bluuebunny/crossref_metadata_2025 huggingface dataset:
Two components of blood‐brain barrier disruption in the rat
Authors: Easton A. S. , Sarker M. H. , Fraser P. A.
Published: September 2004

Permeability of pial venular capillaries to Lucifer Yellow (Ply) was measured using the single microvessel occlusion technique.

P

ly
was extremely low, when measured shortly after the removal of the meninges, consistent with an intact blood‐brain barrier, but rose spontaneously to (1.65 ± 060)× 10−6cms−1 (mean ±s.d.) within 20–‐60 min. This first phase of spontaneous disruption lasted 44–164 min. A second phase started when Ply rose sharply, and was characterized by rapid permeability fluctuations with a mean of (12.31 ± 15.14) × 1CT−6 cm s−1.

The first phase could be mimicked by applying the divalent cation ionophore A23187 in the presence of Ca2+, when Ply rose by (1.47±0–25) × 10−6 cm s−1 (mean±s.e.m.). Application of histamine (10 μm) to tight vessels increased Ply by (2.41 ± 0.22) × 10−6 cm s−1.

Substances that raised intraendothelial cAMP of vessels during the first phase of disruption reduced Ply to the initial blood‐brain barrier level.

The second phase could be prevented by applying catalase. Similar high and fluctuating Ply values could be produced reversibly by applying arachidonic acid or NH4C1.

This is the first report of two distinct types of permeability increase in the cerebral microvasculature, and reasons for this are discussed.
Onset of Bursting in Oxytocin Cells in Suckled Rats
Authors: Brown David , Moos Françoise
Published: September 2004

We tested whether firing characteristics are associated with the onset of bursting in oxytocin cells during suckling. Extracellular electrical activity of paraventricular and supraoptic oxytocin cells was recorded in lactating rats from the beginning of suckling up to the first milk‐ejection burst, which occurred either within less than 1 h of suckling (bursting cells) or after injecting facilitatory drugs (non‐bursting cells).

Significant differences in the distributions of firing rate (using low, intermediate and high categories, ≤ 1, 1–3 and > 3 spikes s−1, respectively) of bursting and non‐bursting cells were observed during suckling. Thirty minutes after pup application, most bursting cells (56%) had an intermediate firing rate, whereas non‐bursting cells had either a low (36%) or high (40%) firing rate.

Thirty minutes after applying the pups, probability of bursting was highest for cells firing at 1–4 spikes s−1, and lowest for those firing above 5 spikes s−1.

Bursting cells with high initial firing rate decreased their firing rate substantially during suckling while most of those with low initial firing rate showed increases. For non‐bursting cells, activity was maintained.

There were marked differences in firing rate and its evolution between paired bursting cells. The firing rates for non‐bursting cell pairs were similar (mostly either low or high), and remained stable during suckling.

In conclusion, we suggest that, as suckling proceeds, probability of bursting is related to the firing rate of oxytocin cells within the whole population, more specifically to the proportion of cells within the animal initially or subsequently displaying a critical firing rate (between 1 and 3 spikes s−1). The firing rates of cells which eventually burst and which are firing outside this range change in a direction which brings them into the preferred range for bursting.

We suggest that when bursting occurs unaided, a majority of oxytocin cells fire in the preferred bursting range and facilitate the bursting of cells outside this range. Since such cooperativity does not develop between non‐bursting cells, it might be due to centrally released oxytocin.
Different roles for GABAA and GABAB receptors in visual processing in the rat superior colliculus
Authors: Binns K. E. , Salt T. E.
Published: September 2004

The superficial grey layer of the superior colliculus (SGS) contains a high proportion of GABAergic inhibitory neurones. We have investigated the role of GABA receptors in synaptic transmission of aspects of visual activity in the SGS that may be driven by inhibitory mechanisms, such as surround inhibition and response habituation.

Multi‐barrel glass iontophoretic pipettes were used to record single neuronal activity in the SGS of urethane‐anaesthetized rats. Visual stimulation was provided by the display of moving bars and stationary spots of light on a monitor placed in the receptive field.

Both ejection of GABA and the GABAB agonist baclofen reduced responses to moving bars (interstimulus intervals ≥ 8 s). The effects of GABA were reversed by the GABAA antagonist bicuculline, and the effects of baclofen were antagonized by the GABAB antagonist CGP 35348.

Surround inhibition was estimated by plotting the response to flashed spots of increasing diameter. In controls, expanding the spot diameter beyond the excitatory receptive field caused a decrease in the response. This inhibitory surround was reversibly reduced by bicuculline, but CGP 35348 had no effect.

Response habituation is the progressive reduction in the visual response during repetitive stimulus presentation. In controls, the visual response was reduced to 44 ± 3% of its initial level when a stimulus (moving bar) was presented 5 times with an interstimulus interval of 0.5 s. During CGP 35348 ejection, response habituation was reversibly reduced. Bicuculline had no effect on response habituation.

The effects of bicuculline on surround inhibition in the superior colliculus are consistent with similar studies in the lateral geniculate nucleus which indicate that GABAA receptors mediate this effect. The function of GABAB receptors in the visual system is less well researched. The reduction of response habituation with CGP 35348 demonstrates that, at least in the SGS, GABAB receptors have an important role in visual transmission which is distinct from that of GABAA receptors.
Identification of a Zn2+ binding site on the marine gABAA receptor complex: Dependence on the Second transmembrane domain of β subunits
Authors: Wooltorton Julian R. A. , McDonald Bernard J. , Moss Stephen J. , Smart Trevor G.
Published: September 2004

Whole‐cell currents were recorded from Xenopus laevis oocytes expressing wild‐type and mutant recombinant GABAA receptors to locate a binding site for Zn2+ ions in the β3 subunit.

