Period four oscillations in chlorophyll a fluorescence

The discovery of period four oscillations of the fluorescence yield under flashing light demonstrated that not only the redox state of the Photosystem II (PS II) electron acceptor Q_A, but also the oxygen evolving cycle (described by the S states) modulates the fluorescence yield of chlorophyll (Chl). The positive charges accumulated on the donor side of PS II act on the fluorescence yield (measured in the Q_A ⁻ state during a strong flash) through the concentration of the quencher P₆₈₀ ⁺, the oxidized form of PS II reaction center Chl a. However, the period four oscillations of the fluorescence yield detected 1 s after a strong flash (in the P₆₈₀Q_A state) have not yet been fully explained. This revised version was published online in June 2006 with corrections to the Cover Date.     

ATP-induced calcium oscillations and change of P2Y subtypes with culture conditions in HeLa cells

ATP, UTP, ADP and UDP induced intracellular Ca(2+) responses and oscillations in HeLa cells that sometimes lasted over 1 h. The response is due to the activation of P2Ys, G-protein coupled ATP receptors, because the oscillations persisted for several minutes even in Ca(2+)-free solution, and suramin and PPADS, antagonists of ATP receptors, partially inhibited the response. The potency of these nucleotides varied with the culture or cell conditions, i.e. UTP was generally most potent but in some cases UDP was more potent; responses to UDP were variable while those to ATP were constant. In addition, Ca(2+) responses to ATP and UDP were additive. These findings suggested the existence of two or more subtypes of P2Ys in HeLa cells. RT-PCR experiments revealed the existence of P2Y(2), P2Y(4) and P2Y(6). Recovery from starvation (culture in FBS-free medium overnight and re-addition of FBS) increased the responses to UTP and UDP but not to ATP, suggesting that the number or activity of P2Y(6) and/or P2Y(4) receptors may increase with cell proliferation in HeLa cells.     

Structured population on two patches: modeling dispersal and delay

We derive from the age-structured model a system of delay differential equations to describe the interaction of spatial dispersal (over two patches) and time delay (arising from the maturation period). Our model analysis shows that varying the immature death rate can alter the behavior of the homogeneous equilibria, leading to transient oscillations around an intermediate equilibrium and complicated dynamics (in the form of the coexistence of possibly stable synchronized periodic oscillations and unstable phase-locked oscillations) near the largest equilibrium.     

Current Oscillations Under Voltage-Clamp Conditions: An Interplay of Series Resistance and Negative Slope Conductance

Using the patch-clamp technique, we observed profound oscillations of the whole-vacuole outward current across the tonoplast of Mesembryanthemum crystallinum L. (common ice plant). These current oscillations showed a clear voltage dependence and appeared at membrane potentials more positive than 90–100 mV. This paper describes the oscillations in terms of two separate mechanisms. First, the Mesembryanthemum vacuolar membrane shows a negative slope conductance at membrane potentials more positive than 100–120 mV. The fact that the oscillations and the negative slope conductance show a similar threshold potential suggests that (part of) the same mechanism is involved in both phenomena. The second mechanism involved is the voltage drop across the series resistance. As a result, the potential actually experienced by the vacuolar membrane deviates from the command potential defined by the patch-clamp amplifier. This deviation depends in an Ohmic manner on the current magnitude. We suggest that the interplay of the negative slope conductance and the voltage drop across the series resistance can cause a positive feedback which is responsible for the current oscillations. Received: 30 April 1999/Revised: 9 September     

Frequency Control in Synchronized Networks of Inhibitory Neurons

We analyze the control of frequency for a synchronized inhibitory neuronal network. The analysis is done for a reduced membrane model with a biophysically based synaptic influence. We argue that such a reduced model can quantitatively capture the frequency behavior of a larger class of neuronal models. We show that in different parameter regimes, the network frequency depends in different ways on the intrinsic and synaptic time constants. Only in one portion of the parameter space, called phasic, is the network period proportional to the synaptic decay time. These results are discussed in connection with previous work of the authors, which showed that for mildly heterogeneous networks, the synchrony breaks down, but coherence is preserved much more for systems in the phasic regime than in the other regimes. These results imply that for mildly heterogeneous networks, the existence of a coherent rhythm implies a linear dependence of the network period on synaptic decay time and a much weaker dependence on the drive to the cells. We give experimental evidence for this conclusion.     

Synchronization and Oscillatory Dynamics in Heterogeneous, Mutually Inhibited Neurons

We study some mechanisms responsible for synchronous oscillations and loss of synchrony at physiologically relevant frequencies (10–200 Hz) in a network of heterogeneous inhibitory neurons. We focus on the factors that determine the level of synchrony and frequency of the network response, as well as the effects of mild heterogeneity on network dynamics. With mild heterogeneity, synchrony is never perfect and is relatively fragile. In addition, the effects of inhibition are more complex in mildly heterogeneous networks than in homogeneous ones. In the former, synchrony is broken in two distinct ways, depending on the ratio of the synaptic decay time to the period of repetitive action potentials (_s/T), where T can be determined either from the network or from a single, self-inhibiting neuron. With _s/T > 2, corresponding to large applied current, small synaptic strength or large synaptic decay time, the effects of inhibition are largely tonic and heterogeneous neurons spike relatively independently. With _s/T < 1, synchrony breaks when faster cells begin to suppress their less excitable neighbors; cells that fire remain nearly synchronous. We show numerically that the behavior of mildly heterogeneous networks can be related to the behavior of single, self-inhibiting cells, which can be studied analytically.     

