Analysis of the peak-to-peak ratio of extracellular potentials in the proximity of excitable fibres

In a previous work we studied the ratio between the amplitudes of the second and first phases (which we call PPR, after peak-to-peak ratio) of the single fibre action potential (SFAP) for a collection of fibrillation potentials (FPs) extracted from two pathological muscles. These FPs showed a wider PPR range than the Dimitrov–Dimitrova (D–D) convolutional model could provide. We proposed a modification of the D–D intracellular action potential (IAP) in order to obtain a range of PPRs comparable to that observed in our FPs. This paper extends that study to a large number of SFAPs extracted from the tibialis anterior muscle of normal subjects. The estimation of the average PPR range of non-diseased muscles in non-fatigued conditions is important since it can be used as a reference to establish a comparison with PPR ranges from muscles suffering some disorder or from fibres that are fatigued. Other aspects of the PPR, as its sensitivity with volume conductor parameters or to what extent changes in the SFAP PPR reflects changes in IAP spatial profile are also examined. We found that the PPR of experimental SFAPs ranges from 0.3 to 2.5 in all subjects and that all PPR histograms contain a well-defined single peak around the PPR value 1.0.     

Identification procedure in a model of single fibre action potential – Part II: Global approach and experimental results

The present paper describes a global procedure for estimating all the synthesis parameters that generate a single fibre action potential (SFAP) in the Dimitrov–Dimitrova (D–D) convolutional model. We call this inverse problem Identification Procedure, and it is presented in two parts, this paper being the second. The procedure incorporates the candidate pair (CP) method developed in Part I, which provides the values of radial distance r and fibre diameter d of the simulated SFAP that best matches a potential under study. The CP-method required prior knowledge of all the excitation parameters. However, since the Identification Procedure makes no assumption about the excitation, multiple combinations of the synthesis parameters result in very similar SFAPs whose shape is close the signal under study. Analysis of the possible combinations reveals that r and d can be modelled as two jointly Gaussian random variables. The interest of the Identification Procedure is that, for a certain SFAP, it provides estimates of r and d, along with estimates of different parameters that determine the IAP waveform. Moreover, the procedure is able to determine the degree of error that accompanies the estimation of r and d.     

Effects of changes in the shape of the intracellular action potential on the peak-to-peak ratio of single muscle fibre potentials

In situ recording of the intracellular action potential (IAP) of human muscle fibres is not yet feasible, and consequently, knowledge about certain IAP characteristics of these IAPs is still limited. The ratio between the amplitudes of the second and first phases (the so-called peak-to-peak ratio, PPR) of a single fibre action potential (SFAP) is known to be closely related to the IAP profile. The PPR of experimentally recorded SFAPs has been found to be largely independent of changes in the fibre-to-electrode (radial) distance. The main goal of this paper is to analyze the effect of changes in different aspects of the IAP spike on the relationship between PPR and radial distance. Based on this analysis, we hypothesize about the characteristics of IAPs obtained experimentally. It was found that the sensitivity of the SFAP PPR to changes in radial distance is essentially governed by the duration of the IAP spike. Assuming that, for mammals, the duration of the IAP rising phase lies within the range 0.2-0.4 ms, we tentatively suggest that the duration of the IAP spike should be over approximately 0.75 ms, with the shape of the spike strongly asymmetric. These IAP characteristics broadly coincide with those observed in mammal IAPs.     

The morphology of single muscle fibre potentials – Part I: Simulation study of the distortion introduced by the distant-interfering potentials