The Cl−‐selective current, spontaneously gated by β3 subunit homomers, was enhanced by pentobarbitone and inhibited by picrotoxinin. The potencies of these agents were minimally affected by mutating histidine (H) 292 to alanine (A) in the second transmembrane domain (TM2).

Zn2+ inhibited the β3 subunit‐gated conductance (IC50, 0.31 μm); the inhibition was voltage insensitive. The H292A mutation in β3 subunits caused a 1000‐fold reduction in Zn2+ potency (IC50, 307 μm).

GABA‐activated responses recorded from heteromeric α1β3 GABAA receptors were also inhibited by Zn2+ (IC50, 0.11 μm). This inhibition was reduced by mutating H292A in the β3 subunit (IC50, 22.8 μm).

H292 in TM2 of the β3 subunit is an important determinant of a Zn2+ binding site on the GABAA receptor. Its location in the presumed ion channel lining suggests that Zn2+ can penetrate into an anion‐selective channel and that the ionic selectivity filter and channel gate are located beyond H292.
Nitric Oxide Contributes to Substance P‐Induced Increases in Lung Rapidly Adapting Receptor Activity in Guinea‐Pigs
Authors: Joad Jesse P. , Kott Kayleen S. , Bonham Ann C.
Published: September 2004

Substance P induces fluid flux via nitric oxide, and fluid flux stimulates lung rapidly adapting receptors (RARs). We therefore proposed that nitric oxide contributes to substance P‐evoked increases in RAR activity. Since substance P decreases dynamic compliance (Cdyn), which can stimulate RARs, we also determined whether nitric oxide contributed to substance P‐induced effects on pulmonary function.

In anaesthetized guinea‐pigs, the effects of substance P on RAR activity, Cdyn, pulmonary resistance (RL), and arterial blood pressure were measured before and after i.v. infusion of NG‐methyl‐l‐arginine (l–NMMA; a nitric oxide synthase inhibitor), or l–NMMA followed by l‐arginine (a nitric oxide precursor which reverses the effects of l–NMMA).

Substance P‐evoked increases in RAR activity were blunted by l–NMMA (P= 0.006) but not by l–NMMA–l‐arginine (P= 0.42).

Substance P‐evoked decreases in Cdyn were slightly inhibited by l–NMMA (P= 0.02) and slightly enhanced by l‐NMM A–l‐arginine (P= 0.004). However, at the time at which l–NMMA maximally reduced substance P‐induced RAR stimulation (the first 30 s), it did not change substance P‐induced decreases in Cdyn.

Substance P‐evoked increases in RL were not changed by l–NMMA (P= 0.10) and were enhanced by l‐NMM A–l‐arginine (P= 0.03).

l–NMMA‐evoked increases in mean arterial blood pressure were reversed by l‐arginine. Substance P‐evoked decreases in mean arterial blood pressure were not changed by l–NMMA or by l–NMMA–l‐arginine.

We conclude that nitric oxide contributes to substance P‐evoked increases in RAR activity and that the increases are most probably independent of decreases in Cdyn.
Fluid transport across cultures of human tracheal glands is altered in cystic fibrosis
Authors: Jiang C. , Finkbeiner W. E. , Widdicombe J. H. , Miller S. S.
Published: September 2004

There is evidence that defective submucosal gland secretion contributes to the airway pathology of cystic fibrosis (CF). Using a capacitance probe technique, we have compared fluid transport across submucosal gland cultures from individuals with and without CF.

Under baseline conditions, ∼60% of non‐CF cultures secreted fluid; the rest absorbed. In secreting tissues, amiloride increased secretion, whereas in absorbing tissues it reduced or reversed absorption. 5‐Mtro‐2‐(3‐ phenylpropylamino)‐benzoate (NPPB) a blocker of the CF transmembrane conductance regulator (CFTR), converted secretion to absorption. Thus, the direction and magnitude of baseline fluid movement depended on a balance between active absorption of Na+ and cAMP‐dependent secretion of CI−.

8‐(4‐Chlorophenylthio)‐adenosine 3',5'‐cyclic monophosphate (CPT‐cAMP), methacholine and luminal uridine 5'‐triphosphate (UTP) all induced or increased fluid secretion across non‐CF cultures. Results with NPPB and with 4,4'‐diisothiocyanatostilbene‐2,2'‐disulphonate (DIDS), a blocker of Ca2+ ‐activated CI− channels, suggested that fluid secretion induced by CPT‐cAMP was mediated primarily by CFTR; UTP acted entirely via Ca2+ ‐activated CI− channels, and methacholine activated both pathways.

All CF cultures showed baseline fluid absorption, which was abolished by amiloride.

CF cultures showed a normal secretory response to UTP, a reduced response to methacholine, and no response to CPT‐cAMP.

Thus, the absorptive processes of airway glands are retained in CF, but the cAMP‐dependent secretory process is lost. This would markedly reduce the water content of gland secretions. The resulting change in viscosity would contribute to the accumulation of airway mucus which is characteristic of this disease.
Inhibition of ATP‐activated current by zinc in dorsal root ganglion neurones of bullfrog
Authors: Li Chaoying , Peoples Robert W. , Weight Forrest F.
Published: September 2004

The effect of Zn2+ on ATP‐activated current was studied in bullfrog dorsal root ganglion (DRG) neurones using the whole‐cell patch‐clamp technique.

