Sensitivity of monosynaptic test reflexes to facilitation and inhibition as a function of the test reflex size: a study in man and the cat

In parallel experiments on humans and in the cat it was investigated how the sensitivity of monosynaptic test reflexes to facilitation and inhibition varies as a function of the size of the control test reflex itself. In man the monosynaptic reflex (the Hoffmann reflex) was evoked in either the soleus muscle (by stimulation of the tibial nerve) or the quadriceps muscle (by stimulation of the femoral nerve). In the decerebrate cat monosynaptic reflexes were recorded from the nerves to soleus and medial gastrocnemius muscles; they were evoked by stimulation of the proximal ends of the sectioned L7 and S1 dorsal roots. Various excitatory and inhibitory spinal reflex pathways were used for conditioning the test reflexes (e.g. monosynaptic Ia excitation, disynaptic reciprocal inhibition, cutaneous inhibition, recurrent inhibition, presynaptic inhibition of the Ia fibres mediating the test reflex). It was shown that the additional number of motoneurones recruited in a monosynaptic test reflex by a constant excitatory conditioning stimulus was very much dependent on the size of the test reflex itself. This dependency had the same characteristic pattern whatever the conditioning stimulus. With increasing size of the test reflex the number of additionally recruited motoneurones first increased, then reached a peak (or plateau) and finally decreased. A similar relation was also seen with inhibitory conditioning stimuli. The basic physiological factors responsible for these findings are discussed. Finally, the implications for the interpretation of experiments in man with the H-reflex technique are considered.

Electrical resistance of brain microvascular endothelium

A newly developed technique for determination of the electrical resistance of the capillary wall was applied to microvessels at the surface of the frog brain. Current was injected into a capillary or venule via a microelectrode and the ensuing intravascular potential profile away from the current source was determined with a second microelectrode placed at various positions along the capillary. The membrane resistance was calculated according to the theory for leaky cables used in determinations of axon membrane resistance. The average resistance was 1870 omega . cm2. Since the surface vessels of the frog brain are devoid of glial investment but otherwise similar to brain parenchymal vessels, the results prove that the endothelium is the site of the blood-brain barrier. The electrical resistance is similar to that of a 'tight' epithelium.

The spinal pathophysiology of spasticity--from a basic science point of view

Spasticity is a term, which was introduced to describe the velocity-sensitive increased resistance of a limb to manipulation in subjects with lesions of descending motor pathways. This distinguishes spasticity from the changes in passive muscle properties, which are often seen in these patients, but are not velocity-sensitive. Increased excitability of the stretch reflex is thus a central component of the definition of spasticity. This review describes changes in cellular properties and transmission in a number of spinal reflex pathways, which may explain the increased stretch reflex excitability. The review focuses mainly on results derived from the use of non-invasive electrophysiological techniques, which have been developed during the past 20-30 years to investigate spinal neuronal networks in human subjects, but work from animal models is also considered. The reflex hyperexcitability develops over several months following the primary lesion and involves adaptation in the spinal neuronal circuitries caudal to the lesion. In animal models, changes in cellular properties (such as 'plateau potentials') have been reported, but the relevance of these changes to human spasticity has not been clarified. In humans, numerous studies have suggested that reduction of spinal inhibitory mechanisms (in particular that of disynaptic reciprocal inhibition) is involved. The inter-subject variability of these mechanisms and the lack of objective quantitative measures of spasticity have impeded disclosure of a clear causal relationship between the alterations in the inhibitory mechanisms and the stretch reflex hyperexcitability. Techniques which make such a quantitative measure possible as well as longitudinal studies where development of reflex excitability and changes in the inhibitory mechanisms are followed over time are in great demand.

