bluuebunny/crossref_metadata_2025 huggingface dataset:
Effects of Batrachotoxin on Membrane Potential and Conductance of Squid Giant Axons
Authors: Narahashi Toshio , Albuquerque Edson X. , Deguchi Takehiko
Published: May 2004

The effects of batrachotoxin (BTX) on the membrane potential and conductances of squid giant axons have been studied by means of intracellular microelectrode recording, internal perfusion, and voltage clamp techniques. BTX (550–1100 nM) caused a marked and irreversible depolarization of the nerve membrane, the membrane potential being eventually reversed in polarity by as much as 15 mv. The depolarization progressed more rapidly with internal application than with external application of BTX to the axon. External application of tetrodotoxin (1000 nM) completely restored the BTX depolarization. Removal or drastic reduction of external sodium caused a hyperpolarization of the BTX-poisoned membrane. However, no change in the resting membrane potential occurred when BTX was applied in the absence of sodium ions in both external and internal phases. These observations demonstrate that BTX specifically increases the resting sodium permeability of the squid axon membrane. Despite such an increase in resting sodium permeability, the BTX-poisoned membrane was still capable of undergoing a large sodium permeability increase of normal magnitude upon depolarizing stimulation provided that the membrane potential was brought back to the original or higher level. The possibility that a single sodium channel is operative for both the resting sodium, permeability and the sodium permeability increase upon stimulation is discussed.
Studies of Cardiac Muscle with a High Permeability to Calcium Produced by Treatment with Ethylenediaminetetraacetic Acid
Authors: Winegard Saul
Published: May 2004

Thin strips of frog ventricle were isolated and bathed for 15 min in a solution containing 140 mM KCl, 5 mM Na2ATP, 3 mM EDTA, and 10 mM Tris buffer at pH 7.0. The muscle was then exposed to contracture solutions containing 140 mM KCl, 5 mM Na2ATP, 1 mM MgCl2, 10 mM Tris, 3 mM EGTA, and CaCl2 in amounts to produce concentrations of free calcium from 10-4.8 M to 10-9 M. The muscles developed some tension at approximately 10-8 M, and maximum tension was achieved in 10-5 M Ca++. They relaxed in Ca++ concentrations less than 10-8 M. The development of tension by the EDTA-treated muscles was normalized by comparison with twitch tension at a stimulation rate of 9 per min before exposure to EDTA. In 10-5 M Ca++ tension was always several times the twitch tension and was greater than the contracture tension of a frog ventricular strip in KCl low Na-Ringer. Tension equal to half-maximum was produced at approximately 10-6.2 M Ca++. Intracellular recording of membrane potential indicated that after EDTA treatment the resting potential of cells in Ringer solution with 10-5 M Ca or less was between 5 and 20 mv. Contracture solutions did not produce tension without prior treatment with EDTA. The high permeability of the membrane produced by EDTA was reversed and the normal resting and action potentials restored in 1 mM Ca-Ringer. Similar studies of EDTA-treated rabbit right ventricular papillary muscle produced a similar tension vs. Ca++ concentration relation, and the high permeability state reversed with exposure to normal Krebs solution.
Active Cation Transport and Ouabain Binding in High Potassium and Low Potassium Red Blood Cells of Sheep
Authors: Dunham Philip B. , Hoffman Joseph F.
Published: May 2004

Red cells from high K sheep contained 82 mM K/liter cells and had a pump flux of 0.86 mM K/liter cells x hr; similarly, LK cells had 16.5 mM K/liter cells and a pump flux of 0.12 mM K/liter cells x hr. Using [3H]-ouabain, the relation between the number of ouabain molecules bound per cell and the concomitant per cent inhibition of the pump was found to be approximately linear for both HK and LK cells. The number of glycoside molecules necessary for 100 % inhibition of the pump was 42 for HK cells and 7.6 for LK cells, after correction for six nonspecific binding sites for each type of cell. The ratio of ouabain molecules/cell at 100 % inhibition was 5.5, HK to LK, and the ratio of the normal K pump fluxes was 7.2, HK to LK. The similarity of these ratios suggests that an important difference between HK and LK cells, determining the difference in pump fluxes, is the number of pump sites. The turnover times (ions/site x min) are 6000 and 4800 for HK and LK cells, respectively. The results also indicate a high specificity of binding of ouabain to pump sites.
Effect of Changes in Transepithelial Transport on the Uptake of Sodium across the Outer Surface of the Frog Skin
Authors: Biber Thomas U. L.
Published: May 2004

The unidirectional sodium, uptake at the outer surface of the frog skin was measured by the method described by Biber and Curran (8). With bathing solutions containing 6 mM NaCl there is a good correlation between sodium uptake and short-circuit current (SCC) measured simultaneously except that the average uptake is about 40% higher than the average SCC. The discrepancy between uptake and SCC increases approximately in proportion to an increase in sodium concentration of the bathing solutions. Amiloride inhibits the unidirectional sodium uptake by 21 and 69% at a sodium concentration of 115 and 6 mM, respectively. This indicates that amiloride acts on the entry step of sodium but additional effects cannot be excluded. The sodium, uptake is not affected by 10-4 M ouabain at a sodium concentration of 115 mM but is inhibited by 40% at a sodium concentration of 6 mM. Replacement of air by nitrogen leads to a 40% decrease of sodium uptake at a sodium concentration of 6 mM. The results support the view proposed previously (8) that the sodium uptake is made up of two components, a linear component which is, essentially, not involved in transepithelial movement of sodium and a saturating component which reflects changes in transepithelial transport. Amiloride, seems largely to affect the saturating component.
S-Potentials in the Skate Retina
Authors: Dowling John E. , Ripps Harris
Published: May 2004