Zn2+ (2–800 μm) inhibited current activated by submaximal concentrations of ATP. The Zn2+ concentration that produced 50% inhibition (IC50) of current activated by 2.5 μm ATP was 61 ± 9.8 μm. When ATP concentrations were adjusted to account for chelation of Zn2+, the IC50 of Zn2+ was 86 ± 18 μm.

The inhibitory action of Zn2+ on ATP‐gated channels did not appear to be due to a decrease in the concentration of one or more species of ATP.

Zn2+ inhibition of ATP‐activated current was independent of membrane potential between −80 and +40 mV, and did not involve a shift in the reversal potential of the current.

Zn2+ (100 μm) shifted the ATP concentration‐response curve to the right in a parallel manner, increasing the EC50 for ATP from 2.5 ± 0.5 μm to 5.5 ± 0.4 μm.

Zn2+ decreased the time constant of deactivation of ATP‐gated ion channels without affecting the time constant of activation or desensitization.

Dithiothreitol (DTT) reversed Zn2+ inhibition of ATP‐activated current.

2‐Methylthio ATP, α,β‐methylene ATP and ADP activated current with EC50 values of 2.4 ± 0.3, 50.1 ± 5.8 and 303.1 ± 53.9 μm, respectively. Adenosine, AMP or β,γ‐methylene ATP did not evoke detectable current.

Reactive Blue 2 and pyridoxal‐phosphate‐6‐azophenyl‐2′,4′‐disulphonic acid inhibited ATP‐activated current.

The results suggest that Zn2+ can inhibit P2X purinoceptor function by decreasing the affinity of the binding site for ATP. These observations provide the first evidence for this action of Zn2+ on a neurotransmitter‐gated ion channel. Furthermore, the receptor‐channel in these neurones appears to be a novel member of the P2X purinoceptor class.
Desensitization shortens the high‐quantal‐content endplate current time course in frog muscle with intact cholinesterase
Authors: Giniatullin R. A. , Talantova M. , Vyskočil F.
Published: September 2004

The desensitization induced by bath‐applied carbachol or acetylcholine (ACh) and potentiated by proadifen (SKF 525A) was studied in the frog sartorius with intact synaptic acetylcholinesterase (AChE).

The reduction in the density and number of postsynaptic receptors produced by desensitization lowered the amplitude of the endplate currents (EPCs) and shortened the EPC decay when the quantal content (m) of the EPC was about 170 and when multiple release of quanta at single active zones was highly probable. The shortening of high‐quantal‐content EPCs persisted for at least 15 min after the wash‐out of agonists, at a time when the amplitude had recovered fully.

The decay times of the low‐quantal‐content EPCs recorded from preparations pretreated with 5 mm Mg2+ (m≈ 70) and single‐quantum miniature endplate currents (MEPCs) were not affected by carbachol, ACh or proadifen.

The desensitization of ACh receptors potentiated by proadifen, prevented completely the 6‐ to 8‐fold prolongation of EPC which was induced by neostigmine inhibition of synaptic AChE.

It is assumed that high‐quantal‐content EPCs increase the incidence of multiple quanta release at single active zones and the probability of repetitive binding of ACh molecules which leads to EPC prolongation. The shortening which persists after complete recovery of the amplitude during wash‐out of the exogenous agonist is probably due to ‘trapping’ of ACh molecules onto rapidly desensitized receptors and the reduced density of functional AChRs during the quantum action.
Hyaluronan Secretion into the Synovial Cavity of Rabbit Knees and Comparison with Albumin Turnover
Authors: Coleman P. J. , Scott D. , Ray J. , Mason R. M. , Levick J. R.
Published: September 2004

Hyaluronan is not only a lubricant but also enhances the synovial lining's resistance to fluid outflow. This finding led to the proposal that hyaluronan (> 2 × 106Da, ∼210 nm radius) may escape across the synovial lining less freely than smaller solutes (e.g. albumin, 6.7 × 104 Da, 3.6 nm radius) or water. Here multiple washouts were used to measure intra‐articular hyaluronan mass and secretion rate in rabbit knees, leading to an estimate of hyaluronan turnover time. Plasma albumin permeation into the joint cavity was also measured to enable comparison of turnover times between molecules of very disparate size.

Endogenous hyaluronan mass in the joint cavity, analysed by high performance liquid chromatography of joint washes, was 182 ± 9.9 μg (mean ± S.E.M; n= 21). Since hyaluronan concentration in synovial fluid averages 3.62 ± 0.19 μg μl−1, the endogenous synovial fluid volume was calculated to be 50 μl (mass/concentration), about double the aspiratable volume.

The hyaluronan secretion rate over 4h was 4.80 ± 0.77μg h−1 (n= 5). The rate was significantly higher in contralateral joints expanded by 2 ml Ringer solution (5.80 ± 0.84 μg h −1, n= 5, P= 0.01, Student's paired t test), indicating a stretch/hydration sensitive secretory mechanism. The newly secreted chains ((2.05–2.48) × 108 Da) were not significantly different in length from the endogenous chains (2.95 × 108 Da).

