bluuebunny/crossref_metadata_2025 huggingface dataset:
Temporal patterns and selectivity in the unitary responses of olfactory receptors in the tiger salamander to odor stimulation.
Authors: Baylin F
Published: May 2004

Temporal patterns and selectivity in unitary responses of 100 single olfactory receptors in the tiger salamander to odor stimulation were investigated. An olfactometer which permitted control of stimulus concentration, duration, and flow rate was calibrated with a gas chromatograph. Stimulus pulses were monitored by recording the electroolfactogram from the surface of the olfactory epithelium. Both diphasic and triphasic spikes were recorded extracellularly. No discernible differences in types of responses, reproducibility of responses, and cross-unit distribution of spontaneous rates distinguished diphasic from triphasic units. The cross-unit selectivity in responses to the seven olfactory stimulants used and the range of odorant concentrations which effectively evoked these responses suggest variations in types and number of types of receptive sites on each cell. Temporal patterns in the unitary responses were generally less complex than those observed in the olfactory bulb. Phasic stimulations evoked phasic patterns. Tonic stimulations evoked phasic/tonic patterns. Occasionally poststimulus depressions or elevations in firing rates were observed. The nature of these patterns varied somewhat with odorant concentration for a particular unit.
Sodium and calcium interactions in vascular smooth muscle cells of the rabbit ear artery.
Authors: Droogmans G , Casteels R
Published: May 2004

The effects of Na-free and of K-free solutions on the membrane potential, on tension development, and on 45Ca exchange have been investigated in rabbit ear artery. The contraction induced by Na-free solutions and the tension which develops in K-free solutions after a delay of about 1 h are both submaximal. Exposure for 4 h to K-free solutions does not affect the membrane potential, whereas Na-free solutions depolarize the cells by 10-20 mV, depending on the Na-substitute. Neither the amplitude nor the rate constant of the slowly exchanging 45Ca-fraction is affected by these experimental procedures. Substituting external Na by choline or TMA induces a transient increase of the 45Ca-efflux rate which does not occur in a Ca-free efflux medium, and which can be blocked with La. K readmission to Na-enriched tissues hyperpolarizes the cells up to -100 mV and induces a relaxation, without exerting any effect on the 45Ca efflux rate. The release of Ca from intracellular stores, induced by histamine and FCCP, and its subsequent extrusion through the plasma membrane produce a transient stimulation of the 45Ca efflux, which is not affected by the reduction of the Na gradient. The transient contraction induced by histamine in Ca-free solutions is affected in a different way by different Na substitutes. The results do not fit the Na-Ca exchange hypothesis but are consistent with an effect of the Na gradient on the passive Ca influx.
The effect of strychnine, bicuculline, and picrotoxin on X and Y cells in the cat retina.
Authors: Kirby A W
Published: May 2004

The effect of intravenous strychnine and the GABA antagonists picrotoxin and bicuculline upon the discharge pattern of center-surround-organized cat retinal ganglion cells of X and Y type were studied. Stimuli (mostly scotopic, and some photopic) were selected such that responses from both on and off-center cells were either due to the center, due to the surround, or clearly mixed. Pre-drug control responses were obtained, and their behavior following administration of the antagonists was observed for periods up to several hours. X-cell responses were affected in a consistent manner by strychnine while being unaffected by GABA antagonists. All observed changes following strychnine were consistent with a shift in center-surround balance of X cells in favor of the center. For Y-cell responses to flashing annuli following strychnine, there was either no shift or a relatively small shift in center-surround balance. Compared to X-cell responses to flashing lights, those of Y cells were very little affected by strychnine and in most cases were unaffected. It thus appears that glycine plays a similar role in receptive field organization of X cells as does GABA in Y cells (Kirby and Enroth-Cugell, 1976. J. Gen. Physiol. 68:465-484).
The active tension-length curve of vascular smooth muscle related to its cellular components.
Authors: Mulvany M J , Warshaw D M
Published: May 2004

The active and passive isometric tension-length (internal circumference) relation of vascular smooth muscle has been investigated using a 100-200-micron lumen diameter artery from the rat mesenteric bed. Conditions were established under which maximal activation was obtained at all lengths. Below L0 (the length at which maximum tension, delta T0, was developed) the active tension fell with decreasing length along a line which extrapolated to 0.38 L0; below 1.1 L0 the relation was reversible regardless of the protocol used. Above L0 the active tension fell linearly with increasing length along a line which extrapolated to zero tension at 1.82 L0. At the longer lengths investigated (up to 1.6 L0) the passive tension upon which the active responses were superimposed was as high as 4.4 delta T0. However, measurements of the dynamic characteristics of the preparation (with a time resolution of 2 ms) suggest that the active tension measured is nevertheless a measure of the active properties of the contractile apparatus. Direct light microscopic observation of the effect of length change on the cells within the walls of the preparation showed that changes in vessel length produced, on average, the same percentage change in cell length. Histological examination showed no signs of cell destruction following large extensions. The results suggest that the decrease in tension with extension above L0 is due to changes in the properties of the contractile apparatus, rather than to cellular damage.
Ionic and osmotic equilibria of human red blood cells treated with nystatin.
Authors: Freedman J C , Hoffman J F
Published: May 2004

