bluuebunny/crossref_metadata_2025 huggingface dataset:
Electrotonic Interaction between Muscle Fibers in the Rabbit Ventricle
Authors: Tille J.
Published: May 2004

Transmembrane potentials were recorded simultaneously from pairs of ventricular fibers in an isolated, regularly beating preparation. A double-barrelled microelectrode was used to record the potentials from, and to polarize, one fiber. A single microelectrode was used to record from a distant fiber. The existence of two systems of fibers, termed P and V, was confirmed. Histological evidence for the existence of two types of fibers is also presented. Electrotonic current spread was observed within both systems, electrotonic interaction between the two systems was rare and always weak. In the case of those pairs of fibers showing electrotonic interaction, the distance for an e-fold decrease in magnitude of the electrotonic potentials was found to be from 300 to 600 µ in P fibers and from 100 to 300 µ in V fibers. However, no electrotonic interaction could be observed in the majority of V fiber pairs. Moreover, the magnitude of the electrotonic potential did not decay monotonically with distance in any one direction. It is concluded that the rabbit ventricle cannot be regarded as a single freely interconnected syncytium.
The Spatiotemporal Analysis of Odorants at the Level of the Olfactory Receptor Sheet
Authors: Mozell Maxwell Mark
Published: May 2004

Activity in two separate regions of the frog olfactory mucosa was sampled by simultaneously recording the summated neural discharges from the olfactory nerve branches originating from them. The difference in the activity from these two regions in response to a stimulus was measured by: (a) the ratio of the response amplitude recorded from the lateral nerve branch to that recorded from the medial nerve branch (LB/MB ratio), (b) the latency difference (or time interval) between these two responses. Equal concentrations of four different odorants were drawn into the nose by an artificially produced sniff of known dimensions. At each concentration in every animal the four chemicals were ranked in order of the magnitudes of their LB/MB ratios and again in order of their latency differences. Regardless of their concentration, the same chemicals fell into the same ranks in different animals. In addition, for each chemical the magnitudes of the ratios and latency differences showed only minimal changes with concentration. Thus, spatiotemporal patterns of relative response magnitudes and latency differences across the mucosa differentially represented the odorants. Such a spatiotemporal code, together with physicochemical considerations, suggested that the nose separates vapors in a manner similar to a gas chromatograph. This is further supported by the previously observed reversal of the ratio patterns with reversal of air flow direction through the olfactory sac.
Nonelectrolyte Penetration and Sodium Fluxes through the Axolemma of Resting and Stimulated Medium Sized Axons of the Squid Doryteuthis plei
Authors: Villegas Raimundo , Villegas Gloria M. , Blei Margarita , Herrera Francisco C. , Villegas Jorge
Published: May 2004

The penetration of 14C-labeled erythritol, mannitol, and sucrose through the axolemma was determined in medium sized paired axons, one at rest and the other stimulated 25 times per sec. The resting permeabilities, in 10-7 cm/sec, are erythritol, 2.9 ± 0.3 (mean ± SEM); mannitol, 2.3 ± 0.4; and sucrose 0.9 ± 0.1. In the stimulated axons they are: erythritol, 5.2 ± 0.3; mannitol, 4.0 ± 0.5; and sucrose, 1.8 ± 0.3. Thus, the calculated permeabilities during activity (1 msec per impulse), in the same units, are: 100, 75, and 38, respectively. These changes in permeability are reversible. The effects of external potassium and sodium concentrations on erythritol penetration were also studied. At rest, erythritol penetration is independent of potassium and sodium concentrations. In the stimulated axons, erythritol penetration decreases when the extracellular sodium is diminished. Sodium influx (not the efflux) decreases during rest and activity when the extracellular sodium is diminished. The diminution during activity of erythritol and sodium entries in low sodium solutions may be related to a decrease of a drag effect of sodium ions on the nonelectrolyte molecules or to independent effects of the sodium concentration on sodium influx and the nonelectrolyte pathways. The axolemma discriminates among erythritol, mannitol, sucrose, and the different ionic species during rest and activity.
The Measurement of Sodium Concentration in Human Red Blood Cells
Authors: Beilin L. J. , Knight G. J. , Munro-Faure A. D. , Anderson J.
Published: May 2004

Experiments are described which indicate that iodinated human serum albumin underestimates the amount of extracellular sodium trapped in the packed layer of red blood cells, when cells and plasma are separated by centrifugation. Sucrose-14C also underestimates the amount of trapped extracellular sodium, but the difference between the percentages of sucrose-14C and extracellular sodium trapped is constant and independent of mean relative centrifugal force. It is concluded that human red blood cell sodium concentration can be measured with accuracy (a) if trapped plasma sodium is estimated with radioisotopes of sodium and a correction made for entry of sodium into the cells, providing cells and plasma can be separated rapidly; (b) by the use of sucrose as a standard plasma marker to derive the amount of trapped plasma sodium; (c) by washing the cells with sodium-free solutions. Reported values for red blood cell sodium concentration in healthy adults are critically reviewed.
The Entry of Sodium into Human Red Blood Cells in Vivo
Authors: Beilin L. J. , Eyeions D. , Hatcher G. , Knight G. J. , Munro-Faure A. D. , Anderson J.
Published: May 2004

