bluuebunny/crossref_metadata_2025 huggingface dataset:
Delayed production of adenosine underlies temporal modulation of swimming in frog embryo
Authors: Dale Nicholas
Published: September 2004

To investigate the dynamics of adenosine production in the spinal cord during motor activity, and its possible contribution to the temporal modulation of motor patterns, a sensor sensitive to adenosine at concentrations as low as 10 nM was devised.

When pressed against the outside of the spinal cord, the sensor detected slow changes in the levels of adenosine during fictive swimming that ranged from 10 to 650 nM. In four embryos where particularly large signals were recorded due to favourable probe placement, the adenosine levels continued to rise for up to a minute following cessation of activity before slowly returning to baseline. In the remaining thirteen embryos, levels of adenosine started to return slowly to baseline almost immediately after activity had stopped.

Inhibitors of adenosine uptake increased the magnitude of the signal recorded and slowed the recovery following cessation of activity.

A realistic computational model of the spinal circuitry was combined with models of extracellular breakdown of ATP to adenosine. ATP and adenosine inhibited, as in the real embryo, the voltage‐gated K+ and Ca2+ currents, respectively. The model reproduced the temporal run‐down of motor activity seen in the real embryo suggesting that synaptic release of ATP together with its extracellular breakdown to adenosine is sufficient to exert time‐dependent control over motor pattern generation.

The computational analysis also suggested that the delay in the rise of adenosine levels is likely to result from feed‐forward inhibition of the 5′‐ectonucleotidase in the spinal cord. This inhibition is a key determinant of the rate of run‐down.
Evidence that a transcortical pathway contributes to stretch reflexes in the tibialis anterior muscle in man
Authors: Petersen N. , Christensen L. O. D. , Morita H. , Sinkjær T. , Nielsen J.
Published: September 2004

In human subjects, stretch applied to ankle dorsiflexors elicited three bursts of reflex activity in the tibialis anterior (TA) muscle (labelled M1, M2 and M3) at mean onset latencies of 44, 69 and 95 ms, respectively. The possibility that the later of these reflex bursts is mediated by a transcortical pathway was investigated.

The stretch evoked a cerebral potential recorded from the somatosensory cortex at a mean onset latency of 47 ms in nine subjects. In the same subjects a compound motor‐evoked potential (MEP) in the TA muscle, evoked by magnetic stimulation of the motor cortex, had a mean onset latency of 32 ms. The M1 and the M2 reflexes thus had too short a latency to be caused by a transcortical pathway (minimum latency, 79 ms (47 + 32)), whereas the later part of the M2 and all of the M3 reflex had a sufficiently long latency.

When the transcranial magnetic stimulation was timed so that the MEP arrived in the TA muscle at the same time as the M1 or M2 reflexes, no extra increase in the potential was observed. However, when the MEP arrived at the same time as the M3 reflex a significant (P < 0.01) extra‐facilitation was observed in all twelve subjects investigated.

Peaks evoked by transcranial magnetic stimulation in the post‐stimulus time histogram of the discharge probability of single TA motor units (n= 28) were strongly facilitated when they occurred at the same time as the M3 response. This was not the case for the first peaks evoked by electrical transcranial stimulation in any of nine units investigated.

We suggest that these findings are explained by an increased cortical excitability following TA stretch and that this supports the hypothesis that the M3 response in the TA muscle is ‐ at least partly ‐ mediated by a transcortical reflex.
Neutralization of pyrogen‐induced tumour necrosis factor by its type 1 soluble receptor in guinea‐pigs: effects on fever and interleukin‐6 release
Authors: Roth Joachim , Martin Dave , Störr Birgit , Zeisberger Eugen
Published: September 2004

A soluble form of the tumour necrosis factor (TNF) type 1 receptor (referred to as TNF binding protein, TNF‐bp) at a dose of 1 mg per animal, or an equivalent volume of solvent, was injected together with 10 μg kg−1 lipopolysaccharide (LPS) or 50 μg kg−1 muramyl‐dipeptide (MDP) directly into the arterial circulation of guinea‐pigs and the effects on circulating TNF or interleukin‐6 (IL‐6) and on abdominal temperature were studied.

At 15 or 60 min after injection, LPS‐induced and MDP‐induced circulating TNF was below the detection limit of the assay and thus completely neutralized in animals treated with TNF‐bp. In the control group, TNF was still below the limit of detection in most animals 15 min after LPS was injected; in some animals small traces of TNF could already be detected at that time. However, 60 min after administration of LPS, large amounts of TNF (19508 ± 4682 pg ml−1) were measured in the control group. MDP‐induced TNF in plasma was below the limit of detection 15 min after MDP was injected, and rose to 10862 ± 3029 pg ml−1 60 min after injection.

Low levels of circulating IL‐6 (20‐40 international units (IU) ml−1) were measured in all groups of animals 15 min after injection of LPS or MDP. This value corresponds to the baseline activity of IL‐6 in plasma of guinea‐pigs. One hour after administration of LPS, IL‐6 rose to 5442 ± 1662 IU ml−1 in the control group and to a significantly lower value of 1485 ± 179 IU ml−1 in guinea‐pigs treated with TNF‐bp. One hour after injection of MDP, circulating IL‐6 was 2614 ± 506 IU ml−1 in the control group, while the corresponding value in animals treated with TNF‐bp again was significantly lower (873 ± 312 IU ml−1).

The second phase of the characteristic biphasic LPS fever in guinea‐pigs was significantly attenuated in animals treated with TNF‐bp. The shorter first phase of the febrile response to LPS was identical in both groups of animals.