Methodological implications of the post activation depression of the soleus H-reflex in man

A long lasting inhibition (greater than 8 s) of the soleus Hoffmann reflex (H-reflex) was evoked by a preceding soleus H-reflex, by a brief voluntary ankle flexor or extensor muscle contraction or by a tap applied to the Achilles tendon. The time course of this long lasting inhibition was similar in all these cases, suggesting that the same spinal mechanism is involved. Furthermore, it was shown that the post-activation depression may interfere with the determination of inhibitory or facilitatory effects on the H-reflex. It is stressed that when the onset of inhibitory or facilitatory effects on the soleus H-reflex is to be determined in relation to start of an ankle movement, either very long stimulus intervals (greater than 8 s) must be used, or the onset must be determined in relation to a reference value of the soleus H-reflex, which may be influenced by the long lasting inhibitory effect, but not yet by the succeeding muscle contraction.

Reciprocal Ia inhibition between ankle flexors and extensors in man

1. Reciprocal inhibition between antagonist muscle groups at the ankle has been investigated in sixty healthy subjects. Hoffmann reflexes (H reflexes) in the soleus and tibialis anterior muscles were used to assess changes in reciprocal inhibition evoked by electrical stimulation of antagonist muscle nerves. 2. Inhibition of the soleus H reflex was evoked by a single conditioning stimulus to the common peroneal nerve, and inhibition of the tibialis anterior H reflex was elicited by one conditioning stimulus to the posterior tibial nerve. Symmetrical central connections between the antagonist flexors and extensors were assumed and under this assumption the central delay for the inhibition, in addition to the delay for monosynaptic Ia excitation, was calculated to be about 1 ms. The inhibition was evoked by weak stimuli to the nerves from antagonist muscle groups; the threshold for the inhibition was around 0.6 X threshold for a direct motor response (M-threshold). Furthermore, tendon taps to the Achilles tendon facilitated the soleus H reflex and inhibited the tibialis anterior reflex at short latencies. The short central delay, the low electrical threshold and the.actions of Achilles tendon taps strongly suggest that the early reciprocal inhibition is homologous to the disynaptic Ia inhibition previously studied in animal experiments. 3. With the test soleus H reflex kept at 15-25% of the maximum directly evoked motor response (M-response) and the strength of the conditioning peroneal nerve stimulation kept at 1.0 X M-threshold, the inhibition from the peroneal nerve ranged between 0 and 40% (mean, 14.9%) at rest. 4. Changes in the amount of reciprocal inhibition from the peroneal nerve were studied both during tonic and dynamic dorsi- and plantarflexion. During tonic dorsiflexion there was no significant change of inhibition as compared to rest, while inhibition decreased during tonic plantarflexion. However, during ramp-and-hold dorsiflexion there was a transient increase in reciprocal inhibition of the soleus H reflex. This increase in inhibition from the peroneal nerve could be seen 50 ms prior to the onset of contraction. The increase in inhibition before and at the very beginning of the contraction cannot be due to sensory feed-back during contraction, but must depend on a supraspinal control of the spinal cord. 5. At conditioning-test intervals of 4-6 ms, the inhibition of the soleus H reflex from the peroneal nerve was considerably larger during tonic dorsiflexion than at rest. Thus, tonic dorsiflexion revealed an inhibition with long latency from the peroneal nerve, which was not seen at rest.(ABSTRACT TRUNCATED AT 400 WORDS)

Induction processes in blood-brain transfer of ketone bodies during starvation

Fed and starved rats were studied on successive days during a 5-day starvation period. The ability of ketone bodies to pass the blood-brain barrier was estimated by single common carotid injections of labeled ketone bodies and water, and results were expressed as the ratio between the normalized activities of tracers in tissue and blood, the brain uptake index (BUI). BUI of D-3-hydroxybutyrate and acetoacetate decreased as their total concentrations increased in the injectate bolus: BUI of D-3-hydroxybutyrate decreased significantly from 8% at 0.2 mM to 3--4% at 20.2 mM in fed rats and from 11.5% at 0.2 mM to 6% at 20.2 mM in starved rats, indicating saturation of the uptake mechanism. The BUI of both ketone bodies increased significantly with increasing duration of starvation, indicating adaptation to ketonemia. Enzymatic kinetics explained the uptake behavior of D-3-hydroxybutyrate in both fed and starved rats and involved a rise of Km and Vmax during starvation consistent with a doubling of the transport rate at the degree of ketonemia found in starved rats. The uptake of glucose was not influenced by starvation or ketonemia.