The kinetics of sodium, movement into human red blood cells has been studied in vivo with 24Na. When human serum albumin-131I is used to measure the percentage of plasma trapped in the packed red blood cells after centrifugation, approximately 30 % of red blood cell sodium is found to equilibrate immediately with plasma. It is concluded that this immediately exchangeable compartment of red blood cell sodium is an experimental artefact, associated with the use of labeled albumin for measuring plasma trapping. This immediately exchangeable fraction disappears when sucrose-14C is used to measure plasma trapping. The experimental results were examined by compartmental analysis, using an analogue computer. The results obtained, when plasma trapping was measured with sucrose-14C could be simulated by the use of models containing two compartments, arranged in series or in parallel. The errors of the techniques used and the possible physical basis for the results are discussed.
Sodium, Potassium, and Chloride Fluxes in Intercostal Muscle from Normal Goats and Goats with Hereditary Myotonia
Authors: Lipicky R. J. , Bryant S. H.
Published: May 2004

In isolated bundles of external intercostal muscle from normal goats and goats with hereditary myotonia the following were determined: concentrations and unidirectional fluxes of Na+, K+, and Cl-, extracellular volume, water content, fiber geometry, and core-conductor constants. No significant difference between the two groups of preparations was found with respect to distribution of fiber size, intracellular concentrations of Na+ or Cl-, fiber water, resting membrane potential, or overshoot of action potential. The intracellular Cl- concentration in both groups of preparations was 4 to 7 times that expected if Cl- were distributed passively between intracellular and extracellular water. The membrane permeability to K (PK) calculated from efflux data was (a) at 38°C, 0.365 x 10-6 cm sec-1 for normal and 0.492 x 10-6 for myotonic muscle, and (b) at 25°C, 0.219 x 10-6 for normal and 0.199 x 10-6 for myotonic muscle. From Cl- washout curves of normal muscle usually only three exponential functions could be extracted, but in every experiment with myotonic muscle there was an additional, intermediate component. From these data PPcl could be calculated; it was 0.413 x 10-6 cm sec-1 for myotonic fibers and was ⩾ 0.815 x 10-6 cm sec-1 for normal fibers. The resting membrane resistance of myotonic fibers was 4 to 6 times greater than that of normal fibers.
Some Factors Influencing Sodium Extrusion by Internally Dialyzed Squid Axons
Authors: Mullins L. J. , Brinley F. J.
Published: May 2004

Squid giant axons were internally dialyzed by a technique previously described. In an axon exposed to cyanide seawater for 1 hr and dialyzed with an ATP-free medium, the Na efflux had a mean value of 1.3 pmole/cm2sec when [Na]i was 88 mM, in quantitative agreement with flux ratio calculations for a purely passive Na movement. When ATP at a concentration of 5–10 mM was supplied to the axoplasm by dialysis, Na efflux rose almost 30-fold, while if phosphoarginine, 10 mM, was supplied instead of ATP, the Na efflux rose only about 15-fold. The substitution of Li for Na in the seawater outside did not affect the Na efflux from an axon supplied with ATP, while a change to K-free Na seawater reduced the Na efflux to about one-half. When special means were used to free an axon of virtually all ADP, the response of the Na efflux to dialysis with phosphoarginine (PA) at 10 mM was very small (an increment of ca. 3 pmole/cm2sec) and it can be concluded that more than 96% of the Na efflux from an axon is fueled by ATP rather than PA. Measurements of [ATP] in the fluid flowing out of the dialysis tube when the [ATP] supplied was 5 mM made it possible to have a continuous measurement of ATP consumption by the axon. This averaged 43 pmole/cm2sec. The ATP content of axons was also measured and averaged 4.4 mM. Estimates were made of the activities of the following enzymes in axoplasm: ATPase, adenylate kinase, and arginine phosphokinase. Values are scaled to 13°C.
Permeability Studies on Red Cell Membranes of Dog, Cat, and Beef
Authors: Rich G. T. , Sha'afi R. I. , Barton T. C. , Solomon A. K.
Published: May 2004

Water permeability coefficients of dog, cat, and beef red cell membranes have been measured under an osmotic pressure gradient. The measurements employed a rapid reaction stop flow apparatus with which cell shrinking was measured under a relative osmotic pressure gradient of 1.25 to 1.64 times the isosmolar concentration. For the dog red cell the osmotic permeability coefficient is 0.36 cm4/(sec, osmol). The water permeability coefficient for the dog red cell under a diffusion gradient was also measured (rate constant = 0.10/msec). The ratio between the two permeabilities was used to calculate an equivalent pore radius of 5.9 A. This value agrees welt with an equivalent pore radius of 6.2 A obtained from reflection coefficients of nonelectrolyte water-soluble molecules, and is consistent with data on the permeability of the dog red cell membrane to glucose. These data provide evidence supporting the existence of equivalent pores in single biological membranes.
The Contractile and Control Sites of Natural Actomyosin
Authors: Levy Harvey M. , Ryan Elizabeth M.
Published: May 2004