Human red blood cells have been incubated in the presence of nystatin, which allows Na and K, as well as Cl and pH to equilibrate rapidly when cell volume is set with external impermeant sucrose. The intracellular mean ionic activity coefficients, relative to values in the extracellular solution, for KCl and NaCl are 1.01 +/- 0.02 and 0.99 +/- 0.02 (SD, n = 10), respectively, and are independent of external pH, pH o, and of [sucrose]o. With nystatin the dependence of red cell volume on [sucrose]o deviates from ideal osmotic behavior by as much as a factor of three. A virial equation for the osmotic coefficient, phi, of human hemoglobin, Hb, accounts for the cell volumes, and is the same as that which describes Adair's measurements of phi Hb for Hb isolated from sheep and ox bloods. In the presence of nystatin the slope of the acid-base titration curve of the cells is independent of cell volume, implying that the charge on impermeant cellular solutes is independent of Hb concentration at constant pH. By modifying the Jacobs-stewart equations (1947. J. Cell. Comp. Physiol. 30: 79--103) with the osmotic coefficients of Hb and of salts, a nonideal thermodynamic model has been devised which predicts equilibrium Donnan ratios and red cell volume from the composition of the extracellular solution and from certain parameters of the cells. In addition to accounting for the dependence of cell volume on osmotic pressure, the model also describes accurately the dependence of Donnan ratios and cell volumes on pHo either in the presence or absence of nystatin.
Gustatory responses of eel palatine receptors to amino acids and carboxylic acids.
Authors: Yoshii K , Kamo N , Kurihara K , Kobatake Y
Published: May 2004

The gustatory receptors of the eel palate were found to be extremely sensitive to amino acids and carboxylic acids. The results obtained are as follows: (a) 11 amino acids which are among naturally occurring amino acids elicited responses in the palatine nerve, but 9 amino acids did not elicit a response even at a high concentration. The effect of D-amino acids was always much less than that of their corresponding L-isomers. There was no appreciable difference in the effectiveness of an alpha-amino acid (alpha-alanine) and beta-amino acid (beta-alanine). (b) The threshold concentrations of the most potent amino acids (arginine, glycine) were between 10(-8) and 10(-9) M. A linear relation between the magnitude of the response and log stimulus concentration held for a wide concentration range for all the amino acids examined. (c) The palatine receptors responded sensitively to various carboxylic acid solutions whose pH was adjusted to neutral. The threshold concentrations varied between 10(-4) and 10(-7) M. The magnitude of the response at 10(-2) M increased with an increase of carbon chain length. (d) The extent of cross-adaptation was examined with various combinations of amino acids. A variety of the response patterns showing complete cross-adaptation, no cross-adaptation, or synergetic interaction was observed. The synergetic interaction was also observed when one amino acid below its threshold concentration was added to the other amino acid below its threshold concentration was added to the other amino acid. No cross-adaptation was observed between amino acids and fatty acids. (e) The treatment of the palate with papain led to loss of the responses to arginine, glycine, and histidine without affecting those to proline and acetic acid. The treatment with pronase E eliminated selectively the response to proline. The possibility that the eel gustatory receptors are responsible for sensing food at a distance was discussed.
Anion transport in dog, cat, and human red cells. Effects of varying cell volume and Donnan ratio.
Authors: Castranova V , Weise M J , Hoffman J F
Published: May 2004

Membrane potential and the rate constants for anion self-exchange in dog, cat, and human red blood cells have been shown to vary with cell volume. For dog and cat red cells, the outward rate constants for SO4 and Cl increase while the inward rate constant for SO4 decreases as cells swell or shrink. These changes coincide with the membrane potential becoming more negative as a result of changes in cell volume. Human red cells exhibit a similar change in the rate constants for SO4 and Cl efflux in response to cell swelling, but shrunken cells exhibit a decreased rate constant for SO4 efflux and a more positive membrane potential. Hyperpolarization of shrunken dog and cat red cells is due to a volume-dependent rate constant for SO4 efflux and a more positive membrane potential. Hyperpolarization of shrunken dog and cat red cells is due to a volume-dependent increase in PNa. If this increase in PNa is prevented by ATP depletion or if the outward Na gradient is removed, the response to shrinking is identical to human red cells. These results suggest that the volume dependence of anion permeability may be secondary to changes in the anion equilibrium ratio which in red cells is reflected by the membrane potential. When the membrane potential and cell volume of human red cells were varied independently by a method involving pretreatment with nystatin, it was found that the rate of anion transport (for SO4 and Cl) does not vary with cell volume but rather with membrane potential (anion equilibrium ratio); that is, the rate constant for anion efflux is decreased and that for influx is increased as the membrane potential becomes more positive (internal anion concentration increases) while the opposite is true with membrane hyperpolarization (a fall in internal anion concentration).
The effect of calcium ions on the secretion of quanta evoked by an impulse at nerve terminal release sites.
Authors: Bennett M R , Lavidis N A
Published: May 2004