Oscillations of the energy gap for the initial electron-transfer step in bacterial reaction centers

Oscillations in the electrostatic energy gap [V_elec(t)] for electron transfer from the primary electron donor (P) to the adjacent bacteriochlorophyll (B) in photosynthetic bacterial reaction centers are examined by molecular-dynamics simulations. Autocorrelation functions of V_elec in the reactant state (PB) include prominent oscillations with an energy of 17 cm^–1. This feature is much weaker if the trajectory is propagated in the product state P⁺B^–. The autocorrelation functions also include oscillations in the regions of 5, 80 and 390 cm^–1 in both states, and near 25 and 48 cm^–1 in P⁺B^–. The strong 17-cm^–1 oscillation could involve motions that modulate the distance between P and B, because a similar oscillation occurs in the direct electrostatic interactions between the electron carriers.     

COMT polymorphism modulates the resting‐state EEG alpha oscillatory response to acute nicotine in male non‐smokers

Performance improvements in cognitive tasks requiring executive functions are evident with nicotinic acetylcholine receptor (nAChR) agonists, and activation of the underlying neural circuitry supporting these cognitive effects is thought to involve dopamine neurotransmission. As individual difference in response to nicotine may be related to a functional polymorphism in the gene encoding catechol‐O‐methyltransferase (COMT), an enzyme that strongly influences cortical dopamine metabolism, this study examined the modulatory effects of the COMT Val158Met polymorphism on the neural response to acute nicotine as measured with resting‐state electroencephalographic (EEG) oscillations. In a sample of 62 healthy non‐smoking adult males, a single dose (6 mg) of nicotine gum administered in a randomized, double‐blind, placebo‐controlled design was shown to affect α oscillatory activity, increasing power of upper α oscillations in frontocentral regions of Met/Met homozygotes and in parietal/occipital regions of Val/Met heterozygotes. Peak α frequency was also found to be faster with nicotine (vs. placebo) treatment in Val/Met heterozygotes, who exhibited a slower α frequency compared to Val/Val homozygotes. The data tentatively suggest that interindividual differences in brain α oscillations and their response to nicotinic agonist treatment are influenced by genetic mechanisms involving COMT.     

肱动脉内皮依赖性舒张功能鉴别左室舒张功能假性正常化的价值

目的:研究冠心病患者左室舒张功能假性正常化与肱动脉内皮依赖性舒张功能的关系。方法:将75例行选择性冠状动脉造影的患者按冠状动脉病变程度分为单/双支病变组和三支病变组两组,选取48例健康志愿者作为对照组。检测左室舒张功能指标二尖瓣口血流频谱E峰、A峰、E/A比值,同时观察休息时肱动脉反应性充血后内径变化率。结果:单/双支病变组(第一组)E峰、E/A比值下降,肱动脉反应性充血后内径变化率低于正常对照组(P<0.05);三支病变组(第二组)E峰、E/A值无明显改变,肱动脉反应性充血后内径变化率明显低于对照组(P<0.01)。结论:肱动脉内皮依赖性舒张功能可作为鉴别冠心病左室舒张功能假性正常的指标。     

Oscillations in photosynthesis are initiated and supported by imbalances in the supply of ATP and NADPH to the Calvin cycle

Oscillations in the rate of photosynthesis of sunflower (Helianthus annuus L.) leaves were induced by subjecting leaves, whose photosynthetic apparatus had been activated, to a sudden transition from darkness or low light to high-intensity illumination, or by transfering them in the light from air to an atmosphere containing saturating CO₂. It was found that at the first maximum, light-and CO₂-saturated photosynthesis can be much faster than steady-state photosynthesis. Both Q_A in the reaction center of PS II and P₇₀₀ in the reaction center of PS I of the chloroplast electron-transport chain were more oxidized during the maxima of photosynthesis than during the minima. Maxima of P₇₀₀ oxidation slightly preceded maxima in photosynthesis. During a transition from low to high irradiance, the assimilatory force F_A, which was calculated from ratios of dihydroxyacetone phosphate to phosphoglycerate under the assumption that the reactions catalyzed by NADP-dependent glyceraldehydephosphate dehydrogenase, phosphoglycerate kinase and triosephosphate isomerase are close to equilibrium, oscillated in parallel with photosynthesis. However, only one of its components, the calculated phosphorylation potential (ATP)/(ADP)(P_i), paralleled photosynthesis, whereas calculated NADPH/NADP ratios exhibited antiparallel behaviour. When photosynthetic oscillations were initiated by a transition from low to high CO₂, the assimilatory force F_A declined, was very low at the first minimum of photosynthesis and increased as photosynthesis rose to its second maximum. The observations indicate that the minima in photosynthesis are caused by lack of ATP. This leads to overreduction of the electron-transport chain which is indicated by the reduction of P₇₀₀. During photosynthetic oscillations the chloroplast thylakoid system is unable to adjust the supply of ATP and NADPH rapidly to demand at the stoichiometric relationship required by the carbonreduction cycle.Abbreviations PGA 3-phosphoglycerate - DHAP dihydroxyacetone phosphate - P₇₀₀ electron-donor pigment in the reaction enter of PS I - Q_A quinone acceptor in the reaction center of PS II This work received support from the Estonian Academy of Sciences, the Bavarian Ministry of Science and Art and the Sonderforschungsbereich 251 of the University of Würzburg. We are grateful for criticism by D.A. Walker, Robert Hill Institute, University of Sheffield, U.K. and by Mark Stitt, Institute of Botany, University of Heidelberg, FRG.