Coordination of pancreatic islet rhythmic activity by delayed negative feedback

Secretion of insulin from the pancreas is pulsatile, driven by intrinsic oscillations within individual islets of Langerhans. The secretions are coordinated among the many islets distributed throughout the pancreas producing a synchronized rhythm in vivo that is essential for maintaining normal glucose levels. One hypothesized mechanism for the coordination of islet activity is negative feedback, whereby sequestration of glucose in response to elevated insulin leads to a reduction in the blood glucose level that is sensed by the islet population. This global signal of glucose then coordinates the individual islets. In this study, we tested how this coordination mechanism is affected by time delays in the negative feedback, using a microfluidic system to monitor Ca2+ levels in a small population of islets and implementing glucose control through a negative feedback system. We found that islet synchronization occurs even with time delays in the feedback of up to 7 min. We also found that a second, slower closed-loop oscillation period is produced during delayed feedback in which islet oscillations are clustered into episodes. The period of this second oscillatory mode increases with the time delay and appears to be a second stable behavior that coexists with the faster synchronized oscillation. The general conclusion is that islet coordination through negative feedback is a viable means of islet coordination that is robust to delays in the timing of the feedback, and could complement other potential coordination mechanisms such as entrainment by pancreatic ganglia.NEW & NOTEWORTHY Insulin secretion from islets of Langerhans is rhythmic, and these rhythms are coordinated to produce oscillatory plasma insulin levels. Using a combination of microfluidics and computational modeling, we demonstrate that coordination can occur through negative feedback of the type provided by the liver, even if that feedback is delayed by several minutes. We also demonstrate that a second, slower, mode of oscillations can occur when feedback is delayed where faster oscillations are grouped into episodes.

Pulsatile Basal Insulin Secretion Is Driven by Glycolytic Oscillations

In fasted and fed states, blood insulin levels are oscillatory. While this phenomenon is well studied at high glucose levels, comparatively little is known about its origin under basal conditions. We propose a possible mechanism for basal insulin oscillations based on oscillations in glycolysis, demonstrated using an established mathematical model. At high glucose, this is superseded by a calcium-dependent mechanism.

Melt growth and properties of bulk BaSnO3 single crystals

We present the first-time growth of bulk BaSnO₃ single crystals from the melt by direct solidification, their basic electrical and optical properties as well as their structural quality. Our measurement of the melting point (MP) of BaSnO₃ amounts to 1855 °C  ±  25 K. At this temperature an intensive decomposition and non-stoichiometric evaporation takes place as the partial pressure of SnO(g) is about 90 times higher than that of BaO(g). X ray powder diffraction identified only the BaSnO₃ perovskite phase, while narrow rocking curves having a full width at half maximum of 26 arcsec and etch pit densities below 10⁶ cm^-2 confirm a high degree of structural perfection of the single crystals. In this respect they surpass the structural properties of those single crystals that were reported in the literature. The electrical conductivity of nominally undoped crystals depends on the growth conditions and ranges from insulating to medium n-type conductivity. After post-growth annealing in an oxidizing atmosphere undoped crystals are generally insulating. Doping the crystals with lanthanum during growth results in a high n-type conductivity. For a La doping concentration of 0.123 wt.% we measured an electron concentration of 3.3  ×  10¹⁹ cm^-3 and an electron mobility of 219 cm² V^-1 s^-1. Based on optical absorption measurements we determined an energy of 3.17  ±  0.04 eV at 5 K and of 2.99  ±  0.04 eV at 297 K for the indirect band gap of BaSnO₃.

Nanoscopic yttrium oxide fluorides: non-aqueous fluorolytic sol-gel synthesis and structural insights by 19F and 89Y MAS NMR

Nanoscopic yttrium acetate fluorides Y(CH(3)COO)(3-z)F(z) and yttrium oxide fluorides YO(3-z)/(2)F(z )were prepared with tunable Y/F molar ratios via the fluorolytic sol-gel route. All samples were characterized by X-ray diffraction, elemental analysis and thermal analysis. In addition, local structures of all samples were studied by (19)F MAS, (19)F-(89)Y CP MAS and (1)H-(89)Y CP MAS NMR spectroscopy and the respective chemical shifts are given. For both classes of compounds, only the fluorination using one equivalent of F (z = 1) leads to defined, well crystalline matrices: yttrium acetate fluoride Y(CH(3)COO)(2)F and r-YOF.