A study has been made of the effects of calcium ions on the number of quanta secreted from all the release sites at an amphibian motor nerve terminal recorded with an intracellular microelectrode (m) compared with the number secreted simultaneously from a small number of release sites recorded with an extracellular microelectrode (me). If the endplate potential was made subthreshold by lowering the external calcium concentration ([Ca]o less than or equal to 0.4 mM), it was possible to find small groups of release sites for which me was comparable to m, indicating considerable nonuniformity in the probability of release of a quantum at different groups of release sites (Pe) in a given [Ca]o. Increasing [Ca]o in the range from 0.25 to 0.4 mM increased the probability of release of a quantum at groups of release sites (Pe), independent of the initial value of Pe, and the dependence of Pe on [Ca]o followed a fourth power relationship. A conditioning impulse enhanced the probability of release of a quantum by a subsequent test impulse at release sites, if Pe was less than 1.0 during the conditioning impulse. It is shown that the present observations regarding the dependence of Pe on [Ca]o and on conditioning impulses can be quantitatively predicted from previous observations regarding the dependence of the binomial parameters m, p, and n on [Ca]o and on conditioning impulses determined with intracellular electrodes, if the probability of secretion of a quantum at a release site (Pj) is different for different release sites and Pj is distributed as a beta random variable.
Inotropic effects and changes in sodium and calcium contents associated with inhibition of monovalent cation active transport by ouabain in cultured myocardial cells.
Authors: Biedert S , Barry W H , Smith T W
Published: May 2004

Cultured monolayers of spontaneously contracting chick embryo ventricular cells were perfused with culture medium containing ouabain. Contractile state was monitored by an optical-video system recording amplitude and velocity of cell wall motion. Positive inotropic effects of 2.5 x 10(-7) to 10(-6) M ouabain were manifest within 1.5-2 min, and reached a stable plateau within 5-6 min. The inotropic effect was fully reversed within 5 min after washout of ouabain. Inhibition of uptake of 42K+ (or the K+ analog 86Rb+) and efflux of 24Na+ occurred 1.5-2 min after exposure to ouabain. The degree of inhibition of transport was closely related to the magnitude of the positive inotropic effect throughout the ouabain concentration range 10(-7) to 10(-6) M. After washout of ouabain from monolayers, the monovalent cation active transport rate returned to normal within 1 min. Thus, both the onset and offset of inotropic action of ouabain were closely related temporally to inhibition of the sodium pump. Exposure to ouabain caused significant increases in exchangeable Na and Ca contents that appeared to be developed within 5 min. These data support the hypothesis that inhibition of monovalent cation active transport by ouabain is causally related to the development of positive inotropy and are consistent with modulation of Ca content by intracellular Na+ via the Na+-Ca2+ exchange carrier mechanism.
Sucrose uptake by pinocytosis in Amoeba proteus and the influence of external calcium.
Authors: Prusch R D , Hannafin J A
Published: May 2004

The relationship between Ca++ and pinocytosis was investigated in Amoeba proteus. Pinocytosis was induced with 0.01% alcian blue, a large molecular weight dye which binds irreversibly to the cell surface. The time-course and intensity of pinocytosis was monitored by following the uptake of [3H]SUCROSE. When the cells are exposed to 0.01% alcian blue, there is an immediate uptake of sucrose. The cells take up integral of 10% of their initial volume during the time-course of pinocytosis. The duration of pinocytosis in the amoeba is integral of 50 min, with maximum sucrose uptake occurring 15 min after the induction of pinocytosis. The pinocytotic uptake of sucrose is reversibly blocked at 3 degrees C and a decrease in pH increases the uptake of sucrose by pinocytosis. The process of pinocytosis is also dependent upon the concentration of the inducer in the external medium. The association between Ca++ and pinocytosis in A. proteus was investigated initially by determining the effect of the external Ca++ concentration on sucrose uptake induced by alcian blue. In Ca++-free medium, no sucrose uptake is observed in the presence of 0.01% alcian blue. As the Ca++ concentration is increased, up to a maximum of 0.1 mM, pinocytotic sucrose uptake is also increased. Increases in the external Ca++ concentration above 0.1 mM brings about a decrease in sucrose uptake. Further investigations into the association between Ca++ and pinocytosis demonstrated that the inducer of pinocytosis displaces surface calcium in the amoeba. It is suggested that Ca++ is involved in two separate stages in the process of pinocytosis; an initial displacement of surface calcium by the inducer which may increase the permeability of the membrane to solutes and a subsequent Ca++ influx bringing about localized increases in cytoplasmic Ca++ ion activity.
Arterial bifurcations in the human retina.
Authors: Zamir M , Medeiros J A , Cunningham T K
Published: May 2004

The branching angles and relative diameters of blood vessels in 51 arterial bifurcations in the retina of a normal human eye were measured. In eight other bifurcations, only the total branching angles were measured. The results are compared with theoretical predictions in an attempt to understand the physiological principles governing branching in the cardiovascular system.
Specificity of serotoninergic inhibition in Limulus lateral eye.
Authors: Adolph A R , Kass L
Published: May 2004