Hyaluronan turnover time, calculated as mass/secretion rate, was 31.4–37.9 h. This is more than an order of magnitude longer than turnover time for intra‐articular albumin. The latter, determined from the intra‐articular albumin mass and plasma‐to‐cavity permeation rate was 1.8 h (95% confidence intervals 1.2–3.5 h, n= 9). The big difference in turnover times support the view that, relative to albumin and water, hyaluronan is partially sieved out and retained in the joint cavity by the synovial lining. The lining cell layer is discontinuous, so it appears that interstitial matrix itself acts as a leaky size‐selective molecular filter.
Evidence for the involvement of histaminergic neurones in the regulation of the rat oxytocinergic system during pregnancy and parturition
Authors: Luckman Simon M. , Larsen Philip J.
Published: September 2004

Previous studies have shown that histaminergic neurones of the tuberomammillary nucleus project directly to hypothalamic magnocellular nuclei and that intracerebroventricular administration of histamine increases the synthetic activity of magnocellular oxytocin neurones.

Histaminergic neurones of the dorsomedial tuberomammillary nucleus that project to the magnocellular region of the paraventricular nucleus are activated during late pregnancy and lactation, as measured by an increase in mRNA for the synthetic enzyme histidine decarboxylase.

There is a concomitant increase in oxytocin mRNA in magnocellular neurones of the paraventricular nucleus. This increase in mRNA contributes to an accumulation of oxytocin before birth and to continued oxytocin synthesis during lactation.

Intracerebroventricular administration of mepyramine, a specific antagonist of the H1histamine receptor, causes a delay in the birth of subsequent pups if given to the mother during parturition. Vehicle or the H2 receptor antagonist cimetidine has no effect. Thus, histamine acts centrally, via H1 receptors, during parturition and may have an excitatory effect on oxytocin release.

These results suggest that afferent histaminergic neurones show increased activity during pregnancy and may be responsible for the increase of synthesis in magnocellular oxytocin neurones at this time. If, as previously reported, these histamine neurones can reduce the electrical activity of oxytocin neurones via H2 receptors, then they may have a dual effect, increasing the synthesis of oxytocin while inhibiting its premature release. At term, any inhibitory effects of histamine are overcome to allow oxytocin secretion.
Block of open channels of recombinant AMPA receptors and native AMPA/kainate receptors by Adamantane derivatives
Authors: Magazanik L. G. , Buldakova S. L. , Samoilova M. V. , Gmiro V. E. , Mellor I. R. , Usherwood P. N. R.
Published: September 2004

The effects of two adamantane derivatives, 1‐trimethylammonio‐5‐(l‐adamantane‐methylammoniopentane dibromide) (IEM‐1460) and 1‐ammonio‐5‐(l‐adamantane‐methylammoniopentane dibromide) (IEM‐1754) on kainate‐induced currents were studied in Xenopus oocytes expressing recombinant ionotropic glutamate receptors and in freshly isolated neurones from rat hippocampal slices.

The adamantane derivatives caused use‐and voltage‐dependent block of open channels of recombinant AMPA receptors. This antagonism was dependent on receptor subunit composition; channels gated by recombinant, homomeric GluRl and GluR3 receptors exhibited a higher sensitivity to block than those gated by receptors containing edited GluR2 subunits. In the former cases, IEM‐1460 had an IC50 of 1.6 μm at a holding potential (Vh) of −80 mV and IEM‐1754 was 3.8 times less potent than IEM‐1460. In contrast, 100 μm IEM‐1460 inhibited responses to 100 μm kainate of receptors containing edited GluR2 subunits by only 7.8 ± 2.4% (n= 5 oocytes) at a Vh of −80 mV.

Native AMPA/kainate receptors in isolated hippocampal cells were inhibited by adamantane derivatives in a use‐ and voltage‐dependent manner. This antagonism was dependent on cell type: pyramidal neurones were less sensitive to IEM‐1460 (IC50= 1617 μm at Vh=−80mV) than interneurones (IC50= 1.6 μm at Vh=−80 mV). IEM‐1460 and IEM‐1754 were equipotent when applied to pyramidal neurones, but IEM‐1754 was less potent (∼3 times) than IEM‐1460 when applied to interneurones.

It is concluded that the presence of the edited GluR2 subunit in recombinant AMPA receptors and native AMPA/kainate receptors inhibits channel block by organic cations and that adamantane derivatives are potentially valuable tools for identifying classes of AMPA/kainate receptors and their roles in synaptic transmission.
Renal tubular lithium reabsorption in potassium‐depleted rats
Authors: Shirley D. G. , Walter S. J.
Published: September 2004

In order to identify the tubular sites responsible for the reduced fractional excretion of lithium (FELi) during potassium depletion, free‐flow micropuncture was performed in anaesthetized rats that had been fed a low potassium (low‐K+) diet or a control diet for 5–6 days. FELi in low‐K+ rats was 0.09 ± 0.02, compared with 0.25 ± 0.01 in control animals.

Fractional water reabsorption in proximal convoluted tubules was enhanced in potassium‐depleted rats. However, fractional lithium reabsorption was not. Consequently, the tubular fluid‐to‐plasma lithium concentration ratio at the late proximal convoluted tubule was raised in the low‐K+ animals (1.50 ± 0.03 vs. 1.18 ± 0.02; P < 0.001).

Fractional lithium delivery to the early distal tubule in low‐K+ rats (0.31 + 0.01) was similar to that in control animals (0.30 + 0.02). However, whereas in control rats there was no significant difference between early and late distal tubular deliveries of lithium, late distal fractional lithium delivery in the low‐K+ group was reduced markedly (to 0.10 ± 0.01).

Treatment of potassium‐depleted rats with amiloride had no effect on lithium reabsorption in the proximal convoluted tubule or loop of Henle. However, fractional lithium delivery to the end of the distal tubule was increased slightly (to 0.15 + 0.02; P < 0.05) and FELi was increased substantially (to 0.22 ± 0.01; P < 0.001).

