bluuebunny/crossref_metadata_2025 huggingface dataset:
Variations in contractile properties of rabbit single muscle fibres in relation to troponin T isoforms and myosin light chains.
Authors: Greaser M L , Moss R L , Reiser P J
Published: December 2014

1. The maximal velocity of shortening (Vmax), tension‐pCa relationships and the contractile and regulatory protein composition were determined in single, chemically skinned fibres from adult rabbit plantaris muscles. 2. Three groups of fibres were identified based on their protein compositions. One group had exclusively the slow‐type myosin heavy chain (MHC) and myosin light chains (LC) and had low velocities. Another group of fibres had mixtures of fast‐type and slow‐type MHCs and LCs and had intermediate shortening velocities. The third group of fibres had fast‐type myosin heavy and light chains and high velocities. 3. The low‐velocity fibres had a mean velocity (+/‐ S.E.M.) of 0.86 +/‐ 0.03 muscle lengths/s (ML/s) at 15 degrees C. The remaining fibres formed a continuum with respect to Vmax from 1.37 to 3.94 ML/s. These results indicate that a much greater diversity exists among single fibres from adult mammalian skeletal muscle than previously recognized. The intermediate‐ and high‐velocity fibres formed a continuum (from slow to fast) with respect to the amount of myosin light chain 3 (LC3). That is, Vmax increased with the relative LC3 content in single fibres in the intermediate‐ and high‐velocity groups in a quantitative, statistically significant manner. 4. Three isoforms of fast‐type troponin T were identified among the intermediate‐ and high‐velocity fibres. These fibres also contained fast‐type troponin C and troponin I. As was the case with the relative LC3 content, these fibres also formed a continuum with respect to the relative proportions of the three isoforms of fast‐type troponin T. It appears that different isoforms of troponin T are responsible for a slightly higher Ca2+ sensitivity of tension development in the high‐velocity fibres compared to the intermediate fibres. The continuum in troponin T isoform composition paralleled an increase in Vmax among these fibres. 5. The low‐velocity fibres had the highest Ca2+ sensitivity of the three groups and had exclusively the slow‐type isoforms of the regulatory proteins in the troponin complex. 6. The co‐ordinated variations in troponin T and LC3 compositions among the intermediate‐ and high‐velocity fibres are discussed as a possible means for the further differentiation of the contractile properties of the fibres in these two groups, beyond that provided by myosin heavy chain isoforms alone.
Metabolic flux of cyclic GMP and phototransduction in rabbit retina.
Authors: Ames A , Barad M
Published: December 2014

1. Rabbit retinas were isolated and subjected in vitro to shifts between light and darkness in the presence or absence of four concentrations of the phosphodiesterase inhibitor 3‐isobutyl‐1‐methylxanthine (IBMX). Changes in the rate of cyclic GMP hydrolysis (determined by 18O labelling of guanine nucleotide alpha‐phosphoryls) and in total cyclic GMP content (determined by radioimmunoassay) were compared with the changes in the electrical potential across the retina. The experiments were designed so that the changes in potential would reflect changes in the light‐sensitive conductance of the photoreceptors. 2. IBMX at 27‐730 microM caused dose‐related reductions in cyclic GMP hydrolysis in both light and darkness. The reductions in hydrolysis were associated with almost equal reductions in synthesis, so that there was little increase in the total content of cyclic GMP despite large changes in its metabolic flux. 3. Shifting from light (2.3 x 10(3) photons microns‐2 s‐1) to darkness also caused large reductions in the metabolic flux of cyclic GMP, with little increase in its total content. 4. Reductions in cyclic GMP flux were always associated with increases in the vitreous‐positive transretinal potential, which was used as a measure of photoreceptor outer segment conductance, and the inverse correlation between flux and potential was closely maintained (r = 0.98) under all conditions examined. The correlation between total cyclic GMP content and transretinal potential was much less close. 5. Since IBMX and darkness acted similarly and additively, the combination of IBMX and darkness caused large decreases, of up to 21‐fold, in cyclic GMP flux and large increases, of up to 23‐fold, in the transretinal potential. 6. Kinetic analysis of the data indicated that the great majority (about 95%) of the light‐sensitive conductance was closed under physiological conditions in darkness. 7. The data appear to be consistent with a system in which much of the cyclic GMP is bound, in which the binding is increased by light, and in which the free cyclic GMP acts co‐operatively with a Hill coefficient of 3 to open outer segment conductance and to inhibit guanylate cyclase.
Agonist‐induced changes in cell membrane capacitance and conductance in dialysed pancreatic acinar cells of rats.
Authors: Maruyama Y
Published: December 2014