Establishment of a Cre recombinase based mutagenesis protocol for markerless gene deletion in Streptococcus suis

The lack of knowledge about pathogenicity mechanisms of Streptococcus (S.) suis is, at least partially, attributed to limited methods for its genetic manipulation. Here, we established a Cre-lox based recombination system for markerless gene deletions in S. suis serotype 2 with high selective pressure and without undesired side effects.

Two neural streams, one voice: pathways for theme and variation in the songbird brain

Birdsong offers a unique model system to understand how a developing brain - once given a set of purely acoustic targets - teaches itself the vocal-tract gestures necessary to imitate those sounds. Like human infants, to juvenile male zebra finches (Taeniopygia guttata) falls the burden of initiating the vocal-motor learning of adult sounds. In both species, adult caregivers provide only a set of sounds to be imitated, with little or no information about the vocal-tract gestures used to produce the sounds. Here, we focus on the central control of birdsong and review the recent discovery that zebra finch song is under dual premotor control. Distinct forebrain pathways for structured (theme) and unstructured (variation) singing not only raise new questions about mechanisms of sensory-motor integration, but also provide a fascinating new research opportunity. A cortical locus for a motor memory of the learned song is now firmly established, meaning that anatomical, physiological, and computational approaches are poised to reveal the neural mechanisms used by the brain to compose the songs of birds.

Investigating heterogeneity of intracellular calcium dynamics in anterior pituitary lactotrophs using a combined modelling/experimental approach

Cell responses are commonly heterogeneous, even within a subpopulation. In the present study, we investigate the source of heterogeneity in the Ca(2+) response of anterior pituitary lactotrophs to a Ca(2+) mobilisation agonist, thyrotrophin-releasing hormone. This response is characterised by a sharp increase of cytosolic Ca(2+) concentration as a result of mobilisation of Ca(2+) from intracellular stores, followed by a decrease to an elevated plateau level that results from Ca(2+) influx. We focus on heterogeneity of the evoked Ca(2+) spike under extracellular Ca(2+) free conditions. We introduce a method that uses the information provided by a mathematical model to characterise the source of heterogeneity. This method compares scatter plots of features of the Ca(2+) response obtained experimentally with those made from the mathematical model. The model scatter plots reflect random variation of parameters over different ranges, and matching the experimental and model scatter plots allows us to predict which parameters are most variable. We find that a large degree of variation in Ca(2+) efflux is a likely key contributor to the heterogeneity of Ca(2+) responses to thyrotrophin-releasing hormone in lactotrophs. This technique is applicable to any situation in which the heterogeneous biological response is described by a mathematical model.

A tale of two rhythms: the emerging roles of oxytocin in rhythmic prolactin release

Hormone secretion often occurs in a pulsatile manner. In this review, we discuss two rhythms of in vivo prolactin release in female rats and the ongoing research that we and others have performed aiming to understand the mechanisms underlying them. The peptide hormone oxytocin appears to play an important role in both rhythms. One rhythm occurs during the first half of pregnancy, but can also be induced in ovariectomised rats. This is characterised by a circadian pattern with two prolactin surges per day. Two methods for triggering this rhythm are discussed, each utilising a unique physiological pathway that includes oxytocin action, presumably on pituitary lactotrophs. The second rhythm occurs during the oestrous cycle and is characterised by a surge of prolactin on the afternoon of pro-oestrus. We discuss recent findings that oxytocin is more effective at stimulating prolactin release from lactotrophs taken from animals on the afternoon of pro-oestrus than from those of animals on the morning of dioestrus 1, raising the possibility that this hormone plays a physiological role in the regulation of prolactin secretion during the oestrous cycle.

Synchronization of pancreatic islet oscillations by intrapancreatic ganglia: a modeling study

Plasma insulin measurements from mice, rats, dogs, and humans indicate that insulin levels are oscillatory, reflecting pulsatile insulin secretion from individual islets. An unanswered question, however, is how the activity of a population of islets is coordinated to yield coherent oscillations in plasma insulin. Here, using mathematical modeling, we investigate the feasibility of a potential islet synchronization mechanism, cholinergic signaling. This hypothesis is based on well-established experimental evidence demonstrating intrapancreatic parasympathetic (cholinergic) ganglia and recent in vitro evidence that a brief application of a muscarinic agonist can transiently synchronize islets. We demonstrate using mathematical modeling that periodic pulses of acetylcholine released from cholinergic neurons is indeed able to coordinate the activity of a population of simulated islets, even if only a fraction of these are innervated. The role of islet-to-islet heterogeneity is also considered. The results suggest that the existence of cholinergic input to the pancreas may serve as a regulator of endogenous insulin pulsatility in vivo.

