bluuebunny/crossref_metadata_2025 huggingface dataset:
Kinetics of the Opening and Closing of Individual Excitability-Inducing Material Channels in a Lipid Bilayer
Authors: Ehrenstein Gerald , Blumenthal Robert , Latorre Ramon , Lecar Harold
Published: May 2004

The kinetics of the opening and closing of individual ion-conducting channels in lipid bilayers doped with small amounts of excitability-inducing material (EIM) are determined from discrete fluctuations in ionic current. The kinetics for the approach to steady-state conductance during voltage clamp are determined for lipid bilayers containing many EIM channels. The two sets of measurements are found to be consistent, verifying that the voltage-dependent conductance of the many-channel EIM system arises from the opening and closing of individual EIM channels. The opening and closing of the channels are Poisson processes. Transition rates for these processes vary exponentially with applied potential, implying that the energy difference between the open and closed states of an EIM channel is linearly proportional to the transmembrane electric field. A model incorporating the above properties of the EIM channels predicts the observed voltage dependence of ionic conductance and conductance relaxation time, which are also characteristic of natural electrically excitable membranes.
Calcium-Activated Tension of Skinned Muscle Fibers of the Frog
Authors: Godt Robert E.
Published: May 2004

The influence of MgATP on the Ga++-activated isometric tension of skinned frog muscle fibers was examined in solutions containing: Mg++ = 5 mM, creatine phosphate (CP) = 14.5 mM, creatinephosphokinase (CPK) = 1 mg/ml, total EGTA = 7 mM, CaCl2, KCl, imidazole ≥ 20 mM so that ionic strength = 0.15, pH = 7.00, and MgATP = 2 mM, 0.1 mM, or 20 µM. CP and CPK were necessary for these experiments as determined experimentally by their effect on the tension-Ca++ relation, which was saturated for CP ≥ 14.5 mM. This was interpreted to mean that sufficient CP was present to effectively buffer MgATP intracellularly. Decreasing MgATP shifts the tension-pCa curve to higher pCa (-log Ca++) so that, for half-maximal tension: pCa1/2 = 4.5 for MgATP = 2 mM, pCa1/2 = 5.1 for MgATP = 0.1 mM, and pCa1/2 = 5.8 for MgATP = 20 µM; maximum isometric tension is the same in all cases, however. If MgATP was decreased to 1 µM, tension at Ga++ > 10–8 M was 84% of the maximum Ca-+-activated tension in 2 mM MgATP and increased only slightly to 90% for pCa = 4.5. Weber (1970, In The Physiology and Biochemistry of Muscle as Food, Volume 2, E. J. Briskey, R. G. Cassens, and B. B. Marsh, University of Wisconsin Press, Madison, Wis.), using similar solutions, observed similar shifts in half-maximal calcium activation of rabbit myofibril ATPase rates. In explanation, Weber and Bremel (1971, In Contractility of Muscle Cells and Related Processes, R. J. Podolsky, editor, Prentice-Hall, Inc., Englewood Cliffs, N.J.; Bremel and Weber, 1972, Nat. New Biol., 238:97) have described a mechanism whereby, at low ATP, "rigor complexes" are formed between myosin and thin filament actin and, in turn, alter the calcium affinity of one class of the two Ca++-binding sites on troponin, so that the thin filament is "turned on" for contraction at lower Ca++ levels. Tension data from skinned fibers substantially supports this hypothesis. A stability constant for CaEGTA of 2.62 x 1010 M–1 was determined, with the help of F. N. Briggs, in solutions similar to those used for skinned fibers and was the same for 100 and 300 mM KCl.
Fast Electrical Potential from a Long-Lived, Long-Wavelength Photoproduct of Fly Visual Pigment
Authors: Pak William L. , Lidington Kellie J.
Published: May 2004

A rapid electrical potential, which we have named the M-potential, can be obtained from the Drosophila eye using a high energy flash stimulus. The potential can be elicited from the normal fly, but it is especially prominent in the mutant norp AP12 (a phototransduction mutant), particularly if the eye color pigments are genetically removed from the eye. Several lines of evidence suggest that the M-potential arises from photoexcitation of long-lived metarhodopsin. Photoexcitation of rhodopsin does not produce a comparable potential. The spectral sensitivity of the M-potential peaks at about 575 nm. The M-potential pigment (metarhodopsin) can be shown to photoconvert back and forth with a "silent pigment(s)" absorbing maximally at about 485 nm. The silent pigment presumably is rhodopsin. These results support the recent spectrophotometric findings that dipteran metarhodopsin absorbs at much longer wavelengths than rhodopsin. The M-potential probably is related to the photoproduct component of the early receptor potential (ERP). Two major differences between the M-potential and the classical ERP are: (a) Drosophila rhodopsin does not produce a rapid photoresponse, and (b) an anesthetized or freshly sacrificed animal does not yield the M-potential. As in the case of the ERP, the M-potential appears to be a response associated with a particular state of the fly visual pigment. Therefore, it should be useful in in vivo investigations of the fly visual pigment, about which little is known.
Passive Electrical Properties of Toad Urinary Bladder Epithelium
Authors: Reuss Luis , Finn Arthur L.
Published: May 2004

