Biomedical subjects

G Dupont

Publications and source records attributed to G Dupont.

At least 19 recordsLinked to original sources

PubMed2003

Ca2+ oscillations in hepatocytes do not require the modulation of InsP3 3-kinase activity by Ca2+.

Receptor-mediated production of inositol 1,4,5-trisphosphate (InsP(3)) initiates Ca(2+) release and is responsible for cytosolic Ca(2+) oscillations. InsP(3) oscillations have also been observed in some cells. One of the enzymes controlling InsP(3) catabolism, the InsP(3) 3-kinase, is stimulated by Ca(2+); this regulation is presumably part of the reason for InsP(3) oscillations that have been observed in some cells. Here, we investigate the possible role of Ca(2+)-activated InsP(3) catabolism on the characteristics of the InsP(3)-induced Ca(2+) oscillations. Numerical simulations show that if it is assumed that the Ca(2+)-independent InsP(3) catabolism is predominant, Ca(2+) oscillations remain qualitatively unchanged although the relative amplitude of the oscillations in InsP(3) concentrations becomes minimal. We tested this prediction in hepatocytes by masking the Ca(2+)-dependent InsP(3) catabolism by 3-kinase through the injection of massive amounts of InsP(3) 5-phosphatase, which is not stimulated by Ca(2+). We find that in such injected hepatocytes, Ca(2+) oscillations generated by modest agonist levels are suppressed, presumably because of the decreased dose in InsP(3), but that at higher doses of agonist, oscillations reappear, with characteristics similar to those of untreated cells at low agonist doses. Altogether, these results suggest that oscillations in InsP(3) concentration due to Ca(2+)-stimulated InsP(3) catabolism do not play a major role for the oscillations in Ca(2+) concentration.

Animals↗

PubMed2003

Time spent at VO2max: a methodological issue.

This study was designed to propose a standardised procedure to determine the time spent at VO2max (tVO2max) based on the VO2max of the day (i. e. the VO2max value measured the day of the test). Ten male subjects first performed a graded field test, followed by a continuous running exercise to exhaustion, at the velocity of the Université de Montréal Track Test (V(UMTT)) plus 1 km x h(-1) (V(UMTT)(+1)). The second test consisted of an exhaustive run at 100 % of V(UMTT), followed by a V(UMTT)(+1) test. Different methods were used to compare time spent at VO2max, based on the VO2max of the graded field test, and time spent at VO2max, based on the VO2max of the day, during an exhaustive run at 100 % of V(UMTT). Results have shown that V(UMTT)(+1) tests were of sufficient intensity and duration to identify the VO2max of the day. Time spent at VO2max ranged from 25 +/- 53 s to 139 +/- 76 s according to the method used. However, the tVO2max method based on the sum of each value higher than 95 % of VO2max of the day appeared more robust than methods based on the time to exhaustion minus time to reach VO2 reference value, or the method based on the sum of values higher than VO2max minus 2.1 ml x kg(-1) x min(-1).

Adult↗

PubMed2003

Plasma lactate and plasma volume recovery in adults and children following high-intensity exercises.

AIM: To compare plasma lactate concentration recovery kinetics when measured and corrected for variations in plasma volume between children and adults. METHODS: Nine boys (11.3 +/- 1.1 y) and 8 men (21.9 +/- 1.9 y) performed a maximal and a supramaximal exercise until exhaustion. Plasma lactate concentrations, haemoglobin and haematocrit were measured at rest, immediately on completion of exercise and after the 2nd, 5th, 12th and 30th minute of recovery. The plasma lactate concentrations and the rate of recovery were corrected for variations in plasma volume. RESULTS: The maximal decreases in plasma volume were significantly higher in adults than in children for maximal exercise (-18.7 +/- 2.6% vs -14.5 +/- 3.2%; p < 0.05), but similar for the supramaximal exercise (-16.9 +/- 3.4% vs -15.2 +/- 3.4%). During recovery, measured and corrected plasma lactate concentrations were significantly higher in adults. The rate of plasma lactate recovery was higher in adults for maximal exercise only. The same results were obtained when the rates of plasma lactate decrease were calculated from corrected plasma lactate concentrations. CONCLUSION: The correction of the plasma lactate concentrations for variations in plasma volume did not influence the comparison of the concentrations obtained in adults and children, or their rate of recovery.