The receptor specificity for synaptically mediated lateral inhibition in Limulus lateral eye retina was studied by structure-activity correlations of the action of the putative indoleaminergic neurotransmitter, serotonin (5-HT), and its isomers and structural analogs, tryptamine (TRYP), 6-hydroxytryptamine (6HT), 5,6-dihydroxytryptamine (5,6-DHT), 5-hydroxydimethyltryptamine (5-HDMT), and 5-hydroxytryptophan (5-HTP). The 5-HT blockers, lysergic acid diethylamide (LSD), bromo-LSD (BOL), and cinanserin, were also tested. The inhibitory action of the indoleaminergic agonists is highly structure-specific. An hydroxyl group in the 5 position of the indole nucleus, sterically unencumbered by hydroxyls in neighboing positions, is essential. In order of decreasing potency, 5-HT, 5-HDMT, and 5-HTP are active agonists; TRYP, 6-HT, and 5,6-DHT are inactive. Configuration and mobility of the side chains of the active agonists also affect the interaction, and these side-chain characteristics correlate with agonist potency. The receptors for inhibitory action and for transmembranal transport in reuptake are different. Both active agonists and inactive analogs appear to be taken up (Adolph and Ehinger, 1975. Cell Tissue Res. 163:1-14). LSD and BOL have bimodal actions: direct inhibition and agonist blockade. These actions may be mediated via low-specificity presynaptic uptake receptor sites rather than highly specific, postsynaptic, agonist receptor sites.
Retinochrome and rhodopsin in the extraocular photoreceptor of the squid, Todarodes.
Authors: Hara T , Hara R
Published: May 2004

The deep-sea squid, Todarodes pacificus, possesses well-developed parolfactory vesicles as extraocular photoreceptors connected with the brain. The ventral set of vesicles forms a thread approximately 3mm long and looks orange owing to photopigments. The vesicle mainly consists of receptor cells, each of which is similar in structure to the visual cell, carrying rhabdomeres in the distal process and lamellated myeloid bodies in the proximal part. Recently we noticed that a crude extract of the vesicles is capable of isomerizing retinal from all-trans to the 11-cis form in the light, and confirmed that the vesicles in fact contained retinochrome in addition to rhodopsin. This is the first time that retinochrome has been detected in any place other than ocular tissues. The optical and chemical nature of these photopigments is the same as that we have observed in the Todarodes retina. Quantitative extractions have shown that the total yield of photopigments is approximately 0.0006 in absorbance at lambda max (light path, 10 mm) per milliliter per thread of vesicles, and that the amount of retinochrome in the vesicles is roughly equivalent to that of rhodopsin. Whereas rhodopsin is located in the rhabdomal membranes, retinochrome is probably associated with lamellated structures and their derivatives in the cytoplasm. In the parolfactory vesicles, retinochrome may also cooperate with rhodopsin in the same way as has been discussed for retinal photoreception.
Control of ion distribution in isolated smooth muscle cells. I. Potassium.
Authors: Scheid C R , Fay F S
Published: May 2004

We describe a technique for examining unidirectional ion movements in suspensions of enzymatically disaggregated smooth muscle cells derived from stomach muscle of the toad. This technique has been used to analyze the movement of 42K across these cells. This analysis was greatly simplified by the finding that the cells were in a steady state with respect to K+ distribution after isolation. The potassium contents of the isolated cells were identical to those of intact smooth muscle (131 mM/liter intracellular fluid) and stable for over 4 h; moreover, the unidirectional influx and efflux rates were equal. An additional simplification was provided by the finding that virtually all the K+ exchanges in a manner predicted for a simple two-compartment system consisting of an extracellular and an intracellular space. Transmembrane K+ flux in these cells averaged 1.2 pmol.cm-2.s-1 at room temperature. A large portion (approximately 80%) of 42K influx appeared to be mediated by a saturable transport system with an apparent Km of 0.6 mM and an apparent Vmax of 1.3 pmol.cm-2.s-1. The calculated resting membrane permeability to K+ in these isolated smooth muscle cells, assuming a membrane potential of -50 mV, was 2.9 X 10(-8) cm/s. The calculated gK+ was 2.7 mumho/cm2 constituting only a small fraction of the total membrane conductance as measured electrophysiologically. The latter finding suggests that the resting membrane potential in the isolated cells must be determined by ions in addition to K+. We propose that these methods for studying ion movements in smooth muscle should aid in unraveling the mechanisms responsible for controlling the distribution of ions both at rest, as in the present study, as well as in response to neurotransmitters.
Fusion of phospholipid vesicles with planar phospholipid bilayer membranes. II. Incorporation of a vesicular membrane marker into the planar membrane.
Authors: Cohen F S , Zimmerberg J , Finkelstein A
Published: May 2004

Fusion of multilamellar phospholipid vesicles with planar phospholipid bilayer membranes was monitored by the rate of appearance in the planar membrane of an intrinsic membrane protein present in the vesicle membranes. An essential requirement for fusion is an osmotic gradient across the planar membrane, with the cis side (the side containing the vesicles) hyperosmotic to the opposite (trans) side; for substantial fusion rates, divalent cation must also be present on the cis side. Thus, the low fusion rates obtained with 100 mM excess glucose in the cis compartment are enhanced orders of magnitude by the addition of 5-10 mM CaCl2 to the cis compartment. Conversely, the rapid fusion rates induced by 40 mM CaCl2 in the cis compartment are completely suppressed when the osmotic gradient (created by the 40 mM CaCl2) is abolished by addition of an equivalent amount of either CaCl2, NaCl, urea, or glucose to the trans compartment. We propose that fusion occurs by the osmotic swelling of vesicles in contact with the planar membrane, with subsequent rupture of the vesicular and planar membranes in the region of contact. Divalent cations catalyze this process by increasing the frequency and duration of vesicle-planar membrane contact. We argue that essentially this same osmotic mechanism drives biological fusion processes, such as exocytosis. Our fusion procedure provides a general method for incorporating and reconstituting transport proteins into planar phospholipid bilayer membranes.
Cyclic nucleotide regulation of the contractile proteins in mammalian cardiac muscle.
Authors: McClellan G B , Winegrad S
Published: May 2004