The late phase of MDP‐induced fever (7‐22 h after injection) was depressed by treatment with TNF‐bp, while the first phase of MDP‐induced fever (0‐7 h after injection) was significantly enhanced by the neutralization of TNF by TNF‐bp.
Mechanisms underlying reduced maximum shortening velocity during fatigue of intact, single fibres of mouse muscle
Authors: Westerblad Håkan , Dahlstedt Anders J. , Lännergren Jan
Published: September 2004

The mechanism behind the reduction in shortening velocity in skeletal muscle fatigue is unclear. In the present study we have measured the maximum shortening velocity (V0) with slack tests during fatigue produced by repeated, 350 ms tetani in intact, single muscle fibres from the mouse. We have focused on two possible mechanisms behind the reduction in V0: reduced tetanic Ca2+ and accumulation of ADP.

During fatigue V0 initially declined slowly, reaching 90 % of the control after about forty tetani. The rate of decline then increased and V0 fell to 70 % of the control in an additional twenty tetani. The reduction in isometric force followed a similar pattern.

Exposing unfatigued fibres to 10 μM dantrolene, which reduces tetanic Ca2+, lowered force by about 35 % but had no effect on V0.

In order to see if ADP might increase rapidly during ongoing contractions, we used a protocol with a tetanus of longer duration bracketed by standard‐duration tetani. V0 in these three tetani were not significantly different in control, whereas V0 was markedly lower in the longer tetanus during fatigue and in unfatigued fibres where the creatine kinase reaction was inhibited by 10 μM dinitrofluorobenzene.

We conclude that the reduction in V0 during fatigue is mainly due to a transient accumulation of ADP, which develops during contractions in fibres with impaired phosphocreatine energy buffering.
Altered respiratory responses to hypoxia in mutant mice deficient in neuronal nitric oxide synthase
Authors: Kline David D. , Yang Tianen , Huang Paul L. , Prabhakar Nanduri R.
Published: September 2004

The role of endogenous nitric oxide (NO) generated by neuronal nitric oxide synthase (NOS‐1) in the control of respiration during hypoxia and hypercapnia was assessed using mutant mice deficient in NOS‐1.

Experiments were performed on awake and anaesthetized mutant and wild‐type control mice. Respiratory responses to varying levels of inspired oxygen (100, 21 and 12 % O2) and carbon dioxide (3 and 5 % CO2 balanced oxygen) were analysed. In awake animals, respiration was monitored by body plethysmograph along with oxygen consumption (VO2), CO2 production (VCO2) and body temperature. In anaesthetized, spontaneously breathing mice, integrated efferent phrenic nerve activity was monitored as an index of neural respiration along with arterial blood pressure and blood gases. Cyclic 3′,5′‐guanosine monophosphate (cGMP) levels in the brainstem were analysed by radioimmunoassay as an index of nitric oxide generation.

Unanaesthetized mutant mice exhibited greater respiratory responses during 21 and 12 % O2 than the wild‐type controls. Respiratory responses were associated with significant decreases in oxygen consumption in both groups of mice, and the magnitude of change was greater in mutant than wild‐type mice. Changes in CO2 production and body temperature, however, were comparable between both groups of mice.

Similar augmentation of respiratory responses during hypoxia was also observed in anaesthetized mutant mice. In addition, five of the fourteen mutant mice displayed periodic oscillations in respiration (brief episodes of increases in respiratory rate and tidal phrenic nerve activity) while breathing 21 and 12 % O2, but not during 100 % O2. The time interval between the episodes decreased by reducing inspired oxygen from 21 to 12 % O2.

Changes in arterial blood pressure and arterial blood gases were comparable at any given level of inspired oxygen between both groups of mice, indicating that changes in these variables do not account for the differences in the response to hypoxia.

Respiratory responses to brief hyperoxia (Dejours test) and to cyanide, a potent chemoreceptor stimulant, were more pronounced in mutant mice, suggesting augmented peripheral chemoreceptor sensitivity.

cGMP levels were elevated in the brainstem during 21 and 12 % O2 in wild‐type but not in mutant mice, indicating decreased formation of nitric oxide in mutant mice.

The magnitude of respiratory responses to hypercapnia (3 and 5 % CO2 balanced oxygen) was comparable in both groups of mice in the awake and anaesthetized conditions.

These observations suggest that the hypoxic responses were selectively augmented in mutant mice deficient in NOS‐1. Peripheral as well as central mechanisms contributed to the altered responses to hypoxia. These results support the idea that nitric oxide generated by NOS‐1 is an important physiological modulator of respiration during hypoxia.
Autoregulation of renal blood flow in the conscious dog and the contribution of the tubuloglomerular feedback
Authors: Just Armin , Wittmann Uwe , Ehmke Heimo , Kirchheim Hartmut R.
Published: September 2004

The aim of this study was to investigate the autoregulation of renal blood flow under physiological conditions, when challenged by the normal pressure fluctuations, and the contribution of the tubuloglomerular feedback (TGF).

The transfer function between 0.0018 and 0.5 Hz was calculated from the spontaneous fluctuations in renal arterial blood pressure (RABP) and renal blood flow (RBF) in conscious resting dogs. The response of RBF to stepwise artificially induced reductions in RABP was also studied (stepwise autoregulation).