Adult↗

PubMed2002

Developing a study method for producing 400 microm spheroids.

The aim of this work was to obtain 400 microm spheroids that can be sprinkled on food to improve patient compliance particularly in the case of children and old people. A methodology to select wet masses for extrusion-spheronization through a 400 microm orifice was developed. The first step was to define the parameters that make it possible to assess the qualities required by the wet mass and the extrudates and evaluation norms: plasticity, cohesiveness, brittleness of the mass and the extrudates, and appearance of extrudates. A feasibility assay was then performed on the cylinder extruder, showing that extrusion of the lactose/Avicel PH 101/water (50/50/60) mass is not feasible through the 400 microm orifice. Precirol ato 5 and Gelucire 50/02 wetted with a sodium lauryl sulfate solution at 0.5% show plastic flow through the 400 microm diameter orifice. The presence of Avicel PH 101 does not improve plasticity for this orifice. Micropellets of 400 microm have been proved feasible as long as excipients with suitable pharmaceutical technological properties are used. After proving the feasibility of 400 microm spheroids of Gelucire 50/02, we considered the association of a drug with it.

Cellulose↗

PubMed2002

Effects of high intensity intermittent training on peak VO(2) in prepubertal children.

This study was designed to examine peak VO(2) responses of prepubescent children following a 7-week aerobic training. Twenty-three boys and thirty girls (9.7 +/- 0.8 years) were divided into a high intensity experimental group (HIEG: 20 girls and 13 boys) and a control group (CG: 10 girls and 10 boys). A graded 20-m shuttle run with measurement of gas exchange values was performed prior to and after the 7-week training program. The test consisted of a 3-min run at 7 km x h(-1) to determine energy cost of running, immediately followed by a 20-meter shuttle run test. HIEG had two 30 min-sessions of short intermittent aerobic training per week at velocities ranging from 100 up to 130 % of the maximal aerobic speed. For HIEG, absolute peak VO(2)(9.1 %) and relative to body mass peak VO(2)(8.2 %) increased significantly (p < 0.001); it was unchanged in the CG. Similarly, maximal shuttle run improved significantly in HIEG (5.1 %, p < 0.001). In contrast, there was no significant change for CG. For both groups energy cost of running remained unchanged. These findings show that prepubescent children could significantly increase their peak VO(2) and maximal shuttle velocity with high intensity short intermittent aerobic exercises.

Age Factors↗

PubMed2001

Effects of two low-dose oral contraceptives containing ethinylestradiol and either desogestrel or levonorgestrel on serum lipids and lipoproteins with particular regard to LDL size.

This study was designed to determine the effects of two low-dose oral contraceptives, most frequently given in our area, monophasic desogestrel/ethinylestradiol (DG/EE) and triphasic levonorgestrel/ethinylestradiol (LNG/EE), on lipoprotein parameters, especially LDL particle size and HDL subclass distribution (determined by lipid-stained 2%-20% polyacrylamide gradient gel electrophoresis) in 37 healthy normolipidemic women aged 19 to 27 years. Lipid and lipoprotein parameters were measured before the start of treatment and in the third month of oral contraceptive use. Results reflected the estrogen-progestin balance. As compared with baseline values, with both formulations, plasma total cholesterol, phospholipids, and HDL3 cholesterol increased, and LDL-predominant peak size decreased, with a translation of LDL pattern A towards pattern I. With DG/EE, plasma triglycerides, apolipoproteins AI and B increased. With LNG/EE, LDL cholesterol increased, and HDL2 cholesterol decreased. All these modifications were moderate, within threshold limits. Estrogen-dominant monophasic DG/EE appears to be more favorable than progestin-dominant triphasic LNG/EE, since the reduction in LDL-predominant peak size is not associated with an increase in LDL cholesterol or with a decrease in HDL2 cholesterol.