It is concluded that two factors contribute to the reduced FELi seen in potassium‐depleted rats: lithium reabsorption in the superficial distal tubules and amiloride‐sensitive lithium reabsorption in the collecting ducts. The data also suggest heterogeneity with respect to lithium handling between superficial and deep nephrons during potassium depletion.
Partial depletion of sarcoplasmic reticulum calcium does not prevent calcium sparks in Rat Ventricular myocytes
Authors: Song Long‐Sheng , Stern Michael D. , Lakatta Edward G. , Cheng Heping
Published: September 2004

The exact nature of calcium sparks in the heart remains highly controversial. We sought to determine whether calcium sparks arise from a single or multiple calcium release channels/ryanodine receptors in the sarcoplasmic reticulum (SR). If their genesis involves a calcium‐coupled recruitment of multiple channels, calcium sparks might be abolished by a modest depletion of SR. calcium (because of the decrease in unitary calcium flux and hence a decrease in the gain of local calcium‐induced calcium release). If, on the other extreme, calcium sparks are produced despite severe SR depletion, the single‐channel origin will be preferred.

Spontaneous calcium sparks were studied in rat ventricular myocytes using confocal microscopy and the fluorescent calcium probe fluo‐3. A computer algorithm was developed to count and measure objectively calcium sparks in linescan images.

Thapsigargin (25–150 nm) depleted caffeine‐releasable SR calcium by up to 64%, in a dose‐and time‐dependent manner, without altering the resting cytosolic calcium level. During SR depletion, calcium sparks were robustly observed, albeit at reduced frequency (≥ 30% of control) and amplitude (≥ 60% of control).

Due to the reduced detectability of small sparks against noise background, the observed data would overestimate reduction in spark frequency but underestimate amplitude reduction. After correction for this detection bias, we found that the spark frequency was independent of SR load, whereas the amplitude was proportional to load.

We conclude that, although spark amplitude depends on SR filling status, the frequency of spark generation is independent of SR calcium load, and therefore independent of the local calcium release rate. This implies that sparks are single‐channel events, or collective events that are well above threshold for local regeneration. Additionally, our results suggest that intraluminal SR calcium, at normal or low loads, does not play a major role in the regulation of on‐gating of the ryanodine receptor.
Sympathetic nerve traffic correlates with the release of nitric oxide in humans: implications for blood pressure control
Authors: Skarphedinsson Jon O. , Elam Mikael , Jungersten Lennart , Wallin B. Gunnar
Published: September 2004

Resting human sympathetic vasoconstrictor traffic displays large reproducible inter‐individual differences which are similar in nerves to muscle, heart and kidney. In spite of this there is no correlation between levels of blood pressure and sympathetic traffic. To test the hypothesis that the pressor effect of the vasoconstrictor activity is counteracted by a circulating dilating factor we measured muscle nerve sympathetic activity (MSA) and an indicator of nitric oxide release (plasma nitrate) in healthy young males.

Sympathetic activity was recorded with the microneurographic technique in the peroneal nerve and a forearm venous plasma sample was obtained in twenty‐one normotensive males aged 21–28 years. Plasma nitrate was analysed by gas chromatography and mass spectrometry.

There was a positive linear correlation between the plasma nitrate concentration and the strength of MSA both when the nerve activity was expressed as bursts per minute and bursts per 100 heart beats (r= 0.51, P= 0.02 and r= 0.46, P= 0.04, respectively).

The data suggest that the stronger the sympathetic activity the higher the release of the dilating substance, nitric oxide. This would be expected to counteract vasoconstrictor effects of the nerve traffic and thereby contribute to the lack of relationship between resting levels of MSA and blood pressure. We speculate that altered coupling between sympathetic traffic and nitric oxide release may cause abnormal peripheral resistance, e.g. in hypertension.
Evidence from Motoneurone Synchronization for Disynaptic Pathways in the Control of Inspiratory Motoneurones in the Cat
Authors: Vaughan C. W. , Kirkwood P. A.
Published: September 2004

Motoneurone synchronization was measured by cross‐correlation between paired inspiratory discharges in external and internal intercostal nerves or their intramuscular branches (T3 to T8) or in the phrenic nerve (C5 root or both C5 and C6 roots independently) in anaesthetized, paralysed cats.

All cross‐correlation histograms showed central peaks, for which the durations at half‐amplitude (half‐widths) from internal nerve pairs in adjacent segments were all less than for external nerve pairs in adjacent segments or within a segment (means, 1.6 ms vs. 3.4 ms for adjacent segments). Values for external‐internal pairs covered the ranges for both these two. Lowest values came from two phrenic pairs (1.2 and 1.4 ms).

The peaks from ipsisegmental external–internal pairs were usually asymmetric and the maximum of the peak was often displaced to a lag of about –1 ms (external nerve providing the reference spikes), whereas peaks from external–external pairs were always symmetrical and centred on zero. Phrenic–internal peaks gave maxima with lags about 1 ms less than for phrenic–external peaks from the same segments.

Two explanations were considered possible for the differences in duration and timing: an extra synapse on the pathway to the external nerve motoneurones, or a correlation kernel for a monosynaptic connection to the external nerve motoneurones that had a slower time course than that for the internal or phrenic nerve motoneurones. Computer simulations, assuming the extra synapse, gave a good fit to the observed time courses of the correlation peaks for all categories of nerve pairs using single values of parameters (e.g. EPSP rise time) consistent with those in the literature. This could not be achieved with the different correlation kernel model. The timing of high‐frequency oscillation (HFO), which was sometimes present in the correlations, was also better predicted with the extra synapse model.

