PubMed1985

Latency dependence of colour-based target vs nontarget discrimination by the saccadic system.

First saccade responses to sudden presentations of a target/nontarget stimulus consisting of green and red spots of light have been investigated. This paradigm, which avoids certain ambiguities present in earlier experiments with identical double stimuli, leads to remarkably similar conclusions. We found, again, that the saccadic system had short-latency compromising responses (averaging response mode) when the stimulus pair had a modest direction difference (delta phi). When delta phi was enlarged, first saccades were either directed near the green or the red spot (bistable response mode). In a second series of experiments different instructions, emphasizing either speed or accuracy of response, have been given to investigate the relation between saccade accuracy and latency. It appears that, independent of instruction, there is a fixed relation between saccade metrics and latency. The only way to avoid errors, such as averaging responses, is to delay the saccade. Hypothetical mechanisms underlying this relation are discussed against recent neurophysiological studies.

Color Perception↗
PubMed1988

Stimulus conditions that enhance anticipatory slow eye movements.

Anticipatory slow eye movements are predictive responses that occur prior to both ramp and step target motions. These low velocity eye movements are enhanced and can be studied in isolation by transient target disappearance before ramp motion onset. Slow eye velocities also decrease prior to the termination of target motion. In experiments using a bistable apparent motion stimulus, it was found that perceived motion is a stimulus for anticipatory slow eye movements. This relationship between motion perception and anticipatory slow eye movements can explain previously noted differences between these predictive movements and the predictive component of smooth pursuit.

Adult↗
PubMed1994

Simultaneity in the Ternus configuration: psychophysical data and a computer model.

The Ternus configuration is an apparent motion display which is typically described as being bistable; subjects usually describe seeing either element motion or group motion, depending upon temporal properties of the display. The results of an experiment are reported in which subjects are also permitted to report seeing four stationary display elements (simultaneity). It was found that simultaneity was produced when both frame durations and interstimulus intervals (ISIs) were brief. A weaker than expected effect of stimulus onset asynchrony (SOA) was revealed because this third type of judgment was obtained. Furthermore, statistical analyses indicated that SOA was not by itself the best predictor of judgment type. The interaction between duration and ISI was also an important predictor. This suggested that a complete account of the Ternus configuration requires two mechanisms: a visible persistence mechanism, governed by an SOA law, and a motion correspondence mechanism, governed by an ISI law. These two mechanisms were added to Dawson's (1991) [Psychological Review, 98, 569-603] autoassociative network for motion correspondence processing. The resulting model could generate each of the three interpretations of the Ternus configuration at appropriate combinations of frame duration and ISI.

Association↗
PubMed1995

A comparison of varieties of "second-order" motion.

Bistable apparent-movement displays were created using four different kinds of "second-order" stimuli in which figures were defined by binocular disparity, spatial phase shifts of periodic luminance distributions, relative motion, and texture-element orientation differences. For each display, characteristics of the local structure of the figures, backgrounds, or both were varied. For each experimental condition, the type of apparent movement seen as a function of interstimulus interval was measured, and it was found that the relationship between perceived apparent movement and interstimulus interval differed across the types of displays viewed. The results suggest that the transformations between first-order stimulus properties and second-order motion may be too complex to imply a single uniform class of second-order motion detectors. Alternative physiological accounts of the results are discussed.

Adolescent↗
PubMed1996

Perceptual stability and the selective adaptation of perceived and unperceived motion directions.

Adaptation was studied in a paradigm in which the adapting stimulus was a variably biased version of a bistable apparent motion stimulus, a motion quartet, and the post-adaptation test stimulus was a "neutral" motion quartet. Either horizontal or vertical motion was perceived, never both at the same time. When only one of these was perceived during the entire adaptation phase of a trial, and the perceived motion was highly stable, adaptation effects were greater for the perceived than the unperceived motion directions (i.e., adaptation was selective to the perceived motion). However, when the perceived motion during adaptation was relatively unstable (i.e., when the perceived motion was more likely to spontaneously change directions), similar levels of adaptation were obtained for perceived as well as unperceived, but possible motion directions. Thus, adaptation occurs prior to the determination of which of the competing motion directions will be perceived. The relationship between the stability of the adapting percept and the selectivity of adaptation is explained in terms of differences in the activation of mutually inhibitory horizontal and vertical motion detectors.

Adaptation, Ocular↗
PubMed1977

Three steady state situation in an open chemical reaction system I.

In an isothermal continuously stirred tank reactor (open chemical reaction system) fed by sulphuric acid solutions of bromate, bromide and cerium (III) bistability (three steady state situation) is experimentally observed. This remarkable behavior, based on the instability of one steady state, has important consequences for the understanding of excitability and biochemical control mechanisms. The mass-balance equations for the reactor and the chemical mechanism of the reaction are combined into a simple mathematical model. The behavior of the resulting nonlinear differential equations is examined analytically and by a graphical integration procedure (method of isoclines). Using realistic kinetic data, the model shows the same behavior as observed in the experiment.

Bromates↗
PubMed1983

Experimental study of synchronization phenomena under periodic light irradiation of a nonlinear chemical system.