Adult↗

PubMed2001

Investigation of the roles of Ca(2+) and InsP(3) diffusion in the coordination of Ca(2+) signals between connected hepatocytes.

Glycogenolytic agonists induce coordinated Ca(2+) oscillations in multicellular rat hepatocyte systems as well as in the intact liver. The coordination of intercellular Ca(2+) signals requires functional gap-junction coupling. The mechanisms ensuring this coordination are not precisely known. We investigated possible roles of Ca(2+) or inositol 1,4,5-trisphosphate (InsP(3)) as a coordinating messengers for Ca(2+) spiking among connected hepatocytes. Application of ionomycin or of supra-maximal concentrations of agonists show that Ca(2+) does not significantly diffuse between connected hepatocytes, although gap junctions ensure the passage of small signaling molecules, as demonstrated by FRAP experiments. By contrast, coordination of Ca(2+) spiking among connected hepatocytes can be favored by a rise in the level of InsP(3), via the increase of agonist concentrations, or by a shift in the affinity of InsP(3) receptor for InsP(3). In the same line, coordination cannot be achieved if the InsP(3) is rapidly metabolized by InsP(3)-phosphatase in one cell of the multiplet. These results demonstrate that even if small amounts of Ca(2+) diffuse across gap junctions, they most probably do not play a significant role in inducing a coordinated Ca(2+) signal among connected hepatocytes. By contrast, coordination of Ca(2+) oscillations is fully dependent on the diffusion of InsP(3) between neighboring cells.

Animals↗

PubMed2001

Predicting sprint kinematic parameters from anaerobic field tests in physical education students.

The relations among kinematic parameters measured during the first 10 seconds of 100-m sprint and anaerobic tests were studied in 22 male physical education students. During the first 10 seconds of the sprint, the position of the runners was "continuously" measured with a laser telemeter. Maximal acceleration (Amax), maximal velocity (Vmax), and time to reach Vmax (tVmax) were derived from position data. In addition, the subjects performed anaerobic tests: squat jump (SJ), countermovement jump (CMJ), and force-velocity test to measure maximal power, maximal theoretical cranking velocity (VO), maximal theoretical isometric force, and the Wingate anaerobic test (30 seconds). The mean 100-m run time of the subjects was 12.6 +/- 0.9 seconds. The highest correlations were calculated between Amax and V0 (r = 0.55, p < 0.01) and CMJ (r = 0.48, p < 0.05) and Vmax and SJ (r = 0.63, p < 0.01) and CMJ (r = 0.56, p < 0.05). The tVmax was uncorrelated to other tests. Because the CMJ was the anaerobic performance best correlated to the different kinematic parameters of the run, our results fail to identify one anaerobic test that specifically explains one sprint kinematic parameter.

Adult↗

PubMed2000

Activation of the liver glycogen phosphorylase by Ca(2+)oscillations: a theoretical study.

Cytosolic calcium plays a crucial role as a second messenger in cellular signalling. Various cell types, including hepatocytes, display Ca(2+)oscillations when stimulated by an extracellular signal. However, the biological relevance of this temporal organization remains unclear. In this paper, we investigate theoretically the effect of Ca(2+)oscillations on a particular example of cell regulation: the phosphorylation-dephosphorylation cycle controlling the activation of glycogen phosphorylase in hepatocytes. By modelling periodic sinusoidal variations in the intracellular Ca(2+)concentration, we show that Ca(2+)oscillations reduce the threshold for the activation of the enzyme. Furthermore, as the activation of a given enzyme depends on the kinetics of its phosphorylation-dephosphorylation cycle, specificity can be encoded by the oscillation frequency. Finally, using a model for signal-induced Ca(2+)oscillations based on Ca(2+)-induced Ca(2+)release, we show that realistic Ca(2+)oscillations can potentiate the response to a hormonal stimulation. These results indicate that Ca(2+)oscillations in hepatocytes could contribute to increase the efficiency and specificity of cellular signalling, as shown experimentally for gene expression in lymphocytes (Dolmetsch et al., 1998).