It is concluded that most of the synchronization between external nerve motoneurones is derived from disynaptic common inputs and that any motoneurone synchronization peak with a half‐width greater than about 2.2 ms should be assumed to be likely to contain di‐ or oligosynaptically derived components.
The interplay of central and peripheral factors in limiting maximal O2 consumption in man after prolonged bed rest
Authors: Ferretti Guido , Antonutto Guglielmo , Denis Christian , Hoppeler Hans , Minetti Alberto E. , Narici Marco V. , Desplanches Dominique
Published: September 2004

The effects of bed rest on the cardiovascular and muscular parameters which affect maximal O2 consumption (VO2,max) were studied. The fractional limitation of VO2,max imposed by these parameters after bed rest was analysed.

The VO2,max, by standard procedure, and the maximal cardiac output (Q̇max), by the pulse contour method, were measured during graded cyclo‐ergometric exercise on seven subjects before and after a 42‐day head‐down tilt bed rest. Blood haemoglobin concentration ([Hb]) and arterialized blood gas analysis were determined at the highest work load.

Muscle fibre types, oxidative enzyme activities, and capillary and mitochondrial densities were measured on biopsy samples from the vastus lateralis muscle before and at the end of bed rest. The measure of muscle cross‐sectional area (CSA) by NMR imaging at the level of biopsy site allowed computation of muscle oxidative capacity and capillary length.

The VO2max was reduced after bed rest (−16.6%). The concomitant decreases in Q̇max (−30.8%), essentially due to a change in stroke volume, and in [Hb] led to a huge decrease in O2 delivery (−39.7%).

Fibre type distribution was unaffected by bed rest. The decrease in fibre area corresponded to the significant reduction in muscle CSA (−17%). The volume density of mitochondria was reduced after bed rest (−16.6%), as were the oxidative enzyme activities (−11%). The total mitochondrial volume was reduced by 28.5%. Capillary density was unchanged. Total capillary length was 22.2% lower after bed rest, due to muscle atrophy.

The interaction between these muscular and cardiovascular changes led to a smaller reduction in VO2max than in cardiovascular O2 transport. Yet the latter appears to play the greatest role in limiting VO2max after bed rest (>70% of overall limitation), the remaining fraction being shared between peripheral O2 diffusion and utilization.
Mass spectrometric determination of HCO3− permeability and carbonic anhydrase activity in intact guinea‐pig colon epithelium
Authors: Böllert Petra , Peters Thomas , von Engelhardt Wolfgang , Gros Gerolf
Published: September 2004

A mass spectrometric method originally used in red blood cells was applied to suspensions of isolated colonocytes and intact colonic epithelium to measure the exchange of 18O between HCO3−, CO2 and H2O to determine intracellular carbonic anhydrase activity (Ai) and membrane bicarbonate permeability (P).

In suspensions of isolated guinea‐pig colon epithelial cells, colonocytes, we found significantly higher values of Ai and P for cells derived from the proximal colon than for cells from the distal colon. In the case of Ai, this confirms earlier reports.

When the 18O exchange process was observed across the mucosal (apical) side of intact colon mucosa, the estimated values of Ai were identical to those obtained for isolated colonocytes, for both the proximal and the distal part of the colon. This is considered to be strong evidence that this method can be applied to a layer of intact epithelium as well as to cell suspensions.

The values of P obtained from the apical side of intact colon mucosa were 6 times higher than those estimated from measurements with isolated colonocytes. This indicates that the basolateral membrane of colon epithelium, which participates in the 18O exchange process in isolated colonocytes but not in the 18O exchange process across the apical side of intact mucosa, has a markedly lower bicarbonate permeability than the apical membrane.

When the 18O exchange process was observed across the serosal (basolateral) side of intact colon mucosa, the P values, as expected, were low compared with the apical side of intact mucosa. However, rather unexpectedly, the Ai values derived from these measurements were 2–3 times lower than those obtained with isolated colonocytes. It appears possible that the latter finding is an artifact due to the submucosal tissue markedly slowing down CO2 diffusion from the bathing medium into the epithelial cells, thus causing an apparent fall in Ai.

A
i decreased and P increased with increasing temperature, as expected, when studied on the mucosal side of intact colon. This provides additional support for the validity of the method.
Medium‐Latency Stretch Reflexes of Foot and Leg Muscles Analysed by Cooling the Lower Limb in Standing Humans
Authors: Schieppati Marco , Nardone Antonio
Published: September 2004

In standing subjects, an ankle‐dorsiflexing perturbation of the supporting surface evokes a short‐latency response (SLR) and a medium‐latency response (MLR) to stretch in both soleus (Sol) and flexor digitorum brevis (FDB) muscles. The SLR is the counterpart of the monosynaptic reflex, whilst the MLR might be either mediated by Ia fibres, the delay being due to a long‐loop central circuit, or by fibres of slower conduction velocity. Since small afferents are slowed more than large ones by low temperature, a greater latency increment for the MLR than the SLR induced by cooling of the limb would point to a peripheral origin of the MLR.

In nine subjects, one limb was cooled by circulating water in a tube wrapped around it for about 120 min. Perturbations were delivered to the same limb prior to and during cooling, and after rewarming. EMG was recorded by surface electrodes from the Sol and FDB muscles.