The various contractile and control sites of natural actomyosin gel were studied by comparing the kinetics of ATP hydrolysis with those of gel contraction, measured as an increase in turbidity. Contraction of actomyosin gel seems to require the cooperative reaction of ATP (with Mg) at two different sites. One of these sites catalyzes the hydrolysis of ATP and most probably contributes the driving force for contraction; the binding of ATP to the other site appears to break certain links that retard movement of the gel components. At limiting concentrations of ATP, the rate of contraction seems to depend on the rate of breaking these links as well as on the rate of ATP hydrolysis. But when both sites are saturated, the rate of contraction appears to be limited only by the rate of ATP hydrolysis. In addition to these two contractile sites, there are also two different control sites. At one, the relaxing site, the binding of ATP with Mg inhibits ATP hydrolysis and gel contraction. At the other, the binding of calcium activates contraction by overcoming the inhibitory action of Mg and ATP at the relaxing site. This control system—inhibition by substrate and disinhibition by calcium—can be selectively inactivated by heat and reactivated by dithiothreitol, a disulfide-reducing agent. These observations on the isolated contractile system are discussed in relation to the contraction and relaxation of muscle.
Correlation of Transmitter Release with Membrane Properties of the Presynaptic Fiber of the Squid Giant Synapse
Authors: Kusano Kiyoshi , Livengood David R. , Werman Robert
Published: May 2004

Depolarization of the presynaptic terminal by current produced a postsynaptic potential (PSP) which increased with increasing presynaptic polarization and then reached a plateau. Iontophoretic injection of tetraethylammonium ions (TEA) into the presynaptic axon near the terminal produced a prolonged presynaptic spike. The resulting PSP is increased in size and its time course closely followed that of the presynaptic spike. The presynaptic fiber no longer exhibited rectification and strong depolarizations revealed that the PSP reached a maximum with about 110 mv depolarization. Further depolarization produced a decrease in PSP amplitude and finally transmission was blocked. However, a PSP then always appeared on withdrawal of the depolarizing current. Under the conditions of these experiments, the PSP could be considered a direct measure of transmitter release. Bathing the TEA-injected synapse with concentrations of tetrodotoxin (TTX) sufficient to block spike activity in both pre- and postsynaptic axons did not greatly modify postsynaptic electrogenesis. However, doubling TTX concentration reversibly blocked PSP. Thus the permeability changes to Na and K accompanying the spike do not appear necessary for transmitter release. Some other processes related to the level of presynaptic polarization must be involved to explain the data. The inhibition of transmitter release by strong depolarizations appears to be related to Ca action. A membrane Ca current may also be necessary for normal transmitter release.
The Effects of External Sodium and Potassium Concentration on the Membrane Potential of Atrioventricular Fibers of the Toad
Authors: Kanno Tomio , Matsuda Kojiro
Published: May 2004

Intracellular records were made from fibers in the A-V conducting system of isolated toad hearts. The A-V region was perfused with Ringer's solution of various K and Na concentrations. Resting potential in 2.8 mM [K]o was about 60 mv. Over the range 0.28 to 28 mM, resting potential diminished with increasing [K]o. Spontaneous action potentials appeared when [K]o was increased to 11.2 mM, and when resting potential had fallen to about 40 to 50 mv. Changes in [Na]o over the range 22 to 110 mM had a little effect on resting potential, but there was a linear relation between the peak value of the action potential and log [Na]o Wenckebach periodicity was observed when [Na]o was lowered.
Energetics of Coupled Active Transport of Sodium and Chloride
Authors: Martin Duncan W. , Diamond Jared M.
Published: May 2004

A Clark electrode was used to measure oxygen consumption by the gall bladder, in which there is a direct and one-to-one linkage between active Na and active Cl transport. O2 uptake was reversibly depressed when Cl in the mucosal bathing solution was replaced by a poorly transported anion, such as sulfate. This effect of Cl was abolished by ouabain or in Na-free solutions. When the anion was chloride, treatment with ouabain or replacement of Na by a poorly transported cation depressed Qoo2, more than did replacement of Cl. However, ouabain or removal of Na also depressed Qoo2, in Na2SO4 solutions, in which salt transport is minimal. It is concluded that oxygen uptake in the gall bladder consists of three fractions: 9% requires both Na and Cl, is inhibited by ouabain, and is linked to the NaCl pump; 36% requires Na but not Cl, is inhibited by ouabain, and possibly is linked to the cellular K uptake mechanism; and 55% represents basal uptake. If the extra oxygen uptake observed during transport supplies all the energy for transport, then 25 Na + 25 Cl ions are transported actively per O2 consumed; i.e., twice as many ions as in epithelia which transport only Na actively. This extra uptake is more than sufficient to supply the energy for overcoming internal membrane resistance under the experimental conditions used.
Stimulation of the Salt Receptor of the Blowfly
Authors: Gillary Howard L.
Published: May 2004