1. Single acinar cells enzymatically isolated from the rat pancreas were subjected to tight‐seal whole‐cell recordings. Changes in cell membrane capacitance and conductance were simultaneously recorded using a phase‐sensitive detection method. 2. Acetylcholine (ACh, 0.05‐0.5 microM) and cholecystokinin octapeptide (CCK‐8, 10‐50 pM) concomitantly induced transient increases in cell membrane current, capacitance and conductance only when cytosolic Ca2+ was weakly chelated by EGTA (70 microM). These responses were prolonged when the cells were dialysed with a solution containing GTP gamma S (a stable analogue of GTP, 50‐100 microM), whereas they were inhibited by dialysing with that containing GDP beta S (a stable analogue of GDP). These results suggest that a type of guanine‐nucleotide‐binding protein (G‐protein) could be involved in ACh‐ or CCK‐receptor signalling. 3. The ACh‐ or CCK‐induced responses (with or without GTP gamma S in the cytosol) were all abolished when a high dose of EGTA (1‐2 mM) was injected into the acinar cells. In addition, A23187, a calcium ionophore, induced sustained responses when the cytosolic Ca2+ was weakly buffered by 70 microM‐EGTA. These results suggest that the secretagogues regulate the changes in cell membrane capacitance and conductance via an increase and decrease of cytosolic Ca2+ concentration. 4. Oscillatory changes in cell membrane conductance and capacitance were consistently observed even without applying secretagogues when the cells were dialysed with a solution containing GTP gamma S (50‐100 microM) and cytosolic free Ca2+ ions weakly buffered at about 10(‐6) M with a low dose of EGTA and CaCl2. 5. The peak amplitude of changes in cell membrane capacitance induced by ACh or CCK‐8, with or without GTP gamma S in the cytosol, varied between 200 and 1000 fF, thereby suggesting that 20‐100 zymogen granules can fuse with the luminal cell membrane in response to these agonists in rat exocrine pancreatic acinar cells.
Calcium transients evoked by electrical stimulation of smooth muscle from guinea‐pig ileum recorded by the use of Fura‐2.
Authors: Ito Y , Kuriyama H , Parker I
Published: December 2014

1. Intracellular free calcium levels were recorded in strips of longitudinal smooth muscle from guinea‐pig ileum, by the use of the fluorescent calcium indicator Fura‐2. 2. The resting intracellular free calcium concentration was estimated to be 210 nM. Many muscle strips showed spontaneous bursts of contractions, accompanied by bursts of calcium transients. Following these the calcium level often fell transiently below the resting level. The spontaneous transients were unaffected by tetrodotoxin (TTX) and atropine. 3. Field electrical stimulation of muscle strips evoked a series of calcium transients comprising: (i) an initial rise in free calcium, reaching a peak within 20‐30 ms of stimulation, (ii) a second rise in calcium, beginning after a few hundred milliseconds, and finally (iii) a decline in calcium to below the resting level, persisting for a few seconds. The mean peak increase in free calcium above the resting level during components (i) and (ii) was, respectively, 130 and 200 nM. The mean decrease in free calcium during the third component was to 20 nM below the resting level. 4. The short‐latency calcium transient required relatively long stimuli for activation, and was not blocked by TTX and atropine. The long‐latency transient was selectively activated by brief stimuli, and was abolished by TTX and atropine. Thus, the short‐latency component probably arose because of direct electrical stimulation of muscle fibres, while the long‐latency component was due to stimulation of muscarinic nerves. 5. The first detectable increase in tension began about 100 ms after the peak of the initial calcium transient. Contractions associated with the long‐latency calcium transient were much larger than those associated with the short‐latency transient, even in muscle strips where the calcium levels were similar for both transients. 6. Removal of calcium in the bathing solution caused the resting intracellular calcium level to fall, following an initial rise accompanied by increased spontaneous transients. Electrically evoked contractions and calcium transients were abolished in calcium‐free solution, and by the addition of verapamil or diltiazem to normal Krebs solution.
Somatostatin induces an inward rectification in rat locus coeruleus neurones through a pertussis toxin‐sensitive mechanism.
Authors: Inoue M , Nakajima S , Nakajima Y
Published: December 2014