The S-potentials recorded intracellularly from the all-rod retina of the skate probably arise from the large horizontal cells situated directly below the layer of receptors. These cells hyperpolarize in response to light, irrespective of stimulus wavelength, and the responses in photopic as well as scotopic conditions were found to be subserved by a single photopigment with λmax = 500 nm. The process of adaptation was studied by recording simultaneously the threshold responses and membrane potentials of S-units during both light and dark adaptation. The findings indicate that the sensitivity of S-units, whether measured upon steady background fields or in the course of dark adaptation, exhibits changes similar to those demonstrated previously for the ERG b-wave and ganglion cell discharge. However, the membrane potential level of the S-unit and its sensitivity to photic stimulation varied independently for all the adapting conditions tested. It appears, therefore, that visual adaptation in the skate retina occurs before the S-unit is reached, i.e., at the receptors themselves.
Characterization and Analysis of Frog Photoreceptor Membranes
Authors: Bownds Deric , Gordon-Walker Ann , Gaide-Huguenin Anne-Claude , Robinson William
Published: May 2004

Frog photoreceptor membranes contain 54,000 g of protein per mole of visual pigment chromophore, virtually all of it insoluble membrane protein. Acrylamide gel electrophoresis indicates one major polypeptide class, most likely the visual pigment apoprotein. Suspensions of these photoreceptor membranes accumulate calcium ions when ATP is present, a characteristic that may play a part in visual excitation.
Water Permeability of Isolated Muscle Fibers of a Marine Crab
Authors: Sorenson A. L.
Published: May 2004

This report deals with the diffusional and nondiffusional water fluxes of muscle fibers of the crab, Chionoecetes bairdi. Graphical analysis of the deuterium exchange indicates that two fiber compartments exist for water. The first, comprising about 60–70% of the fiber water, probably represents the sarcoplasm which is bounded externally by the plasma membrane. The second compartment might represent intracellular organelles. The ratio between the nondiffusional and diffusional fluxes is very much larger than that found earlier for erythrocytes and for the giant axon of the squid. A ratio of such size is unlikely to be caused by unstirred layers and more accurate determinations of the water flux must include study of the influence of the complex morphology of these muscle fibers.
Studies on Calcium and Sodium in Uterine Smooth Muscle Excitation under Current-Clamp and Voltage-Clamp Conditions
Authors: Anderson Nels C. , Ramon Fidel , Snyder Ann
Published: May 2004

The objective of these studies was to define the roles of calcium and sodium in uterine smooth muscle excitation. The double sucrose-gap technique was used for current-clamp and voltage-clamp experiments. It was shown that neither sodium nor calcium alone is capable of supporting excitation in estrogen-dominated uterine smooth muscle. Calcium dependence was explained in part by increased membrane "leakage" current in calcium-free solution and calcium control of the voltage dependence of the early transient conductance. High concentrations of TTX did not affect the magnitude of the peak transient current while La+++, Mn++, and Co++ greatly reduced or abolished it and decreased the steady-state current. From these and other data it was concluded that the regenerative mechanism in uterine smooth muscle has the functional characteristics of a single transient conductance channel whose activation requires the presence of both sodium and calcium. Insensitivity to TTX indicates that the molecular structure of the channel is unlike that in certain sodium-dependent systems, while the effects of La+++, Mn++, Co++, and Ca++ reveal a similar dependence of conductances on extracellular polyvalent cations.
Adenosine Triphosphate-Dependent Calcium Uptake by Rat Submaxillary Gland Microsomes
Authors: Alonso G. L. , Bazerque P. M. , Arrigó D. M. , Tumilasci O. R.
Published: May 2004

Microsomes from rat submaxillary glands are able to take up calcium from the suspension media. Calcium uptake is greatly increased by the presence of ATP. This effect of ATP is not detected at 0°C. ADP cannot replace ATP to potentiate calcium uptake. ATP-dependent calcium uptake is not observed in the absence of magnesium. ATP-dependent calcium uptake is enhanced by oxalate and, to a lesser degree, by inorganic phosphate. Total calcium per milligram of microsomal protein observed when tests were performed without oxalate closely parallels the amounts for skeletal and cardiac muscles reported by several authors. Calcium uptake in salivary gland microsomes is slower than in muscle microsomes. Speculations are considered about the role of ATP-dependent calcium uptake. It is suggested that a decrease in intracellular free calcium levels returns these cells to the resting state after secretion.
Two Inward Currents in Frog Atrial Muscle
Authors: Tarr Merrill
Published: May 2004