Comparison of skeletal muscle strength between cardiac patients and age-matched healthy controls

The purpose of the present study was to compare muscular strength of knee extensors and arm flexor muscles of cardiac patients (n = 638) and healthy controls (n = 961) in different age groups. Isometric torques were measured in a sitting position with the elbow, hip, and knee flexed to 90(0). For statistical analysis, age groups were pooled in decades from the age of 30 to 90 years. Additionally, the influence of physical lifestyle prior to disease on muscular strength was obtained in the patients. For statistical analysis three-way ANOVA (factors age, gender, and physical activity level) was used.Both in patients and in controls a significant age-dependent decline in maximal torque could be observed for arm flexors and knee extensors. Maximal leg extensor muscle showed statistically significant differences between healthy controls and cardiac patients as well as between subgroups of patients: Physically inactive patients showed lowest torques (male: 148 +/- 18 Nm; female: 82 +/- 25 Nm) while highest values were measured in control subjects (male: 167 +/- 16 Nm; female: 93 +/- 17 Nm). In contrast, arm flexor muscles did not show any significant influence of health status or sports history.This qualitative difference between weight-bearing leg muscles and the muscle group of the upper extremity suggest that lower skeletal muscle strength in heart patients is mainly a consequence of selective disuse of leg muscles rather than any pathological skeletal muscle metabolism. Since a certain level of skeletal muscle strength is a prerequisite to cope with everyday activities, strength training is recommended as an important part of cardiac rehabilitation.

Continuity conditions and torsion angles from ssNMR orientational restraints

The backbone torsion angle pair (varphi,psi) at each amino acid of a polypeptide is a descriptor of its conformation. One can use chemical shift and dipolar coupling data from solid-state NMR PISEMA experiments to directly calculate the torsion angles for the membrane-spanning portion of a protein. However, degeneracies inherent in the data give rise to multiple potential torsion angles between two adjacent peptide planes (a diplane). The molecular backbone structure can be determined by gluing together the consecutive diplanes, as in the PIPATH algorithm [T. Asbury, J.R. Quine, S. Achuthan, J. Hu, M.S. Chapman, T.A. Cross, R. Bertram, PIPATH: an optimized alogrithm for generating alpha-helical structures from PISEMA data, J. Magn. Reson. 183 (2006) 87-95.]. The multiplicities in torsion angles translate to multiplicities in diplane orientations. In this paper, we show that adjacent diplanes can be glued together to form a permissible structure only if they satisfy continuity conditions, described quantitatively here. These restrict the number of potential torsion angle pairs. We rewrite the torsion angle formulas from [J.R. Quine, M.T. Brenneman, T.A. Cross, Protein structural analysis from solid-state NMR-drived orientational constraints, Biophys. J. 72 (1997) 2342-2348.] so that they automatically satisfy the continuity conditions. The reformulated torsion angle formulas have been applied recently in the PIPATH algorithm [T. Asbury, J.R. Quine, S. Achuthan, J. Hu, M.S. Chapman, T.A. Cross, R. Bertram, PIPATH: an optimized alogrithm for generating alpha-helical structures from PISEMA data, J. Magn. Reson. 183 (2006) 87-95.] and will be helpful in other applications in which diplane gluing is used to construct a protein backbone model.

PIPATH: an optimized algorithm for generating alpha-helical structures from PISEMA data

An optimized algorithm for finding structures and assignments of solid-state NMR PISEMA data obtained from alpha-helical membrane proteins is presented. The description of this algorithm, PIPATH, is followed by an analysis of its performance on simulated PISEMA data derived from synthetic and experimental structures. pipath transforms the assignment problem into a path-finding problem for a directed graph, and then uses techniques of graph theory to efficiently find candidate assignments from a very large set of possibilities.

Intensity and mosaic spread analysis from PISEMA tensors in solid-state NMR

The solid-state NMR experiment PISEMA, is a technique for determining structures of proteins, especially membrane proteins, from oriented samples. One method for determining the structure is to find orientations of local molecular frames (peptide planes) with respect to the unit magnetic field direction, B0. This is done using equations that compute the coordinates of this vector in the frames. This requires an analysis of the PISEMA function and its degeneracies. As a measure of the sensitivity of peptide plane orientations to the data, we use these equations to derive a formula for the intensity function in the powder pattern. With this function and other measures, we investigate the effect of small changes in peptide plane orientations depending on the location of the resonances in the powder pattern spectrum. This gives us an indication of the change in lineshape due to mosaic spread and a way to interpret these in terms of an orientational error bar.