Under control conditions (n= 12 dogs), the gain of the transfer function started to decrease, indicating improving autoregulation, below 0.06‐0.15 Hz (t= 7‐17 s). At 0.027 Hz a prominent peak of high gain was found. Below 0.01 Hz (t> 100 s), the gain reached a minimum (maximal autoregulation) of ‐6.3 ± 0.6 dB. The stepwise autoregulation (n= 4) was much stronger (‐19.5 dB). The time delay of the transfer function was remarkably constant from 0.03 to 0.08 Hz (high frequency (HF) range) at 1.7 s and from 0.0034 to 0.01 Hz (low frequency (LF) range) at 14.3 s, respectively.

Nifedipine, infused into the renal artery, abolished the stepwise autoregulation (‐2.0 ± 1.1 dB, n= 3). The gain of the transfer function (n= 4) remained high down to 0.0034 Hz; in the LF range it was higher than in the control (0.3 ± 1.0 dB, P< 0.05). The time delay in the HF range was reduced to 0.5 s (P < 0.05).

After ganglionic blockade (n= 7) no major changes in the transfer function were observed.

Under furosemide (frusemide) (40 mg + 10 mg h−1 or 300 mg + 300 mg h−1 i.v.) the stepwise autoregulation was impaired to ‐7.8 ± 0.3 or ‐6.7 ± 1.9 dB, respectively (n= 4). In the transfer function (n= 7 or n= 4) the peak at 0.027 Hz was abolished. The delay in the LF range was reduced to ‐1.1 or ‐1.6 s, respectively. The transfer gain in the LF range (‐5.5 ± 1.2 or ‐3.8 ± 0.8 dB, respectively) did not differ from the control but was smaller than that under nifedipine (P < 0.05).

It is concluded that the ample capacity for regulation of RBF is only partially employed under physiological conditions. The abolition by nifedipine and the negligible effect of ganglionic blockade show that above 0.0034 Hz it is almost exclusively due to autoregulation by the kidney itself. TGF contributes to the maximum autoregulatory capacity, but it is not required for the level of autoregulation expended under physiological conditions. Around 0.027 Hz, TGF even reduces the degree of autoregulation.
Cutaneous receptive fields and topography of mossy fibres and climbing fibres projecting to cat cerebellar C3 zone
Authors: Garwicz Martin , Jörntell Henrik , Ekerot Carl‐Fredrik
Published: September 2004

The topographical organization of mossy fibre input to the forelimb area of the paravermal C3 zone in cerebellar lobules IV and V was investigated in barbiturate‐anaesthetized cats and compared with the previously described microzonal organization of climbing fibre input to the same part of the cortex. Recordings were made in the Purkinje cell and granule cell layers from single climbing fibre and mossy fibre units, respectively, and the organization of cutaneous receptive fields was assessed for both types of afferents.

Based on spatial characteristics, receptive fields of single mossy fibres could be systematized into ten classes and a total of thirty‐two subclasses, mainly in accordance with a scheme previously used for classification of climbing fibres. Different mossy fibres displayed a substantial range of sensitivity to natural peripheral stimulation, responded preferentially to phasic or tonic stimuli and were activated by brushing of hairs or light tapping of the skin.

Overall, mossy fibres to any given microzone had receptive fields resembling the climbing fibre receptive field defining that microzone. However, compared with the climbing fibre input, the mossy fibre input had a more intricate topographical organization. Mossy fibres with very similar receptive fields projected to circumscribed cortical regions, with a specific termination not only in the mediolateral, but also in some cases in the rostrocaudal and dorsoventral, dimensions of the zone. On the other hand, mossy fibre units with non‐identical, albeit usually similar, receptive fields were frequently found in the same microelectrode track.
Harmonic distortion on the basilar membrane in the basal turn of the guinea‐pig cochlea
Authors: Cooper Nigel P.
Published: September 2004

Mechanical responses to pure‐tone stimuli were recorded from the basilar membrane in the basal turn of the guinea‐pig cochlea using a displacement‐sensitive laser interferometer. The harmonic content of the responses was evaluated using Fourier analysis.

Harmonic distortion products were observed in many of the basilar membrane responses. Response components locked to twice the frequency of the stimulus (i.e. 2F0) were the largest of the distortion products.

The second harmonic responses showed a bimodal frequency distribution at low to moderate sound pressure levels: one peak occurred around the preparation's best or most sensitive frequency (i.e. when F0≈ 17 kHz), and another occurred around one‐half of the best frequency (when F0≈ 8.5 kHz).

The absolute levels of most distortion products increased progressively with increasing stimulus strength. When expressed with respect to the levels of the fundamental responses, however, the distortion levels usually decreased with increasing stimulus strength.

The levels of the distortion decreased (in both absolute and relative terms) with deterioration in the physiological condition of the cochlea.

Maximum second harmonic distortion levels amounted to ≈3.5 and ≈28 % of the fundamental responses to tones near and below the best frequency, respectively.

The above findings are shown to be consistent with a highly simplified model of cochlear mechanics which incorporates an asymmetric, saturating non‐linearity in a positive feedback loop.
Models of ensemble firing of muscle spindle afferents recorded during normal locomotion in cats
Authors: Prochazka Arthur , Gorassini Monica
Published: September 2004

The aim of this work was to compare the ability of several mathematical models to predict the firing characteristics of muscle spindle primary afferents recorded chronically during normal stepping in cats.

Ensemble firing profiles of nine hamstring spindle primary (presumed group Ia) afferents were compiled from stored data from 132 step cycles. Three sets of profiles corresponding to slow, medium and fast steps were generated by averaging groups of step cycles aligned to peak muscle length in each cycle.