Some morphologic aspects of human single fibre action potentials (SFAPs) are not sufficiently well-known. This uncertainty especially concerns the declining negative phase and the final positive phase (third phase) of SFAPs, as these parts are significantly affected by distant electrical activity. The incomplete characterisation of the SFAP shape is also explained by the limited knowledge of human intracellular action potentials (IAPs). The objectives of this study are to assess the morphologic features of human SFAPs and to derive information about the characteristics of human IAPs. To achieve this, the study has been divided into two parts. The present paper, Part I, aims to analyse the changes in the SFAP time-course introduced by distant-interfering potentials and to evaluate how these changes depend on the spike duration of the corresponding IAP. It was found that, for fibre-to-electrode distances shorter than about 0.2 mm, SFAPs generated by short-spike IAPs have a declining negative phase with a steep approximately constant slope that is largely unaffected by the potentials from distant fibres. For the same distances, SFAPs resulting from wide-spike IAPs have a declining negative phase with a slow return towards the baseline that is highly sensitive to distant-interfering potentials. The third phase of an SFAP is considerably distorted by distant potentials irrespective of the spike duration of the IAP.     

Identification Procedure in a model of single fibre action potential – Part I: Estimation of fibre diameter and radial distance

The Dimitrov–Dimitrova (D–D) model generates a single fibre action potential (SFAP) as the convolution of an excitation function and a filter impulse function. We propose a method to estimate the parameters involved in these functions from a SFAP waveform (inverse problem) and call it Identification Procedure. The Identification Procedure comprises two parts. The present paper, Part I, is centred on the estimation of the radial distance, r, and the fibre diameter, d, of a given SFAP. To this end we develop a technique which we call the candidate pair method (CP-method), and we test it on fibrillation potentials (FPs) as experimental data. We found that the D–D model cannot synthesize all the SFAP waveforms observed in the experimental recordings, but in the cases where it can, the CP-method then provides the values of r and d that are more likely to have synthesized the SFAP. Having a method that provides information about the fibre diameter straight from a SFAP waveform is very desirable as this parameter has clinical and physiological relevance. Moreover, the CP-method plays a major role in the Identification Procedure that is carried out in Part II.     

The morphology of single muscle fibre potentials – Part II: Experimental findings

Studies dealing with single fibre action potentials (SFAPs) have been more interested in obtaining quantitative data of certain parameters of the SFAP waveform than in the analysis of its morphologic features. The characterization of the SFAP morphology is highly valuable as it will allow to obtain information about in vivo intracellular action potentials (IAPs). However, the SFAP final portion is highly sensitive to distant electrical activity, as shown in Part I of this study. The present paper, Part II, is aimed at analysing the morphologies found in human SFAPs and deriving data of the associated IAPs. It was found that, for most SFAPs (97%), the declining negative phase starts decreasing steeply up to a certain point (slope-discontinuity point), from which it returns more slowly towards the baseline. This return may be further slowed down due to the contamination from distant potentials, but the slope-discontinuity point seems to be an intrinsic feature of human SFAPs. The third phase of SFAPs was, either absent (65%), or of rather small amplitude and prolonged duration. The slope-discontinuity point was apparent for the SFAPs of larger amplitude and vanished gradually as the SFAPs got smaller. Several lines of evidence strongly suggest that the spike duration of human IAPs should be about 1.0 ms.     

An analysis of extracellular single muscle fibre action potential field —Modelling results

The radial variability of extracellular single muscle fibre action potential (SFAP) field was studied on the basis of an SFAP theoretical model originally constructed by Rosenfalck (1969). Amplitude and time parameters of the SFAP as a function of the radial distance r10 mm from the fibre were described by simple mathematical expressions. The results obtained were compared with existing experimental data from different authors and discussed in detail.     

Action of extremely low frequency electric fields on the cytosolic calcium concentration of differentiated HL-60 cells: Nonactivated cells

The effect of sinusoidal electric fields on the cytosolic free [Ca²⁺]_i concentration in differentiated HL-60 cells was measured. The calcium concentration was measured in a fluorescence spectrometer using the fluorescence sample fluo-3. In the fluorescence spectrometer two samples can be measured simultaneously, one as the sham-exposed control and the other as the field-exposed sample. The effects of an external field, applied using two capacitor plates outside the cuvettes, and a field applied directly to the medium, using two platinum electrodes inside the cuvettes, were measured at selected frequencies between 0 and 100 Hz and field strengths from 1 to 2000 V_pp/m (external field) and from 0.1 to 1000 V_pp/m (in medium). No significant effects of the fields on the cytosolic free [Ca²⁺]_i concentration in HL-60 cells have been observed at the measured frequencies and field strengths. Bioelectromagnetics 19:32–40, 1998. © 1998 Wiley-Liss, Inc.     