1. Membrane properties and somatostatin effects were studied in cultured locus coeruleus neurones from neonatal rats by using the whole‐cell version of the patch clamp technique. 2. The current‐voltage relationship of the resting cell revealed an inward‐going rectification. The inward currents developed almost instantaneously upon hyperpolarizing the membrane under voltage clamp, and at large negative potentials the inward current showed a time‐dependent inactivation. Extracellularly applied Cs+ or Ba2+ (0.1 mM) inhibited the inward current in a voltage‐dependent manner. 3. Application of somatostatin (0.01‐1 microM) produced an increase in membrane conductance. Somatostatin‐induced currents were calculated by subtracting the control current from the current during the somatostatin‐induced response. The somatostatin‐induced current developed almost instantaneously with hyperpolarization and did not show any time‐dependent inactivation. The current‐voltage relationship of the somatostatin‐induced current exhibited a rectification in the inward direction and showed a reversal potential. The reversal potentials were close to the K+ equilibrium potential. 4. Extracellular Cs+ or Ba2+ (0.1 mM) inhibited the somatostatin‐induced currents in a voltage‐dependent manner, the effectiveness increasing with hyperpolarization. The somatostatin‐induced hyperpolarization was not affected by apamin (20 nM) or by charybdotoxin (100 nM). 5. These results indicate that the somatostatin‐induced conductance is very similar to the inward‐rectification conductance. Because the somatostatin‐induced inward rectification did not exhibit a time‐dependent inactivation, this rectification and the inward rectification in the control neurones may arise from two different channels. 6. Pre‐treatment of neurones with pertussis toxin abolished the somatostatin‐induced response, but did not affect the resting inward rectification. When GTP gamma S was applied intracellularly, somatostatin produced an irreversible activation of the inward rectification conductance. The somatostatin‐induced hyperpolarization may therefore be mediated through a pertussis toxin‐sensitive GTP‐binding protein.
Heterogeneous calcium and adenosine triphosphate distribution in calcium‐permeabilized human red cells.
Authors: García‐Sancho J , Lew V L
Published: December 2014

1. Calcium permeabilization of inosine‐fed human red cells using the divalent cation ionophore A23187 induces pump‐leak steady states in which the mean total calcium content of the cells may be held below electrochemical equilibrium for hours. A new method developed to detect and separate cells with different calcium contents revealed a striking heterogeneity of calcium contents in subpopulations of cells in pump‐leak steady state (García‐Sancho & Lew, 1988a). Most of the mean total cell calcium was found within a fraction of cells rendered dense by the separation procedure (H cells), with relatively little within the remaining light cells (L cells). The experiments in this paper were designed to study the nature and origin of the observed heterogeneity. 2. The fraction of steady‐state H cells increased, and the mean ATP content of the cells fell, both linearly, as calcium influx was increased. The H/L divide is therefore the result of a continuous variation in cell properties. When calcium influx was above about 30 mmol/(l cells.h), all cells became dense, calcium distribution was at or near equilibrium, and cell ATP was 0.1‐0.2 mmol/l cells. 3. Inosine‐fed cells, subjected to ionophore‐mediated net calcium influx of 13‐15 mmol/(l cells.h), attained a steady state with mean calcium contents far below equilibrium. After ionophore removal and reincubation in calcium‐free media, the initial calcium efflux was only a fraction of that required to sustain the previous steady state (less than 25% for H cells, and less than 2% for L cells). The ATP content of L cells was normal whereas that of H cells was irreversibly reduced. These results revealed a paradoxical discrepancy between leak influx and calcium pump efflux in H and L cells which were supposed to have been in steady‐state pump‐leak balance. 4. The changes in cell calcium and ATP were followed in time after calcium permeabilization to characterize the development of steady‐state heterogeneity. Calcium influx triggered a sharp peak in the H cell fraction within 15 s of permeabilization. The mean calcium content of H cells increased towards steady‐state values as their fraction decreased; most other cells transferred from H to L density fractions (HL cells) within the first 5 min of permeabilization. 5. In substrate‐starved cells calcium influx triggered an immediate fall in cell ATP, steeper in H cells than in L cells. The initial calcium and density transients were unattected.(ABSTRACT TRUNCATED AT 400 WORDS)
Properties of the residual calcium pools in human red cells exposed to transient calcium loads.
Authors: García‐Sancho J , Lew V L
Published: December 2014