The double sucrose-gap voltage-clamp technique was applied to frog atrial tissue to investigate the ionic currents responsible for the action potential in this tissue. Membrane depolarization elicited two distinct components of inward current when the test node was exposed to normal Ringer solution: a fast inward current and a slow inward current. The fast inward current appeared to be carried by sodium ions, since it was rapidly abolished by exposure of the fiber to Na+-free solution or tetrodotoxin but persisted on exposure to Ca++-free solution. In contrast, in the majority of the preparations the slow inward current appeared to be primarily carried by calcium ions, since it was abolished on exposure of the fiber to Ca++-free solution but persisted on exposure to Na+-free solution. Action potential data supported the voltage-clamp findings. The normal action potential shows two distinct components in the upstroke phase: an initial rapid phase of depolarization followed by a slower phase of depolarization reaching the peak of the action potential. Abolition of the fast inward current resulted in abolition of the initial rapid phase of depolarization. Abolition of the slow inward current resulted in abolition of the slow phase of depolarization. These data support the hypothesis that two distinct and different ionic mechanisms contribute to the upstroke phase of the action potential in frog atrial tissue.
Intensity Characteristics of the Noctuid Acoustic Receptor
Authors: Adams William B.
Published: May 2004

Spiking activity of the more sensitive acoustic receptor is described as a function of stimulus intensity. The form of the intensity characteristic depends strongly on stimulus duration. For very brief stimuli, the integral of stimulus power over stimulus duration determines the effectiveness. No response saturation is observed. With longer stimuli (50 msec), a steady firing rate is elicited. The response extends from the spontaneous rate of 20–40 spikes/sec to a saturated firing rate of nearly 700 spikes/sec. The characteristic is monotonic over more than 50 db in stimulus intensity. With very long stimuli (10 sec), the characteristics are nonmonotonic. Firing rates late in the stimulus decrease in response to an increase in stimulus intensity. The non-monotonic characteristics are attributed to intensity-related changes in response adaptation.
Effects of Divalent Ions and Drugs on Synaptic Transmission in Phasic Electroreceptors in a Mormyrid Fish
Authors: Steinbach A. B. , Bennett M. V. L.
Published: May 2004

We recorded impulses in afferent nerve fibers innervating two kinds of phasic electroreceptors in a mormyrid fish. We used an isolated preparation of skin, receptors, and sensory nerves to estimate synaptic delays, and to change solution in contact with the receptor-nerve synapse. The minimum delays between stimuli and sensory nerve responses, which must be slightly larger than synaptic delays, are about 0.7 msec in medium receptors and about 0.25 msec in large receptors. This result supports previous suggestions that transmission is chemically mediated in medium receptors and electrically mediated in large receptors. Furthermore, Mg+2 depresses synaptic transmission in medium receptors, and has little effect on transmission in large receptors. A complex dependence of response on both Mg+2 and Ca+2 masks divalent ion dependence of transmission, but a large excess of Mg+2 cannot completely block transmission in medium electroreceptors. L-glutamate, and not cholinergic drugs, produces a sequence of excitation and depression of medium receptor response which indicates that a similar chemical is the transmitter in the afferent synapse.
Resistance of Erythrocytes of Hibernating Mammals to Loss of Potassium during Hibernation and during Cold Storage
Authors: Kimzey S. L. , Willis J. S.
Published: May 2004

In two species of hibernators, hamsters and ground squirrels, erythrocytes were collected by heart puncture and the K content of the cells of hibernating individuals was compared with that of awake individuals. The K concentration of hamsters did not decline significantly during each bout of hibernation (maximum period of 5 days) but in long-term bouts in ground squirrels (i.e. more than 5 days) the K concentration of cells dropped significantly. When ground squirrels were allowed to rewarm the K content of cells rose toward normal values within a few hours. Erythrocytes of both hamsters and ground squirrels lose K more slowly than those of guinea pigs (nonhibernators) when stored in vitro for up to 10 days at 5°C. In ground squirrels the rate of loss of K during storage is the same as in vivo during hibernation, and stored cells taken from hibernating ground squirrels also lose K at the same rate. The rate of loss of K from guinea pig cells corresponded with that predicted from passive diffusion unopposed by transport. The actual rate of loss of K from ground squirrel cells was slower than such a predicted rate but corresponded with it when glucose was omitted from the storage medium or ouabain was added to it. Despite the slight loss of K that may occur in hibernation, therefore, the cells of hibernators are more cold adapted than those of a nonhibernating mammal, and this adaptation depends in part upon active transport.
Energetics of Sodium Transport in Frog Skin
Authors: Vieira F. L. , Caplan S. R. , Essig A.
Published: May 2004

Studies were made of the dependence of the rate of oxygen consumption, Jr, on the electrical potential difference, Δψ, across the frog skin. After the abolition of sodium transport by ouabain the basal oxygen consumption was independent of Δψ. In fresh skins Jr was a linear function of Δψ over a range of at least ±70 mv. Treatment with aldosterone stimulated the short-circuit current, Io, and the associated rate of oxygen consumption, Jro, and increased their stability; linearity was then demonstrable over a range of ±160 mv. Brief perturbations of Δψ (±30–200 mv) did not alter subsequent values of Io. Perturbations for 10 min or more produced a "memory" effect both with and without aldosterone: accelerating sodium transport by negative clamping lowered the subsequent value of Io; positive clamping induced the opposite effect. Changes in Jro were more readily detectable in the presence of aldosterone; these were in the same direction as the changes in Io. The linearity of Jr in Δψ indicates the validity of analysis in terms of linear nonequilibrium thermodynamics—brief perturbations of Δψ appear to produce no significant effect on either the phenomenological coefficients or the free energy of the metabolic driving reaction. Hence it is possible to evaluate this free energy.
Biophysical Effects of Adrenaline on the Smooth Muscle of the Rabbit Common Carotid Artery
Authors: Mekata F. , Niu H.
Published: May 2004