The electrical resistances of the transcellular and paracellular pathways across the toad urinary bladder epithelium (a typical "tight" sodium-transporting epithelium) were determined by two independent sets of electrophysiological measurements: (a) the measurement of the total transepithelial resistance, the ratio of resistance of the apical to the basal cell membrane, and cable analysis of the voltage spread into the epithelium; (b) the measurement of the total transepithelial resistance and the ratio of resistances of both cell membranes before and after replacing all mucosal sodium with potassium (thus, increasing selectively the resistance of the apical membrane). The results obtained with both methods indicate the presence of a finite transepithelial shunt pathway, whose resistance is about 1.8 times the resistance of the transcellular pathway. Appropriate calculations show that the resistance of the shunt pathway is almost exclusively determined by the zonula occludens section of the limiting junctions. The mean resistance of the apical cell membrane is 1.7 times that of the basal cell membrane. The use of nonconducting materials on the mucosal side allowed us to demonstrate that apparently all epithelial cells are electrically coupled, with a mean space constant of 460 µm, and a voltage spread consistent with a thin sheet model.
Light-Induced Changes in Photoreceptor Membrane Resistance and Potential in Gecko Retinas
Authors: Pinto L. H. , Pak W. L.
Published: May 2004

The time-course of light-induced changes in membrane voltage and resistance were measured in single photoreceptors in eyecup preparations of Gekko gekko. A small circular stimulus directed toward the impaled receptor produced membrane hyperpolarization. Application of a steady annular light to the receptor periphery resulted in diminution of the receptor's response to the stimulus. The effects of illumination of the surrounding receptors were isolated by directing a small, steady desensitizing light to the impaled receptor and then applying a peripheral stimulus. Brief stimuli produced a transient decrease in resistance with rapid onset and offset, a time-course similar to that of the response diminution. For some cells a depolarization that coincided with the resistance decrease was seen. During illumination with prolonged stimuli the resistance decrease was followed by a slow increase. After offset resistance rose transiently above the original value and then returned slowly to its original value. The slow resistance changes were not accompanied by changes in membrane voltage. The response diminution, resistance decrease, and depolarization were not observed in retinas treated with aspartate or hypoxia. It is therefore concluded that these effects are mediated by horizontal cells. The diminution is achieved by shunting the receptor potential and may play a role in field adaptation.
Osmosis in Cortical Collecting Tubules
Authors: Schafer James A. , Troutman Susan L. , Andreoli Thomas E.
Published: May 2004

The present experiments were designed to evaluate the effects of varying the osmolality of luminal solutions on the antidiuretic hormone (ADH)-independent water and solute permeability properties of isolated rabbit cortical collecting tubules. In the absence of ADH, the osmotic water permeability coefficient (cm s–1) Pfl→b, computed from volume flows from hypotonic lumen to isotonic bath, was 20 ± 4 x 10–4 (SEM); the value of Pfb→l in the absence of ADH, computed from volume flows from isotonic bath to hypertonic lumen, was 88 ± 15 x 10–4 cm s–1. We also measured apparent urea permeability coefficients (cm s–1) from 14C-urea fluxes from lumen to bath (PDDureal→b) and from bath to lumen (PDDureab→l). For hypotonic luminal solutions and isotonic bathing solutions, PDDureal→b was 0.045 ± 0.004 x 10–4 and was unaffected by ADH. The ADH-independent values of PDDureal→b and Pureab→l were, respectively, 0.216 ± 0.022 x 10–4 cm s–1 and 0.033 ± 0.002 x 10–4 cm s–1 for isotonic bathing solutions and luminal solutions made hypertonic with urea, i.e., there was an absolute increase in urea permeability and asymmetry of urea fluxes. Significantly, PDDureal→b did not rise when luminal hypertonicity was produced by sucrose; and, bathing fluid hypertonicity did not alter tubular permeability to water or to urea. We interpret these data to indicate that luminal hypertonicity increased the leakiness of tight junctions to water and urea but not sucrose. Since the value of Pfb→l in the absence of ADH, when tight junctions were open to urea, was approximately half of the value of Pfl→b in the presence of ADH, when tight junctions were closed to urea, we conclude that tight junctions are negligible paracellular shunts for lumen to bath osmosis with ADH. These findings, together with those in the preceding paper, are discussed in terms of a solubility-diffusion model for water permeation in which ADH increases water solubility in luminal plasma membranes.
Characteristics of Sodium Flux from Serosa to Mucosa in Rabbit Ileum
Authors: Desjeux Jehan-F. , Tai Y.-H. , Curran Peter F.
Published: May 2004

Sodium flux from serosa to mucosa, JsmNa in rabbit ileum in vitro has been studied as a function of applied electrical potential at equal sodium concentrations in the bathing solutions. The results indicate that JsmNa involves two pathways, a diffusional flux through a paracellular shunt pathway and a flux that is independent of applied potential and presumably involves a transcellular pathway. The latter pathway comprises approximately 25 % of JsmNa in Ringer's solution containing 10 mM glucose and 25 mM bicarbonate. It is stimulated significantly by theophylline unaffected by removal of glucose or addition of ouabain but is reduced to negligible values by anoxia, dinitrophenol, and replacement of all chloride and bicarbonate by isethionate. Thus this component of JsmNa has a number of characteristics consistent with involvement in a specific secretory process mediating an electrically neutral secretory transport of sodium plus anion from serosa to mucosa. In addition to stimulating this process, theophylline significantly reduced the permeability of the paracellular shunt pathway to sodium.
Effects of Epinephrine on the Pacemaker Potassium Current of Cardiac Purkinje Fibers
Authors: Tsien Richard W.
Published: May 2004