Blood-brain glucose transfer: repression in chronic hyperglycemia

Diabetic patients with increased plasma glucose concentrations may develop cerebral symptoms of hypoglycemia when their plasma glucose is rapidly lowered to normal concentrations. The symptoms may indicate insufficient transport of glucose from blood to brain. In rats with chronic hyperglycemia the maximum glucose transport capacity of the blood-brain barrier decreased from 400 to 290 micromoles per 100 grams per minute. When plasma glucose was lowered to normal values, the glucose transport rate into brain was 20 percent below normal. This suggests that repressive changes of the glucose transport mechanism occur in brain endothelial cells in response to increased plasma glucose.

Appearance of reciprocal facilitation of ankle extensors from ankle flexors in patients with stroke or spinal cord injury

The purpose of the present study was to investigate the pathophysiological role of reciprocal facilitation between antagonistic motoneuron pools in spasticity. The soleus H-reflex was conditioned by prior stimulation of the peroneal nerve in 15 healthy subjects, six hemiplegic patients and 11 spinal cord injured (SCI) patients. The hemiplegic patients were tested from soon after the onset of hemiplegia and up to 2 years later. Whereas stimulation of the peroneal nerve produced short-latency inhibition of the soleus H-reflex in healthy subjects, it produced facilitation in spastic SCI and hemiplegic patients. This facilitation was demonstrated to have a low threshold compatible with activation of group I afferents and was most likely mediated by an oligosynaptic (reciprocal) excitatory pathway. The facilitation appeared in parallel with the development of hyperactive Achilles tendon reflexes, which was the only clinical finding that could be correlated positively with the facilitation. It is suggested that the appearance of reciprocal excitation plays a role in the pathophysiology of spasticity.

H-reflexes are smaller in dancers from The Royal Danish Ballet than in well-trained athletes

The size of the maximal H-reflex (Hmax) was measured at rest and expressed as a percentage of the maximal M-response (Mmax) in 17 untrained subjects, 27 moderately trained subjects, 19 well-trained subjects and 7 dancers from the Royal Danish Ballet. The Hmax/Mmax was significantly larger in the moderately and well-trained subjects than in the untrained subjects but smaller in the ballet dancers. It is therefore suggested that both the amount and the type of habitual activity may influence the excitability of spinal reflexes.

Maintained changes in motoneuronal excitability by short-lasting synaptic inputs in the decerebrate cat

1. During investigation of the tonic stretch reflex in the unanaesthetized decerebrate cat we observed that a short train of impulses in Ia afferents from the soleus muscle (or its synergists) may cause a prolonged activity in the soleus muscle as judged by EMG and tension recordings. This excitability increase, which outlasted the stimulus train, could stay virtually constant during long periods (even minutes), but could be terminated at any time by a train of impulses in, for example, the peroneal nerve. 2. Gradation of the strength of stimulation and the duration of the train of impulses show that the amount of maintained excitability increase depends-within some limits-on the total amount of Ia impulses. 3. In paralysed preparations a short train of impulses in Ia afferents from any part of the triceps surae, caused a maintained increase of the efferent activity in the nerves to triceps surae and a maintained increase of the triceps surae monosynaptic test reflex. These experiments demonstrate the existence of a central mechanism (in the spinal cord and/or the brain stem), which is responsible for the maintained excitability increase seen in motoneurones to the homonymous and synergic muscles. 4. In acute spinal preparations it was not possible to demonstrate any long-lasting excitability increase by a train of Ia impulses. Following intravenous administration of the serotonin precursor 5-hydroxytryptophan, mimicking the tonic activity of these pathways in the decerebrate state, it was again possible to elicit the long-lasting excitability increase by a train of impulses in Ia afferents. A subsequent I.V. injection of methysergide (a serotonin receptor blocker) abolished the long-lasting excitability increase. This set of experiments demonstrates that the basic mechanism responsible for the maintained excitability increase is located at segmental level, and involves serotonergic systems. 5. It was demonstrated that activation of several ipsilateral and crossed reflex pathways by trains of impulses in cutaneous or high-threshold muscle afferents could trigger a maintained excitability increase of those motoneurone pools which were activated by the stimulation. Trains of stimuli to facilitatory regions in the brain stem could also cause a long-lasting excitability increase of motoneurones. Furthermore, activation of all reflex pathways which mediate postsynaptic inhibition to a motor nucleus (including recurrent inhibition via Renshaw cells) could terminate the prolonged excitability increase of that particular motor nucleus.(ABSTRACT TRUNCATED AT 400 WORDS)