[Comparison of dorso-lateral and dorso-ventral stabilization procedures in the treatment of vertebral fractures]

AIM

The radiological and clinical outcome of surgical treated thoracolumbal fractures were followed up after 6.7 years.

METHOD

The study encompassed 97 patients. The degrees of the bony deformation and the local kyphosis were measured on the lateral view X-ray at 4 different time points: post trauma, after the operation, before implant removal and at the follow up examination. The clinical outcome was evaluated by the Oswestry score.

RESULTS

74 fractures were treated with a fixateur intern and a dorsal fusion by apposition of autologous bone postero-laterally. 23 fractures were fused in a combined dorso-ventral manner by intervertebral fusion with tricortical autologous bone. The lateral X-rays showed a loss of correction up to 48 % in the dorsal fused group and 10 % loss of correction in the combined group. The local kyphosis increased up to 84 % in the dorsal operated group due to degeneration of the injured disk. The clinical Oswestry score showed no significant difference in both groups.

CONCLUSION

The additional ventral surgery should be evaluated carefully.

Calcium-activated K+ channels of mouse beta-cells are controlled by both store and cytoplasmic Ca2+: experimental and theoretical studies

A novel calcium-dependent potassium current (K(slow)) that slowly activates in response to a simulated islet burst was identified recently in mouse pancreatic beta-cells (Göpel, S.O., T. Kanno, S. Barg, L. Eliasson, J. Galvanovskis, E. Renström, and P. Rorsman. 1999. J. Gen. Physiol. 114:759-769). K(slow) activation may help terminate the cyclic bursts of Ca(2+)-dependent action potentials that drive Ca(2+) influx and insulin secretion in beta-cells. Here, we report that when [Ca(2+)](i) handling was disrupted by blocking Ca(2+) uptake into the ER with two separate agents reported to block the sarco/endoplasmic calcium ATPase (SERCA), thapsigargin (1-5 microM) or insulin (200 nM), K(slow) was transiently potentiated and then inhibited. K(slow) amplitude could also be inhibited by increasing extracellular glucose concentration from 5 to 10 mM. The biphasic modulation of K(slow) by SERCA blockers could not be explained by a minimal mathematical model in which [Ca(2+)](i) is divided between two compartments, the cytosol and the ER, and K(slow) activation mirrors changes in cytosolic calcium induced by the burst protocol. However, the experimental findings were reproduced by a model in which K(slow) activation is mediated by a localized pool of [Ca(2+)] in a subspace located between the ER and the plasma membrane. In this model, the subspace [Ca(2+)] follows changes in cytosolic [Ca(2+)] but with a gradient that reflects Ca(2+) efflux from the ER. Slow modulation of this gradient as the ER empties and fills may enhance the role of K(slow) and [Ca(2+)] handling in influencing beta-cell electrical activity and insulin secretion.

Differential filtering of two presynaptic depression mechanisms

The filtering of input signals carried out at synapses is key to the information processing performed by networks of neurons. Two forms of presynaptic depression, vesicle depletion and G-protein inhibition of Ca2+ channels, can play important roles in the presynaptic processing of information. Using computational models, we demonstrate that these two forms of depression filter information in very different ways. G-protein inhibition acts as a high-pass filter, preferentially transmitting high-frequency input signals to the postsynaptic cell, while vesicle depletion acts as a low-pass filter. We examine how these forms of depression separately and together affect the steady-state postsynaptic responses to trains of stimuli over a range of frequencies. Finally, we demonstrate how differential filtering permits the multiplexing of information within a single impulse train.

The role of derivational morphology in vocabulary acquisition: get by with a little help from my morpheme friends

This study explores the role of morphology in vocabulary knowledge of 3rd and 6th grade Finnish elementary school children. In a word definition task, children from both grades performed overall better on derived words than on monomorphemic words. However, the results were modified by the factors Frequency and Productivity. Most strikingly, performance on monomorphemic words was disproportionately weaker than on derived words at the low frequency range. At the high-frequency range, derived words with low-productive suffixes yielded poorest performance. We partly make an appeal to the lexical-statistical properties of the Finnish language to explain the interaction of Frequency and Word Structure. At any rate, the results suggest that Finnish elementary school children benefit significantly from utilizing morphology in determining word meanings.