Automatic VMAT planning for post-operative prostate cancer cases using particle swarm optimization: A proof of concept study

ObjectiveTo investigate the potential of Particle Swarm Optimization (PSO) for fully automatic VMAT radiotherapy (RT) treatment planning.Material and MethodsIn PSO a solution space of planning constraints is searched for the best possible RT plan in an iterative, statistical method, optimizing a population of candidate solutions. To identify the best candidate solution and for final evaluation a plan quality score (PQS), based on dose volume histogram (DVH) parameters, was introduced.Automatic PSO-based RT planning was used for N = 10 postoperative prostate cancer cases, retrospectively taken from our clinical database, with a prescribed dose of EUD = 66 Gy in addition to two constraints for rectum and one for bladder. Resulting PSO-based plans were compared dosimetrically to manually generated VMAT plans.ResultsPSO successfully proposed treatment plans comparable to manually optimized ones in 9/10 cases. The median (range) PTV EUD was 65.4 Gy (64.7–66.0) for manual and 65.3 Gy (62.5–65.5) for PSO plans, respectively. However PSO plans achieved significantly lower doses in rectum D_2% 67.0 Gy (66.5–67.5) vs. 66.1 Gy (64.7–66.5, p = 0.016). All other evaluated parameters (PTV D_98% and D_2%, rectum V_40Gy and V_60Gy, bladder D_2% and V_60Gy) were comparable in both plans. Manual plans had lower PQS compared to PSO plans with −0.82 (−16.43–1.08) vs. 0.91 (−5.98–6.25).ConclusionPSO allows for fully automatic generation of VMAT plans with plan quality comparable to manually optimized plans. However, before clinical implementation further research is needed concerning further adaptation of PSO-specific parameters and the refinement of the PQS.     

The Source Parameters of Echolocation Clicks from Captive and Free-Ranging Yangtze Finless Porpoises (Neophocaena asiaeorientalis asiaeorientalis)

The clicks of Yangtze finless porpoises (Neophocaena asiaeorientalis asiaeorientalis) from 7 individuals in the tank of Baiji aquarium, 2 individuals in a netted pen at Shishou Tian-e-zhou Reserve and 4 free-ranging individuals at Tianxingzhou were recorded using a broadband digital recording system with four element hydrophones. The peak-to-peak apparent source level (ASL__pp) of clicks from individuals at the Baiji aquarium was 167 dB re 1 μPa with mean center frequency of 133 kHz, -3dB bandwidth of 18 kHz and -10 dB duration of 58 μs. The ASL__pp of clicks from individuals at the Shishou Tian-e-zhou Reserve was 180 dB re 1 μPa with mean center frequency of 128 kHz, -3dB bandwidth of 20 kHz and -10 dB duration of 39 μs. The ASL__pp of clicks from individuals at Tianxingzhou was 176 dB re 1 μPa with mean center frequency of 129 kHz, -3dB bandwidth of 15 kHz and -10 dB duration of 48 μs. Differences between the source parameters of clicks among the three groups of finless porpoises suggest these animals adapt to their echolocation signals depending on their surroundings.     

Kinetics of net nitrification associated with soil aggregates under conventional and no-tillage in a subtropical rice soil