1. Inosine‐fed human red cells, pre‐loaded with calcium with the use of the ionophore A23187, and reincubated at 37 degrees C after ionophore and external calcium removal, pumped calcium out initially fast and then substantially slower when the residual calcium was still very high (García‐Sancho & Lew, 1988 a, b). Particularly surprising was the finding of such residual calcium pools in a subpopulation of cells (L cells, García‐Sancho & Lew, 1988b) with normal ATP, high active calcium pumping rates, and K+ channels which remained inactive during the density separation procedure (SCN‐ treatment; García‐Sancho & Lew, 1988a). The purpose of the present experiments was to investigate the properties and possible origins of the residual calcium pools. 2. At each ionophore concentration, the pool size increased with the duration and magnitude of the calcium load. At comparable calcium loads, the pool size was smaller the higher the ionophore concentration used. 3. In cells from the same sample, residual calcium pools were present in cells that became dense after SCN‐ treatment (H cells) as well as those that remained in the light‐cell fraction (L cells). Residual calcium was always higher in H cells than in L cells. 4. Re‐exposure of cells to ionophore in Ca2+‐free media could rapidly extract over 99% of their residual calcium. Residual calcium is therefore in a rapidly mobilizable form within a membrane‐bound compartment. 5. Iodoacetamide‐induced ATP depletion of H cells with high residual calcium‐stimulated calcium loss. Such stimulation could only occur if ATP depletion inhibited a calcium‐retaining process within the cells. 6. L cells with high residual calcium may have failed to dehydrate during SCN‐ treatment because of irreversible K+ channel inactivation or because K+ permeabilization would no longer generate a dehydrating net cation loss. These possibilities were tested and ruled out since it was found that virtually all cells became dense in low‐K+, SCN‐ media after valinomycin addition or re‐exposure to A23187 + calcium. 7. The results suggest that part if not all of the residual calcium is contained within compartments with the properties of endocytic inside‐out vesicles capable of ATP‐dependent calcium accumulation, such as those found in normal and abnormal human red cells (Lew, Hockaday, Sepúlveda, Somlyo, Somlyo, Ortiz & Bookchin, 1985).
Maximal calcium extrusion capacity and stoichiometry of the human red cell calcium pump.
Authors: Dagher G , Lew V L
Published: December 2014