Effects of adrenaline on the smooth muscle of the rabbit common carotid artery were studied by the partitional chamber method. The experiments on excitation-contraction coupling were carried out in isotonic Krebs solution; the other experiments were carried out in hypertonic Krebs solution. Adrenaline (10-7 g/ml) caused rhythmical electrical and mechanical activity of arterial strips in isotonic Krebs solution. By addition of adrenaline (10-5 g/ml), the membrane was depolarized by about 10 mv and the amplitude of the electrotonic potential was decreased by 40–50% of the control in hypertonic Krebs solution. Present experimental results suggest that the depolarization of the membrane and the decrease of the amplitude of the electrotonic potential in the artery are due to the increase of Na and Cl conductance. Contraction appeared in all preparations exposed to 10-8 g/ml adrenaline; at that concentration membrane potential and membrane resistance showed little or no change.
The Ionic Requirements for the Initiation of Action Potentials in Insect Muscle Fibers
Authors: Washio Hiroshi
Published: May 2004

Electrical properties of locust leg muscle fibers were studied by means of intracellular electrodes. In most fibers, a depolarizing current pulse initiated a local response. A delayed decrease in membrane resistance appeared with more than about 10 mv depolarization. In some fibers a regenerative response also was found. Membrane constants were measured, applying the short cable model. The value of the space constant λ was 1.6 mm and the calculated value of Rm was about 1750 ohm cm2. Action potentials could be elicited when the bathing fluid contained more than 2–5 mM Ba or Sr. Similar responses were seen with 2 mM Ca in the presence of tetraethylammonium (TEA). The overshoot of these action potentials increased with increasing [Ca++]o, [Sr++]o, or [Ba++]o, the increment for a 10-fold increase being about 29 mv for Ca and Sr and between 40 and 50 mv for Ba. These action potentials were inhibited by Mn ions but were not affected by tetrodotoxin or procaine. In solutions containing Ba or Sr, action potentials generated were suppressed by addition of Ca. The removal of Na ions did not change the configuration of the action potential. The results suggest that an increase in permeability to Ca, Ba, or Sr ions makes a major contribution to the initiation of action potentials in this tissue.
Fluorometric Studies of Oxidative Metabolism in Isolated Papillary Muscle of the Rabbit
Authors: Chapman J. Brian
Published: May 2004

The fluorometric technique for measuring the levels of reduced pyridine nucleotides was used to study oxidative metabolism in isolated rabbit papillary muscle at 23°C. The 100% standard level of tissue fluorescence was defined as that measured for muscles resting in oxygenated 10 mM pyruvate solution. This level increased 15% with anoxia and decreased 45% with stimulation in substrate-free solution. Thus, about one-half of the standard tissue fluorescence was metabolically labile and this labile fraction is suggested to be mitochondrial in origin. Decreased tissue fluorescence following mechanical activity was identified with increased oxidation of mitochondrial reduced nicotinamide adenine dinucleotide (NADH) owing to stimulation by adenosine diphosphate (ADP), released during activity, of mitochondrial respiration. The kinetics of the fluorescence transients were slowed fourfold by removal of pyruvate. This effect was not significantly reversed by addition of 10 mM glucose. The time integrals of the fluorescence transients were linearly related to the amounts of mechanical activity in the presence, but not in the absence, of pyruvate. A positive correlation was observed between the steady-state peak tension at constant stimulus rate and the resting level of reduction of pyridine nucleotides in various media. The fluorometric results are interpreted to be indicative of the steady and transient states established by the substrate dehydrogenases and the respiratory chain during oxidative phosphorylation in mitochondria.
The Orientation of Rhodopsin and Other Pigments in Dry Films
Authors: Wright Woodring E. , Brown Paul K. , Wald George
Published: May 2004

When rhodopsin in a gelatin film is dried, the rhodopsin chromophores orient primarily in the plane of the film. When the film is wetted, the chromophores disorient. These changes are reversible. When rhodopsin in a wet film. is bleached in the presence of hydroxylamine and redried, the retinal oxime which results is oriented more perpendicularly to the plane of the film. These orientations in dry gelatin films resemble those in the disc membranes of rod outer segments. A variety of other proteins are similarly oriented in dry gelatin films: methemoglobin, cytochrome c, phycocyanin. Films of methemoglobin and cytochrome c display prominently the high Soret band near 410 nm when measured with unpolarized light passing through the face of the fim, but display no Soret band at all with light passing through the edge of the film. All of these orientations imply a large asymmetry of the protein micelles, perhaps conferred upon them by linear polymerization in the course of drying. Such asymmetry can be demonstrated directly with rhodopsin. A wet paste of rhodopsin-digitonin micelles, sheared between glass slides, becomes highly oriented, the rhodopsin chromophores lining up in the direction of shear, the retinal oxime produced by bleaching orienting more perpendicularly to the shear.
Conduction of the Cardiac Impulse
Authors: Cranefield Paul F. , Wit Andrew L. , Hoffman Brian F.
Published: May 2004