The response of the salt receptor of the blowfly increased with increasing ambient temperature. At constant ambient temperature, the response increased as the relative humidity was raised. At low relative humidity, the temperature of the stimulating solution near its air interface was markedly below ambient temperature, due to evaporation. Warming and cooling the tip of the sensillum while recording from its side respectively raised and lowered the frequency of "spontaneous" action potentials of the receptor. The results indicate that the response of the salt receptor is dependent on the temperature of the stimulus. The Q10 is probably several times greater than one.
Cytodifferentiation and Membrane Transport Properties in LK Sheep Red Cells
Authors: Lee Ping , Woo Anna , Tosteson D. C.
Published: May 2004

Young cells produced in LK sheep during rapid hematopoiesis after massive hemorrhage contain more K than the cells which are normally released into the circulation. The K content in these new cells falls to that characteristic of mature LK cells after a few days in the circulation. K transport properties in young and old cells before and after massive bleeding were studied. Young and old cells were separated by means of a density gradient centrifugation technique. Evidence showing that younger cells are found in the lower density fractions is presented. Active transport of K in the lightest fraction as measured by strophanthidin-sensitive influx was four to five times greater in red cells drawn 6 days after massive bleeding while the K leak as measured by strophanthidin-insensitive influx was only slightly larger. No change after bleeding was observed in older cells which had been present in the circulation prior to the hemorrhage. It is concluded that the high K content of young cells produced in LK sheep after bleeding is due to temporary retention of membrane K transport properties characteristic of HK cells. Thus, genetically determined modification of membrane transport properties has been shown to occur in nondividing circulating red cells.
The Effect of High Sodium Concentration on the Action Potential of the Skate Heart
Authors: Seyama Issei , Irisawa Hiroshi
Published: May 2004

It already has been well documented that the maximum rate of depolarization and amplitude of action potentials are directly dependent on [Na+]o in the vertebrate myocardium. Almost all studies have been carried out at low sodium concentration ranges by substituting NaCl for other substances. Action potentials should be demonstrable in higher sodium concentrations, but cells are inevitably damaged by osmotic changes. The blood of elasmobranchs is nearly isosmotic with sea water, but NaCl accounts for 54.5% of the osmotic pressure and 38.7% of it is maintained by urea molecules. Utilizing this special situation in elasmobranchs, the effect of high sodium concentration was studied up to 170% of normal sodium concentration, while still retaining isosmotic condition. The rate of depolarization, amplitude, and duration of the myocardial action potential all increased in direct proportion to [Na+]o, and no depressant effect on transmembrane action potentials was observed in solutions of high sodium concentration. With regard to depolarization rate, the regression curve fitted by the least squares method passed through zero within two standard errors. At high sodium levels, the overshoot changed as expected theoretically, but at lower ranges it deviated from the theoretical values. [Na+]i, and [K+]i, in this tissue have been determined, and these data are explained on the basis of the Na theory.
Light Metabolism and Chloroplast Structure in Chlorophyll-Deficient Tobacco Mutants
Authors: Schmid Georg H. , Gaffron Hans
Published: May 2004

In tobacco mutants which contain ⅛ to 1/30 of the normal chlorophyll content per leaf area the content of yellow pigments (carotenoids) is also diminished but less in proportion to the chlorophyll content. The pale yellow-green mutant grows and matures provided that light intensity and temperature make up for the chlorophyll deficiency. In most green plants and algae light saturation of photosynthesis is reached between 5000 and 12,000 ergs/sec·cm2. The mutants continue to give higher photosynthetic rates until the incident intensity reaches 50,000 ergs/sec·cm2. While often unable to compensate their respiration at intensities at which the normal green plant approaches saturation, the pale yellow-green leaves are able to provide the mutant plant with two to three times the absolute amount of carbon dioxide assimilation per hour and leaf area at 50,000 ergs/sec·cm2 and 20° to 25°C. These observations are valid for red light λ > 600 mµ. In blue light λ < 575 mµ (below saturation levels) the mutants separate into two classes, one in which absorption by some carotenoid enhances the photosynthetic rate and the other in which the absorbing pigments are inactive and therefore depress the rate strongly. The unusual kinetics of photosynthesis in these chlorophyll-deficient tobacco mutants is reflected in the structure of their chloroplasts which we found to be of a kind thus far not described for healthy, normally growing, higher plants.
Surface Density of Calcium Ions and Calcium Spikes in the Barnacle Muscle Fiber Membrane
Authors: Hagiwara Susumu , Takahashi Kunitaro
Published: May 2004

The effects of various divalent cations in the external solution upon the Ca spike of the barnacle muscle fiber membrane were studied using intracellular recording and polarizing techniques. Analysis of the maximum rate of rise of the spike potential indicates that different species of divalent cations bind the same membrane sites competitively with different dissociation constants. The overshoot of the spike potential is determined by the density of Ca (Sr) ions in the membrane sites while the threshold membrane potential for spike initiation depends on the total density of divalent cations. The order of binding among different divalent and trivalent cations is the following: La+++, UO2++ > Zn++, Co++, Fe++ > Mn++ > Ni++ > Ca++ > Mg++, Sr++
The Transport of Salt and Water across Isolated Rat Ileum
Authors: Clarkson T. W.
Published: May 2004