The mean increase in latency of MLRs was significantly greater than that of SLRs in both muscles. On average, the Sol SLR increased from 42.4 to 47.0 ms and the Sol MLR from 72.0 to 82.3 ms. The FDB SLR increased from 58.1 to 66.5 ms and the FDB MLR from 94.9 to 110.5 ms. The mean difference (MLR minus SLR) increased from 29.6 to 35.2 ms for Sol, and from 36.8 to 43.9 ms for FDB at the end of cooling. After 30 min of rewarming, the responses of both muscles recovered towards control values.

The greater latency increment of the MLRs than of the SLRs favours the hypothesis of a slower conduction velocity of the responsible afferent fibres. The most likely candidate fibres are the spindle group II afferents.
Control of motor units in human flexor digitorum profundus under different proprioceptive conditions
Authors: Garland S. Jayne , Miles Timothy S.
Published: September 2004

Changing the posture of the human fingers can functionally ‘disengage’ the deep finger flexor muscle from its normal action on the terminal phalanx of the fourth (or third) finger. This enables the activity of the muscle to be studied both with and without its normal proprioceptive inputs.

Spike trains of long duration from pairs of concurrently active motor units in this muscle were recorded in both the engaged and disengaged hand postures. Subjects voluntarily kept one of the motor units (the ‘controlled’ unit) discharging at the same target frequency in both postures. The strength of short‐term synchrony, the strength of common drive, and the variability of discharge of these pairs of motor units were determined in both postures.

All subjects reported that the effort required to activate the motor units in the disengaged hand posture was substantially greater than in the normal engaged posture.

Short‐term synchrony, which is a function of common corticospinal inputs to pairs of motor units, was similar in both hand postures. However, the strength of common drive was significantly decreased when the muscle was disengaged. Although the neural substrate for common drive is not known, this observation suggests that proprioceptive feedback is involved either directly or indirectly.

Although the discharge rate of the ‘uncontrolled’ motor units increased when the muscle was disengaged, the variability of discharge of these and the ‘controlled’ motor units increased significantly. This supports the idea that the precision with which fine motor tasks can be performed is improved when proprioceptive feedback is intact.
Adenosine formation in contracting primary rat skeletal muscle cells and endothelial cells in culture
Authors: Hellsten Y. , Frandsen U.
Published: September 2004

The present study examined the capacity for adenosine formation, uptake and metabolism in contracting primary rat muscle cells and in microvascular endothelial cells in culture.

Strong and moderate electrical simulation of skeletal muscle cells led to a significantly greater increase in the extracellular adenosine concentration (421 ± 91 and 235 ± 30 nmol (g protein)−1, respectively; P < 0.05) compared with non‐stimulated muscle cells (161 ± 20 nmol (g protein)−1). The ATP concentration was lower (18%; P < 0.05) in the intensely contracted, but not in the moderately contracted muscle cells.

Addition of microvascular endothelial cells to the cultured skeletal muscle cells enhanced the contraction‐induced accumulation of extracellular adenosine P < 0.05), whereas endothelial cells in culture alone did not cause extracellular accumulation of adenosine.

Skeletal muscle cells were found to have ecto‐forms of several enzymes involved in nucleotide metabolism, including ATPases capable of converting extracellular ATP to ADP and AMP.

Adenosine added to the cell medium was taken up by muscle cells and incorporated into the adenine nucleotide pool so that after 30 min of incubation, over 95% of the adenosine label was present in ATP, ADP and AMP. A similar extent of incorporation of adenosine into the nucleotide pool was evident in the endothelial cells.

The present data suggest that contracting muscle cells induce an elevation in the extracellular adenosine concentration. Addition of endothelial cells to muscle cells enhances the contraction‐induced formation of adenosine. Adenosine taken up by muscle and endothelial cells from the extracellular space is not likely to be used for storage in intracellular pools, but may serve to regulate muscle extracellular adenosine levels.
Firing pattern of type‐identified wrist extensor motor units during wrist extension and hand clenching in humans
Authors: Sturm Hendrik , Schmied Annie , Vedel Jean‐Pierre , Pagni Simone
Published: September 2004

Single motor unit activity was investigated in the extensor carpi radialis muscles during voluntary isometric contraction involving either the coactivation of the wrist agonist extensor muscles (wrist extension) or the coactivation of the wrist and finger antagonist extensor and flexor muscles (hand clenching).

The motor units were found to be activated at a similar level of motoneurone pool drive during both wrist extension and hand clenching, as indicated by the fact that the EMG activity at which they were recruited was practically the same in both cases (mean ±s.d.: 20 ± 26 and 21 ± 25 mV, respectively). In addition, the net excitatory drive exerted on the motoneurones, as assessed from the mean interspike intervals, did not differ significantly between the two tasks (mean ±s.d.: 104.57 ± 17.24 and 103.01 ± 16.26 ms, for wrist extension and hand clenching, respectively).

However, the discharge variability, in terms of the coefficient of variation of the interspike intervals, was slightly but significantly greater during hand clenching than during wrist extension (0.213 ± 0.049 and 0.198 ± 0.045, respectively). This increase involved all types of motor units, regardless of their contractile force.

We suggest that the greater motoneurone discharge variability observed during hand clenching may be attributable to an increase in the synaptic noise. This increase might be due to the activation of numerous afferent pathways mediating reciprocal interactions between antagonist motoneurone pools, as well as to the activation of hand cutaneous receptors that play a major role in the regulation of handling and gripping motor activities.
Luminal and basolateral endothelin inhibit chloride reabsorption in the mouse thick ascending limb via a Ca2+‐Independent Pathway
Authors: de Jesus Ferreira Marie Celeste , Bailly Claire
Published: September 2004

The recent localization of endothelin synthesis and receptors in the thick ascending limb (TAL) prompted us to investigate a possible autocrine and/or paracrine effect of this agent. The net chloride flux (JCl) has been determined in isolated cortical and medullary TALs by the in vitro microperfusion technique.