Epinephrine promotes spontaneous activity in cardiac Purkinje fibers through its action on the pacemaker potassium current (iKK2). The mechanism of the acceleratory effect was studied by means of a voltage clamp technique. The results showed that the hormone speeds the deactivation of iKK2 during pacemaker activity by displacing the kinetic parameters of iKK2 toward less negative potentials. This depolarizing voltage shift is the sole explanation of the acceleratory effect since epinephrine did not alter the rectifier properties of iKK2, or the underlying inward leakage current, or the threshold for iNNa. The dose dependence of the voltage shift in the iKK2 activation curve was similar in 1.8 and 5.4 mM [Ca]o. The maximal voltage shift (usually ∼20 mV) was produced by epinephrine concentrations of > 10-6 M. The half-maximal effect was evoked by 60 nM epinephrine, nearly an order of magnitude lower than required for half-maximal effect on the secondary inward current (Carmeliet and Vereecke, 1969). The ß-blocker propranolol (10-6 M) prevented the effect of epinephrine (10-7M) but by itself gave no voltage shift. Epinephrine shifted the activation rate coefficient α8 to a greater extent than the deactivation rate coefficient ß8, and often steepened the voltage dependence of the steady-state activation curve. These deviations from simple voltage shift behavior were discussed in terms of possible mechanisms of epinephrine's action on the iKK2 channel.
Resting Sarcomere Length-Tension Relation in Living Frog Heart
Authors: Winegrad Saul
Published: May 2004

The sarcomere pattern and tension of isolated resting frog atrial trabeculae were continuously monitored. In the absence of any resting tension the sarcomere lengths varied with the diameter of the trabeculae. In over 75 % of the trabeculae the value exceeded 2.05 µm, the estimated in vivo length of the thin filaments, and it was never less than 1.89 µm. When the trabeculae were stretched the increase in length of the central undamaged portion could be completely accounted for by an increase in sarcomere length. The width of the A band was constant only at sarcomere lengths between 2.3 and 2.6 µm it decreased at smaller and increased at larger sarcomere lengths. A group of spontaneously active cells stretched the sarcomeres in cells in series to longer lengths than could be produced by passive tension applied to the ends of the trabeculae, but they did not influence the sarcomeres of adjacent cells. It is proposed that the connective tissue is a major factor in determining sarcomere length and that there are interactions between thick and thin filaments in resting muscles.
Functional Separation of the Na-K Exchange Pump from the Volume Controlling Mechanism in Enlarged Duck Red Cells
Authors: Kregenow Floyd M. ,   With the technical assistance of Mary Frances Spears.
Published: May 2004

Previous publications have described a "volume controlling mechanism" in duck erythrocytes that returns both enlarged and shrunken cells to their original isotonic volume. Enlarged cells return to their original size by readjusting their K content. To study the specificity of this aspect of the mechanism for K, we prepared enlarged cells with various Na and K contents. Only cells containing a high K content resume their original size in the standard isotonic medium. The process of regulation resembles that described above. In contrast, cells containing a high Na content fail to reestablish this volume, but shrink instead until they reach a limiting minimal volume (four-fifths of normal). Here, another mechanism, the cation pump rather than the volume controlling mechanism, removes Na and is responsible for the changes in cell size. Enlarged cells with an intermediate Na and K content utilize both mechanisms to reduce their cation content. Only if Na is prevented from leaving the cell and sufficient K is present initially, will these cells reestablish their original size. These studies demonstrate that the cation pump and volume controlling mechanism function independently and, when cells enlarge, only K can effectively traverse the pathway associated with the volume controlling mechanism. This route differs from the one used by the cation pump to eject Na.
Potassium Flux and Leaf Movement in Samanea saman
Authors: Satter R. L. , Geballe G. T. , Galston A. W.
Published: May 2004

Phytochrome, a membrane-localized biliprotein whose conformation is shifted reversibly by brief red or far-red light treatments, interacts with the rhythmic oscillator to regulate leaflet movement and potassium flux in pulvinal motor cells of Samanea. Darkened pinnae exposed briefly to red light (high Pfr level) have less potassium in motor cells in the extensor region, more potassium in motor cells in the flexor region, and smaller angles than those exposed to far-red light (low Pfr level). Increase in temperature from 24° to 37° increases the differential effect of the light treatments during opening (the energetic phase) but not during closure, implying that phytochrome controls an energetic process. It seems likely that phytochrome interacts with rhythmically controlled potassium pumps in flexor and extensor cells. During nyctinastic closure of white-illuminated pinnae, exposure to far-red light before darkening results in larger angles than does exposure to red. As in rhythmic opening, the angles of all pinnae and the differential effect of the light treatments increases with increasing temperature.
Acetylcholine Receptors
Authors: Fambrough Douglas M.
Published: May 2004

The number of extrajunctional acetylcholine receptors (125I-labeled α-bungarotoxin binding sites) per unit length of muscle fiber and the average fiber circumference were determined for rat diaphragm muscle fibers denervated 0, 2, 4, 7, 10, and 14 days. From these data receptor densities (sites per square micrometer of surface) were calculated. Values thus obtained were considerably lower than those estimated previously by autoradiography. Receptor density increased from < 6 sites/µm2 in innervated muscle to 635 ± 29 sites/µm2 14 days after denervation. The form of the relationship between receptor density and acetylcholine sensitivity and the time-course of change in receptor density after denervation are as previously reported.
Equilibrium Potential for the Postsynaptic Response in the Squid Giant Synapse
Authors: Llinás R. , Joyner R. W. , Nicholson C.
Published: May 2004