The flows of sodium, potassium, and chloride under electrical and chemical gradients and of salt and water in the presence of osmotic pressure gradients are described by phenomenological equations based on the thermodynamics of irreversible processes. The aim was to give the simplest possible description, that is to postulate the least number of active transport processes and the least number of separate pathways across the intestine. On this basis, the results were consistent with the following picture of the intestine: Two channels exist across this tissue, one allowing only passive transport of ions and the other only active. In the passive channel, the predominant resistance to ion flow is friction with the water in the channel. The electroosmotic flow indicates that the passive channel is lined with negative fixed charged groups having a surface charge density of 3000 esu cm-2. The values of the ion-water frictional coefficients, and the relationship between ionic concentrations and flows indicate that the passive channel is extracellular. The active channel behaves as two membranes in series, the first membrane being semipermeable but allowing active transport of sodium, and the second membrane being similar to the passive channel. Friction with the ions in the second "membrane" is the predominant resistance to water flow.
Oxidative and Glycolytic Recovery Metabolism in Muscle
Authors: Jöbsis Frans F. , Duffield James C.
Published: May 2004

The average degree of reduction of mitochondrial NAD has been measured in the intact toad sartorius by a fluorometric technique. It has been shown that cytoplasmic NADH does not interfere materially with these measurements. The percentage reduction of this respiratory coenzyme has been determined in a number of physiological steady states which are well correlated with fluorometrically determined levels of NADH in suspensions of mitochondria from the hind leg musculature of the toad. In addition, these findings are closely comparable to similar, spectrophotometric measurements on mitochondria from other sources. In the presence of an adequate O2 level a single twitch produces a decrease in fluorescence from the resting steady state which is followed by a slow return to the base line condition. This cycle indicates the intensity and the time course of the oxidative recovery metabolism. The area under this curve is directly related to the number of twitches up to three or four. Greater activity produces a curtailment of oxidative recovery due to glycolysis. In the presence of iodoacetate the linear relation holds for five to seven twitches. At still higher levels of activity a curtailment of the change in NAD level sets in, probably due to the removal of AMP by catabolic reactions.
The Role of Calcium in the Regulation of the Steady-State Levels of Sodium and Potassium in the HeLa Cell
Authors: Morrill Gene A. , Robbins Elliott
Published: May 2004

Studies on HeLa cells in spinner culture at pH 7.0 and 37° have shown that [Na]i decreased and [K]i increased with increasing [Ca]o. In Na-free (choline) medium [K]i remained high whether or not Ca was present in the medium. [Na]i and [K]i approached a new steady state within 1 min after transfer to Ca-free medium and returned to the initial values within 15 min upon readdition of Ca. 40% of the cell Ca exchanged within 1 min followed by a slow exchange of the remaining Ca over several hours. [Ca]i increased with decreasing [Na]o but was independent of [K]o. Equimolar Mg did not substitute for Ca in maintaining low [Na]i and high [K]i. Under steady-state conditions about 50% of the cell Na exchanged in accordance with a single rate constant. The initial Na influx was 270, 100, and 2.5 µM/liter of cell water/sec for 0, 0.10, and 1.0 mM [Ca]o, respectively. When Na transport was inhibited with strophanthidin and [Na]i and [K]i allowed to reach a steady state, Na influx was more rapid for cells incubated in Ca-free medium than for cells incubated in medium containing 1.0 mM Ca. These results suggest that Ca competes with Na at the cell membrane and thus controls the passive diffusion of Na into the cell.
Electrical Properties of the Pacemaker Neurons in the Heart Ganglion of a Stomatopod, Squilla oratoria
Authors: Watanabe Akira , Obara Shosaku , Akiyama Toyohiro , Yumoto Katsuto
Published: May 2004

In the Squilla heart ganglion, the pacemaker is located in the rostral group of cells. After spontaneous firing ceased, the electrophysiological properties of these cells were examined with intracellular electrodes. Cells respond to electrical stimuli with all-or-none action potentials. Direct stimulation by strong currents decreases the size of action potentials. Comparison with action potentials caused by axonal stimulation and analysis of time relations indicate that with stronger currents the soma membrane is directly stimulated whereas with weaker currents the impulse first arises in the axon and then invades the soma. Spikes evoked in a neuron spread into all other neurons. Adjacent cells are interconnected by electrotonic connections. Histologically axons are tied with the side-junction. B spikes of adjacent cells are blocked simultaneously by hyperpolarization or by repetitive stimulation. Experiments show that under such circumstances the B spike is not directly elicited from the A spike but is evoked by invasion of an impulse or electrotonic potential from adjacent cells. On rostral stimulation a small prepotential precedes the main spike. It is interpreted as an action potential from dendrites.
Depolarization of the Internal Membrane System in the Activation of Frog Skeletal Muscle
Authors: Costantin L. L. , Podolsky R. J.
Published: May 2004