The contractile system of rat cardiac muscle that has been made hyperpermeable by soaking the tissue in EGTA (McClellan and Winegrad. 1978. J. Gen. Physiol. 72:737-764) can be probed directly with Ca buffer from the bathing solution without significant interference from either sarcoplasmic reticulum or mitochondria on the Ca concentration. Changes in Ca-activated force are due therefore to changes in the properties of the contractile system itself and not to regulation of Ca concentration. The addition of cAMP, cGMP, and GTP, guanylyl imidodiphosphate (GMP-PNP), or epinephrine to the bath does not alter maximum Ca-activated force, but when these drugs are added with 1% nonionic detergent to the bath, contractility increases by as much as 180%. An inhibitor of phosphodiesterase must be present for the inotropic effect of cAMP but not cGMP, GTP, GMP-PNP, or epinephrine. The inotropic response to cAMP is independent of the Ca sensitivity of the contractile system, but guanine nucleotides enhance contractility only when Ca sensitivity is not high. The inotropic effect of epinephrine is inhibited to a large extent by cGMP but not by GMP-PNP. These data can be explained by a model in which contractility is enhanced by a cAMP-regulated phosphorylation that can be controlled through the beta-receptor adenylate cyclase complex in the sarcolemma. The regulation involves two reactions, one a phosphorylation and a second that occurs in the presence of detergent. Phosphorylation of neither the myosin light chain nor the inhibitory subunit of troponin appears to be involved in this mechanism for regulating contractility.
Fast electrical potentials arising from activation of metarhodopsin in the fly.
Authors: Minke B , Kirschfeld K
Published: May 2004

The cellular origin and properties of fast electrical potentials arising from activation of Calliphora photopigment were investigated. It was found by intracellular recordings that only the corneal-negative M1 phase of fly M potential arises in the photoreceptors' membrane. This M1 phase has all the accepted characteristics of an early receptor potential (ERP). It has no detectable latency, it survives fixation with glutaraldehyde, it is linear with light intensity below pigment saturation, and it is linear with the amount of metarhodopsin activated by light. The Calliphora ERP was found, however, to be exceptional because activation of rhodopsin, which causes the formation of metarhodopsin in 125 microsecond (25 degrees C), was not manifested in the ERP. Also, the extracellularly recorded ERP was not proportional to the rate of photopigment conversion. The corneal-positive M2 phase of the M potential was found to arise from second-order lamina neurons (L neurons). Intracellular recordings from these cells showed a fast hyperpolarizing potential, which preceded the normal hyperpolarizing transient of these cells. This fast potential appeared only when metarhodopsin was activated by a strong flash. The data indicate that the intracellularly recorded positive ERP, which arises from activation of metarhodoposin, elicits a hyperpolarizing fast potential in the second-order neuron. This potential is most likely the source of the corneal-positive M potential.
The initial inward current in spherical clusters of chick embryonic heart cells.
Authors: Ebihara L , Shigeto N , Lieberman M , Johnson E A
Published: May 2004

The rapid inward sodium current in spherical clusters of 11-d-old embryonic chick heart cells, ranging in size between 65 and 90 micron diameter, was studied using the two-microelectrode voltage-clamp technique. Using these preparations, it was possible to resolve the activation phase of the rapid inward current for potentials negative to -25 mV at 37 degrees C. The rapid inward current exhibited a voltage and time dependence similar to that observed in other excitable tissues. It was initiated at potential steps more positive than -45 mV. The magnitude of the current reached its maximum value at a potential of approximately -20 mV. The measured reversal potential was that predicted by the Nernst equation for sodium ions. The falling phase of the current followed a single exponential time-course with a time constant of inactivation, tau h, ranging between 2.14 ms at -40 mV and 0.18 ms at -5 mV. The time constant of inactivation, tau h, determined by a single voltage-step protocol was compared to the constant, tau c, determined by a double voltage-step protocol and no significant different between the two constants of inactivation was found. Furthermore, the time constants of inactivation and reactivation at the same potential in the same preparation were similar. The results of this study demonstrate that the sodium current of heart cells recorded at 37 degrees C can be described by Hodgkin-Huxley kinetics with speeds approximately four times faster than the squid giant axon at 15 degrees C.
Voltage dependence of desensitization. Influence of calcium and activation kinetics.
Authors: Fiekers J F , Spannbauer P M , Scubon-Mulieri B , Parsons R L
Published: May 2004