The excitability of short segments (5–7 mm) of bundles of canine Purkinje fibers was depressed by exposure to 15–18 mM K+, to 15–18 mM K+ plus 5 x 10-6 epinephrine or norepinephrine, to low K+, and to low Na+. The depressed segment was in the center chamber of a three-chamber bath; the ends of the bundle were exposed to normal Tyrode solution. Each method of depression resulted in slow and probably decremental conduction with an effective conduction velocity in the middle chamber of about 0.05 m/sec, or one-way block, or two-way block with summation of the graded responses in the depressed region. The action potential in the depressed segment (the slow response) differs from the normal action potential in its response to applied stimuli. A second active depolarization can be evoked by cathodal stimulation during much of the slow response. The response in the depressed segment is graded. The response of depressed fibers may depend on excitatory events similar to those responsible for the slow component of the cardiac action potential. It is suggested that the slow response can propagate, at least decrementally, in fibers in which the rapid, Na+-dependent upstroke is absent, and can cause reentrant excitation by so doing.
Activation of the Adenosine Triphosphatase of Limulus polyphemus Actomyosin by Tropomyosin
Authors: Lehman William , Szent-Györgyi Andrew G.
Published: May 2004

Purified actin does not stimulate the adenosine triphosphatase (ATPase) activity of Limulus myosin greatly. The ATPase activity of such reconstituted preparations is only about one-fourth the ATPase of myofibrils or of natural actomyosin. Actin preparations containing tropomyosin, however, activate Limulus myosin fully. Both the tropomyosin and the actin preparations appear to be pure when tested by different techniques. Tropomyosin combines with actin but not with myosin and full activation is reached at a tropomyosin-to-actin ratio likely to be present in muscle. Tropomyosin and actin of several different animals stimulate the ATPase of Limulus myosin. Tropomyosin, however, is not required for the ATPases of scallop and rabbit myosin which are fully activated by pure actin alone. Evidence is presented that Limulus myosin, in the presence of ATP at low ionic strength, has a higher affinity for actin modified by tropomyosin than for pure actin.
Rhodopsin of the Larval Mosquito
Authors: Brown Paul K. , White Richard H.
Published: May 2004

Larvae of the mosquito Aedes aegypti have a cluster of four ocelli on each side of the head. The visual pigment of each ocellus of mosquitoes reared in darkness was characterized by microspectrophotometry, and found to be the same. Larval mosquito rhodopsin (λmax = 515 nm) upon short irradiation bleaches to a stable photoequilibrium with metarhodopsin (λmax = 480 nm). On long irradiation of glutaraldehyde-fixed tissues or in the presence of potassium borohydride, bleaching goes further, and potassium borohydride reduces the product, retinal, to retinol (vitamin A1). In the presence of hydroxylamine, the rhodopsin bleaches rapidly, with conversion of the chromophore to retinaldehyde oxime (λmax about 365 nm).
Action Potential Parameters Affecting Excitation-Contraction Coupling
Authors: Taylor Stuart R. , Preiser Hanna , Sandow Alexander
Published: May 2004

In quantifying type B potentiation effects, given earlier merely qualitatively, it is found that Zn2+, 1—50 µM, causes increases in action potential duration, twitch tension, and twitch contraction period time, which are all directly proportional to the log of the concentration. Hence, the duration of the action potential, i.e. the magnitude of its mechanically effective period, is a causal factor quantitatively determining the degree of mechanical activation in the isometric twitch. In higher concentrations of Zn2+ up to 1000 µM, the spike duration and the contraction time continue to increase but the twitch tension is disproportionately smaller, evidently because the high zinc (500—1000 µM) raises the mechanical threshold of excitation-contraction (E—C) coupling and reduces the intrinsic strength of the contractile system. Eserine (1.5 mM) and also high Zn2+ not only cause type B potentiation effects, but also slow the rise of the spike, thus causing retardation of the very onset of tension production, which is even greater for high Zn2+ because of the raised mechanical threshold. This retardation is then succeeded by the faster tension output characteristic of type B potentiation resulting from spike prolongation. Thus, the changes in the consecutive, rising and falling phases of the action potential explicitly register their separate effects in the respective very earliest and directly following periods of twitch output; i.e., each phase of the action potential produces its own mechanical "transform." These transforms, and other effects, suggest that the release of activator Ca2+ from the sarcoplasmic reticulum during E—C coupling can be graded in both the rate and the total amount of the release.
Temperature Dependence of Vasopressin Action on the Toad Bladder
Authors: Eggena Patrick
Published: May 2004

Toad bladders were challenged with vasopressin at one temperature, fixed on the mucosa with 1% glutaraldehyde, and then subjected to an osmotic gradient at another temperature. Thus, the temperature dependence of vasopressin action on membrane permeability was distinguished from the temperature dependence of osmotic water flux. As the temperature was raised from 20° to 38°C, there was a substantial increase in the velocity of vasopressin action, but osmotic flux was hardly affected. In this range of temperature the apparent energy of activation for net water movement across the bladder amounted to only 1.2 kcal/mole, a value well below the activation energy for bulk water viscosity. It is suggested that osmotic water flux takes place through narrow, nonpolar channels in the membrane. When the temperature was raised from 4° to 20°C, both vasopressin action as well as osmotic water flux were markedly enhanced. Activation energies for net water movement were now 8.5 kcal/mole (4°–9°C) and 4.1 kcal/mole (9°–20°C), indicating that the components of the aqueous channel undergo conformational changes as the temperature is lowered from 20°C. At 43°C bladder reactivity to vasopressin was lost, and irreversible changes in selective permeability were observed. The apparent energy of activation for net water movement across the denatured membrane was 6.6 kcal/mole. Approximately 1 µosmol of NaCl was exchanged for 1 µl of H2O across the denatured membrane.
Microspectrophotometry of Arthropod Visual Screening Pigments
Authors: Strother G. K. , Casella A. J.
Published: May 2004