1. The uphill calcium efflux through calcium‐saturated pumps in intact red cells was investigated with the aid of a new method, in initial conditions of uniform ionophore A23187‐induced calcium distribution among the cells. The method is based on findings by Tiffert, García‐Sancho & Lew (1984) which show that cobalt can suddenly arrest passive calcium transport by the ionophore and expose, without noticeable interference, uphill calcium extrusion by the pump. The results comprise methodological aspects and questions concerning interactions between inner pump sites, ATP and Ca2+, and the calcium: ATP stoichiometry of the calcium‐saturated pump. 2. Ionophore‐induced calcium influx was set to be far in excess of the maximal calcium pump capacity. This secured a uniform calcium distribution among the cells, and Ca2+ equilibration by 2 min or less of calcium permeabilization. Cobalt was added between 15 s and 5 min after ionophore addition. The calcium and ATP content of the cells was followed during ionophore‐induced influx and cobalt‐exposed efflux. 3. The external cobalt concentrations required to block completely ionophore‐mediated calcium transport were similar or only marginally higher than those of calcium. 4. The reproducibility of independent cobalt‐exposed calcium efflux measurements from single blood samples was within an 8% range. 5. During cobalt‐exposed calcium efflux, the calcium content of subpopulations of cells, with and without active Ca2+‐sensitive K+ channels, investigated by post‐incubation of samples in low‐K+, thiocyanate (SCN‐) media (modified from García‐Sancho & Lew, 1988a), was similar. This is consistent with the maintenance of a uniform calcium distribution among the cells during uphill calcium extrusion. 6. Cobalt‐exposed calcium efflux was similar in the interval from 15 s to 5 min after calcium permeabilization although cell ATP levels had fallen by over 50% in that period. Therefore, cell ATP concentrations within the physiological range do not seem to be regulatory for calcium‐saturated pumps in the intact red cell. 7. All cobalt‐exposed calcium efflux curves were linear in time, at least until total cell calcium contents reached levels below 100 mumol/l cells. This suggests that internal calcium is not inhibitory for calcium‐saturated efflux in intact cells in the 0.1‐1 mmol/l cells range. 8. The cobalt‐exposed calcium fluxes were in the range from 4 to 24 mmol/(1 cells.h) for fresh cells and from 10 to 18 mmol/1 cells. h) for samples from the Blood Bank.(ABSTRACT TRUNCATED AT 400 WORDS)
Temperature and synaptic efficacy in frog skeletal muscle.
Authors: Adams B A
Published: December 2014

1. Intracellular recording and voltage‐clamp techniques were used to measure synaptic efficacy and the safety factor for neuromuscular transmission in frog skeletal muscle. All measurements were made in normal Ringer solution, in the absence of presynaptic or postsynaptic blocking agents. 2. Over a broad temperature range (10‐30 degrees C), a small percentage of sartorius fibres (about 6%) could be found which produced only subthreshold end‐plate potentials and no action potential in response to single, supramaximal nerve shock. At lower temperatures the proportion of such fibres increased; 42% of the fibres had subthreshold transmission at 5 degrees C, and 59% were subthreshold at 2.5 degrees C. 3. Threshold current, measured by intracellularly injecting short pulses of depolarizing current at end‐plate regions, was independent of temperature between 2.5 and 20 degrees C. Thus, the reduced synaptic efficacy observed at low temperatures was not due to decreased electrical excitability of the postsynaptic membrane. 4. The amplitude of evoked end‐plate currents (EPCs) decreased with cooling. At temperatures below 10 degrees C, the evoked EPCs at many end‐plates were too small to initiate action potentials. The decline in EPC amplitude was due to three factors: a decrease in the amplitude of single quantum currents (MEPCs), an increase in the temporal dispersion of transmitter release, and (below 5 degrees C) a decrease in quantal content. 5. The safety factor for neuromuscular transmission decreased dramatically as temperature was lowered. At 30 degrees C average safety factor was large and positive (540 nA), but at 2.5 degrees C it was negative (‐78 nA). 6. The quantal content of evoked transmitter release was independent of temperature change between 5 and 30 degrees C, the average value over this range being 180. However, at temperatures below 5 degrees C, quantal content fell off sharply (average value = 37). 7. The thermal independence of transmitter release may be an important mechanism in allowing poikilothermic animals to maintain physiological function over a wide range of body temperatures.
Electrophysiological responses to somatostatin of rat hypophysial cells in somatotroph‐enriched primary cultures.
Authors: Chen C , Israel J M , Vincent J D
Published: December 2014