The reversal potential for the EPSP in the squid giant synapse has been studied by means of an intracellular, double oil gap technique. This method allows the electrical isolation of a portion of the axon from the rest of the fiber and generates a quasi-isopotential segment. In order to make the input resistance of this nerve segment as constant as possible, the electroresponsive properties of the nerve membrane were blocked by intracellular injection of tetraethylammonium (TEA) and local extracellular application of tetrodotoxin (TTX). Thus, EPSP's could be evoked in the isolated segment with a minimal amount of electroresponsive properties. The reversal potential for the EPSP (EEPSP) was measured by recording the synaptic potential or the synaptic current during voltage clamping. The results indicate that EEPSP may vary from +15 to +25 mV, which is more positive than would be expected for a 1:1 conductance change for Na+ and K+ (approximately -15 mV) and too negative for a pure Na+ conductance (+40 mV). This latter value (ENa) was directly determined in the voltage clamp experiments. The results suggest that the synaptic potential is probably produced by a permeability change to Na+ to K+ in a 4:1 ratio. No change in time-course was observed in the synaptic current at clamp levels of -100 and +90 mV. The implications of a variable ratio for Na+-K+ permeability in subsynaptic-postsynaptic membranes are discussed.
Relationship of Cation Influxes and Effluxes in Yeast
Authors: Rothstein Aser
Published: May 2004

The Na+ efflux from Na+-rich yeast cells into a cation-free medium is largely balanced by the excretion of organic anions. In the presence of Rb+, K+, or high levels of H+ (pH 3–4), the Na+ efflux is increased and the organic anion excretion is suppressed so that stoichiometric cation exchanges occur. H+ participates in the exchanges, moving into or out of the cells depending on the external pH and on the concentration of external Rb+(K+). The total cation efflux is dependent on the external Rb+ concentration in a "saturation" relationship, but the individual cations in the efflux stream are not. The discrimination factor in the efflux pathway between H+ and Na+ is very large (of the order of 10,000), and between Na+ and K+ considerable (of the order of 50). For the latter pair, the recycling of K+ from the cell wall space is an important factor in the discrimination. In addition, the Na+ efflux as a function of Na+ content follows a sigmoidal curve so that the discrimination factor is increased at high levels of cellular Na+. Although the influx and efflux pathways behave as a tightly coupled system, the mechanism of coupling is not entirely clear. A single system with different cation specificities and kinetic behaviors on the inside and outside faces of the membrane could account for the data.
Red Cell Membrane Permeability Deduced from Bulk Diffusion Coefficients
Authors: Redwood W. R. , Rall E. , Perl W.
Published: May 2004

The permeability coefficients of dog red cell membrane to tritiated water and to a series of[14C]amides have been deduced from bulk diffusion measurements through a "tissue" composed of packed red cells. Red cells were packed by centrifugation inside polyethylene tubing. The red cell column was pulsed at one end with radiolabeled solute and diffusion was allowed to proceed for several hours. The distribution of radioactivity along the red cell column was measured by sequential slicing and counting, and the diffusion coefficient was determined by a simple plotting technique, assuming a one-dimensional diffusional model. In order to derive the red cell membrane permeability coefficient from the bulk diffusion coefficient, the red cells were assumed to be packed in a regular manner approximating closely spaced parallelopipeds. The local steady-state diffusional flux was idealized as a one-dimensional intracellular pathway in parallel with a one-dimensional extracellular pathway with solute exchange occurring within the series pathway and between the pathways. The diffusion coefficients in the intracellular and extracellular pathways were estimated from bulk diffusion measurements through concentrated hemoglobin solutions and plasma, respectively; while the volume of the extracellular pathway was determined using radiolabeled sucrose. The membrane permeability coefficients were in satisfactory agreement with the data of Sha'afi, R. I., C. M. Gary-Bobo, and A. K. Solomon (1971. J. Gen. Physiol. 58:238) obtained by a rapid-reaction technique. The method is simple and particularly well suited for rapidly permeating solutes.
One-way fluxes of alpha-aminoisobutyric acid in Ehrlich ascites tumor cells. Trans effects and effects of sodium and potassium.
Authors: Jacquez J A
Published: May 2004

One-way fluxes in the steady state and one-way influxes at zero intracellular concentrations were measured for alpha-aminoisobutyric acid (AIB) in Ehrlich ascites tumor cells at 32 degrees C. The one-way fulxes show trans effects in the concentration of AIB and are dependent on sodium levels. The one-way fluxes for initial influx and for the steady state were fitted with the equations derived for the frequently used two-state carrier model. Estimates of the parameters of these equations were obtained with use of nonlinear least squares. These gave relatively good fits of the flux data and the data on steady-state distribution ratios. The two-state carrier model predicted a trans inhibition of one-way influx and a trans stimulation of one-way efflux. The former phenomenon has been demonstrated for AIB transport in Ehrlich ascites cells and there is evidence, through less firm, for the latter.
Squid retinochrome.
Authors: Sperling L , Hubbard R
Published: May 2004