Five models obtained from the literature were compared. Each of these models was used to predict the spindle firing profiles from the averaged muscle length signals. The models were also used in the reverse direction, namely to predict muscle length from the firing profiles. A sixth model incorporating some key aspects of the other models was also included in the comparisons.

Five of the models predicted spindle firing well, with root mean square (r.m.s.) errors lower than 14% of the modulation depth of the target profiles. The key variable in achieving good predictions was muscle velocity, the best fits being obtained with power‐law functions of velocity, with an exponent of 0.5 or 0.6 (i.e. spindle firing rate is approximately proportional to the square root of muscle velocity). The fits were slightly improved by adding small components of EMG signal to mimic fusimotor action linked to muscle activation. The modest relative size of EMG‐linked fusimotor action may be related to the fact that hamstring muscles are not strongly recruited in stepping.

Length was predicted very accurately from firing profiles with the inverse of the above models, indicating that the nervous system could in principle process spindle firing in a relatively simple way to give accurate information on muscle length.

The responses of the models to standard ramp‐and‐hold displacements at 10 mm s−1 were also studied (i.e. velocities that were an order of magnitude lower than that during stepping). In these cases components of spindle primary response related to length as well as velocity were needed for good fits. Because these length‐related components detracted from rather than improved predictions of the step cycle data, an attenuation of length dependence at high muscle velocities emerged as a possibility.

We conclude that in this study we have identified models and parameters that may be used to predict spindle afferent firing from the time course of muscle length in the cat step cycle.
Mitochondrial oxidative function in human saponin‐skinned muscle fibres: effects of prolonged exercise
Authors: Tonkonogi Michail , Harris Beorn , Sahlin Kent
Published: September 2004

The influence of prolonged exhaustive exercise on mitochondrial oxidative function was investigated in ten men.

Muscle biopsies were taken before and after exercise and mitochondrial respiration investigated in fibre bundles made permeable by pretreatment with saponin.

After exercise, respiration in the absence of ADP increased by 18 % (P < 0.01), but respiration at suboptimal ADP concentration (0.1 mM) and maximal ADP‐stimulated respiration (1 mM ADP) remained unchanged.

In the presence of creatine (20 mM), mitochondrial affinity for ADP increased markedly and respiration at suboptimal ADP concentration (0.1 mM) was similar (pre‐exercise) or higher (post‐exercise; P < 0.05) than with 1 mM ADP alone. The increase in respiratory rate with creatine was correlated to the relative type I fibre area (r= 0.84). Creatine‐stimulated respiration increased after prolonged exercise (P < 0.01).

The respiratory control index (6.8 ± 0.4, mean ± s.e.m.) and the ratio between respiration at 0.1 and 1 mM ADP (ADP sensitivity index, 0.63 ± 0.03) were not changed after exercise. The sensitivity index was negatively correlated to the relative type I fibre area (r=−0.86).

The influence of exercise on muscle oxidative function has for the first time been investigated with the skinned‐fibre technique. It is concluded that maximal mitochondrial oxidative power is intact or improved after prolonged exercise, while uncoupled respiration is increased. The latter finding may contribute to the elevated post‐exercise oxygen consumption. The finding that the sensitivity of mitochondrial respiration for ADP and creatine are related to fibre‐type composition indicates intrinsic differences in the control of mitochondrial respiration between fibres.
The effect of dopamine on lung liquid production by in vitro lungs from fetal guinea‐pigs
Authors: Chua B. A. , Perks A. M.
Published: September 2004

The neuroendocrine system of the lungs has no clear function. However, previous studies of one of its products, somatostatin, have implicated it in lung liquid removal at birth. The present study extends this concept by investigating the effects of dopamine, a major product of this system, on lung liquid reabsorption.

The effects of dopamine on fetal lung liquid production and reabsorption were tested on in vitro lungs from fetal guinea‐pigs of 60 ± 2 days of gestation (term = 67 days). Dopamine was placed in the outer bathing saline during the middle hour of 3 h incubations. Fluid movements across the pulmonary epithelium were monitored by a dye dilution method, and changes in rates over 1 h intervals were tested for significance by analysis of variance and regression analysis.

Dopamine was able to reduce fluid production or cause reabsorption (based on 42 preparations). Control preparations and those given 10−8m dopamine showed no significant changes; those given higher concentrations showed significant reductions in production or reabsorption (P < 0.025 to P < 0.0005), according to dose (42.6 ± 10.8 % reduction at 10−7m; 75.4 ± 5.9 % reduction at 10−6m; 92.1 ± 7.0 % reduction at 10−5m and 121.4 ± 12.8 % (reabsorption) at 10−4m dopamine). The linear log dose‐response curve (r= 0.99) showed a theoretical threshold at 1.7 × 10−9m dopamine.

Effects were mediated through specific dopamine receptors (based on 78 preparations). Dopamine at 10−6m was tested together with each of three dopamine receptor antagonists at 10−5m. The general dopamine receptor antagonist haloperidol and the more specific D2 receptor blocker domperidone both abolished responses, but the D1 receptor antagonist SCH 23390 was without effect. This suggested that D2 dopamine receptors mediated the responses, and that responses were not due to conversion of dopamine to adrenaline or noradrenaline.

There was no evidence that responses involved amiloride‐sensitive Na+ transport (based on 54 preparations). Apical amiloride at 10−6, 10−5 or 10−4m, and the more specific Na+ channel blocker benzamil (10−5m), had no effect on responses to dopamine, in contrast to their effects on responses to adrenaline in sheep.