Modulation of presynaptic inhibition and disynaptic reciprocal Ia inhibition during voluntary movement in spasticity

The aim of the study was to investigate whether impaired control of transmission in spinal inhibitory pathways contributes to the functional disability of patients with spasticity. To this end, transmission in the pathways mediating disynaptic reciprocal Ia inhibition and presynaptic inhibition was investigated in 23 healthy subjects and 20 patients with spastic multiple sclerosis during ankle dorsiflexion and plantar flexion. In healthy subjects, but not in spastic patients, the soleus H reflex was depressed at the onset of dorsiflexion (300 ms rise time, 20% of maximal voluntary effort). At the onset of plantar flexion, the soleus H reflex was more facilitated in the healthy subjects than in the patients. The H reflex increased more with increasing level of tonic plantar flexion and decreased more with dorsiflexion in the healthy subjects than in the spastic patients. Transmission in the disynaptic Ia reciprocal inhibitory pathway from ankle dorsiflexors to plantar flexors was investigated by conditioning the soleus H reflex by previous stimulation of the common peroneal nerve (conditioning-test interval 2-3 ms; stimulation intensity 1.05 times the motor response threshold). At the onset of dorsiflexion, stimulation of the common peroneal nerve evoked a significantly larger inhibition than at rest in the healthy subjects but not in the spastic patients. At the onset of plantar flexion the inhibition decreased in the healthy subjects, but because only weak inhibition was observed at rest in the patients it was not possible to determine whether a similar decrease occurred in this group. There were no differences in the modulation of inhibition during tonic plantar flexion and dorsiflexion in the two populations. Presynaptic inhibition of Ia afferents terminating on soleus motor neurones was evaluated from the monosynaptic Ia facilitation of the soleus H reflex evoked by femoral nerve stimulation. Femoral nerve facilitation was decreased at the onset of dorsiflexion and increased at the onset of plantar flexion in the healthy subjects and patients, but the changes were significantly greater in the healthy subjects. There was no difference between the two populations in the modulation of presynaptic inhibition during tonic plantar flexion and dorsiflexion. It is suggested that the abnormal regulation of disynaptic reciprocal inhibition and presynaptic inhibition in patients with spasticity is responsible for the abnormal modulation of stretch reflexes in relation to voluntary movement in these patients. Lack of an increase in reciprocal inhibition and presynaptic inhibition at the onset of dorsiflexion may be responsible for the tendency to elicitation of unwanted stretch reflex activity and co-contraction of antagonistic muscles in patients with spasticity.

Spinal mechanisms in man contributing to reciprocal inhibition during voluntary dorsiflexion of the foot

1. The inhibition of the soleus Hoffmann reflex (H reflex) during voluntary dorsiflexion of the foot--henceforth referred to as 'natural' reciprocal inhibition--was found to be initiated 50 ms before the onset of the EMG activity in the tibialis anterior muscle and to increase gradually during a ramp-and-hold dorsiflexion. There was a positive correlation between strength of tonic dorsiflexion and amount of 'natural' reciprocal inhibition. 2. The change of activity in the disynaptic and a long-latency group Ia inhibitory pathway and the change in presynaptic inhibition of the Ia fibres mediating the soleus H reflex were tested separately during ramp-and-hold dorsiflexion as well as during tonic dorsiflexion of the foot, and the results were compared with the development of the 'natural' reciprocal inhibition of the unconditioned soleus H reflex. 3. The disynaptic group I inhibition of soleus motoneurones was increased, as compared to rest, during the dynamic phase of a ramp-and-hold dorsiflexion movement, but the inhibition generally did not increase during tonic dorsiflexion of the foot. 4. The long-latency group I inhibition was seen only during dorsiflexion of the foot. It appeared around 50 ms before tibial anterior EMG activity and there was a positive correlation between strength of tonic dorsiflexion and amount of this long-latency inhibition. 5. Presynaptic inhibition of Ia afferents terminating on soleus motoneurones was estimated by an indirect method. The increase of presynaptic inhibition started soon after the onset of the ramp-and-hold dorsiflexion, and gradually became more pronounced during the ramp phase. The amount of presynaptic inhibition was positively correlated with strength of tonic dorsiflexion. 6. It is concluded that all investigated mechanisms may contribute to the 'natural' reciprocal inhibition and it seems that the different pathways are used differentially during different types of movement.