Retinochrome is a photosensitive pigment located primarily in the inner portions of the visual cells of cephalopods. Its absorption spectrum resembles that of rhodopsin, but its chromophore is all-trans retinal, which light isomerizes to 11-cis, the reverse of the situation in rhodopsin. The 11-cis photoproduct of retinochrome slowly reverts to retinochrome in the dark. The chromophoric site of retinochrome is more reactive than that of most visual pigments: (a) Hydroxylamine converts retinochrome in the dark to all-trans retinal oxime + retinochrome opsin. (by Sodium borohydride reduces it to N-retinyl opsin. (c) Lambda max of retinochrome shifts from 500 to 515 nm as the pH is raised from 6 to 10, with a loss of absorption above pH 8; meanwhile above this PH a second band appears at shorter wavelengths with lambda max 375 nm. These changes are reversible. (d) If retinochrome is incubated with all-trans 3-dehydroretinal (retinal2) in the dark, some 3-dehydroretinochrome (retinochrome2, lambda max about 515 nm) is formed. Conversely, when retinochrome2, made by adding all-trans retinal2 to bleached retinochrome or retinochrome opsin, is incubated in the dark with all-trans retinal some of it is converted to retinochrome. Retinal and 3-dehydroretinal therefore can replace each other as chromophores in the dark.
Mechanical control of the time-course of contraction of the frog heart.
Authors: Bozler E
Published: May 2004

Changes in load during most phases of an isotonic contraction of the frog and turtle heart increased or decreased the duration of the twitch. It was abbreviated by a maintained increase or by a brief decrease in load. The relaxing effect of these procedures developed with a delay lasting more than a second under some conditions and will be called lengthening deactivation. The reverse procedures, a maintained diminution or a brief increase in load, increased the duration of the twitch. This effect will be called shortening activation. Although the termination of relaxation may be delayed or advanced by the mechanical interventions mentioned, the normal time-course of isotonic relaxation was always resumed later, regardless of the starting level of the load, making it possible to measure accurately changes in the duration of the twitch. The responses to changes in load produce positive feedback during the isotonic contraction and explain, at least in part, the difference in the time-course of isotonic and isometric contraction. The effects of changes in load were much smaller and briefer in the atrium than the ventricle.
A dual synaptic effect on hair cells in Hermissenda.
Authors: Alkon D L
Published: May 2004

Type A photorecptors can produce an initial hyperpolarizing wave followed by a delayed long-lasting increase in firing which is usually accompanied by a small depolarizing wave. The initial hyperpolarizing wave arises from an increase in conductance while the depolarizing wave was shown to arise from a decrease in conductance. The data presented indicate that both effects produced by the type A photoreceptors in ipsilateral hair cells are synaptic.
Action spectra and chromatic mechanisms of cells in the median ocelli of dragonflies.
Authors: Chappell R L , DeVoe R D
Published: May 2004

Spectral sensitivities were recorded intracellulary in median ocelli of Anax junius, Aeschnatuberculifera, and Libellulapulcella. All cells had peak sensitivities at 360 and 500 nm while UV-blue+green cells found only in Anax had a third peak sensitivity at 440 nm. Ratios of UV-to-green sensitivities varied from cell to cell in each ocellus, but no UV-only or green-only cells were recorded. Half of the cells tested had a reverse Purkinje shift: They were more sensitive in the green at low illuminations but more sensitive in the UV at high illuminations; their intensity-response curves at 370 and 520 nm crossed but became parallel for large responses. Wave-lengths 420 nm and shorter elicited a family of low intensity-response curves with one slope; wavelengths 440 nm and longer elicities a family of curves with another slope. Orange-adapting lights selectively adapted sensitivity in the green, but UV-adapting lights had little selective effect. Amounts of log-selective adaptation were proportional to log orange-adapting intensity. It is concluded that two spectral mechanisms can be recorded from each cell, possibly by coupling of UV and green cells or possibly because each cell contains two visual pigments. Selective chromatic adaptations may provide the ocellus with a kind of "authomatic color control," while the reverse Purkinje shift could extend the ocellus' sensitivity to prevailing skylight.
Structural dynamics of frog muscle during isometric contraction.
Authors: Bonner R F , Carlson F D
Published: May 2004

Intensity fluctuation autocorrelation functions of laser light scattered by actively contracting muscle were measured at points in the scattered field. They were reproducible and showed characteristics which depended on the physiological state of the muscle and the parameters of the scattering geometry. The autocorrelation functions had large amplitudes and decay rates that varied significantly with the phase of the contraction-relaxation cycle. The dependence of the autocorrelation function on scattering geometry indicated many elements with diameters on the order of 0.5 mum (presumed to be myofibrillar sarcomeres or their A bands or I bands) undergo independent random changes in their axial positions and their internal distribution of optical polarizability during the plateau of an isometric tetanus. The experimental results are interpreted in terms of a model in which most of the scattering elements in isometrically contracting muscle have random fluctuating axial velocities of average magnitude 20 nm/ms that persist for a few milliseconds at least. In addition to these axial motions there are local fluctuations in polarizability. Similar intensity fluctuation autocorrelation functions were observed throughout the active state on two muscle preparations, whole sartorius muscle and small bundles of single fibers (three to eight) of semitendinosus muscle. These results imply that the tension developed during an isometric tetanus contains a fluctuating component as well as a constant component.
Ionic properties of the acetylcholine receptor in cultured rat myotubes.
Authors: Ritchie A K , Fambrough D M
Published: May 2004