It is suggested that internal release of dopamine by the neuroendocrine system of the lungs may influence lung liquid reabsorption at birth. This system, which also produces somatostatin, another agent active on lung liquid production, is maximally developed and activated at birth; it is also deficient in hyaline membrane disease.
Induction of antioxidant enzyme activity by hyperoxia (60 % O2) in the developing chick embryo
Authors: van Golde Jolanda C. , Borm Paul J. , Wolfs Marion C. , Rhijnsburger Els H. , Blanco Carlos E.
Published: September 2004

At premature birth, man and animals are exposed to relatively high oxygen levels, compared with intra‐uterine conditions, at a time when their antioxidant enzyme (AOE) system is still immature. Using the chick embryo as a study model, we investigated changes in the AOE system in response to hyperoxia applied at different time points during the incubation period. Relations between hyperoxia and AOE activity were studied in selected organs (brain, heart, liver, intestine and lungs) of developing chick embryos (during the second half of the incubation period).

Incubated White Leghorn eggs were divided into four groups: control (n= 100) and three test groups exposed for 48 h to 60 % O2 on day 10 (test group 1, n= 80), day 14 (test group 2, n= 60) and day 18 (test group 3, n= 30). Superoxide dismutase (SOD), catalase and glutathione peroxidase (GPx) enzyme activities were measured in homogenates of the brain, heart, liver, intestine and lungs.

Exposure to hyperoxia at different time points during incubation resulted in a 2‐ to 10‐fold increase in SOD activity in all organs except the brain. Catalase and GPx enzyme activities were only induced in test group 1, 48 h after initiation of hyperoxia.

In the developing chick embryo, hyperoxia can produce a temporary induction of AOE activity, which is dependent on the AOE, organ, incubation time and time point of exposure.
Ensemble firing of muscle afferents recorded during normal locomotion in cats
Authors: Prochazka Arthur , Gorassini Monica
Published: September 2004

The main purpose of this study was to collate population data on the firing characteristics of muscle afferents recorded chronically during normal stepping in cats.

Ensemble firing profiles of forty‐seven muscle spindle and tendon organ afferents were compiled from stored data. The relationships between the firing profiles and the displacement and force signals were analysed with the help of mathematical models of the response characteristics of spindle primary and secondary afferents and tendon organs.

Whereas the firing of hamstring spindle afferents could be predicted with reasonable accuracy from the length and velocity signals alone, the firing profiles of triceps surae spindle afferents deviated from the predicted profiles, particularly during electromyogram (EMG) activity. This indicated that the components of fusimotor action linked to extrafusal muscle activity were significant in triceps surae, possibly because this muscle is more strongly recruited in the cat step cycle.

From the limited data available, it was not possible to identify the ‘best’ or most general mathematical function to predict spindle secondary firing. In the two triceps surae spindle secondary units studied, firing was well predicted by using the simplest possible model, rate proportional to displacement, whereas in the hamstring spindle secondary data, a more complex linear transfer function was needed. The results of modelling the spindle secondary data were consistent with a modest amount of phasic, static fusimotor action linked to EMG activity.

The averaged ensemble of tendon organ afferent activity from the triceps surae gave predictions of whole‐muscle force that agreed well with separate triceps force measurements in normal cat locomotion. This supports the idea that ensembles of tendon organ afferents signal whole‐muscle force.

Our overall conclusion is that to a first approximation, large muscle afferents in the cat hindlimb signal muscle velocity, muscle length and muscle force, at least in movements of the speed and amplitude seen in locomotion.
The effect of repeated acute hypoxaemia on fetal cardiovascular development in the sheep
Authors: Steyn Clare , Hanson Mark A.
Published: September 2004

Hypoxaemia during intrauterine life may be important in the development of cardiovascular diseases in later life. Thus it was the aim of this study to investigate the effect of repeated acute hypoxia on the cardiovascular development and growth of the fetus.

Fourteen fetal sheep (105–109 days gestational age) were instrumented with amniotic and vascular catheters, an electrocardiogram (ECG) electrode and a flow probe around the femoral artery. Seven animals were given repeated acute isocapnic hypoxaemia (Pa,O2 reduced to ca. 13 mmHg) for 1 h every day for 14 days and they were compared to seven animals which remained normoxic throughout with respect to fetal mean arterial blood pressure (MAP), fetal heart rate (FHR), and fetal baro‐ and chemoreflexes.

No differences were found between the two groups of fetuses in FHR, MAP, baro‐ or chemoreflexes, femoral blood flow, femoral vascular resistance or fetal growth.

Repeated acute hypoxaemia of a moderate degree over a period of 2 weeks in late gestation does not affect cardiovascular development or growth in the fetal sheep.
Changes in single motor unit behaviour contribute to the increase in contraction speed after dynamic training in humans
Authors: Van Cutsem Michaël , Duchateau Jacques , Hainaut Karl
Published: September 2004

The adaptations of the ankle dorsiflexor muscles and the behaviour of single motor units in the tibialis anterior in response to 12 weeks of dynamic training were studied in five human subjects. In each training session ten series of ten fast dorsiflexions were performed 5 days a week, against a load of 30–40 % of the maximal muscle strength.

Training led to an enhancement of maximal voluntary muscle contraction (MVC) and the speed of voluntary ballistic contraction. This last enhancement was mainly related to neural adaptations since the time course of the muscle twitch induced by electrical stimulation remained unaffected.

The motor unit torque, recorded by the spike‐triggered averaging method, increased without any change in its time to peak. The orderly motor unit recruitment (size principle) was preserved during slow ramp contraction after training but the units were activated earlier and had a greater maximal firing frequency during voluntary ballistic contractions. In addition, the high frequency firing rate observed at the onset of the contractions was maintained during the subsequent spikes after training.