1. Somatotroph cells were obtained from pituitaries of adult male rats by dissociation, separation and enrichment on a continuous gradient of bovine serum albumin at unit gravity. They were kept in culture for 7‐15 days before electrophysiological experiments. 2. Immunofluorescent staining of the resulting gradient fractions (numbered F2 to F9) indicated that the majority of somatotrophs (75‐85%) were located in the heavy fractions (F8 and F9). However, a small percentage (15‐20%) of cells in these fractions were identified as lactotrophs. 3. Perifusion experiments indicated that on the one hand release of growth hormone from somatotroph‐enriched fractions was stable at the level of 6 ng (2 min)‐1 (10(6) cells)‐1 and was markedly inhibited by somatostatin (1.9 ng (2 min)‐1 (10(6) cells)‐1) but not by dopamine. On the other hand, in the same cell preparations, basal prolactin release (1.6 ng (2 min)‐1 (10(6) cells)‐1) was significantly reduced by dopamine (0.08 ng (2 min)‐1 (10(6) cells)‐1) but remained unchanged by somatostatin treatment. 4. The inhibitory effect of somatostatin on growth hormone release was dose dependent. This effect was not abolished by tetraethylammonium (40 mM) or 4‐aminopyridine (5 mM), but somatostatin decreased high‐potassium‐induced release. 5. In all the cells recorded (n = 187), 14% (n = 26) displayed a low resting potential (less than ‐30 mV) and poor membrane resistance (less than 50 M omega). The recording was unstable and resting potentials decreased regularly to 0 mV in less than 5 min. The other 86% of the cells displayed resting potentials varying from ‐45 to ‐65 mV and had a membrane resistance of more than 150 M omega. Only cells which displayed these membrane characteristics showed clear responses to somatostatin or dopamine, and were therefore chosen for experiments. 6. In all the cells selected for the experiments (n = 161), 78% (n = 126) showed either triggered or spontaneous action potentials. The action potentials remained insensitive to sodium‐free bath solution, but were reversibly blocked by the calcium channel blockers cobalt (5 mM) or nickel (5 mM). 7. When the cells were at resting potential, somatostatin induced a hyperpolarizing response associated with a decrease of membrane resistance. During this response, spontaneous or triggered action potentials were inhibited. The hyperpolarizing response induced by somatostatin was dose‐dependent.(ABSTRACT TRUNCATED AT 400 WORDS)
Enterocyte‐lymphocyte interactions in the follicle‐associated epithelium of the mouse Peyer's patch.
Authors: Cremaschi D , James P S , Rossetti C , Smith M W
Published: December 2014

1. Measurements of membrane potential (Vm) have been carried out in the follicle‐associated epithelium of the mouse Peyer's patch to investigate possible site‐dependent anomalies in enterocyte development. Initial heterogeneity in Vm values obtained from carrying out random impalements in the upper third of the follicle‐associated epithelium could be described as arising from the presence of three different cell populations. 2. The predominant cell type in this epithelium (T1) had a mean Vm of ‐35.3 mV which was partly depolarized by increasing concentrations of K+. These cells were not stained with the vital dye Neutral Red. Two other types of cell (T2L and T2H) were recorded together as consecutive negative jumps in Vm, but only in areas of the epithelium previously stained with Neutral Red. T2L behaved like T1 cells to K+ but with a low Vm of ‐23.3 mV. T2H cells had a high mean Vm of ‐47.5 mV showing transient hyperpolarization to increasing K+ concentration. 3. The electrophysiological and non‐staining properties of T1 cells are similar to those expected from mature enterocytes. The anatomical size, site, staining properties and Vm of T2H cells correspond to those expected for intraepithelial lymphocytes. The low Vm, K+ depolarization and close association of T2L with T2H cells are typical of properties predicted for an immature‐type of antigen‐transporting enterocyte known to be present in this type of epithelium. 4. The possibility that intraepithelial lymphocytes either slow or reverse a more normal process of development of enterocytes in their immediate vicinity is discussed along with wider aspects of enterocyte‐lymphocyte interactions leading to the transfer to enteric antigens across these modified cells.
Intracisternal naloxone and cardiac nerve blockade prevent vasodilatation during simulated haemorrhage in awake rabbits.
Authors: Evans R G , Ludbrook J , Potocnik S J
Published: December 2014