Disynaptic reciprocal inhibition of ankle extensors in spastic patients

The soleus Hoffmann-reflex (H-reflex) was conditioned by a preceding stimulation of the common peroneal nerve in 74 healthy control subjects and 39 patients with spasticity in the lower extremities due to multiple sclerosis. At a conditioning-test interval of 1-3 ms a decrease of the size of the soleus H-reflex was seen in the healthy subjects. The decrease was most likely caused by disynaptic reciprocal Ia inhibition (Crone et al., 1987). In the spastic patients a similar short-latency inhibition was rarely seen. On the contrary, in several patients a facilitation was seen at a conditioning-test interval of 3-4 ms. A short-latency inhibition as pronounced as in healthy subjects was seen in four patients. These four patients did not differ from the other patients regarding the degree of spasticity or any other clinical parameter. However, they all used an external peroneal nerve stimulator daily as a walking aid. It is suggested that the lack of short-latency reciprocal inhibition reflects a deficient control of the interneurons which mediate this inhibitory spinal mechanism between antagonistic muscles in man. This might contribute to the pathophysiology of spasticity and it might be related to the frequent occurrence of co-contraction of functionally antagonistic muscles during gait in spastic patients. The existence of a pronounced reciprocal inhibition in patients receiving frequent stimulation of the peroneal nerve may suggest that regular activation of peripheral nerves is of importance for the maintenance of the activity in spinal pathways.

Endothelial plasmalemmal vesicles as elements in a system of branching invaginations from the cell surface

In electron microscopy studies of the endothelial vesicles in frog mesenteric capillaries, an accidental observation was made concerning vesicular organization. When tannic acid was added to already fixed tissue, the mordant reached apparently free vesicles in the cytoplasm under conditions in which vesicular movement was excluded and in which the impermeability of the cell membranes was preserved. This indicates a spatial continuity between the vesicles and the cell exterior. It is proposed that cytoplasmic vesicles in endothelial cells are elements of branching, permanent or semipermanent invaginations of the plasmalemma.

Changes in transmission across synapses of Ia afferents in spastic patients

The transmission across synapses of Ia afferents on spinal motor neurons was investigated in 30 healthy subjects and 25 spastic multiple sclerosis patients. Slow passive stretch (17 degrees/s of the soleus muscle evoked a pronounced depression of the soleus Hoffmann reflex (H-reflex) lasting for more than 10 s in the healthy subjects. This depression was less pronounced and had a shorter duration in the spastic patients. A tap applied to the biceps femoris tendon also produced an inhibition of the soleus H-reflex, which was larger in the healthy subjects than in the spastic patients. This inhibition only lasted for 300-400 ms. Finally, stimulation of the femoral nerve (FN) produced a facilitation of the soleus H-reflex, which was larger in the spastic patients than in the healthy subjects. The inhibition of the H-reflex evoked by the biceps femoris tendon tap is known to be caused by presynaptic inhibition of the Ia afferents, which mediate the reflex. The facilitation of the soleus H-reflex produced by FN stimulation has also been shown to be influenced by changes in presynaptic inhibition. The increased facilitation from the FN and the decreased inhibition from the biceps femoris tendon tap onto the soleus H-reflex in spastic patients are thus both compatible with a deficient presynaptic inhibition in these subjects. The long lasting depression of the reflex evoked by a previous slow stretch of the soleus muscle is most likely caused by a decrease of the probability of transmitter release from the Ia afferents. The decrease of this depression in spastic patients suggests that mechanisms other than presynaptic inhibition may contribute to changes in the efficiency of transmission across the synapses of Ia afferents in spastic patients and thus contribute to the exaggeration of stretch reflexes seen in these patients.