The voltage dependence of carbachol-induced desensitization has been analyzed in potassium-depolarized frog sartorius muscle preparations with voltage clamp techniques over a wide voltage range (-120 to +40 mV). Desensitization developed exponentially at all voltages with tau, the time constant of desensitization onset, varying as a logarithmic function of membrane voltage. The voltage dependence of tau remained in calcium-deficient solutions and was not altered by elevating either the level of extracellular or intracellular calcium. We have analyzed our results according to a simple sequential kinetic scheme in which the rate-limiting step in the development of desensitization is a transition of the receptor channel complex from the activated conducting state to a desensitized, nonconducting state. We conclude (a) that the observed voltage sensitivity of desensitization primarily resides in the voltage dependence of this transition, and (b) the kinetics of activation appear to have a greater influence on the observed rate of desensitization than on its voltage dependence. The magnitude of the voltage dependence suggests that a greater change in free energy is required for the transition to the desensitized state than for the transition between the open and closed states of the receptor channel complex.
Conductive properties of the proximal tubule in Necturus kidney.
Authors: Anagnostopoulos T , Teulon J , Edelman A
Published: May 2004

The electrical properties of the proximal tubule of the in vivo Necturus kidney were investigated by injecting current (as rectangular waves) into the lumen or into the epithelium of single tubules and by studying the resulting changes of transepithelial (VL) and/or cell membrane potential (VC) at various distances from the source. In some experiments paired measurements of VL and VC were performed at two abscissas x and x'. The luminal length constant of about 1,030 micrometer was shown to provide a good estimate of the transepithelial resistance, specific resistance (RTE = 420 omega.cm2) and/or per unit length (rTE = 1.3 x 10(4) omega.cm). The apparent intraepithelial length constant was subject to distortions arising from concomitant current spread in the lumen. The resistances of luminal membrane (rL), basolateral membrane (rB), and shunt pathway (rS) were estimated by two independent methods at 3.5 x 10(4), 1.2 x 10(4), and 1.7 x 10(4) omega.cm, respectively. The corresponding specific resistances were close to 1,200, 600, and 600 omega.cm2. There are two main conclusions of this study. (a) The resistances of cell membranes and shunt pathway are of the same order of magnitude. The figure of the shunt resistance is at variance with the notion that the proximal tubule of Necturus is a leaky epithelium. (b) A rigorous assessment of the conductive properties of concentric cylindrical double cables (such as renal tubules) requires that electrical interactions arising from one cable to another be taken into account. Appropriate equations were developed to deal with this problem.
Kinetics of phloretin binding to phosphatidylcholine vesicle membranes.
Authors: Verkman A S , Solomon A K
Published: May 2004

The submillisecond kinetics for phloretin binding to unilamellar phosphatidylcholine (PC) vesicles was investigated using the temperature-jump technique. Spectrophotometric studies of the equilibrium binding performed at 328 nm demonstrated that phloretin binds to a single set of independent, equivalent sites on the vesicle with a dissociation constant of 8.0 microM and a lipid/site ratio of 4.0. The temperature of the phloretin-vesicle solution was jumped by 4 degrees C within 4 microseconds producing a monoexponential, concentration-dependent relaxation process with time constants in the 30--200-microseconds time range. An analysis of the concentration dependence of relaxation time constants at pH 7.30 and 24 degrees C yielded a binding rate constant of 2.7 X 10(8) M-1 s-1 and an unbinding constant of 2,900 s-1; approximately 66 percent of total binding sites are exposed at the outer vesicle surface. The value of the binding rate constant and three additional observations suggest that the binding kinetics are diffusion limited. The phloretin analogue, naringenin, which has a diffusion coefficient similar to phloretin yet a dissociation constant equal to 24 microM, bound to PC vesicle with the same rate constant as phloretin did. In addition, the phloretin-PC system was studied in buffers made one to six times more viscous than water by addition of sucrose or glycerol to the differ. The equilibrium affinity for phloretin binding to PC vesicles is independent of viscosity, yet the binding rate constant decreases with the expected dependence (kappa binding alpha 1/viscosity) for diffusion-limited processes. Thus, the binding rate constant is not altered by differences in binding affinity, yet depends upon the diffusion coefficient in buffer. Finally, studies of the pH dependence of the binding rate constant showed a dependence (kappa binding alpha [1 + 10pH-pK]) consistent with the diffusion-limited binding of a weak acid.
Intracellular K+ activity and its relation to basolateral membrane ion transport in Necturus gallbladder epithelium.
Authors: Reuss L , Weinman S A , Grady T P
Published: May 2004

A study of the mechanisms of the effects of amphotericin B and ouabain on cell membrane and transepithelial potentials and intracellular K activity (alpha Ki) of Necturus gallbladder epithelium was undertaken with conventional and K-selective intracellular microelectrode techniques. Amphotericin B produced a mucosa-negative change of transepithelial potential (Vms) and depolarization of both apical and basolateral membranes. Rapid fall of alpha Ki was also observed, with the consequent reduction of the K equilibrium potential (EK) across both the apical and the basolateral membrane. It was also shown that, unless the mucosal bathing medium is rapidly exchanged, K accumulates in the unstirred fluid layers near the luminal membrane generating a paracellular K diffusion potential, which contributes to the Vms change. Exposure to ouabain resulted in a slow decrease of alpha Ki and slow depolarization of both cell membranes. Cell membrane potentials and alpha Ki could be partially restored by a brief (3-4 min) mucosal substitution of K for Na. Under all experimental conditions (control, amphotericin B, and ouabain), EK at the basolateral membrane was larger than the basolateral membrane equivalent emf (Eb). Therefore, the K chemical potential difference appears to account for Eb and the magnitude of the cell membrane potentials, without the need to postulate an electrogenic Na pump. Comparison of the rate of Na transport across the tissue with the electrodiffusional K flux across the basolateral membrane indicates that maintenance of a steady-state alpha Ki cannot be explained by a simple Na,K pump-K leak model. It is suggested that either a NaCl pump operates in parallel with the Na,K pump, or that a KCl downhill neutral extrusion mechanism exists in addition to the electrodiffusional K pathway.
Cationic selectivity and competition at the sodium entry site in frog skin.
Authors: Benos D J , Mandel L J , Simon S A
Published: May 2004