Dynamic training induced brief (2–5 ms) motor unit interspike intervals, or ‘doublets’. These doublets appeared to be different from the closely spaced (±10 ms) discharges usually observed at the onset of the ballistic contractions. Motor units with different recruitment thresholds showed doublet discharges and the percentage of the sample of units firing doublets was increased by training from 5.2 to 32.7 %. The presence of these discharges was observed not only at the onset of the series of spikes but also later in the electromyographic (EMG) burst.

It is likely that earlier motor unit activation, extra doublets and enhanced maximal firing rate contribute to the increase in the speed of voluntary muscle contraction after dynamic training.
The role of endothelin‐A receptors in cardiovascular responses to acute hypoxaemia in the late gestation sheep fetus
Authors: Green L. R. , McGarrigle H. H. G. , Bennet L. , Hanson M. A.
Published: September 2004

In unanaesthetized chronically instumented fetal sheep (118‐121 days gestation) we investigated the effect of acute isocapnic hypoxaemia (arterial Po2, 12.5 ± 0.6 mmHg) on heart rate (FHR), mean systemic arterial blood pressure (MABP), carotid and femoral blood flows (CBF and FBF, respectively), and carotid and femoral vascular resistances (CVR and FVR, respectively) with the infusion of either the endothelin‐A (ETA) receptor antagonist FR139317, or saline vehicle.

During normoxaemia FHR (P < 0.05) and CBF (P < 0.01) were greater, and CVR (P < 0.01) was lower with FR139317 than with vehicle infusion. CVR remained lower with FR139317 than with vehicle infusion during hypoxaemia (P < 0.01) and recovery (P < 0.05). During hypoxaemia the rapid initial bradycardia, the increase in MABP and FVR and the decrease in FBF were similar with vehicle and FR139317 infusion. In both groups plasma endothelin‐1 concentration ([ET‐1]) was unaltered by hypoxaemia. The increase in CBF during hypoxaemia with vehicle (P < 0.01) was absent with FR139317 infusion.

Thus in the late gestation ovine fetus endogenous ET‐1 modulates basal FHR, CBF and CVR via ETA receptors. Modulation of CBF and CVR persists during hypoxaemia but ETA receptors do not appear to contribute to the decrease in femoral blood flow measured during acute hypoxaemia.
Bladder motility and efferent nerve activity during isotonic and isovolumic recording in the cat
Authors: Sasaki Mitsuyoshi
Published: September 2004

Bladder motility and pelvic nerve efferent activity were recorded in anaesthetized cats under isotonic or isovolumic conditions. In isotonic conditions the bladder was connected to a reservoir and fluid entering or leaving the bladder was monitored for 8 min using different heads of pressure (isotonic recording). Under isovolumic conditions fixed volumes were injected into the bladder and pressure changes recorded (micturition contractions).

In isotonic conditions, at low pressures the bladder filled to a roughly constant level, and then developed small amplitude oscillatory contractions. At higher pressures, after the initial filling, the bladder contracted, expelling part of its volume. Depending on the pressure this contraction was either sustained or led to a gradual expansion of the bladder. In both cases transient periods of expansion were superimposed. In contrast after an initial slow build up, the efferent nerve activity increased reaching a sustained level of activity during which transient decreases in rate were seen.

The firing frequency of the efferent nerve activity during the sustained phase increased as the intravesical pressure was raised until a plateau was reached at high pressures. The plateau pressure (Pplateau) in a bladder was positively correlated with the peak pressure reached by the bladder during micturition contractions under isovolumic conditions, suggesting that micturition contractions were produced by the maximum output of the reflex pathway.

Section of the hypogastric nerves had relatively little effect, but additional section of the pelvic nerves abolished the contractile phase of the bladder response to filling, and resulted in continuous filling of the bladder.

The transient bladder expansions (duration, 3‐33 s) seen when the innervation was intact were closely related to the transient decreases in efferent nerve activity, suggesting involvement of a central inhibitory mechanism. The threshold pressure for triggering transient expansions was similar to the pressure shown to activate myelinated afferent fibres.

Under isovolumic conditions the frequency of micturition contractions increased with increasing vesical volume, and the relaxed interval between contractions shortened. This response could also be evoked by stimulation of the central cut end of the pelvic nerves, suggesting that the central inhibitory mechanism could also be active during isovolumic conditions.
Oestrogen deficiency impairs intestinal calcium absorption in the rat
Authors: O'Loughlin Peter D. , Morris Howard A.
Published: September 2004

The effects of ovariectomy on the relationships between calcium consumption and calcium balance and its components were assessed in adult (10‐14 months) sham‐operated and ovariectomized (Ovx) rats fed a semi‐synthetic diet with the calcium content varying between 0·02 and 0·4 %.

Adaptation to dietary calcium restriction was monitored for 47 days from commencement of a 0·02 % Ca diet.

Response to 1,25‐dihydroxyvitamin D (20 ng kg−1 day−1) administration in sham and Ovx rats and oestradiol (E2) (20 μg kg−1 day−1) replacement in Ovx rats was assessed in rats fed a 0·05 % Ca diet.

Ovx rats had lower intercepts for the relationships between calcium consumption and both calcium balance (P < 0·005) and intestinal calcium absorption (P < 0·005) compared with sham rats, but 1,25‐dihydroxyvitamin D was not reduced in Ovx rats.