The phantom burster model for pancreatic beta-cells

Pancreatic beta-cells exhibit bursting oscillations with a wide range of periods. Whereas periods in isolated cells are generally either a few seconds or a few minutes, in intact islets of Langerhans they are intermediate (10-60 s). We develop a mathematical model for beta-cell electrical activity capable of generating this wide range of bursting oscillations. Unlike previous models, bursting is driven by the interaction of two slow processes, one with a relatively small time constant (1-5 s) and the other with a much larger time constant (1-2 min). Bursting on the intermediate time scale is generated without need for a slow process having an intermediate time constant, hence phantom bursting. The model suggests that isolated cells exhibiting a fast pattern may nonetheless possess slower processes that can be brought out by injecting suitable exogenous currents. Guided by this, we devise an experimental protocol using the dynamic clamp technique that reliably elicits islet-like, medium period oscillations from isolated cells. Finally, we show that strong electrical coupling between a fast burster and a slow burster can produce synchronized medium bursting, suggesting that islets may be composed of cells that are intrinsically either fast or slow, with few or none that are intrinsically medium.

Atomic refinement using orientational restraints from solid-state NMR

We describe a procedure for using orientational restraints from solid-state NMR in the atomic refinement of molecular structures. Minimization of an energy function can be performed through either (or both) least-squares minimization or molecular dynamics employing simulated annealing. The energy, or penalty, function consists of terms penalizing deviation from "ideal" parameters such as covalent bond lengths and terms penalizing deviation from orientational data. Thus, the refinement strives to produce a good fit to orientational data while maintaining good stereochemistry. The software is in the form of a module for the popular refinement package CNS and is several orders of magnitude faster than previous software for refinement with orientational data. The short computer time required for refinement removes one of the difficulties in protein structure determination with solid-state NMR.

Transarticular joint reconstruction and indirect plate osteosynthesis for complex distal supracondylar femoral fractures

In a prospective study conducted between 1992 and 1995, displaced Müller type C2-C3 intraarticular fractures of the distal femur were treated using an indirect plate fixation technique and a lateral parapatellar arthrotomy for the direct reduction of the condylar block. There were 6 closed, and 2 open fractures (1 grade II and 1 grade IIIB). Following reconstruction of the articular block, the block was indirectly reduced and fixed to the shaft by a plate inserted by the retrograde method beneath the vastus lateralis. Transcutaneous/transmuscular screws were used to fix the plate to the shaft. Time to bone healing was 12 weeks (median range 8-17 weeks) after surgery without needing primary or secondary bone grafts. There were no infections or refractures. Except for one case which required a corrective osteotomy for 10 degrees of varus malalignment, there were no revisions. According to the Neer score, there were 6 excellent or satisfactory results, 2 unsatisfactory results, and no failures. At follow-up, there were 2 varus-valgus deformities greater than 5 degrees, 2 leg length discrepancies greater than 10 mm, and 2 rotational deformities of 15 degrees. The treatment results for complex supracondylar/intracondylar fractures of the distal femur obtained with this technique compare favourably with other reported series using different techniques without the added morbidity associated with autogenous bone grafting. However, the surgical technique is demanding and special care must be taken to ensure correct axial alignment.

The role of morphological constituents in reading Finnish compound words

The processing of transparent Finnish compound words was investigated in 2 experiments in which eye movements were recorded while sentences were read silently. The frequency of the second constituent had a large influence (95 ms) on gaze duration on the target words, but its influence was relatively late in processing: A clear effect only occurred on the probability of a third fixation. The frequency of the whole compound word had a similar influence on gaze duration (82 ms) and influenced eye movements at least as rapidly as did the frequency of the second constituent. These results, together with an earlier finding that the frequency of the first constituent affected the first fixation duration, indicate that the identification of these compound words involves parallel processing of both morphological constituents and whole-word representations.

Dynamical complexity and temporal plasticity in pancreatic beta-cells

We discuss some of the biological and mathematical issues involved in understanding and modelling the bursting electrical activity in pancreatic beta-cells. These issues include single-cell versus islet behaviour, parameter heterogeneity, channel noise, the effects of hormones, neurotransmitters, and ions, and multiple slow biophysical processes. Some of the key experimental and modelling studies are described, and some of the major open questions are discussed.