The acetylcholine reversal potential (Er) of cultured rat myotubes is -3mV. When activated, the receptor is permeable to K+ and Na+, but not to Cl- ions. Measurement of Er in Tris+-substituted, Na-free medium also indicated a permeability to Tris+ ions. Unlike adult frog muscle the magnitude of Er was insensitive to change in external Ca++ (up to 30 mM) or to changes in external pH (between 6.4 and 8.9). The equivalent circuit equation describing the electrical circuit composed of two parallel ionic batteries (EK and ENa) and their respective conductances (gK and gNa), which has been generally useful in describing the Er of adult rat and frog muscle, could also be applied to rat myotubes when Er was measured over a wide range of external Na+ concentrations. The equivalent circuit equation could not be applied to myotubes bathed in media of different external K+ concentrations. In this case, the Er was more closely described by the Goldman constant field equation. Under certain circumstances, it is known that the receptor in adult rat and frog muscle can be induced to reversibly shift from behavior described by the equivalent circuit equation to that described by the Goldman equation. Attempts to similarly manipulate the responses of cultured rat myotubes were unsussessful. These trials included a reduction in temperature (15 degress C), partial alpha-bungarotoxin blodkade, and activation of responses with the cholinergic agonist, decamethonium.
Sodium chloride transport by rabbit gallbladder. Direct evidence for a coupled NaCl influx process.
Authors: Frizzell R A , Dugas M C , Schultz S G
Published: May 2004

The results of the present study that NaCl transport by in vitro rabbit gallbladder must be a consequence of a neutral coupled carrier-mediated mechanism that ultimately results in the active absorption of both ions; pure electrical coupling between the movements of Na and Cl can be excluded on the grounds of electrphysiologic considerations. Studies on the unidirectional influxes of Na and Cl have localized the site of this coupled mechanism to the mucosal membranes. Studies on the intracellular ion concentrations and the intracellular electrical potential are consistent with the notion that (a) the coupled NaCl influx process results in the movement of Cl from the mucosal solution into the cell against an apparent electrochemical potential difference; (b) the energy for the uphill movement of Cl is derived from the Na gradient across the mucosal membrane which is maintained by an active Na extrusion mechanism located at the basolateral membranes; and (c) Cl exit from the cell across the basolateral membranes is directed down an electrochemical potential gradient and may be diffusional. Finally, as for the case of rabbit ileum, the coupled NaCl influx process is inhibited by elevated intracellular levels of cyclic 3',5'-adenosine monophosphate. A working model for transcellular and paracellular NaCl transport by in vitro rabbit gallbladder is proposed.
Regulation of muscular contraction. Distribution of actin control and myosin control in the animal kingdom.
Authors: Lehman W , Szent-Györgyi A G
Published: May 2004

The control systems regulating muscle contraction in approximately 100 organisms have been categorized. Both myosin control and actin control operate simultaneously in the majority of invertebrates tested. These include insects, chelicerates, most crustaceans, annelids, priapulids, nematodes, and some sipunculids. Single myosin control is present in the muscles of molluscs, brachiopods, echinoderms, echiuroids, and nemertine worms. Single actin control was found in the fast muscles of decapods, in mysidacea, in a single sipunculid species, and in vertebrate striated muscles. Classification is based on functional tests that include measurements of the calcium dependence of the actomyosin ATPase activity in the presence and the absence of purified rabbit actin and myosin. In addition, isolated thin filaments and myosins were also analyzed. Molluscs lack actin control since troponin is not present in sufficient quantities. Even though the functional tests indicate the complete lack of myosin control in vertebrate striated muscle, it is difficult to exclude unambiguously the in vivo existence of this regulation. Both control systems have been found in animals from phyla which evolved early. We cannot ascribe any simple correlation between ATPase activity, muscle structure, and regulatory mechanisms.
Sizes of components in frog skeletal muscle measured by methods of stereology.
Authors: Mobley B A , Eisenberg B R
Published: May 2004

Stereological techniques of point and intersection counting were used to measure morphological parameters from light and electron micrographs of frog skeletal muscle. Results for sartorius muscle are as follows: myofibrils comprise 83% of fiber volume; their surface to volume ratio is 3.8 mum-1. Mitochondria comprise 1.6% of fiber volume. Transverse tubules comprise 0.32% of fiber volume, and their surface area per volume of fiber is 0.22 mum-1. Terminal cisternae of the sarcoplasmic reticulum comprise 4.1% of fiber volume; their surface area per volume of fiber is 0.54 mum-1. Longitudinal sarcoplasmic reticullum comprises 5.0% of fiber volume, and its surface area per volume of fiber is 1.48 mum-1. Longitudinal bridges between terminal cisternae on either side of a Z disk were observed infrequently; they make up only 0.035% of fiber volume and their surface area per volume of fiber is 0.009 mum-1. T-SR junction occurs over 67% of the surface of transverse tubules and over 27% of the surface of terminal cisternae. The surface to volume ratio of the caveolae is 48 mum-1; caveolae may increase the sarcolemmal surface area by 47%. Essentially the same results were obtained from semitendinosus fibers.
Magnitude and location of surface charges on Myxicola giant axons.
Authors: Begenisich T
Published: May 2004