The magnitude of adaptation to dietary calcium restriction was unaffected by ovariectomy.

Intestinal calcium absorption was stimulated by an equivalent amount in sham and Ovx rats following 1,25‐dihydroxyvitamin D administration, although this did not reach statistical significance for sham (sham, t= 1·91, n.s.; Ovx, t= 3·18, P < 0·05).

Oestradiol replacement in Ovx rats induced a marked increase in intestinal calcium absorption (t= 8·25, P < 0·005), without stimulating circulating 1,25‐dihydroxyvitamin D levels and led to a marked increase in calcium balance (t= 6·89, P < 0·005).

These data indicate that the impairment of intestinal calcium absorption following ovariectomy is not the result of reduced circulating 1,25‐dihydroxyvitamin D or reduced intestinal responsiveness to 1,25‐dihydroxyvitamin D. Moreover E2 stimulates intestinal calcium absorption probably by a direct effect on the intestine.
Modulation of reconstituted ATP‐sensitive K+ channels by GTP‐binding proteins in a mammalian cell line
Authors: Sánchez Jorge A. , Gonoi Tohru , Inagaki Nobuya , Katada Toshiaki , Seino Susumu
Published: September 2004

The action of GTP‐binding proteins on ATP‐sensitive potassium (KATP) channels was investigated. KATP channels were expressed in a mammalian cell line (COS‐1 cells) by cotransfecting vectors carrying the sulphonylurea receptor (SUR1) and BIR (Kir6.2), a member of the inward rectifier K+ channel family. G proteins were also tested on KATP channels composed of an isoform of SUR1, SUR2A, in combination with Kir6.2.

The α and βγ subunits of the GTP binding protein Gi were tested separately in inside‐out patches under continuous recording. Gα‐i1 increases the activity of SUR1‐Kir6.2 and SUR2A‐Kir6.2 channels by 200 and by 30 %, respectively.

Gα‐i2 does not increase the activity of SUR1‐Kir6.2 channels, but increases the activity of SUR2A‐Kir6.2 channels by 30 %.

Control experiments showed that GTPγS, a specific activator of G proteins, and heat‐ inactivated Gα‐i1 do not increase the single channel activity.

No effects of the other subunits (βγ) from either Gi1 or Gi2 on the single channel activity were observed.

The protein kinase C inhibitors H7 and an inhibitory peptide (FARKGALRQKNV) had no effect on the modulatory action of Gα‐i1 on SUR1‐Kir6.2 channels.

We conclude that both types of reconstituted KATP channels are modulated by G proteins.
Opposite modulation of NMDA receptors by lysophospholipids and arachidonic acid: common features with mechanosensitivity
Authors: Casado Mariano , Ascher Philippe
Published: September 2004

Two classes of amphiphilic compounds, lysophospholipids and arachidonic acid, have been suggested to produce opposite deformations of the lipid bilayer. We have found that their effects on N‐methyl‐D‐aspartate (NMDA) responses are opposite, and resemble those of mechanical deformations of the plasma membrane.

Lysophospholipids inhibited NMDA responses both in nucleated patches taken from cultured neurons and in cells expressing recombinant NMDA receptors. This inhibition was reversible, voltage independent and stronger at non‐saturating doses of agonist. It was not linked to the charge of the polar head, and was not mimicked by lysophosphatidic acid or phosphatidylcholine. In outside‐out patches, lysophospholipids reduced the open probability of NMDA‐activated channels without changing their single‐channel conductance.

The inhibition produced by lysophospholipids occluded that produced by a mechanical compression induced by changes in osmotic or hydrostatic pressure.

The potentiation of NMDA responses by arachidonic acid was observed both in native and recombinant receptors, including those in which the putative ‘fatty acid binding domain’ had been deleted. This suggests that, like lysophospholipids, arachidonic acid alters the NMDA receptor by insertion into the lipid bilayer.

Recombinant receptors in which the cytoplasmic tails had been modified or deleted were still sensitive to mechanical deformation. A linkage to the cytoskeleton is therefore not required for NMDA receptor mechanosensitivity.

The fact that the NMDA responses are depressed similarly by compression and lysophospholipids, and potentiated similarly by stretch and arachidonic acid supports the notion that the modulation of NMDA receptor activity by asymmetrical amphiphilic compounds involves pressure changes transmitted through the lipid bilayer. Compounds with a large hydrophilic head mimic the effects of a compression, and compounds with a small hydrophilic head mimic the effects of stretch.
An ether ‐à‐go‐go K+ current, Ih‐eag, contributes to the hyperpolarization of human fusion‐competent myoblasts
Authors: Bijlenga Philippe , Occhiodoro Teresa , Liu Jian‐Hui , Bader Charles R. , Bernheim Laurent , Fischer‐Lougheed Jacqueline
Published: September 2004

Two early signs of human myoblast commitment to fusion are membrane potential hyperpolarization and concomitant expression of a non‐inactivating delayed rectifier K+ current, IK(NI). This current closely resembles the outward K+ current elicited by rat ether‐à‐go‐go (r‐eag) channels in its range of potential for activation and unitary conductance.

It is shown that activation kinetics of IK(NI), like those of r‐eag, depend on holding potential and on [Mg2+]o, and that IK(NI), like r‐eag, is reversibly inhibited by a rise in [Ca2+].

Forced expression of an isolated human ether‐à‐go‐go K+ channel (h‐eag) cDNA in undifferentiated myoblasts generates single‐channel and whole‐cell currents with remarkable similarity to IK(NI).

h‐eag current (Ih‐eag) is reversibly inhibited by a rise in [Ca2+]i, and the activation kinetics depend on holding potential and [Mg2+]o.