1. Acute haemorrhage was simulated in five unanaesthetized rabbits, by inflating a cuff on the inferior vena cava so that cardiac output fell by 8.3% of its resting level per minute. Simulated haemorrhage was performed after sham treatment, after graded doses of intravenous and intracisternal naloxone, and after cardiac nerve blockade with intrapericardial procaine. 2. After sham treatment, the haemodynamic response to simulated haemorrhage was biphasic. During the first phase, systemic vascular conductance fell steadily, heart rate rose steadily, and arterial pressure fell only slightly. A second decompensatory phase began abruptly when cardiac output had fallen to approximately 55% of its resting level. Vascular conductance rose steeply, heart rate fell slowly, and arterial pressure fell precipitately. 3. Treatment with naloxone (intravenous, 0.04‐0.4 mg kg‐1; intracisternal, 0.2‐2 micrograms kg‐1) did not affect either phase of the haemodynamic response to simulated haemorrhage. 4. After treatment with larger doses of naloxone (intravenous, 4‐8 mg kg‐1; intracisternal, 4‐69 micrograms kg‐1), the first phase was unaffected, but the second phase no longer occurred. Throughout simulated haemorrhage, systemic vascular conductance fell steadily, heart rate rose, and arterial pressure was well maintained. The dose of intracisternal naloxone which prevented the second phase was 90‐900 times less than the corresponding intravenous dose. The second phase was also prevented by cardiac nerve blockade. 5. We conclude that an endogenous opiate mechanism is responsible for the haemodynamic decompensation that occurs when cardiac output falls to a critical level. The mechanism is located within the central nervous system. It is triggered by a signal from the heart.
Multiple‐site optical monitoring of neural activity evoked by vagus nerve stimulation in the embryonic chick brain stem.
Authors: Kamino K , Katoh Y , Komuro H , Sato K
Published: December 2014

1. Electrical activity in the embryonic chick brain stem has been monitored optically. The vagus‐brain stem preparations isolated from 7‐day‐old chick embryos were stained with voltage‐sensitive merocyanine‐rhodanine dyes. 2. Voltage‐related optical absorption signals evoked by vagus nerve stimulation with depolarizing and hyperpolarizing pulses using a suction electrode were recorded simultaneously from 127 adjacent loci in the brain stem using a 12 x 12‐element photodiode array. 3. The optical signals evoked by the stimulation appeared to be concentrated longitudinally in the central region and in the lateral region, both on the stimulated side of the brain stem, and they did not spread to the opposite side. In addition, the evoked optical responses were detected from small areas on the dorsal surface of the stimulated side, in experiments using transverse slices of brain stem. 4. The optical action potential signals evoked by the brief depolarizing stimulus were conducted slowly and were blocked completely by tetrodotoxin. With relatively long‐duration depolarizing and hyperpolarizing stimulations, electrotonic responses were recorded. 5. When 2 microA/2 ms hyperpolarizing pulse stimulations were applied, anode‐break excitation signals were detected, and these signals were also blocked by tetrodotoxin. 6. On the basis of the data obtained from these experiments, we constructed maps of the electrical response area and demonstrated the spatial pattern of the vagus dorsal nucleus in the 7‐day‐old embryonic chick brain stem.
Two types of nicotinic acetylcholine receptor channels at slow fibre end‐plates of the garter snake.
Authors: Dionne V E
Published: December 2014

1. Two different types of acetylcholine receptor channels can be detected on the post‐junctional membrane of slow muscle fibres in garter snakes. Here they are designated T‐type and S‐type channels. Only T‐type channels can also be found at twitch fibre neuromuscular junctions. 2. The physiological properties of slow fibre T‐type channels are similar to those of acetylcholine receptor channels in end‐plates of twitch fibres in these animals. 3. S‐type channels had a smaller conductance than T‐type channels (32 vs. 49 pS), but a similar reversal potential near 0 mV. 4. Both S‐ and T‐type channels were found together in most patches of slow fibre end‐plate membrane, but some patches displayed just one type or the other. 5. The activity of both S‐ and T‐type channels desensitized in the presence of micromolar concentrations of acetylcholine. S‐type channels desensitized less rapidly and less completely than did T‐type channels. 6. Desensitized channels of both types recovered and produced bursts of activity, then went silent again. During a burst, channels did not appear to change type. 7. The activation of channels of either type was not correlated with activity in channels of the other type. 8. The open‐duration distribution of S‐type channels required two exponential components to be well fitted, with time constants in the range of 1‐2 ms and 3‐10 ms. In contrast, the open‐duration distribution of T‐type channels was a single exponential with a time constant similar in magnitude to the slower S‐type component. 9. Desensitization‐resistant S‐type acetylcholine receptor channels could allow slow muscle end‐plates to retain their sensitivity to acetylcholine during periods of heavy use. Under non‐desensitizing conditions, differences in the decay properties of slow fibre end‐plate currents compared to those in twitch fibres can be explained by the activation kinetics of S‐type channels.
Sodium‐dependent calcium extrusion and sensitivity regulation in retinal cones of the salamander.
Authors: Nakatani K , Yau K W
Published: December 2014