Tillage effects on soil nitrification kinetics at the aggregate scale were studied for a subtropical rice soil. Soil samples were separated into large aggregates (>2.0 mm), macro-aggregates (2.0–0.25 mm), micro-aggregates (0.25–0.053 mm) and silt + clay fractions (<0.053 mm) by wet-sieving. The net nitrification process was simulated by a zero- and first kinetics model. Conventional tillage (CT) increased the proportion of the silt + clay fraction by 60% and decreased large-aggregates by 35% compared to ridge with no-till (RNT). Regression analysis showed that the time-dependent kinetics of net nitrification were best fitted by a zero-order model for the large-aggregates and silt + clay fraction but a first-order kinetic model for macro- and microaggregates and whole soil, regardless of tillage regime. Both potential nitrification rates (V _p ) and net nitrification rates (V _a ) were higher for macroaggregates than microaggregates. The potential nitrification (N _p ) for whole soil under RNT was 38.7% higher than CT. The V _p and V _a for whole soil was 88.5% and 64.7% higher under RNT than CT, respectively. Although nitrification was stimulated under RNT, the kinetics model of nitrification was not affected by tillage. This inferred that the interaction between substrates and enzymes involved in nitrification associated with aggregates was not altered by tillage. For this soil, nitrifying microorganisms were mainly associated with macro- and microaggregates rather than large-aggregates and silt + clay fractions.     

Increasing the synthesis/hydrolysis ratio of aminoacylase 1 by site-directed mutagenesis

Aminoacylase-1 from pig kidney (pAcy1) catalyzes the highly stereoselective acylation of amino acids, a useful conversion for the preparation of optically pure N-acyl-l-amino acids. The kinetic of this thermodynamically controlled conversion is determined by maximal velocities for synthesis (V_mS) and hydrolysis (V_mH) of the N-acyl-l-amino acid. To investigate which parameter affects maximal velocities, we focused on the proton acceptor potential of the catalytic base, E146, and studied the influence of the active site architecture on its contribution to the pKa of residue E146. The modeled structure of pAcy1 identified residue D346 as having the strongest impact on the electrostatic features of the catalytic base. Substitutions of D346 generally decreased enzymatic activities but also altered both the pH-dependency of hydrolytic activity and the V_mS/V_mH ratio of pAcy1. A reduced theoretical pKa value and a lowered experimental pH optimum of hydrolytic rates for the D346A mutant were associated with a 9-fold increase in V_mS/V_mH. This supports the importance of electrostatic contributions of D346 to the acid-base properties of E146 and demonstrates for the first time the possibility of engineering the V_mS/V_mH ratio of pAcy1.     

Monte Carlo simulations of structure and entanglements in polymer melts

Atomistic models of short chain branched (SCB) polyethylene melts have been equilibrated at 450 K using a connectivity altering Monte Carlo method. Quantities related to the chain dimensions and entanglements have been determined. The simulated tube diameters, 〈a_pp〉, of SCB melts are found to scale with the backbone weight fraction, ?, as 〈a_pp〉～?^? 0.46, close to the scaling predicted by the binary contact model, 〈a_pp〉～?^? 0.5. Similar relationships are observed experimentally for polymer solutions, and reproduced by the present methods.     

Evidence for the role of a guanine nucleotide-binding regulatory protein in the zona pellucida-induced mouse sperm acrosome reaction

Recently, it has been demonstrated that mouse sperm contain a protein with properties similar to the inhibitory guanine nucleotide-binding regulatory protein, G_i (Kopf, G. S., Woolkalis, M. J., and Gerton, G. L. 1986. J. Biol. Chem., 261, 7327–7331). Since sperm-zona pellucida interaction represents a specialized form of intercellular communication and signal transduction we examined the role of the mouse sperm G_i-like protein in the zona pellucida-induced acrosome reaction using mechanically isolated, structurally intact zonae pellucidae. Sperm capacitated for 90 min in the presence of increasing concentrations of islet-activating protein (IAP) bind to the zona pellucida to a similar extent as control sperm incubated in the absence of this toxin. The zona pellucida-induced acrosome reaction, however, is inhibited in a concentration dependent manner by IAP, with half-maximal effects at 0.1-1.0 ng/ml IAP. IAP does not affect the ability of the sperm to become capacitated, but inhibits the cells from progressing into an intermediate stage prior to the completion of the acrosome reaction. When sperm are capacitated in the presence of 100 μM guanosine-5′-O-(3-thiotriphosphate) for 60 min prior to the addition of IAP during the final 30 min, the IAP-induced inhibition of the zona pellucida-induced acrosome reaction is abolished; capacitation in the presence of 100 μM guanosine-5′-O-(2-thiodiphosphate) does not abolish the inhibitory effects of IAP. The target of the IAP effect on intact sperm appears to be at the level of the G_i-like protein since IAP-catalyzed ³²P-ADP-ribosylation of the M_r = 41,000 substrate in detergent extracts of sperm is reduced when intact sperm are preincubated with IAP during capacitation. These data suggest that the mouse sperm G_i-like protein plays an intermediary role in the zona pellucida-induced acrosome reaction.     