Electrical resistance of a capillary endothelium

The electrical resistance of consecutive segments of capillaries has been determined by a method in which the microvessels were treated as a leaky, infinite cable. A two-dimensional analytical model to describe the potential field in response to intracapillary current injection was formulated. The model allowed determination of the electrical resistance from four sets of data: the capillary radius, the capillary length constant, the length constant in the mesentery perpendicular to the capillary, and the relative potential drop across the capillary wall. Of particular importance were the mesothelial membranes covering the mesenteric capillaries with resistances several times higher than that of the capillary endothelium. 27 frog mesenteric capillaries were characterized. The average resistance of the endothelium was 1.85 omega cm2, which compares well with earlier determinations of the ionic permeability of such capillaries. However, heterogeneity with respect to resistance was observed, that of 10 arterial capillaries being 3.0 omega cm2 as compared with 0.95 omega cm2 for 17 mid- and venous capillaries. The average in situ length constant was 99 micrometers for the arterial capillaries and 57 micrometers for the mid- and venous capillaries. It is likely that the ions that carry the current must move paracellularly, through junctions that are leaky to small solutes.

Fluid absorption by rat lung in situ: pathways for sodium entry in the luminal membrane of alveolar epithelium

1. The purpose of the investigation was to characterize the luminal membrane and the paracellular pathway of rat lung alveolar epithelium. Experiments were performed on lungs in situ instilled with isotonic, buffered Ringer solution and perfused with blood from a donor rat using cross-circulation technique. 2. The rate of active Na+ transport was 4.4 pmol/(cm2s). The fluid absorption was 156 nl/s, and was unaffected by the presence of protein in the instillate (166 nl/s). In the absence of Na+, fluid absorption was zero. Amiloride (10(-3) M) reduced fluid absorption by 60%. Amiloride, combined with absence of D-glucose, arrested fluid absorption completely. Phloridzin at the luminal side reduced fluid absorption whilst phloretin had no effect. Amiloride together with phloridzin (10(-3) M) also arrested absorption. Thus, there are two entry systems for Na+ in the luminal membrane: Na+ channels and a Na+-D-glucose symport. These results show that alveolar fluid absorption is due to cellular activity. 3. Substitution of Cl- with gluconate not only stopped fluid absorption, but led to slight reversal of net fluid movement. 4. Passive unidirectional flux of Na+, determined with 22Na+, was 9.9 pmol/(cm2s) and that of Cl-, determined with 36Cl-, was 12.4 pmol/(cm2s). These fluxes were based on an assumed alveolar surface area of 5000 cm2. Transference numbers calculated from these figures are close to those in free solution, suggesting a neutral or weakly charged intercellular junctional pathway. The D-mannitol permeability in the paracellular pathway was 1.7 X 10(-8) cm/s. 5. It is a consequence of the proposed mechanism for fluid absorption that it becomes inoperative if the normally high reflexion coefficients for Na+ and Cl- are lowered in pathological states. In such conditions pulmonary oedema may develop depending on the net balance of passive mechanical and colloid-osmotic forces. 6. An explanation of the reversal of fluid transport at the time of birth is presented.