Animals↗

PubMed2000

Hierarchical organization of calcium signals in hepatocytes: from experiments to models.

The proper working of the liver largely depends on the fine tuning of the level of cytosolic Ca(2+) in hepatocytes. Thanks to the development of imaging techniques, our understanding of the spatio-temporal organization of intracellular Ca(2+) in this - and other - cell types has much improved. Many of these signals are mediated by a rise in the level of inositol 1,4,5-trisphosphate (InsP(3)), a second messenger which can activate the release of Ca(2+) from the endoplasmic reticulum. Besides the now well-known hepatic Ca(2+) oscillations induced by hormonal stimulation, intra- and intercellular Ca(2+) waves have also been observed. More recently, subcellular Ca(2+) increases associated with the coordinated opening of a few Ca(2+) channels have been reported. Given the complexity of the regulations involved in the generation of such processes and the variety of time and length scales necessary to describe those phenomena, theoretical models have been largely used to gain a precise and quantitative understanding of the dynamics of intracellular Ca(2+). Here, we review the various aspects of the spatio-temporal organization of cytosolic Ca(2+) in hepatocytes from the dual point of view provided by experiments and modeling. We first focus on the description and the mechanism of intracellular Ca(2+) oscillations and waves. Second, we investigate in which manner these repetitive Ca(2+) increases are coordinated among a set of hepatocytes coupled by gap junctions, a phenomenon known as 'intercellular Ca(2+) waves'. Finally, we focus on the so-called elementary Ca(2+) signals induced by low InsP(3) concentrations, leading to Ca(2+) rises having a spatial extent of a few microns. Although these small-scale events have been mainly studied in other cell types, we theoretically infer general properties of these localized intracellular Ca(2+) rises that could also apply to hepatocytes.

Animals↗

PubMed2000

[Marchiafava-Bignani disease of good prognosis. Two cases].

BACKGROUND: Marchiafava-Bignami disease or corpus callosum necrosis, is an uncommon complication of chronic alcoholism. Prognosis is severe. CASE REPORTS: Two young adults with chronic alcoholism who developed Marchiafava-Bignami disease initially improved clinically. DISCUSSION: Diagnosis has been facilitated with MRI. An MRI exploration should be performed in chronic alcoholic patients presenting chronic encephalopathy with dementia, dysarthria, hypertonia, and ataxia, or acute encephalopathy with confusion, dumbness, seizures, and hypertonia. Necrosis of the corpus callosum gives the diagnosis of Marchiafava-Bignami disease.

Adult↗

PubMed2000

The frequency encoding of pulsatility.

Examples of pulsatile signalling abound in intercellular communication, suggesting that this phenomenon represents a major function of biological rhythms. Pulsatile signals can be encoded in terms of their frequency and prove more efficient than monotonous ones whenever constant stimulation induces desensitization of target cells. We address the main examples of frequency encoding of pulsatility, besides those of neuronal nature. Considered in turn are cAMP oscillations in the slime mould Dictyostelium discoideum, the pulsatile secretion of hormones such as gonadotropin-releasing hormone or growth hormone, intracellular Ca2+ oscillations, and circadian rhythms. Models based on receptor desensitization show the possibility of optimizing cellular responses to cAMP signals in Dictyostelium or to pulsatile hormonal stimulation. The models indicate how the optimal duration of the pulsatile signal and the optimal interval between successive pulses vary as a function of the rates or receptor desensitization and resensitization and of the maximum ligand level during stimulation. The frequency encoding of intracellular Ca2+ oscillations appears to rely on another molecular mechanism. Models based on protein phosphorylation by a Ca(2+)-calmodulin activated kinase show that the mean level of phosphorylated protein increases with the frequency of calcium spikes--which itself rises with the degree of stimulation--and that distinct levels of different phosphorylated proteins can be reached for a Ca2+ signal of given frequency.