The effects of changes in the concentration of calcium in solutions bathing Myxicola giant axons on the voltage dependence of sodium and potassium conductance and on the instantaneous sodium and potassium current-voltage relations have been measured. The sodium conductance-voltage relation is shifted along the voltage axis by 13 mV in the hyperpolarizing direction for a fourfold decrease in calcium concentration. The potassium conductance-voltage relation is shifted only half as much as that for sodium. There is no effect on the shape of the sodium and potassium instantaneous current-voltage curves: the normal constant-field rectification of potassium currents is maintained and the normal linear relationship of sodium currents is maintained. Considering that shifts in conductances would reflect the presence of surface charges near the gating machinery and that shape changes of instantaneous current-voltage curves would reflect the presence of surface charges near the ionic pores, these results indicate a negative surface charge density of about 1 electronic charge per 120 A2 near the sodium gating machinery, about 1 e/300 A2 for the potassium gating machinery, and much less surface charge near the sodium or potassium pores. There may be some specific binding of calcium to these surface charges with an upper limit on the binding constant of about 0.2 M-1. The differences in surface charge density suggest a spatial separation for these four membrane components.
Avoidance response, house response, and wind responses of the sporangiophore of Phycomyces.
Authors: Cohen R J , Jan Y N , Matricon J , Delbrück M
Published: May 2004

Avoidance response: An object placed 1 mm from the growing zone of a Phycomyces sporangiophore elicits a tropic response away from the object. The dependence of this response on the size of the object and its distance from the specimen is described, as well as measurements which exclude electric fields, electromagnetic radiation, temperature, and humidity as avoidance-mediating signals. This response is independent of the composition and surface properties of the object and of ambient light. House Response: A house of 0.5- to 10-cm diameter put over a sporangiophore elicits a transient growth response. Avoidance responses inside closed houses are slightly smaller than those in the open. Wind responses: A transverse wind elicits a tropic response into the wind, increasing with wind speed. A longitudinal wind, up or down, elicits a transient negative growth response to a step-up in wind speed, and vice versa. It is proposed that all of the effects listed involve wind sensing. This proposal is supported by measurements of aerodynamic effects of barriers and houses on random winds. The wind sensing is discussed in terms of the hypothesis that a gas is emitted by the growing zone (not water or any normal constituent of air), the concentration of which is modified by the winds and monitored by a chemical sensor. This model puts severe constraints on the physical properties of the gas.
Relation of Na-K-ATPase to acute changes in renal tubular sodium and potassium transport.
Authors: Katz A I , Lindheimer M D
Published: May 2004

Renal Na-K-ATPase activity changes adaptively in response to chronic alterations in sodium reabsorption or potassium secretion, but the role of this enzyme in rapid adjustments of renal tubular Na and K transport is not known. To evaluate this question, microsomal Na-K-ATPase specific activity and kinetics were determined in the rat and guinea pig kidney after massive but short-term (3 h) sodium or potassium loading. In other experiments renal sodium handling was evaluated in hydropenic and saline-loaded rats in which enzyme synthesis was prevented by the concurrent administration of actinomycin D or cycloheximide. Saline loading increased net sodium reabsorption in both rats and guinea pigs, but microsomal Na-K-ATPase from the outer medulla (where the reabsorptive increment is greatest) did not change significantly in either species. In vitro [3H]ouabain bidint to guinea pig microsomes and apparent Km for sodium of rat microsomal Na-K-ATPase, both from outer medulla, were also unaltered. Actinomycin D and cycloheximide failed to increase sodium excretion and microsomal Na-K-ATPase remained unchanged. KCL loading resulted in a 10-fold increase in K excretion but again Na-K-ATPase specific activity (in cortex, outer medulla, and papilla), and its apparent Km for potassium were not affected. Taken together these results suggest that rapid adjustments in remal tubular Na or K transport are mediated by mechanisms that do not involve the Na-K-ATPase enzyme system.
Reflection coefficients of permeant molecules in human red cell suspensions.
Authors: Owen J D , Eyring E M
Published: May 2004

The Staverman reflection coefficient, sigma for several permeant molecules was determined in human red cell suspensions with a Durrum stopped-flow spectrophotometer. This procedure was first used with dog, cat, and beef red cells and with human red cells. The stopped-flow technique used was similar to the rapid-flow method used by those who originally reported sigma measurements in human red cells for molecules which rapidly penetrate the red cell membrane. The sigma values we obtained agreed with those previously reported for most of the slow penetrants, except malonamide, but disagreed with all the sigma values previously reported for the rapid penetrants. We were unable to calculate an "equivalent pore radius" with our sigma data. The advantages of our equipment and our experimental procedure are discussed. Our sigma data suggest that sigma is indirectly proportional to the log of the nonelectrolyte permeability coefficient, omega. Since a similar trend has been previously shown for log omega and molar volume of the permeant molecules, a correlatioo was shown between sigma and molar volume suggesting the membrane acts as a sieve.
Characterization of the effects of Mg2+ on Ca2+- and Sr2+-activated tension generation of skinned skeletal muscle fibers.
Authors: Donaldson S K , Kerrick W G
Published: May 2004