"Skinned" muscle fibers, single fibers from the frog semitendinosus muscle in which the sarcolemma had been removed, could be reversibly activated by electrical stimulation. Electrical responsiveness was abolished when the skinned fiber was prepared from a muscle exposed to a cardiac glycoside, and the development of responsiveness was delayed when the muscle was bathed in high potassium solution. The findings were taken as evidence that active sodium-potassium exchange across the internal membranes restored electrical excitability, after the sarcolemma had been removed, by establishing a potential gradient across the internal membranes. In general, the contractions were graded with the strength of the applied current. On occasion, however, "all-or-none" type responses were seen, raising the possibility that the internal membranes were capable of an electrically regenerative response. Activation could also be produced by an elevation of the intracellular chloride ion concentration or a decrease in the intracellular potassium, ion concentration, suggesting that depolarization of some element of the internal membrane system, that is, a decrease in the potential of the lumen of the internal membrane system relative to the potential of the myofibrillar space, was responsible for activation in these experiments. The distribution of both the electrically induced contractions and those produced by changes in the intracellular ion concentrations indicated that the responsive element of the internal membrane system was electrically continuous over many sarcomeres.
Small Intestinal Glucose Transport
Authors: Rider Alan K. , Schedl Harold P. , Nokes George , Shining Streeter
Published: May 2004

Proximal and distal small intestinal segments of the rat were perfused in situ at two different rates with isotonic solutions containing glucose in concentrations ranging from 25 to 600 mg/100 ml. Absorption was measured as glucose disappearance rate from the lumen. Glucose absorption had not previously been studied at intraluminal concentrations above and below blood glucose. Absorption was more rapid from the proximal segment. In both segments absorption was independent of perfusion rate and of whether glucose was analyzed by counting 14C or by the Somogyi method. The latter finding suggests that of the unidirectional fluxes, flux out of the bowel is much greater than flux into the bowel. In contrast to the findings in previous studies neither segment showed rate-limiting kinetics, and the Michaelis-Menten analysis was not applicable. The form of the curve depicting absorption rate in relation to concentration differed between the two segments. At the higher concentrations absorption rate continued to increase much more rapidly in the proximal than in the distal segment. The observations could not be explained by known mechanisms of glucose transport and illustrate the difficulties of achieving biochemically and physiologically meaningful in vivo studies of intestinal absorption.
Determination of Extracellular Space in Amphibian Muscle
Authors: Bozler Emil
Published: May 2004

The volumes of distribution of inulin and dextran in the sartorius, stomach, and cardiac muscle of the frog agree rather closely. That these spaces represent the volume of extracellular water is supported by the observation that efflux of sucrose can be divided into a fast and a slow phase and that the fast-moving fraction corresponds closely with inulin space determined in the same muscle. These and other findings confirm that sugars and related substances penetrate slowly into part of the fiber water and that, therefore, their volume of distribution does not accurately represent the volume of extracellular water. The kinetics of efflux of sucrose is consistent with the assumption that the movement of sugars is determined by the resistance of the cell surface as well as by internal diffusion. In connective tissue, sucrose and inulin are excluded only from a small part of the total water.
The Regulation of Catch in Molluscan Muscle
Authors: Twarog Betty M.
Published: May 2004

Molluscan catch muscles are smooth muscles. As with mammalian smooth muscles, there is no transverse ordering of filaments or dense bodies. In contrast to mammalian smooth muscles, two size ranges of filaments are present. The thick filaments are long as well as large in diameter and contain paramyosin. The thin filaments contain actin and appear to run into and join the dense bodies. Vesicles are present which may be part of a sarcoplasmic reticulum. Neural activation of contraction in Mytilus muscle is similar to that observed in mammalian smooth muscles, and in some respects to frog striated muscle. The relaxing nerves, which reduce catch, are unique to catch muscles. 5-Hydroxytryptamine, which appears to mediate relaxation, specifically blocks catch tension but increases the ability of the muscle to fire spikes. It is speculated that Mytilus muscle actomyosin is activated by a Ca++-releasing mechanism, and that 5-hydroxytryptamine may reduce catch and increase excitability by influencing the rate of removal of intracellular free Ca++.
Is the Cell Membrane a Universal Rate-Limiting Barrier to the Movement of Water between the Living Cell and Its Surrounding Medium?
Authors: Ling Gilbert N. , Ochsenfeld Margaret M. , Karreman George
Published: May 2004

With the use of a newly introduced technique, the "influx profile analysis," we studied the diffusion of tritiated water in and out of frog ovarian eggs at 25°C. The results show that the rate-limiting step in the exchange of labeled water is not permeation through the cell membrane but diffusion in the bulk of the intracellular water.
ATPase Activity of Myosin Correlated with Speed of Muscle Shortening
Authors: Bárány Michael
Published: May 2004

Myosin was isolated from 14 different muscles (mammals, lower vertebrates, and invertebrates) of known maximal speed of shortening. These myosin preparations were homogeneous in the analytical ultracentrifuge or, in a few cases, showed, in addition to the main myosin peak, part of the myosin in aggregated form. Actin- and Ca++-activated ATPase activities of the myosins were generally proportional to the speed of shortening of their respective muscles; i.e. the greater the intrinsic speed, the higher the ATPase activity. This relation was found when the speed of shortening ranged from 0.1 to 24 muscle lengths/sec. The temperature coefficient of the Ca++-activated myosin ATPase was the same as that of the speed of shortening, Q10 about 2. Higher Q10 values were found for the actin-activated myosin ATPase, especially below 10°C. By using myofibrils instead of reconstituted actomyosin, Q10 values close to 2 could be obtained for the Mg++-activated myofibrillar ATPase at ionic strength of 0.014. In another series of experiments, myosin was isolated from 11 different muscles of known isometric twitch contraction time. The ATPase activity of these myosins was inversely proportional to the contraction time of the muscles. These results suggest a role for the ATPase activity of myosin in determining the speed of muscle contraction. In contrast to the ATPase activity of myosin, which varied according to the speed of contraction, the F-actin-binding ability of myosin from various muscles was rather constant.
Cell Motility by Labile Association of Molecules
Authors: Inoué Shinya , Sato Hidemi
Published: May 2004