The cation selectivity of the Na entry mechanism located in the outer membrane of the bullfrog (Rana catesbeiana) skin epithelium was studied. This selectivity was determined by measuring the short-circuit current when all of the external sodium was replaced by another cation and, also, by noting the relative degree of inhibition that the alkali metal cations produced on Na influx. The ability of the Group Ia cations to permeate the apical membrane was determined from the tracer uptake experiments. The results demonstrate that (a) only Li and Na are actively transported through the epithelium; (b) the alkali cations K, Rb, and Cs do not enter the epithelium through the apical border and, therefore, Na and Li are the only alkali cations translocated through this membrane; (c) these impermeable cations are competitive inhibitors of Na entry; (d) the cations NH4 and Tl exhibit more complex behavior but, under well-defined conditions, also inhibit Na entry; and (e) the selectivity of the cation binding site is in the sequence Li congruent to Na > Tl > NH4 congruent to K > Rb > Cs, which corresponds to a high field strength site with tetrahedral symmetry.
Separation of the action potential into a Na-channel spike and a K-channel spike by tetrodotoxin and by tetraethylammonium ion in squid giant axons internally perfused with dilute Na-salt solutions.
Authors: Inoue I
Published: May 2004

Squid giant axons internally perfused with a 30 mM NaF solution and bathed in a 100 mM CaCl2 solution, which are known to produce long lasting action potentials in response to pulses of outward current, were investigated. The effects of tetrodotoxin (TTX) and of tetraethylammonium ion (TEA+) on such action potentials were studied. The results are summarized as follows: (a) An addition of 1--3 microM TTX to the external solution altered but did not block the action potentials; it increased the height of the action potential by approximately 15 mV, and it decreased the membrane conductance as the peak of excitation by about two-thirds. (b) Voltage-clamp experiments performed with both NaCl and TTX in the external CaCl2 solution revealed that the TTX-insensitive action potential does not involve a rise in gNa, whereas the experiments performed without TTX showed that the action potential is accompanied by a large rise in gNa. (c) Internally applied TEA+ was shown to selectively block the TTX-insensitive action potential, but it did not block the other component of the action potential, which is accompanied by a rise in gNa, and which is selectively suppressed by TTX. (d) The addition of a small amount of KCl to the external CaCl2 solution containing TTX greatly increased both the maximum peak inward current under voltage clamp and the maximum slope conductance. Furthermore, it was shown that K+ applied on both sides of the axon plays a dominant role in producing the membrane potential in the active state in the presence of TTX, even though a large amount of Ca2+ is presented in the bathing medium. These observations have led me to conclude that the sodium channel is responsible for the production of the TTX-sensitive component of the action potential under the ionic conditions of these experiments, and the potassium channel for the TTX-insensitive component of the action potential.
A single-cell technique for the measurement of membrane potential, membrane conductance, and the efflux of rapidly penetrating solutes in Amphiuma erythrocytes.
Authors: Stoner L C , Kregenow F M
Published: May 2004

We describe a single-cell technique for measuring membrane potential, membrane resistance, and the efflux of rapidly penetrating solutes such as Cl and H2O. Erythrocytes from Amphiuma means were aspirated into a Sylgard (Dow Corning Corp.)-coated capillary. The aspirated cell separated a solution within the capillary from a solution in the bath. Each of these two solutions was contiguous with approximately 5% of the total membrane surface. Microelectrodes placed concentrically within the capillary permit the measurement of intracellular voltage, specific membrane resistance, and the electrical seal between the two solutions. The intracellular voltage averaged -17.7 mV (pH 7.6) and changed as either intra- or extracellular chloride was varied. The average specific membrane resistance measured by passing current across the exposed membrane surface was 110 ohm-cm2. 36Cl and tritiated H2O fluxes (0.84 +/- 0.05 x 10(-6) M . cm-2 . min-1 and 6.4 +/- 1.5 x 10(-3) M . cm-2 . min-1, respectively) were determined by noting the rate at which isotope leaves the cell and crosses the membrane exposed to the bath. Our measured values for the flux of 36Cl and tritiated H2O approximate reported values for free-floating cells. 36Cl efflux, in addition, is inhibited by 4-acetamido-4'-isothiocyano-stilbene 2,2'-disulfonic acid (SITS) and furosemide, known inhibitors of the anion exchange mechanism responsible for the rapid anion fluxes of red blood cells. One can also demonstrate directly that > 89% of 36Cl efflux is "electrically silent" by analyzing the flux in the presence of an imposed transcellular voltage.
Transient adaptation and sensitization in the retina of Necturus.
Authors: Karwoski C J , Proenza L M
Published: May 2004