Evaluation of the criteria to identify single-fibre potentials in human muscle fibres

The criterion normally used to identify a potential generated by a single muscle fibre (SFAP) is that it must have identical shape at consecutive discharges. Technical problems accompanying the recording of single-fibre electromyographic (SFEMG) potentials introduce certain variability in the shape of these potentials, thereby compromising the ability to detect pure SFAPs. This study aims to determine the conditions necessary for two fibres to generate a compound potential that fulfils the single-fibre criterion. This has been done by analysing the alterations in the waveform of compound spikes formed by the summation of two SFAPs whose relative weight in the composite potential can differ considerably. Several factors responsible for this shape variability, with a geometrical, physiological or accidental origin, have been included in our study. It has been shown that a distant interfering component will be hardly detected in the composite potential if it is smaller than approximately 15?% of the main component. For this interfering component to generate a notch in the rising phase of the compound potential, it must be greater than about 30?% of the main component. A compound potential will fulfil the single-fibre criterion if the time dispersion between the individual components is less than 80-120?μs. These results permit the estimation of the amplitude of interfering potentials so they could be useful in fibre density studies. The article also emphasises the inherent variability of SFEMG potentials and the impact of this variability on jitter estimation.     

Model study of muscle spindles subjected to static fusimotor activation

An earlier proposed mechanical model of the de-efferented muscle spindle endings has been developed further in order to study whether the functional effects of activating the static fusimotor fibres may be accounted for by relatively simple mechanical events in the intrafusal muscle fibres. Particular attention has been paid to important controversial problems related to the processes involved in the fusimotor activation of the mammalian muscle spindles. In order to develop an adequate model, preliminary simulation studies of the functional effects of various reasonable modifications of the original “de-efferented” model were first carried out by means of a convenient direct electronic analogue of the mechanical system. The following results apparently reflect pertinent details about the peripheral receptor mechanisms associated with the activation of the static fusimotor fibres:

1. The γ _s-activation is most adequately accounted for by mechanical events occurring in the nuclear chain fibres.
2. Any uniform chain fibre contraction fails to account for the effects of the γ _s-activation.
3. The simulation of a local chain fibre contraction could apparently account for all significant effects of the γ _s-activation.
4. A reasonable increase of the position sensitivity and the vibration sensitivity of both the primary and the secondary endings could be accounted for by increasing the stiffness of about 1/4–1/2 of the length of the simulated nuclear chain fibre by a factor of about 2–10.
5. The powerful modulation of the background discharge of the receptor endings that is associated with the γ _s-activation could be accounted for by an independent contraction force related to the intensity of the static fusimotor activity.
6. The model predicts a decrease of the rate sensitivity of the primary endings (group Ia) and a slight increase of the (moderate) rate sensitivity of the secondary endings.
7. By an adequate selection of the parameters, the simulated γ _s-activation showed a negligible influence on the absolute level of the dynamic phase of the ramp response of the simulated primary endings, in spite of a considerable increase of the steady state (background) discharge before and after the termination of a simulated stretch.