Central control of disynaptic reciprocal inhibition in humans

The disynaptic pathway from muscle spindle Ia afferents to motoneurones of the antagonist muscle is one of the best studied pathways in the spinal cord. Early animal studies--mainly in the cat--have provided a detailed knowledge of the pathway itself and of the integration of segmental and supraspinal convergence at the interneuronal level. Although this knowledge was used to formulate hypotheses on the function of the pathway during natural movements, the reduced animal preparation limited the possibilities of testing these ideas. However, such information has more recently been obtained from human subjects by using indirect electrophysiological techniques. In most of these experiments the disynaptic Ia inhibition was demonstrated as a short-latency depression of a monosynaptic test reflex (H-reflex) following a conditioning stimulation of the antagonist nerve. Changes in the size of this depression during voluntary tasks were then taken as evidence of a central regulation of the pathway. It has for example been demonstrated in this way that the brain regulates the Ia inhibitory interneurones in parallel with their corresponding motoneurones during extension-flexion movements, but not during co-contraction of antagonistic muscles. The importance of the central control of the pathway has also been emphasized by the finding of a disordered regulation of its activity in patients with lesions of the brain. This may possibly contribute to the inappropriate co-contraction of antagonistic muscles observed in some of these patients. It seems reasonable to expect that this kind of experiment in the future may contribute significantly to the knowledge of the central control of spinal motor mechanisms.

Significance of active ion transport in transalveolar water absorption: a study on isolated rat lung

1. Experiments were performed on isolated rat lungs perfused with Ringer solutions containing red cells. The goal was to clarify the role of active transport of Na+ for the absorption of fluid across the alveolar membrane, and to characterize active and passive pathways. 2. Partially degassed lungs were filled with 5 ml of an isotonic Ringer solution containing 125I-labelled albumin in order to calculate the fluid movement, and 22Na+ or 36Cl- for measurement of ion fluxes. Passive non-electrolyte permeability was determined in all experiments using [3H]mannitol. 3. The average rate of fluid absorption in phosphate-buffered instillates was 134 nl/s (S.E., 18.5; n = 14). With ouabain (10(-4) M) in the perfusate the fluid absorption rate fell to 57 nl/s (S.E., 8.2; n = 18). Amiloride (10(-3)-10(-4) M) in the instillate reduced the absorption to 75 nl/s (S.E., 8.6; n = 16). These results show that fluid absorption depends on transcellular transport of Na+ and that alveolar epithelial cells have a Na+ entry system in the luminal membrane and a Na+-K+ pump in the abluminal membrane. 4. The transcellular ion transport operates in parallel with a paracellular, passive leak that allows mannitol to pass with a permeability surface area product of 1.2 X 10(-4) ml/s, corresponding to a permeability coefficient of 2.4 X 10(-8) cm/s, assuming an alveolar surface area of 5000 cm2. 5. The passive fluxes of Na+ were 9.4 pmol/(cm2s) (S.E., 1.3; n = 25) in the direction from alveoli to perfusate and 8.0 pmol/(cm2s) (S.E., 0.86; n = 6) from perfusate to plasma. The passive fluxes of Cl- in the two directions were not significantly different either. Thus the transalveolar electrical potential difference is too small to affect ion movements measurably. 6. The passive permeability to Na+ was 6.7 X 10(-8) cm/s and to Cl- was 10.2 X 10(-8) cm/s (alveolar surface area assumed to be 5000 cm2). The ratio of the permeabilities is close to the ratio of the diffusion coefficients in free solution, suggesting a neutral or weakly charged paracellular channel. 7. We conclude that the alveolar epithelium performs solute-coupled fluid transport from alveoli to plasma, and that it shows many features that are common to other fluid-transporting epithelia; with an approximate surface area of 100 m2 in humans it constitutes one of the largest epithelial surfaces in the body.(ABSTRACT TRUNCATED AT 400 WORDS)

The three-dimensional organization of plasmalemmal vesicular profiles in the endothelium of rat heart capillaries