This article summarizes our current views on the dynamic structure of the mitotic spindle and its relation to mitotic chromosome movements. The following statements are based on measurements of birefringence of spindle fibers in living cells, normally developing or experimentally modified by various physical and chemical agents, including high and low temperatures, antimitotic drugs, heavy water, and ultraviolet microbeam irradiation. Data were also obtained concomitantly with electron microscopy employing a new fixative and through measurements of isolated spindle protein. Spindle fibers in living cells are labile dynamic structures whose constituent filaments (microtubules) undergo cyclic breakdown and reformation. The dynamic state is maintained by an equilibrium between a pool of protein molecules and their linearly aggregated polymers, which constitute the microtubules or filaments. In living cells under physiological conditions, the association of the molecules into polymers is very weak (absolute value of ΔF25°C < 1 kcal), and the equilibrium is readily shifted to dissociation by low temperature or by high hydrostatic pressure. The equilibrium is shifted toward formation of polymer by increase in temperature (with a large increase in entropy: ΔS25°C ≃ 100 eu) or by the addition of heavy water. The spindle proteins tend to polymerize with orienting centers as their geometrical foci. The centrioles, kinetochores, and cell plate act as orienting centers successively during mitosis. Filaments are more concentrated adjacent to an orienting center and yield higher birefringence. Astral rays, continuous fibers, chromosomal fibers, and phragmoplast fibers are thus formed by successive reorganization of the same protein molecules. During late prophase and metaphase, polymerization takes place predominantly at the kinetochores; in metaphase and anaphase, depolymerization is prevalent near the spindle poles. When the concentration of spindle protein is high, fusiform bundles of polymer are precipitated out even in the absence of obvious orienting centers. The shift of equilibrium from free protein molecules to polymer increases the length and number of the spindle microtubules or filaments. Slow depolymerization of the polymers, which can be brought about by low concentrations of colchicine or by gradual cooling, allows the filaments to shorten and perform work. The dynamic equilibrium controlled by orienting centers and other factors provides a plasusible mechanism by which chromosomes and other organelles, as well as the cell surface, are deformed or moved by temporarily organized arrays of microtubules or filaments.
Recent X-ray Diffraction and Electron Microscope Studies of Striated Muscle
Authors: Huxley H. E.
Published: May 2004

The sliding filament model for muscular contraction supposes that an appropriately directed force is developed between the actin and myosin filaments by some process in which the cross-bridges are involved. The cross-bridges between the filaments are believed to represent the parts of the myosin molecules which possess the active sites for ATPase activity and actin-binding ability, and project out sidewise from the backbone of the thick filaments. The arrangement of the cross-bridges is now being studied by improved low-angle X-ray diffraction techniques, which show that in a resting muscle, they are arranged approximately but not exactly in a helical pattern, and that there are other structural features of the thick filaments which give rise to additional long periodicities shown up by the X-ray diagram. The actin filaments also contain helically arranged subunits, and both the subunit repeat and the helical repeat are different from those in the myosin filaments. Diffraction diagrams can be obtained from muscles in rigor (when permanent attachment of the cross-bridges to the actin subunits takes place) and now, taking advantage of the great increase in the speed of recording, from actively contracting muscles. These show that changes in the arrangement of the cross-bridges are produced under both these conditions and are no doubt associated in contraction with the development of force. Thus configurational changes of the myosin component in muscle have been demonstrated: these take place without any significant over-all change in the length of the filaments.
Effects of Some Inhibitors on the Temperature-Dependent Component of Resting Potential in Lobster Axon
Authors: Senft Joseph P.
Published: May 2004

The resting membrane potential of the lobster axon becomes 5–8 mv more negative when the temperature of the perfusion solution is increased 10°C. This potential change is about twice that predicted if the axon membrane potential followed that expected for a potassium ion electrode potential. When the inhibitors, 2, 4-dinitrophenol, sodium cyanide, and sodium azide, were added separately to the perfusion medium the potential change was reduced to about 1.4 times that predicted for a potassium ion electrode potential. Assays of axons exposed to these inhibitors showed that ATP levels were reduced to about one-fourth that obtained for control axons. Ouabain added to the perfusion medium reduced the potential change to that expected for a potassium ion electrode potential. These results suggest that the resting potential changes with temperature as a result of the activity of an electrogenic ion pump.
Standing-Gradient Osmotic Flow
Authors: Diamond Jared M. , Bossert William H.
Published: May 2004