A photosensitive chemical oscillating reaction, i.e., the Briggs-Rauscher (B.R.) reaction, exhibiting a wealth of nonlinear behavior, when performed in a continuous-flow stirred-tank reactor, and subjected to periodic light irradiation, is studied as an experimental example of entrainment phenomena observable in biological systems. The adaptation patterns under periodic light irradiation are elucidated by means of the response of the system to continuous and single-pulse light irradiation. It is shown that self-oscillating states, excitable steady states and bistable systems can exhibit the same types of synchronization patterns when submitted to periodic external forces with appropriate amplitude and time scale conditions.

Journal Article↗
PubMed1992

Steady-state properties of a model ternary substrate cycle: theoretical predictions.

Numerous ternary substrate cycles are metabolically operative in vivo. The relative concentrations of the interconverted substrates are generally correlated with different physiological states. These cycles often include reversible and/or substrate-inhibited enzymic steps. The switch between one steady state (metabolic state) and another may be the consequence of either the effect of an exogeneous metabolite or signal, or the alteration of a cycle internal parameter. The interpretation of results obtained with currently designed experiments on substrate cycles seldom take into account the very dynamic and regulatory properties inherent in the cyclic and often autocatalytic nature of the pathway. In the present report, the various dynamic properties of a model ternary substrate cycle, bounded by moiety conservation, are investigated. Three situations with increasing complexity are considered: (i) the three enzymes are michaelian and catalyse irreversible steps; (ii) one of the enzymic steps is reversible; and (iii) one step is subjected to a destabilizing factor, i.e. inhibition by excess of substrate. The behavior(s) of the whole cycle is mainly controlled by four parameters, that is, ST, the total concentration of the substrate pool, and the three enzyme maximal velocities, VMi (i = 1,2,3). As ST (= S1 + S2 + S3) is constant, the Si steady-state concentrations (stable or not) can be represented in barycentric coordinates in a triangle (simplex). This convenient representation allows us to predict the different states of the system when one enzyme maximal activity is varied. The steady-state concentration dependencies as a function of one or several parameters may be either monostable (possibility of zero-order ultrasensitivity) or bistable (with or without reversible transitions). The physiological and experimental relevances of these observations are emphasized.

Enzyme Stability↗
PubMed1994

An approach to the in vitro study of the UTP/UDPglucose/UDP moiety-conserved cycle.

The kinetic behavior of a moiety-conserved ternary cycle is tested experimentally. This system contains the enzymes UDPglucose pyrophosphorylase, glycogen synthase and nucleoside diphosphokinase, converting respectively UTP into UDPglucose, then into UDP and back to UTP in a cyclic manner. The UDPGlc P2ase and NDPK steps are made irreversible by addition of inorganic pyrophosphatase and phosphocreatine kinase, respectively. In order to predict both the evolution and the steady-state values of the various substrates, a model is derived, which takes into account the actual enzyme rate expressions and parameter values, as determined under our experimental conditions. In that model, the UTP, UDPglucose and UDP are taken as the variables, whereas the total concentration of the substrate pool and the four enzyme maximal activities are chosen as the control parameters. Depending upon the various parameter values, monostability, reversible bistability and irreversible transitions may theoretically occur. However, it turns out that some of these values for which multistability might occur, are not accessible experimentally. Under conditions of monostability, the evolutions of the three substrates as experimentally measured are shown to be in good qualitative and quantitative agreement with the model predictions. The relaxation times between two consecutive steady states when a parameter is varied, are shown to be long-lasting processes (several hours). That such an experimental ternary substrate cycle actually exhibits a low sensitivity to any perturbation, addresses the issue to knowing if the same property is likely to occur in vivo, or, in other words, do large moiety-conserved cycles act as metabolic buffers?

Animals↗
PubMedDate not supplied

Developmental course of general movements in early infancy. II. EMG correlates.

In order to study developmental changes in muscle co-ordination during the first postnatal months, simultaneous polymyographic recordings and video-recordings were made during spontaneous movements of 22 healthy infants, who were followed from birth onwards. During the first 2 months general movements (GM) change from movements with a so-called 'writhing' character, which have a tight appearance, a relatively slow speed and a limited amplitude, into GM with a 'fidgety' character, which consist of an ongoing flow of small, elegant movements. We hypothesized that this transformation would coincide with a change from a pattern of co-contraction of antagonistic muscle groups into a pattern of reciprocal activation. This was not the case, a pattern of co-activation of antagonistic muscle groups remained the prevailing pattern. With increasing age, we found shorter burst durations of phasic activity, an attenuation of burst amplitude and a decrease of tonic background activity. These changes were attributed to a reduction of the sensitivity of the motor units due to spinal and supraspinal reorganization. It is hypothesized that the so-called 'bistable' properties of motoneurones play a central role in the observed phenomena: in neonates motor units are apt at displaying sustained activity, at 2 months of age the threshold for reaching this maintained activity increases, resulting in a low level of excitation of motor units during spontaneous movements. In the third month rapid arm movements ('swipes' and 'swats') develop. The 'swats' are characterized by a consistent pattern of reciprocal activity of antagonistic (shoulder) muscles.

Electromyography↗