Animals↗

PubMed2000

Mechanism of receptor-oriented intercellular calcium wave propagation in hepatocytes.

Intercellular calcium signals are propagated in multicellular hepatocyte systems as well as in the intact liver. The stimulation of connected hepatocytes by glycogenolytic agonists induces reproducible sequences of intracellular calcium concentration increases, resulting in unidirectional intercellular calcium waves. Hepatocytes are characterized by a gradient of vasopressin binding sites from the periportal to perivenous areas of the cell plate in hepatic lobules. Also, coordination of calcium signals between neighboring cells requires the presence of the agonist at each cell surface as well as gap junction permeability. We present a model based on the junctional coupling of several hepatocytes differing in sensitivity to the agonist and thus in the intrinsic period of calcium oscillations. In this model, each hepatocyte displays repetitive calcium spikes with a slight phase shift with respect to neighboring cells, giving rise to a phase wave. The orientation of the apparent calcium wave is imposed by the direction of the gradient of hormonal sensitivity. Calcium spikes are coordinated by the diffusion across junctions of small amounts of inositol 1,4, 5-trisphosphate (InsP(3)). Theoretical predictions from this model are confirmed experimentally. Thus, major physiological insights may be gained from this model for coordination and spatial orientation of intercellular signals.-Dupont, G., Tordjmann, T., Clair, C., Swillens, S., Claret, M., Combettes, L. Mechanism of receptor-oriented intercellular calcium wave propagation in hepatocytes.

Animals↗

PubMed1999

From calcium blips to calcium puffs: theoretical analysis of the requirements for interchannel communication.

In the cytoplasm of cells of different types, discrete clusters of inositol 1,4,5-trisphosphate-sensitive Ca(2+) channels generate Ca(2+) signals of graded size, ranging from blips, which involve the opening of only one channel, to moderately larger puffs, which result from the concerted opening of a few channels in the same cluster. These channel clusters are of unknown size or geometrical characteristics. The aim of this study was to estimate the number of channels and the interchannel distance within such a cluster. Because these characteristics are not attainable experimentally, we performed computer stochastic simulations of Ca(2+) release events. We conclude that, to ensure efficient interchannel communication, as experimentally observed, a typical cluster should contain two or three tens of inositol 1,4,5-trisphosphate-sensitive Ca(2+) channels in close contact.

Animals↗

PubMed1998

Link between fertilization-induced Ca2+ oscillations and relief from metaphase II arrest in mammalian eggs: a model based on calmodulin-dependent kinase II activation.

Mammalian eggs are ovulated in metaphase II of meiosis, in a state characterized by high levels of cyclin B and of active maturation promoting factor (MPF). This arrest is mediated by an activity referred to as cytostatic factor (CSF) which prevents the degradation of cyclin. Fertilization triggers a train of Ca2+ spikes which is responsible for the decrease in activity of both MPF and CSF. The decline in MPF however much precedes that in CSF. Experimental observations on mammalian eggs indicate that the kinetics of cell cycle resumption much depends on the temporal pattern of the repetitive Ca2+ spikes. Here, we propose a theoretical model which accounts for Ca(2+)-induced relief from metaphase II arrest in mammalian eggs. The model is based on the fact that Ca2+/calmodulin kinase II (CaMKII) activation is the primary event leading to inactivation of both CSF and MPF. To account for experimental observations, it has to be assumed that CaMKII activation affects the level of the active form of the anaphase promoting complex (APC), which initiates the degradation of cyclin, through two pathways characterized by different time scales. Thus, we hypothesize that CaMKII activation by Ca2+ leads to the transformation of a mediator protein from a form which stimulates the inactivation of the APC into a form which gradually and indirectly induces the deactivation of CSF. In consequence, a sufficient number of Ca2+ spikes first triggers the decrease of MPF, thus allowing the egg to enter in interphase, and later that of CSF. Finally, when CSF is low and when Ca2+ oscillations have stopped, the level of MPF can increase again, a phenomenon that would correspond to the first mitosis. This model also accounts for the observed dependence of the time of entry in interphase (marked by the appearance of the pronuclei) on the frequency of Ca2+ spikes, as well as for the possible entry in metaphase III arrest, a pathological state of the egg which results from an insufficient activation by Ca2+. This study provides some theoretical prediction as to the time of the first mitosis as a function of the temporal pattern of Ca2+ oscillations.