In both segments, endothelin 1 (ET‐1) at 10−8m in the bath significantly decreased JCl, an effect which was partially reversible and observed at concentrations equal to or greater than 10−13m.

This (JCl inhibition (by 33.9 ± 3.2%) was blocked by BQ788 and was also observed with sarafotoxin 6C and ET‐3, indicating that endothelin receptor B (ETB) are present in TAL.

ET‐1 did not affect cAMP content under basal or hormone‐stimulated conditions. The presence of a prostaglandin synthesis inhibitor also did not prevent the ET‐1 action on JCl.

The ET‐1‐induced inhibition of JCl was prevented by protein kinase C inhibitors (staurosporine or GF 109203) and was reproduced by diacylglycerol analogues (OAG and DiC8). However, ET‐1 failed to increase intracellular Ca2+ concentration.

Addition of ET‐1 or ET‐3 to the apical surface induced a decrease of JCl through ETB receptors, an effect which was not additive with that induced by basolateral ET‐1, and was not concomitant with an increase in intracellular Ca2+ concentration.

It is concluded that the basolateral and luminal inhibitions of JCl by ET‐1 in TAL, through ETB receptors, is mediated by a protein kinase C activation which is independent of intracellular Ca2+ increase.
The effects of neonatal median nerve injury on the responsiveness of Tactile neurones within the cuneate nucleus of the cat
Authors: Murray G. M. , Taub D. R. , Mackie P. D. , Zhang H. Q. , Ghosh S. , Rowe M. J.
Published: September 2004

The capacity of cuneate neurones to attain normal functional properties following neonatal median nerve injury was investigated with single neurone recording in anaesthetized cats, 12–24 months subsequent to a controlled crush injury. Effectiveness of the peripheral nerve injury was confirmed by the abolition of the median nerve compound action potential following the crush.

Cuneate recording was carried out after denervation of the forearm, apart from the median nerve, to ensure that neurones studied had receptive fields within the distribution zone of the regenerated median nerve. Controlled and reproducible tactile stimuli were used to evaluate the functional capacities of neurones to determine whether they were consistent with those reported earlier for cuneate neurones in cats that had normal peripheral nerve development.

Twenty‐two cuneate neurones with well‐defined tactile receptive fields within the distribution zone of the regenerated median nerve were classified according to their adaptation characteristics and functional properties. Slowly adapting neurones responded throughout static skin indentations and had graded and approximately linear stimulus–response relations over indentation ranges up to 1.5 mm. Rapidly adapting neurones responded to the dynamic phases of skin indentations and could be divided into two broad classes, one most sensitive to vibrotactile stimuli at 200–400 Hz which appeared to receive a predominant input from Pacinian corpuscle receptors, and a non‐Pacinian group that included neurones most sensitive to skin vibration at 5–50 Hz which appeared to receive glabrous skin input from the rapidly adapting class of afferent fibres.

Based on the stimulus‐response relations and on measures of phase locking in the responses to vibrotactile stimuli, it appears that the functional properties of cuneate neurones activated from the field of a regenerated median nerve subsequent to a neonatal nerve crush injury were consistent with those reported previously for ‘control’ cuneate neurones. The results indicate that cuneate neurones can acquire normal tactile coding capacities despite the disruption caused by prior crush injury to their peripheral nerve source.
Quantitative analysis of cuneate neurone responsiveness in the cat in association with reversible, partial deafferentation
Authors: Zhang S. P. , Rowe M. J.
Published: September 2004

Partial deafferentation, based on peripheral nerve section or local anaesthetic blockade, has been reported to induce both immediate loss of responsiveness and/or immediate reorganization in receptive fields of neurones in the somatosensory system. In the present study, in anaesthetized cats, we have used a rapid, reversible deafferentation procedure based on cold block of the median nerve in order to evaluate quantitatively the response characteristics of cuneate neurones (n= 39) before, during and after partial deafferentation.

The first hypothesis tested was that cuneate neurones with input from ulnar or superficial radial nerve fields in the vicinity of the median nerve field should undergo, in association with median nerve blockade, an increased level of responsiveness to tactile stimuli within the ulnar or radial nerve zone, and an expansion of their cutaneous receptive fields. However, among eighteen cuneate neurones of this type, there was no evidence for any systematic enhancement of responsiveness nor, in at least sixteen of the eighteen neurones, any evidence for receptive field expansion.

The second hypothesis tested was that cuneate neurones whose input came from both the median nerve and another peripheral nerve source should undergo, in association with median nerve blockade, an increase in responsiveness to the remaining input and an expansion of the receptive field into the field of that remaining nerve source. However, in none of thirteen neurones of this type tested was there evidence of such a change.

The third hypothesis was that cuneate neurones whose ‘control’ receptive fields werewithin the median nerve zone of deafferentation should show an emergence of novel receptive fields and responsiveness from areas around the field of innervation of the median nerve. However, in none of eight neurones of this type was there evidence for such changes in adjacent skin areas.

In conclusion, with the use of cold block of the median nerve for partial deaffferentation, the present study has confirmed previous findings of denervation‐related loss of responsiveness in dorsal column nuclei neurones. The conflicting findings in studies of central nervous system plasticity indicate the need to understand better factors that do and do not lead to acute central changes.