Responses to repetitive stimulation were monitored at several retinal levels in the eyecup of the mudpuppy Necturus maculosus. When alternating sequences of low-intensity small and large spots were presented, two effects were found, which could be localized to the proximal retina: (a) response decrement (RD), in which, after the first small spot response, subsequent small spot responses are decreased in amplitude and (b) transient response enhancement (TRE), in which the first small spot response after a large spot sequence is larger than preceding or subsequent small spot responses. RD and TRE are absent or weak in sustained on or off responses (horizontal and bipolar cells, and ON and OFF ganglion cell post-stimulus time histograms (PSTH) but are particularly well developed in the on/off responses of the proximal retina (proximal negative response, M-wave, PSTHs of ON/OFF ganglion cells, and intracellular responses from on/off neurons and Müller cells). RD and TRE appear to arise from a stimulus-evoked slow depolarization in on/off neurons that interacts with the amplitude of succeeding responses. We conclude that RD and TRE are a form of neural adaptation that is largely specific to the on/off channels of the proximal retina.
Dispersion of latencies in photoreceptors of Limulus and the adapting-bump model.
Authors: Wong F , Knight B W , Dodge F A
Published: May 2004

To light stimuli of very low intensity, Limulus photoreceptors give a voltage response with a fluctuating delay. This phenomenon has been called "latency dispersion." If the generator potential is the superposition of discrete voltage events ("bumps"), and if the effect of light upon bump size is negligible, then the latency dispersion and the bump shape completely characterize the frequency response to sinusoidal flicker. For very low light intensities, the latency dispersion of the bumps, the bump shape, and the frequency response are measured. It is found that for data obtained at 20 degrees C, the frequency response can be accounted for completely by the latency dispersion and by the bump shape derived from steady-state noise characteristics. At 10 degrees C, the time scale of the response of the photoreceptor is lengthened. The dispersion of latencies and the bump shape are found not to have the same temperature dependence. However, just as those measured at 20 degrees C, the bump shape and the dispersion of latencies measured at 10 degrees C can predict the frequency response measured under the same conditions. These results strongly suggest that the major mechanisms involved in the generator potential are the latency process and the bump process. At high light intensities, the time scale of the generator potential shortens. The decrease in time scale of the generator potential can be attributed to the decreases in time scales of the bumps and of the latency dispersion process.
Adapting-bump model for eccentric cells of Limulus.
Authors: Wong F , Knight B W
Published: May 2004

Light-evoked intracellular voltage noise records have been obtained from Limulus eccentric cells, from threshold light intensity to an intensity .10(5) times threshold. These data are analyzed in terms of a simple "adapting-bump" noise model. It is shown how the model yields a data reduction procedure that slightly generalizes the familiar use of Campbell's theorem for Poisson shot noise: the correlative effect of adaptation amends Campbell's theorem by a single multiplicative factor, which may be estimated directly from the power spectrum of the noise data. The model also permits direct estimation of the bump shape from the power spectrum. The bump shape estimated from noise at dim light is in excellent agreement with the average shape of bumps observed directly in the dark. The data yield a bump rate that is linear with light up through about 50 times threshold intensity but that falls short of linearity by a factor of 35 at the brightest light. The bump height decreases as the -0.4 power of light intensity across the entire range. Bump duration decreases by a factor of 2 across the entire range, and the adaptation correlation factor descends from unity to about one-third. The modest change of the adaptation correlation shows that naive application of Campbell's theorem to such data is adequate for rough estimation of the model's physiological parameters. This simple accounting for all the data gives support to the adapting-bump model.
Isosmotic volume reabsorption in rat proximal tubule.
Authors: Warner R R , Lechene C
Published: May 2004

A theoretical model incorporation both active and passive forces has been developed for fluid reabsorption from split oil droplets in rat intermediate and late proximal tubule. Of necessity, simplifying assumptions have been introduced; we have assumed that the epithelium can be treated as a single membrane and that the membrane "effective" HCO3 permeability is near zero. Based on this model with its underlying assumptions, the following conclusions are drawn. Regardless of the presence or absence of active NaCl transport, fluid reabsorption from the split oil droplet is isosmotic. The reabsorbate osmolarity can be affected by changes in tubular permeability parameters and applied forces but is not readily altered from an osmolarity essentially equal to that of plasma. In a split droplet, isosmotic flow need not be a special consequence of active Na transport, is not the result of a particular set of permeability properties, and is not merely a trivial consequence of a very high hydraulic conductivity; isosmotic flow can be obtained with hydraulic conductivity nearly an order of magnitude lower than that previously measured in the rat proximal convoluted tubule. Isosmotic reabsorption is, in part, the result of the interdependence of salt and water flows, their changing in parallel, and thus their ratio, the reabsorbate concentration being relatively invariant. Active NaCl transport can cause osmotic water flow by reducing the luminal fluid osmolarity. In the presence of passive forces the luminal fluid can be hypertonic to plasma, and active NaCl transport can still exert its osmotic effect on volume flow. There are two passive forces for volume flow: the Cl gradient and the difference in effective osmotic pressure; they have an approximately equivalent effect on volume flow. Experimentally, we have measured volume changes in a droplet made hyperosmotic by the addition of 50 mM NaCl; the experimental results are predicted reasonably well by our theoretical model.