Animals↗

PubMed1998

CaM kinase II as frequency decoder of Ca2+ oscillations.

In many cell types, Ca2+ signals are organized in the form of repetitive spikes. The frequency of these intracellular Ca2+ oscillations increases with the level of stimulation, suggesting the existence of a frequency encoding phenomenon. The question arises as to how the frequency of Ca2+ oscillations can be decoded inside the cell. Ca2+/calmodulin kinase II has long been proposed as an attractive candidate, as it is a key target of Ca2+ signals. By immobilizing the Ca2+/calmodulin kinase II and subjecting it to pulses of Ca2+ of variable amplitude, duration, and frequency, De Koninck and Schulman have shown for the first time that the autonomous activity of Ca2+/calmodulin kinase II is highly sensitive to the temporal pattern of Ca2+ oscillations.

Animals↗

PubMed1998

Stochastic simulation of a single inositol 1,4,5-trisphosphate-sensitive Ca2+ channel reveals repetitive openings during 'blip-like' Ca2+ transients.

Confocal microscope studies with fluorescent dyes of inositol 1,4,5-trisphosphate (InsP3)-induced intracellular Ca2+ mobilization recently established the existence of 'elementary' events, dependent on the activity of individual InsP3-sensitive Ca2+ channels. In the present work, we try by theoretical stochastic simulation to explain the smallest signals observed in those studies, which were referred to as Ca2+ 'blips' [Parker I., Yao Y. Ca2+ transients associated with openings of inositol trisphosphate-gated channels in Xenopus oocytes. J Physiol Lond 1996; 491: 663-668]. For this purpose, we assumed a simple molecular model for the InsP3-sensitive Ca2+ channel and defined a set of parameter values accounting for the results obtained in electrophysiological bilayer experiments [Bezprozvanny I., Watras J., Ehrlich B.E. Bell-shaped calcium-response curves of Ins(1,4,5)P3- and calcium-gated channels from endoplasmic reticulum of cerebellum. Nature 1991; 351: 751-754; Bezprozvanny I., Ehrlich B.E. Inositol (1,4,5)-trisphosphate (InsP3)-gated Ca channels from cerebellum: conduction properties for divalent cations and regulation by intraluminal calcium. J Gen Physiol 1994; 104: 821-856]. With a stochastic procedure which considered cytosolic Ca2+ diffusion explicitly, we then simulated the behaviour of a single channel, placed in a realistic physiological environment. An attractive result was that the simulated channel exhibited bursts of activity, arising from repetitive channel openings, which were responsible for transient rises in Ca2+ concentration and were reminiscent of the relatively long-duration experimental Ca2+ blips. The influence of the values chosen for the various parameters (affinity and diffusion coefficient of the buffers, luminal Ca2+ concentration) on the kinetic characteristics of these theoretical blips is analyzed.

Calcium↗

PubMed1998

Theoretical insights into the mechanism of spiral Ca2+ wave initiation in Xenopus oocytes.

Spiral waves of intracellular Ca2+ have often been observed in Xenopus oocytes. Such waves can be accounted for by most realistic models for Ca2+ oscillations taking diffusion of cytosolic Ca2+ into account, but their initiation requires rather demanding and unphysiological initial conditions. Here, it is shown by means of numerical simulations that these spiral Ca2+ waves naturally arise if the cytoplasm is assumed to be heterogeneous both at the level of the synthesis and metabolism of D-myo-inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] and at the level of the distribution of the Ins(1,4, 5)P3 receptors. In such conditions, a spiral can be initiated in the simulations after an increase in Ins(1,4,5)P3 concentration, with the direction of rotation being determined by the position of the region of high receptor density with respect to the locus of Ins(1,4, 5)P3 production.

Animals↗