     

The use and misuse of V c,max in Earth System Models

Earth System Models (ESMs) aim to project global change. Central to this aim is the need to accurately model global carbon fluxes. Photosynthetic carbon dioxide assimilation by the terrestrial biosphere is the largest of these fluxes, and in many ESMs is represented by the Farquhar, von Caemmerer and Berry (FvCB) model of photosynthesis. The maximum rate of carboxylation by the enzyme Rubisco, commonly termed V _c,max, is a key parameter in the FvCB model. This study investigated the derivation of the values of V _c,max used to represent different plant functional types (PFTs) in ESMs. Four methods for estimating V _c,max were identified; (1) an empirical or (2) mechanistic relationship was used to relate V _c,max to leaf N content, (3) V _c,max was estimated using an approach based on the optimization of photosynthesis and respiration or (4) calibration of a user-defined V _c,max to obtain a target model output. Despite representing the same PFTs, the land model components of ESMs were parameterized with a wide range of values for V _c,max (?46 to +77 % of the PFT mean). In many cases, parameterization was based on limited data sets and poorly defined coefficients that were used to adjust model parameters and set PFT-specific values for V _c,max. Examination of the models that linked leaf N mechanistically to V _c,max identified potential changes to fixed parameters that collectively would decrease V _c,max by 31 % in C₃ plants and 11 % in C₄ plants. Plant trait data bases are now available that offer an excellent opportunity for models to update PFT-specific parameters used to estimate V _c,max. However, data for parameterizing some PFTs, particularly those in the Tropics and the Arctic are either highly variable or largely absent.     

The roles of αV integrins in lens EMT and posterior capsular opacification

Posterior capsular opacification (PCO) is the major complication arising after cataract treatment. PCO occurs when the lens epithelial cells remaining following surgery (LCs) undergo a wound healing response producing a mixture of α‐smooth muscle actin (α‐SMA)‐expressing myofibroblasts and lens fibre cells, which impair vision. Prior investigations have proposed that integrins play a central role in PCO and we found that, in a mouse fibre cell removal model of cataract surgery, expression of α_V integrin and its interacting β‐subunits β1, β5, β6, β8 are up‐regulated concomitant with α‐SMA in LCs following surgery. To test the hypothesis that α_V integrins are functionally important in PCO pathogenesis, we created mice lacking the α_V integrin subunit in all lens cells. Adult lenses lacking α_V integrins are transparent and show no apparent morphological abnormalities when compared with control lenses. However, following surgical fibre cell removal, the LCs in control eyes increased cell proliferation, and up‐regulated the expression of α‐SMA, β1‐integrin, fibronectin, tenascin‐C and transforming growth factor beta (TGF‐β)–induced protein within 48 hrs, while LCs lacking α_V integrins exhibited much less cell proliferation and little to no up‐regulation of any of the fibrotic markers tested. This effect appears to result from the known roles of α_V integrins in latent TGF‐β activation as α_V integrin null lenses do not exhibit detectable SMAD‐3 phosphorylation after surgery, while this occurs robustly in control lenses, consistent with the known roles for TGF‐β in fibrotic PCO. These data suggest that therapeutics antagonizing α_V integrin function could be used to prevent fibrotic PCO following cataract surgery.     

Global Validation of a Process-Based Model on Vegetation Gross Primary Production Using Eddy Covariance Observations

Gross Primary Production (GPP) is the largest flux in the global carbon cycle. However, large uncertainties in current global estimations persist. In this study, we examined the performance of a process-based model (Integrated BIosphere Simulator, IBIS) at 62 eddy covariance sites around the world. Our results indicated that the IBIS model explained 60% of the observed variation in daily GPP at all validation sites. Comparison with a satellite-based vegetation model (Eddy Covariance-Light Use Efficiency, EC-LUE) revealed that the IBIS simulations yielded comparable GPP results as the EC-LUE model. Global mean GPP estimated by the IBIS model was 107.50±1.37 Pg C year⁻¹ (mean value ± standard deviation) across the vegetated area for the period 2000–2006, consistent with the results of the EC-LUE model (109.39±1.48 Pg C year⁻¹). To evaluate the uncertainty introduced by the parameter V_cmax, which represents the maximum photosynthetic capacity, we inversed V_cmax using Markov Chain-Monte Carlo (MCMC) procedures. Using the inversed V_cmax values, the simulated global GPP increased by 16.5 Pg C year⁻¹, indicating that IBIS model is sensitive to V_cmax, and large uncertainty exists in model parameterization.