Absorption spectra of visual screening pigments obtained in vitro with a microspectrophotometer using frozen sections are given for the insects Musca domestica, Phormia regina, Libellula luctuosa, Apis mellifera (worker honeybee only), Drosophila melanogaster (wild type only) and the arachnids Lycosa baltimoriana and Lycosa miami. The spectral range covered is 260–700 nm for Lycosa and Drosophila and 310–700 nm for the remainder of the arthropods. A complete description of the instrumentation is given. For the flies, Phormia and Musca, light absorption by the yellow and red pigments is high from 310 to about 610 nm. This implies that for these insects there should be no wavelength shift in electroretinogram (ERG) results due to light leakage among neighboring ommatidia for this wavelength range. The same comment applies to Calliphora erythrocephala, which is known to have similar screening pigments. For some of the insects studied a close correspondence is noted between screening pigment absorption spectra and spectral sensitivity curves for individual photoreceptors, available in the literature. In some cases the screening pigment absorption spectra can be related to chemical extraction results, with the general observation that some of the in vitro absorption peaks are shifted to the red. The Lycosa, Apis, and Libellula dark red pigments absorb strongly over a wide spectral range and therefore prevent chemical identification.
An Electrogenic Sodium Pump in Limulus Ventral Photoreceptor Cells
Authors: Brown J. E. , Lisman J. E.
Published: May 2004

A hyperpolarization can be recorded intracellularly following either a single bright light stimulus or the intracellular injection of Na+. This after-hyperpolarization is abolished by bathing in 5 x 10-6 M strophanthidin or removal of extracellular K+. Both treatments also lead to a small, rapid depolarization of the dark-adapted cell. When either treatment is prolonged, light responses can still be elicited, although with repetitive stimuli the responses are slowly and progressively diminished in size. The rate of diminution is greater for higher values of [Ca++]out; with [Ca++]out = 0.1 mM, there is almost no progressive diminution of repetitive responses produced by either K+-free seawater or strophanthidin. We propose that an electrogenic Na+ pump contributes directly to dark-adapted membrane voltage and also generates the after-hyperpolarizations, but does not directly generate the receptor potential. Inhibition of this pump leads to intracellular accumulation of sodium ions, which in turn leads to an increase in intracellular Ca++ (provided there is sufficient extracellular Ca++). This increase in intracellular calcium probably accounts for the progressive decrease in the size of the receptor potential seen when the pump is inhibited.
The Relationship between the Firing Rate of a Single Neuron and the Level of Activity in a Population of Neurons
Authors: Knight Bruce W.
Published: May 2004

A quantitative comparison is made between experiment and the theoretically predicted dynamics of a neuron population. The experiment confirms the theoretical prediction that under appropriate conditions an enlarged resonant response should appear in the activity of the neuron population, near the frequency at which there is minimum modulation in the instantaneous rate of a single neuron. These findings bear on the relationship between the firing rate of a single neuron and the firing rate of a population of neurons.
Loss of the Plateau of the Cardiac Action Potential in Hypertonic Solutions
Authors: Hermsmeyer Kent , Rulon Russell , Sperelakis Nick
Published: May 2004

The effect of hypertonicity on the electrical properties of vertebrate myocardial cells was studied in ventricular muscle fibers of guinea pig, cat, frog, and chicken. The latter two species do not have a T-tubule system, whereas the former two do. In hypertonic solutions (2 x isotonic) produced by addition of sucrose or excess of NaCl, cell diameter decreased and there was a slight hyperpolarization and decrease in action potential overshoot. In guinea pig and cat, the hypertonic solution caused a decrease in input resistance and the plateau of the action potential to disappear in some of the cells; contractions of the entire ventricle also became depressed. These effects were reversed by returning the muscle fibers to isotonic solution. Addition of 5 mM SrCl2 to the hypertonic solution also caused the plateau component and contraction to reappear. In frog and chick cells, loss of the plateau component and contraction never occurred in hypertonic solution, and input resistance increased. Urea and glycerol hyperosmolarity (2 x) caused no loss of the plateau component or contraction. If the frog and chicken ventricular, and guinea pig atrial myocardial cells (all of which lack T tubules) were to serve as an adequate control for possible effects of hypertonicity on the surface membrane and on contractile proteins, then the results suggest that swelling of the T tubules of mammalian myocardial cells leads to loss of the plateau component.
Biosynthesis of the Egg-Laying Hormone (ELH) in the Bag Cell Neurons of Aplysia californica
Authors: Arch S.
Published: May 2004