The balance of storage and computation in morphological processing: the role of word formation type, affixal homonymy, and productivity

This article is concerned with the way in which the balance of storage-storing and processing words through full-form representations-and computation-storing and processing words through morpheme-based representations-in lexical processing in the visual modality is affected by the following 3 factors: word formation type (roughly, inflection vs. derivation), productivity, and affixal homonymy. Experimental results for 5 different Dutch suffixes, combined with previous results obtained for 4 comparable Finnish suffixes (R. Bertram, M. Laine, & K. Karvinen, 1999) and 2 Dutch suffixes (R. H. Baayen, T. Dijkstra, & R. Schreuder, 1997), show that none of these factors in isolation is a reliable cross-linguistic predictor of the balance of storage and computation. The authors offer a general framework that outlines how morphological processing is influenced by the interaction of word formation type, productivity, and affixal homonymy.

The role of morphological constituents in reading Finnish compound words

The processing of transparent Finnish compound words was investigated in 2 experiments in which eye movements were recorded while sentences were read silently. The frequency of the second constituent had a large influence (95 ms) on gaze duration on the target words, but its influence was relatively late in processing: A clear effect only occurred on the probability of a third fixation. The frequency of the whole compound word had a similar influence on gaze duration (82 ms) and influenced eye movements at least as rapidly as did the frequency of the second constituent. These results, together with an earlier finding that the frequency of the first constituent affected the first fixation duration, indicate that the identification of these compound words involves parallel processing of both morphological constituents and whole-word representations.

[Dorsal lumbar interbody implantation of cages for stabilizing segmental spinal instabilities]

PURPOSE

The technique of posterior lumbar interbody fusion (PLIF) has been critically discussed due to a high degree of complications, including the development of pseudarthrosis. With the recent establishment of intercorporeal implants new aspects have to be contemplated in surgical techniques, especially concerning the posterior approach. In this study we present our first results after intercorporeal stabilisation of segmental spinal instabilities utilising carbon and titanium cages.

METHOD

45 spinal instabilities were surgically stabilised in 42 patients who were evaluated on average for 2.8 years post-operatively. 12 patients had isthmic and 19 patients degenerative instabilities while 11 patients suffered from instabilities resulting from prior spinal surgery.

RESULTS

Assessed according to the Hambly-score, 69% of the patients had an excellent or good result; 2 (4.8%) patients were subjectively worse off than before surgical treatment. After implantation of cages precise radiological evaluation of bony ingrowth is frequently impaired by artefacts. We found that three months after implantation of a titanium cage, which had to be removed after incorrect placement, no bony consolidation was visible. Persisting or recurrent instabilities in fused segments were not recorded.

CONCLUSION

By means of PLIF and implantation of cages the interbody space is reconstructed and jeopardized neural structures are decompressed. In addition to this, the frequently osteochondrotically degenerated segment is immobilized. The posterior approach allows decompression of neural structures and, with comparable results concerning stability, the considerable risks of the ventral approach are avoided.

The balance of storage and computation in morphological processing: the role of word formation type, affixal homonymy, and productivity

This article is concerned with the way in which the balance of storage-storing and processing words through full-form representations-and computation-storing and processing words through morpheme-based representations-in lexical processing in the visual modality is affected by the following 3 factors: word formation type (roughly, inflection vs. derivation), productivity, and affixal homonymy. Experimental results for 5 different Dutch suffixes, combined with previous results obtained for 4 comparable Finnish suffixes (R. Bertram, M. Laine, & K. Karvinen, 1999) and 2 Dutch suffixes (R. H. Baayen, T. Dijkstra, & R. Schreuder, 1997), show that none of these factors in isolation is a reliable cross-linguistic predictor of the balance of storage and computation. The authors offer a general framework that outlines how morphological processing is influenced by the interaction of word formation type, productivity, and affixal homonymy.

Affixal homonymy triggers full-form storage, even with inflected words, even in a morphologically rich language

This paper investigates whether affixal homonymy, the phenomenon that one affix form serves two or more semantic/syntactic functions, affects lexical processing of inflected words in a similar way for a morphologically rich language such as Finnish as for morphologically restricted languages such as Dutch and English. For the latter two languages, there is evidence that affixal homonymy triggers full-form storage for inflected words (Bertram, R., Schreuder, R., and Baayen, R. H. (in press). The balance of storage and computation in morphological processing: the role of word formation type, affixal homonymy, and productivity. Journal of Experimental Psychology: Learning, Memory, and Cognition; Sereno and Jongman (1997). Processing of English inflectional morphology. Memory and Cognition, 25, 425-437). Two visual lexical decision experiments show the same pattern for Finnish. Apparently, the substantially richer morphology in Finnish does not prevent full-form storage for inflected words when the affix is homonymic.