The organization of plasmalemmal vesicular profiles in the endothelium of rat heart capillaries has been reinvestigated. Judged from random thin sections approximately 50% of the vesicles appeared free in the cytoplasm, the rest opening to the surfaces of the endothelial cells--a distribution which corroborates previous studies. However, three-dimensional reconstructions based on ultrathin serial sections (thickness congruent to 12 nm) gave a very different picture. All plasmalemmal vesicular profiles (921 from 5 capillaries) were parts of the surface membrane either as caveolae or as more complex racemose invaginations. This organization has previously been observed in frog mesenteric capillaries ((M. Bundgaard, J. Frøkjaer-Jensen, and C. Crone, 1979, Proc. Nat. Acad. Sci. USA 76, 6439-6442) and (J. Frøkjaer-Jensen, 1980, J. Ultrastruct. Res. 73, 9-20)). It is therefore proposed that absence or extreme rarity of free plasmalemmal vesicles is a general feature of capillary endothelia. Consequently, we suggest that the term "endothelial, plasmalemmal vesicles" be replaced by "endothelial plasmalemmal invaginations." The results imply that trans-endothelial vesicular transport is unlikely to occur and that this membrane system performs other--as yet unknown--functions.

Amplitude of the maximum motor response (Mmax) in human muscles typically decreases during the course of an experiment

It was shown that the amplitude of the soleus Mmax and Hmax responses decreases in the course of long-lasting H-reflex studies. The peak-to-peak amplitudes of the Mmax and Hmax responses in the soleus muscle (and the Mmax in the tibialis anterior muscle and small hand muscles) were measured repeatedly for 1-3 h in 20 subjects. 3-5 Mmax responses and 5-10 Hmax responses were elicited about every 3 min while the subject was at rest. Decreases in the soleus Mmax response of up to 50.5% (mean 20.5% SEM 2.2) and of the soleus Hmax of up to 49.7% (mean 19.1% SEM 3.7) in relation to the amplitudes measured at the beginning of the experiment were seen in 17 subjects. In 3 subjects no Mmax amplitude decrease was seen. The maximum decrease was reached between 10 and 100 min (mean 44.2 min SEM 4.3). An Mmax amplitude decrease was also seen in the tibialis anterior muscle and in two small hand muscles. In some subjects the decrease of the Mmax response seemed to be initiated by the infrequent supramaximal stimulations. The possible causes for this amplitude reduction, as well as the methodological consequences of these findings for H-reflex studies and fatigue studies, are briefly discussed.

K+-permeability of the blood-brain barrier, investigated by aid of a K+-sensitive microelectrode

The K+-permeability of the blood-brain barrier at the capillary level was estimated from determinations of brain extracellular K+-concentration in response to an isotonic bolus containing KCl injected into the carotid artery. A very low permeability appeared from the fact that the extracellular K+-concentration--measured by aid of K+-sensitive microelectrodes--remained unchanged during the passage of the bolus. An upper limit for the blood-brain barrier K+-permeability in the rat was estimated to be 2.8 X 10(-7) cm. with s-1.

Substances that rapidly augment ionic conductance of endothelium in cerebral venules

Classical techniques for studying modulations of microvascular permeability have a time resolution of minutes. A newly developed method allows continuous measurement of the electrical resistance of the microvascular membrane in vivo (Olesen & Crone 1983). The technique exploits microelectrodes impaled into the vascular lumen and is based on cable analysis of the vessel. It was applied to venules on the surface of the frog brain to test the effect on microvascular permeability of a wide variety of substances. The following agents increased ionic permeability reversibly within seconds: 5-hydroxytryptamine, bradykinin, ATP, ADP, AMP, phospholipase A2, arachidonic acid, leukotriene C4, oxygen-derived free radicals, ionophore A23187, and unbound Evans blue dye. An irreversible permeability increase was induced by protamine sulphate, neuraminidase, trypsin, melittin, and snake venoms from Crotalus durissus terrificus and Bothrops atrox. The following substances were without effect within an administration period of 5 min: histamine, epinephrine, putrescine, angiotensin II, vasoactive intestinal polypeptide (VIP), substance P, neurotensin, vasopressin, adenosine, PGE2, PGF2 alpha, prostacyclin (PGI2), leukotriene B4, albumin, heparin, plant cytokinins, hyaluronidase, thrombin, wasp venom. Variations in pH between 5.1 and 8.6 did not change permeability. Three conclusions are drawn from the observations: (1) the permeability of cerebral microvessels can be modulated by specific agents, (2) the agents induced changes in the endothelium within a few seconds, and (3) the rapid permeability increase induced by inflammatory mediators was less than two-fold and reversible within minutes.