Biosynthesis of the egg-laying hormone in the bag cell neurons of Aplysia californica was studied. Bag cells were incubated with leucine-3H in vitro for 30 min and rinsed for variable periods of time in a chase medium. The distribution of incorporated label among proteins within the cells was assayed by electrophoresis of an homogenate on sodium dodecyl sulfate polyacrylamide gels. Results from rinse times shorter than 30 min revealed that the predominant synthetic product is a 25,000 dalton protein. With longer rinse times, this species was reduced and two species of lower molecular weight became prominent. This redistribution of radioactivity was quantitative and was not prevented by inhibition of protein synthesis during the rinse. A 10°C reduction in temperature (from 15°C) blocked the redistribution. These data are interpreted to indicate that the 25,000 dalton molecule is a precursor which is cleaved enzymatically to yield two lower molecular weight products. One product is a 12,000 dalton molecule which remains in the cell bodies. The other is a molecule of <10,000 daltons which is exported from the somata into the neurohemal regions of the connective tissue. Perfusion of these regions with high [K+] medium results in the release of this product into the medium. It is concluded that this product is the 6000 dalton egg-laying hormone (ELH).
Ion Transport Through Excitability-Inducing Material (EIM) Channels in Lipid Bilayer Membranes
Authors: Latorre Ramon , Ehrenstein Gerald , Lecar Harold
Published: May 2004

Two different methods were used to determine the relative permeability and the voltage-dependent conductance of several different cations in excitability-inducing material (EIM)-doped lipid bilayers. In one method, the conductances of individual channels were measured for Li, Na, K, Cs, NH4, and Ca, and in the other method biionic potentials of a membrane with many channels were measured for Li, Na, K, Cs, and Rb. The experimental results for the two methods are in agreement. The relative permeabilities are proportional to the ionic mobilities in free aqueous solution. The voltage dependence of the conductance is the same for all cations measured.
Feedback in the Contractile Mechanism of the Frog Heart
Authors: Bozler Emil
Published: May 2004

Shortening causes a transient decrease, extension an increase, in activity during contractures of the frog ventricle induced by high Ca or by isosmotic K solution. This is shown by the fact that, after the immediate passive shortening, the muscle is extended under isotonic conditions when the load is diminished, and that under isometric conditions quick release causes first a rapid drop, then a further, much slower, fall of tension. Increasing the load or stretching induce the opposite effects. At low temperatures all rapid changes in length produce oscillations of low frequency. These responses are due to a sensitive feedback mechanism similar to that previously demonstrated for insect fibrillar muscle. That this mechanism comes into play in the heart under normal conditions and controls the time-course of the twitch is demonstrated by the observation that relaxation begins earlier the greater the shortening. Thus, during afterloaded isotonic twitches the onset of relaxation is advanced as the load is diminished.
Acetylcholine Receptors
Authors: Hartzell H. Criss , Fambrough Douglas M.
Published: May 2004

Using 125iodine-labeled α-bungarotoxin (α-BGT-125I) and quantitative radioautography, we have studied the time-course of the change in acetylcholine (ACh) receptor distribution and density occurring in rat diaphragm after denervation. In innervated fibers, ACh receptors are localized at the neuromuscular junction and the extrajunctional receptor density is less than five receptors per square micrometer. The extrajunctional receptor density begins to increase between 2 and 3 days after denervation and increases approximately linearly to 1695 receptors/µm2 at 14 days, subsequently decreasing to 529 receptors/µm2 at 45 days. We have isolated plasma membranes from rat leg muscles at various times after denervation and find that the change in concentration of ACh receptors in the membranes measured by α-BGT-125I binding and scintillation counting follows a time-course similar to the change in ACh receptor density measured radioautographically. Furthermore, we have correlated extrajunctional ACh receptor density measured by radioautography with extrajunctional ACh sensitivity measured by iontophoretic application of ACh and intracellular recording and find that the log of ACh receptor density is related to 0.53 times the log of ACh sensitivity. These results are discussed in terms of the electrophysiological experiments on the ACh receptor and the recent, more biochemical approaches to the study of ACh receptor control and function.
Positive and Negative Inotropic Effects of Elevated Extracellular Potassium Level on Mammalian Ventricular Muscle
Authors: Kavaler Frederic , Hyman Paul M. , Lefkowitz Robert B.
Published: May 2004

The effect of moderate elevation in extracellular potassium concentration (up to 12 mM) on contraction of cat ventricular muscle was examined. Isometric force development was recorded from eight excised trabeculae and from six coronary-perfused in situ papillary muscle preparations. Contraction in the steady state was variably affected, sometimes decreasing monotonically, sometimes remaining unchanged, with increasing potassium level. In 11 of these 14 preparations, the steady state was preceded by a transient period in which the contraction was augmented. In addition, eight excised trabeculae were used in an experimental arrangement designed to distinguish between inotropic effects caused by potassium-induced alterations in the action potential and other, more direct, effects of this ion on contraction. The negative inotropic effect is attributable to a potassium-induced reduction in the amplitude and/or duration of the action potential plateau. The positive inotropic effect was found in experimental arrangements where effects of the potassium-rich medium on action potential time-course were effectively "buffered." The positive inotropic effect thus depends on the presence of the elevated potassium concentration and can occur independently of effects on the action potential time-course.
Transport of Axonal Enzymes in Surviving Segments of Frog Sciatic Nerve
Authors: Partlow Lester M. , Ross C. David , Motwani Ratna , McDougal David B.
Published: May 2004