At the ultrastructural level, epithelia performing solute-linked water transport possess long, narrow channels open at one end and closed at the other, which may constitute the fluid transport route (e.g., lateral intercellular spaces, basal infoldings, intracellular canaliculi, and brush-border microvilli). Active solute transport into such folded structures would establish standing osmotic gradients, causing a progressive approach to osmotic equilibrium along the channel's length. The behavior of a simple standing-gradient flow system has therefore been analyzed mathematically because of its potential physiological significance. The osmolarity of the fluid emerging from the channel's open end depends upon five parameters: channel length, radius, and water permeability, and solute transport rate and diffusion coefficient. For ranges of values of these parameters encountered experimentally in epithelia, the emergent osmolarity is found by calculation to range from isotonic to a few times isotonic; i.e., the range encountered in epithelial absorbates and secretions. The transported fluid becomes more isotonic as channel radius or solute diffusion coefficient is decreased, or as channel length or water permeability is increased. Given appropriate parameters, a standing-gradient system can yield hypertonic fluids whose osmolarities are virtually independent of transport rate over a wide range, as in distal tubule and avian salt gland. The results suggest that water-to-solute coupling in epithelia is due to the ultrastructural geometry of the transport route.
The Active State of Mammalian Skeletal Muscle
Authors: Bahler Alan S. , Fales John T. , Zierler Kenneth L.
Published: May 2004

A new technique is proposed for computing the active state of striated muscle, based on the three component model of Fenn and Marsh (8) and of Hill (7). The method permits calculation of the time course of the active state from its peak to the time at which maximum isometric twitch tension is reached. The intormation required for the calculation can be obtained from a single muscle without moving it from its mount in the lever system. The time course of the active state proved to be a function of the length of the muscle. This length dependency led to the predictions that (a) the length at which maximum force is developed during tetanic stimulation is different from that at which it is developed during a twitch, and (b) the tetanus-twitch tension ratio is a function of length. Both predictions were verified in a series of experiments on the rat gracilis anticus muscle at 17.5°C.
Further Observations on Asymmetrical Solute Movement across Membranes
Authors: Franz T. J. , Galey William R. , Van Bruggen J. T.
Published: May 2004

The permeability of frog skin under the influence of urea hyperosmolarity has been studied. Flux ratio asymmetry has been demonstrated again for tracer mannitol. The inhibitors DNP, CN-, and ouabain have been used to eliminate active sodium transport and it was found that urea hyperosmolarity produces asymmetrical mannitol fluxes on frog skins having no short-circuit current. These findings suggest that flux ratio asymmetry is due to solute interaction and is unrelated to sodium transport. Studies with a synthetic membrane show clearly that bulk flow of fluid can produce a "solvent drag" effect and change flux ratios. When bulk flow is blocked and solute gradients allowed their full expression, then solute interaction "solute drag" is easily demonstrable in a synthetic system.
Filtration Coefficient of the Axon Membrane As Measured with Hydrostatic and Osmotic Methods
Authors: Vargas Fernando F.
Published: May 2004

The hydraulic conductivity of the membranes surrounding the giant axon of the squid, Dosidicus gigas, was measured. In some axons the axoplasm was partially removed by suction. Perfusion was then established by insertion of a second pipette. In other axons the axoplasm was left intact and only one pipette was inserted. In both groups hydrostatic pressure was applied by means of a water column in a capillary manometer. Displacement of the meniscus in time gave the rate of fluid flowing across the axon sheath. In both groups osmotic differences across the membrane were established by the addition of a test molecule to the external medium which was seawater. The hydraulic conductivity determined by application of hydrostatic pressure was 10.6 ± 0.8.10-8 cm/sec cm H2O in perfused axons and 3.2 ± 0.6.10-8 cm/sec cm H2O in intact axons. When the driving force was an osmotic pressure gradient the conductivity was 4.5 ± 0.6 x 10-10 cm/sec cm H2O and 4.8 ± 0.9 x 10-10 cm/sec cm H2O in perfused and intact axons, respectively. A comparable result was found when the internal solution was made hyperosmotic. The fluid flow was a linear function of the hydrostatic pressure up to 70 cm of water. Glycerol outflux and membrane conductance were increased 1.6 and 1.1 times by the application of hydrostatic pressure. These increments do not give an explanation of the difference between the filtration coefficients. Other possible explanations are suggested and discussed.
Spontaneous Activity in Isolated Somata of Aplysia Pacemaker Neurons
Authors: Alving Barbara O.
Published: May 2004

Somata of pacemaker and nonpacemaker neurons were isolated by ligatures tied around the axons between the somata and the synaptic regions, and the transmembrane potentials of the isolated somata were recorded. Isolated somata of pacemaker neurons had a spontaneous discharge while isolated somata of nonpacemaker neurons were quiescent. In addition, the time course of accommodation in isolated somata of pacemaker and nonpacemaker neurons was found to be different. In pacemaker neurons, injection of current produced a change in rate of discharge sustained for the duration of current injection, while in nonpacemakers, current injection produced only a transient change in discharge rate. Evidence is presented that the pacemaker locus and spike trigger zone in the intact pacemaker neuron are located on the soma.