Changes in [Mg2+] in a millimolar range have a significant inverse effect on the Ca2+- (or Sr2+)activated tension generation of skeletal muscle fibers. Single frog (Rana pipiens) semitendinosus muscle fibers were "skinned" (sarcolemma removed) and contracted isometrically in bathing solutions of varying [Ca2+] or [Sr2+] and [Mg2+] but a constant pH, [MgATP2-], [K+], [CP2-], [CPK], and ionic strength. Ca2+- (or Sr2+-)activated steady-state tensions were recorded for three [Mg2+]'s: 5 X 10(-5)M, 1 X 10(-3) M, and 2 X 10(-3) M; and these tensions were expressed as the percentages of maximum tension generation of the fibers for the same [Mg2+]. Maximum tension was not affected by [Mg2+] within Ca2+-activating or Sr2+-activating sets of solutions; however, the submaximum Ca2+-(or Sr2+)activated tension is strongly affected in an inverse fashion by increasing [Mg2+]. Mg2+ behaves as a competitive inhibitor of Ca2+ and also affects the degree of cooperativity in the system. At [Mg2+] = 5 X 10(-5)M the shape of tension versus [Ca2+] (or [Sr2+]) curve showed evidence of cooperativity of Ca2+ (or Sr2+) binding or activation of the contractile system. As [Mg2+] increased, the apparent affinity for Ca2+ or Sr2+ and cooperativity of the contractile system declined. The effect on cooperativity suggests that as [Mg2+] decreases a threshold for Ca2+ activation appears.
A component of fluid absorption linked to passive ion flows in the superficial pars recta.
Authors: Schafer J A , Patlak C S , Andreoli T E
Published: May 2004

We studied salt and water absorption in isolated rabbit superficial proximal straight tubules perfused and bathed with solutions providing oppositely directed transepithelial anion gradients similar to those which might obtain in vivo. The perfusing solution contained 138.6 mM Cl- 3.8 mM HCO-3 (pH 6.6) while the bathing solution contained 113.6 mM Cl- and 25 mM HCO-3 (pH 7.4); the system was bubbled with 95% O2-5% CO2. At 37 degrees C, net volume absorption (Jv nl min-1 mm-1) was 0.32 +/- 0.03 (SEM); Ve, the transepithelial voltage (millivolts; lumen to bath), was +3.1 +/- 0.2. At 21 degrees C, Ve rose to +3.7 +/- 0.1 and Jv fell to 0.13 +/- 0.01 (significantly different from zero at P less than 0.001); in the presence of 10(-4)M ouabain at 37 degrees C, Ve rose to +3.8 +/- 0.1 and Jv fell to 0.16 +/- 0.01 (P less than 0.001 with respect to zero). In paired experiments, the ouabain- and temperature-insensitive moieties of Jv and Ve became zero when transepithelial anion concentration gradients were abolished. Titrametric determinations net chloride flux at 21 degrees C or at 37 degrees C with 10(-4) M ouabain showed that chloride was the sole anion in an isotonic absorbate. And, combined electrical and tracer flux data indicated that the tubular epithelium was approximately 18 times more permeable to Cl- than to HCO-3. We interpret these results to indicate that, in these tubules, NaCl absorption depends in part on transepithelial anion concentration gradients similar to those generated in vivo and in vitro by active Na+ absorption associated with absorption to anions other than chloride. A quantitative analysis of passive solute and solvent flows in lateral intercellular spaces indicated that fluid absorption occurred across junctional complexes when the osmolality of the lateral intercellular spaces was equal to or slightly less than that of the perfusing and bathing solutions; the driving force for volume flow under these conditions depended on the fact that sigmaHCO3 exceeded sigmaCl.
Intracellular recordings from gecko photoreceptors during light and dark adaptation.
Authors: Kleinschmidt J , Dowling J E
Published: May 2004

Intracellular recordings were obtained from rods in the Gekko gekko retina and the adaptation characteristics of their responses studied during light and dark adaptation. Steady background illumination induced graded and sustained hyperpolarizing potentials and compressed the incremental voltage range of the receptor. Steady backgrounds also shifted the receptor's voltage-intensity curve along the intensity axis, and bright backgrounds lowered the saturation potential of the receptor. Increment thresholds of single receptors followed Weber's law over a range of about 3.5 log units and then saturated. Most of the receptor sensitivity change in light derived from the shift of the voltage-intensity curve, only little from the voltage compression. Treatment of the eyecup with sodium aspartate at concentrations sufficient to eliminate the beta-wave of the electroretinogram (ERG) abolished initial transients in the receptor response, possibly indicating the removal of horizontal cell feedback. Aspartate treatment, however, did not significantly alter the adaptation characteristics of receptor responses, indicating that they derive from processes intrinsic to the receptors. Dark adaptation after a strongly adapting stimulus was similarly associated with temporary elevation of membrane potential, initial lowering of the saturation potential, and shift of the voltage-intensity curve. Under all conditions of adaptation studied, small amplitude responses were linear with light intensity. Further, there was no unique relation between sensitivity and membrane potential suggesting that receptor sensitivity is controlled at least in part by a step of visual transduction preceding the generation of membrane voltage change.
Effect of cross-reinnervation on physiological parameters and on properties of myosin and sarcoplasmic reticulum of fast and slow muscles of the rabbit.
Authors: Sréter F A , Luff A R , Gergely J
Published: May 2004

Cross-reinnvervation of fast (extensor digitorum longus) and slow (soleus) twitch muscles of the rabbit showed essentially complete fast to slow and slow to fast conversion, respectively, 11-12 mo after surgery with respect to a number of physiological parameters including intrinsic shortening, velocity, and isometric twitch time to peak. There was pronounced bu incomplete biochemical conversion as judged by Ca2+ uptake by sarcoplasmic reticulum, myosin ATPase, alkali lability, and light chain complement. The question of trophic substances of neural origin is discussed in light of the fact that chronic stimulation for 15 wk of a fast muscle produces complete biochemical and physiological conversion to the slow type.