Redistribution of axonal enzymes as a function of time in vitro was studied in an unbranched segment of frog sciatic nerve. Cholinesterase activity moved peripherally at a rate of 99 mm/day and centrally at 19 mm/day. One-quarter of the total nerve content of the enzyme was estimated to be in motion, one-eighth in each direction. Mitochondrial enzymes (hexokinase and glutamic dehydrogenase) moved peripherally at 20–31 mm/day, centrally at 11–20 mm/day. Only 10% of the total content of these mitochondrial enzymes was in motion. No movement of choline acetylase or 6-phosphogluconic dehydrogenase activity was seen even after 4 days in vitro. However, in a 12 day in vivo experiment choline acetylase moved toward the periphery at a rate of 0.34 mm/day. After a day or so in vitro the distal accumulations of cholinesterase and glutamic dehydrogenase decreased, with a concomitant and quantitatively equivalent increase in enzyme activities at the proximal end of the nerve. It is postulated that during incubation a mechanism for reversing the direction of flow develops in the peripheral stump of the nerve. Vinblastine inhibited central and peripheral flow of both cholinesterase and glutamic dehydrogenase. Movement of cholinesterase was not affected by ouabain, thalidomide, or phenobarbital, nor by K+ excess (110 mM) or absence.
Changes in Membrane Properties of Chick Embryonic Hearts during Development
Authors: Sperelakis Nick , Shigenobu K.
Published: May 2004

The electrophysiological properties of embryonic chick hearts (ventricles) change during development; the largest changes occur between days 2 and 8. Resting potential (Em) and peak overshoot potential (+Emax) increase, respectively, from -35 mv and +11 mv at day 2 to -70 mv and +28 mv at days 12–21. Action potential duration does not change significantly. Maximum rate of rise of the action potential (+Vmax) increases from about 20 v/sec at days 2–3 to 150 v/sec at days 18–21; + Vmax of young cells is not greatly increased by applied hyperpolarizing current pulses. In resting Em vs. log [K+]o curves, the slope at high K+ is lower in young hearts (e.g. 30 mv/decade) than the 50–60 mv/decade obtained in old hearts, but the extrapolated [K+]i values (125–140 mM) are almost as high. Input resistance is much higher in young hearts (13 MΩ at day 2 vs. 4.5 MΩ at days 8–21), suggesting that the membrane resistivity (Rm) is higher. The ratio of permeabilities, PNa/PK, is high (about 0.2) in young hearts, due to a low PK, and decreases during ontogeny (to about 0.05). The low K+ conductance (gK) in young hearts accounts for the greater incidence of hyperpolarizing afterpotentials and pacemaker potentials, the lower sensitivity (with respect to loss of excitability) to elevation of [K+]o, and the higher chronaxie. Acetylcholine does not increase gK of young or old ventricular cells. The increase in (Na+, K+)-adenosine triphosphatase (ATPase) activity during development tends to compensate for the increase in gK. +Emax and + Vmax are dependent on [Na+]o in both young and old hearts. However, the Na+ channels in young hearts (2–4 days) are slow, tetrodotoxin (TTX)-insensitive, and activated-inactivated at lower Em. In contrast, the Na+ channels of cells in older hearts (> 8 days) are fast and TTX-sensitive, but they revert back to slow channels when placed in culture.
Active Transport of Chloride by the Giant Neuron of the Aplysia Abdominal Ganglion
Authors: Russell J. M. , Brown A. M.
Published: May 2004

Internal chloride activity, aiCl, and membrane potential, Em, were measured simultaneously in 120 R2 giant neurons of Aplysia californica. aiCl was 37.0 ± 0.8 mM, Em was -49.3 ± 0.4 mv, and ECl calculated using the Nernst equation was -56.2 ± 0.5 mv. Such values were maintained for as long as 6 hr of continuous recording in untreated neurons. Cooling to 1°–4°C caused aiCl to increase at such a rate that 30–80 min after cooling began, ECl equalled Em. The two then remained equal for as long as 6 hr. Rewarming to 20°C caused aiCl to decline, and ECl became more negative than Em once again. Exposure to 100 mM K+-artificial seawater caused a rapid increase of aiCl. Upon return to control seawater, aiCl declined despite an unfavorable electrochemical gradient and returned to its control values. Therefore, we conclude that chloride is actively transported out of this neuron. The effects of ouabain and 2,4-dinitrophenol were consistent with a partial inhibitory effect. Chloride permeability calculated from net chloride flux using the constant field equation ranged from 4.0 to 36 x 10-8 cm/sec.
Negative Conductance Caused by Entry of Sodium and Cesium Ions into the Potassium Channels of Squid Axons
Authors: Bezanilla Francisco , Armstrong Clay M.
Published: May 2004

Internal Cs+, Na+, Li+, and, to a lesser degree, Rb+ interfere with outward current through the K pores in voltage clamped squid axons. Addition of 100 mM NaF to the perfusion medium cuts outward current for large depolarizations about in half, and causes negative conductance over a range of membrane voltages. For example, suddenly reducing membrane potential from +100 to +60 mv increases the magnitude of the outward current. Internal Cs+ and, to a small extent, Li+, also cause negative conductance. Na+ ions permeate at least 17 times less well through the K pores than K+, and Cs+ does not permeate measurably. The results strongly suggest that K pores have a wide and not very selective inner mouth, which accepts K+, Na+, Li+, Cs+, tetraethylammonium ion (TEA+), and other ions. The diameter of the mouth must be at least 8 A, which is the diameter of a TEA+ ion. K+ ions in the mouths probably have full hydration shells. The remainder of the pore is postulated to be 2.6–3.0 A in diameter, large enough for K+ and Rb+ but too small for Cs+ and TEA+. We postulate that Na+ ions do not enter the narrower part of the pore because they are too small to fit well in the coordination cages provided by the pore as replacements for the water molecules surrounding an ion.