Implications of G-protein-mediated Ca2+ channel inhibition for neurotransmitter release and facilitation

G-protein-mediated inhibition of Ca2+ current is ubiquitous in neurons, and in synaptic terminals it can lead to a reduction in transmitter release (presynaptic inhibition). This type of Ca2+ current inhibition can often be relieved by prepulse depolarization, so the disinhibition of Ca2+ current can combine with Ca2+ -dependent mechanisms for activity-induced synaptic facilitation to amplify this form of short-term plasticity. We combine a mathematical model of a G-protein-regulated Ca2+ channel with a model of transmitter secretion to study the potential effects of G-protein-mediated Ca2+ channel inhibition and disinhibition on transmitter release and facilitation. We investigate several scenarios, with the goal of observing a range of behaviors that may occur in different synapses. We find that the effects of Ca2+ channel disinhibition depend greatly on the location and distribution of inhibited channels. Facilitation can be greatly enhanced if all channels are subject to inhibition or if the subpopulation of channels subject to inhibition are located closer to release sites than those insensitive to inhibition, an arrangement that has been suggested by recent experiments (Stanley and Mirotznik, 1997). We also find that the effect of disinhibition on facilitation is greater for longer action potentials. Finally, in the case of homosynaptic inhibition, where Ca2+ channel inhibition occurs through the binding of transmitter molecules to presynaptic autoreceptors, there will be little reduction in transmitter release during the first of two successive bursts of impulses. The reduction of release during the second burst will be significantly greater, and if the unbinding rate of autoreceptors is relatively low, then the effects of G-protein-mediated channel inhibition become more pronounced as the duration of the interburst interval is increased up to a critical point, beyond which the inhibitory effects become less pronounced. This is in contrast to presynaptic depression due to the depletion of the releasable vesicle pool, where longer interburst intervals allow for a more complete replenishment of the pool. Thus, G-protein-mediated Ca2+ current inhibition leads to a reduction in transmitter release, while having a highly variable amplifying effect on synaptic facilitation. The dynamic properties of this form of presynaptic inhibition are very different from those of vesicle depletion.

Modeling study of the effects of overlapping Ca2+ microdomains on neurotransmitter release

Although single-channel Ca2+ microdomains are capable of gating neurotransmitter release in some instances, it is likely that in many cases the microdomains from several open channels overlap to activate vesicle fusion. We describe a mathematical model in which transmitter release is gated by single or overlapping Ca2+ microdomains produced by the opening of nearby Ca2+ channels. This model accounts for the presence of a mobile Ca2+ buffer, provided either that the buffer is unsaturable or that it is saturated near an open channel with Ca2+ binding kinetics that are rapid relative to Ca2+ diffusion. We show that the release time course is unaffected by the location of the channels (at least for distances up to 50 nm), but paired-pulse facilitation is greater when the channels are farther from the release sites. We then develop formulas relating the fractional release following selective or random channel blockage to the cooperative relationship between release and the presynaptic Ca2+ current. These formulas are used with the transmitter release model to study the dependence of this form of cooperativity, which we call Ca2+ current cooperativity, on mobile buffers and on the local geometry of Ca2+ channels. We find that Ca2+ current cooperativity increases with the number of channels per release site, but is considerably less than the number of channels, the theoretical upper bound. In the presence of a saturating mobile buffer the Ca2+ current cooperativity is greater, and it increases more rapidly with the number of channels. Finally, Ca2+ current cooperativity is an increasing function of channel distance, particularly in the presence of saturating mobile buffer.

[Intermediate-term results with the Mecron screw cup acetabulum--a follow-up study]

ISSUE

The short term result of the cementless implanted titan screw cups (type Mec-Ring) has been very encouraging. The study was aimed to show the survival rate in the middle term.

METHOD

The survival rate of 111 implanted acetabular titan screw cups (type Mec-Ring) was determined in a retrospective study. 85 patients (= 76.5%) could be followed up after 6.7 years (5-8.5 years).