Forced expression of h‐eag hyperpolarizes undifferentiated myoblasts from −9 to −50 mV, the threshold for the activation of both Ih‐eag and IK(NI). Similarly, the higher the density of IK(NI), the more hyperpolarized the resting potential of fusion‐competent myoblasts.

It is concluded that h‐eag constitutes the channel underlying IK(NI) and that it contributes to the hyperpolarization of fusion‐competent myoblasts. To our knowledge, this is the first demonstration of a physiological role for a mammalian eag K+ channel.
Renal haemodynamic responses to exogenous and endogenous adenosine in conscious dogs
Authors: Berthold Heike , Just Armin , Kirchheim Hartmut R. , Osswald Hartmut , Ehmke Heimo
Published: September 2004

Adenosine has been suggested to be the mediator of a metabolic feedback mechanism which transfers acute changes in the tubular load into opposite changes in renal blood flow (RBF). The goal of the present experiments was to assess the importance of endogenously formed adenosine as a ‘homeostatic metabolite’ during short‐term changes in metabolic demand.

In nine chronically instrumented conscious foxhounds, both the direct effects of adenosine injections (10, 30 and 100 nmol) into the renal artery and the temporal changes of RBF after short renal artery occlusions (15, 30 and 60 s duration), the most widely used experimental model to study the metabolic feedback mechanism in vivo, were studied.

Intrarenal bolus injections of adenosine (10, 30 and 100 nmol) induced dose‐dependent decreases of RBF (RBF: −34 ± 5, −59 ± 4 and −74 ± 4 %, respectively). This vasoconstrictor effect of adenosine was significantly larger (RBF: −51 ± 4, −68 ± 4 and −83 ± 3 %, respectively) when the dogs received a low salt diet.

The post‐occlusive responses were characterized by a transient hyperaemia with no detectable drop of RBF below the preocclusion level. The post‐occlusive responses were affected neither by changes in local angiotensin II levels, nor by intrarenal infusions of hypertonic NaCl or blockade of A1 adenosine receptors.

When intrarenal adenosine levels were elevated by infusion of the adenosine uptake inhibitor dipyridamole, a transient, although weak, post‐occlusive vasoconstriction was detected.

In summary, the present data demonstrate that adenosine acts as a potent renal vasoconstrictor in the conscious dog. The endogenous production of adenosine during short‐lasting occlusions of the renal artery, however, appears to be too small to induce a post‐occlusive vasoconstrictor response of RBF. These results suggest that a metabolic feedback with adenosine as ‘homeostatic metabolite’ is of minor importance in the short‐term regulation of RBF in the conscious, unstressed animal.
Separable effects of human Kvβ1.2 N‐ and C‐termini on inactivation and expression of human Kv1.4
Authors: Accili E. A. , Kuryshev Y. A. , Wible B. A. , Brown A. M.
Published: September 2004

The Kvβ subunits of voltage‐gated K+ channels alter the functional expression and gating of non‐ or slowly inactivating Kvα1 subunits via two separate domains. To determine how Kvβ subunits modulate a rapidly inactivating Kvα1 subunit, we did two‐microelectrode voltage clamp experiments on human Kv1.4 voltage‐gated K+ channels expressed heterologously in Xenopus oocytes. In addition we tested a slowly inactivating mutant of Kv1.4 lacking amino acids 2–146 of the N‐terminal α‐ball domain (Kv1.4ΔN2‐146). Kv1.4 or Kv1.4ΔN2‐146 were co‐expressed with either rat Kvβ2 or human Kvβ1.2. To separate domain effects, we also used a mutant of Kvβ1.2 lacking the unique 79 amino acid N‐terminal β‐ball domain (Kvβ1‐C).

For the mutant Kv1.4ΔN2‐146 we found that Kvβ1‐C or Kvβ2 increased current amplitude without altering activation or inactivation. By contrast Kvβ1.2 produced rapid inactivation and slowed deactivation due to block produced by the β‐ball. The β‐ball also increased the rate of C‐type inactivation in 5 mm, but not 50 mm, external K+ consistent with an effect of blockade on K+ efflux.

For Kv1.4, Kvβ1‐C produced a voltage‐independent increase in the rate of inactivation and shifted the inactivation curve to more hyperpolarized potentials, but had no effect on deactivation. Kvβ1‐C, Kvβ2 and Kvβ1.2 slowed recovery from inactivation similarly, thereby excluding involvement of the β‐ball. Kvβ1.2 produced an additional more rapid, voltage‐dependent component of inactivation, significantly reduced peak outward current and shifted steady‐state inactivation towards hyperpolarized potentials.

Yeast two‐hybrid studies showed that α‐β interaction was restricted to the N‐terminus of Kv1.4 and the C‐terminus of Kvβ1.2 or Kvβ2. Direct interaction with the α‐ball did not occur. Our interpretation is that Kvβ1‐C and Kvβ2 enhanced N‐type inactivation produced by the Kv1.4 α‐ball allosterically.

We propose that Kvβ1.2 has three effects on Kv1.4, the first two of which it shares with Kvβ2. First, Kvβ1‐C and Kvβ2 have a current‐enhancing effect. Second, Kvβ1‐C and Kvβ2 increase block by the α‐ball allosterically. Third, the β‐ball of Kβ1.2 directly blocks both Kv1.4 and Kv1.4ΔN2‐146. When both α‐ and β‐balls are present, competition for their respective binding sites slows the block produced by either ball.