RESULTS

38 Implants had to be removed during the follow up period (34.2%). Taking into account the radiologic loosening in addition 22 cups were defined as failure (19.8%). Overall 36 (32.4%) cups showed excellent or good clinical results with 85 points according to the Harris-Hip-Score. 15 of the 22 radiologic loose implants showed only little or no pain. Considering the removed implants the probability of survival is 69% after 7 years. Defining the radiologic loose implants as failure the survival rate is 54%.

CONCLUSION

X-rays have to be taken frequently to determine bone loosening early with subsequent revision operation. The survival rate is to low compared to other types of fixation. Therefore acetabular titan screw cups (type Mec-Ring) should not be used any more.

Glucose diffusion in pancreatic islets of Langerhans

We investigate the time required for glucose to diffuse through an isolated pancreatic islet of Langerhans and reach an equilibrium. This question is relevant in the context of in vitro electrophysiological studies of the response of an islet to step changes in the bath glucose concentration. Islet cells are electrically coupled by gap junctions, so nonuniformities in islet glucose concentration may be reflected in the activity of cells on the islet periphery, where electrical recordings are made. Using a mathematical model of hindered glucose diffusion, we investigate the effects of the islet porosity and the permeability of a surrounding layer of acinar cells. A major factor in the determination of the equilibrium time is the transport of glucose into islet beta-cells, which removes glucose from the interstitial spaces where diffusion occurs. This transport is incorporated by using a model of the GLUT-2 glucose transporter. We find that several minutes are required for the islet to equilibrate to a 10 mM change in bath glucose, a typical protocol in islet experiments. It is therefore likely that in electrophysiological islet experiments the glucose distribution is nonuniform for several minutes after a step change in bath glucose. The delay in glucose penetration to the inner portions of the islet may be a major contributing factor to the 1-2-min delay in islet electrical activity typically observed after bath application of a stimulatory concentration of glucose.

A simple model of transmitter release and facilitation

We describe a model of synaptic transmitter release and presynaptic facilitation that is based on activation of release sites by single Ca2+ microdomains. Facilitation is due to Ca2+ that remains bound to release sites between impulses. This model is inherently stochastic, but deterministic equations can be derived for the mean release. The number of equations required to describe the mean release is small, so it is practical to use the model with models of neuronal electrical activity to investigate the effects of different input spike patterns on presynaptic facilitation. We use it in conjunction with a model of dopamine-secreting neurons of the basal ganglia to demonstrate that transmitter release is greater when the neuron bursts than when it spikes continuously, due to the greater facilitation generated by the bursting impulse pattern. Finally, a minimal form of the model is described that is coupled to simple models of postsynaptic receptors and passive membrane to compute the postsynaptic voltage response to a train of presynaptic stimuli. This form of the model is appropriate for neural network simulations.

Evidence that calcium release-activated current mediates the biphasic electrical activity of mouse pancreatic beta-cells

The electrical response of pancreatic beta-cells to step increases in glucose concentration is biphasic, consisting of a prolonged depolarization with action potentials (Phase 1) followed by membrane potential oscillations known as bursts. We have proposed that the Phase 1 response results from the combined depolarizing influences of potassium channel closure and an inward, nonselective cation current (ICRAN) that activates as intracellular calcium stores empty during exposure to basal glucose (Bertram et al., 1995). The stores refill during Phase 1, deactivating ICRAN and allowing steady-state bursting to commence. We support this hypothesis with additional simulations and experimental results indicating that Phase 1 duration is sensitive to the filling state of intracellular calcium stores. First, the duration of the Phase 1 transient increases with duration of prior exposure to basal (2.8 mM) glucose, reflecting the increased time required to fill calcium stores that have been emptying for longer periods. Second, Phase 1 duration is reduced when islets are exposed to elevated K+ to refill calcium stores in the presence of basal glucose. Third, when extracellular calcium is removed during the basal glucose exposure to reduce calcium influx into the stores, Phase 1 duration increases. Finally, no Phase 1 is observed following hyperpolarization of the beta-cell membrane with diazoxide in the continued presence of 11 mm glucose, a condition in which intracellular calcium stores remain full. Application of carbachol to empty calcium stores during basal glucose exposure did not increase Phase 1 duration as the model predicts. Despite this discrepancy, the good agreement between most of the experimental results and the model predictions provides evidence that a calcium release-activated current mediates the Phase 1 electrical response of the pancreatic beta-cell.