1. Membrane current was recorded from an isolated, dark‐adapted salamander cone by sucking its inner segment into a tight‐fitting glass pipette containing Ringer solution. The outer segment of the cell was exposed to a bath solution that could be changed rapidly. 2. After removing Na+ from the bath Ringer solution for a short period of time in darkness (the ‘loading period’), a transient inward current was observed upon restoring it in bright light. A similar but longer‐lasting current was observed when Na+ was restored in the light after a large Ca2+ influx was induced through the light‐sensitive conductance in darkness. 3. The above transient current was not observed if Li+ or guanidinium was substituted for Na+ in the light, or if Ba2+ was substituted for Ca2+ during the dark loading period. However, a current was observed if Sr2+ was the substituting ion for Ca2+ during loading. These observations suggested that the current was associated with an electrogenic Na+‐dependent Ca2+ efflux at the cone outer segment. 4. The saturated amplitude of the exchange current was 12‐25 pA with a mean around 16 pA. This is very comparable to that measured in the outer segment of a salamander rod under similar conditions. 5. By comparing a known Ca2+ load in a cone outer segment to the subsequent charge transfer through the exchange, we estimated that the stoichiometry of the exchange was near 3Na+:1Ca2+. 6. With a small Ca2+ load, or in the presence of Cs+ around the inner segment, the final temporal decline of the Na+‐Ca2+ exchange current was roughly exponential, with a mean time constant of about 100 ms. This decline is about four times faster than that measured in rods. We interpret the shorter time constant in cones to reflect a faster rate of decline of intracellular free Ca2+ in their outer segments resulting from the exchange activity. 7. In the absence of external Na+, and hence any Na+‐dependent Ca2+ efflux, the absolute sensitivity of a cone to a dim flash was several times higher than in normal Ringer solution. 8. A roughly similar increase in light sensitivity was observed for a rod under the same conditions. 9. We conclude that the Na+‐dependent Ca2+ efflux, through lowering intracellular free Ca2+ in the light, has a role in regulating the absolute light sensitivity in cones as it does in rods.(ABSTRACT TRUNCATED AT 400 WORDS)
Variation in the normalized tetanic force of single frog muscle fibres.
Authors: Elzinga G , Howarth J V , Rall J A , Wilson M G , Woledge R C
Published: December 2014

1. The forces produced in maximal fixed‐end tetani of single fibres isolated from the anterior tibialis muscle of the frog Rana temporaria have been measured at sarcomere lengths of 2.2 microns and temperatures near 0 and 10 degrees C. 2. When normalized by either cross‐sectional area or dry weight per unit length at a sarcomere length of 2.2 microns, the forces vary over a twofold range. 3. The normalized force is not significantly correlated with the velocity of unloaded shortening or the twitch characteristics of the fibres. Lack of variability of these two quantities (together with histochemical evidence) suggest that only one fibre type is present in the experimental sample. 4. The steady rate of energy liberation (stable, heart rate) of the fibres during isometric tetani is positively correlated with the normalized force, indicating that extra ATP splitting is required to produce higher forces. 5. Fibres with a higher ratio of dry weight per unit length to cross‐sectional area ('dry density') show a higher force when normalized by area, but not when normalized by dry weight per unit length. 6. Fibres with a more circular cross‐sectional profile produce more force when normalized by either cross‐sectional area or dry weight per unit length. The significance of this correlation is unclear. 7. The contribution of various sources to the total overall variation in normalized force is assessed. It is suggested that a diffusible substance or substances may be involved in modulating fibre force.
Measurement of reversal potential of Na+‐Ca2+ exchange current in single guinea‐pig ventricular cells.
Authors: Ehara T , Matsuoka S , Noma A
Published: December 2014

1. To identify the Na+‐ or Ca2+‐induced current as Na+‐Ca2+ exchange current and to determine the stoichiometry of the Na+‐Ca2+ exchange, the reversal potential was measured in a wide range of external Na+ [( Na+]o) or Ca2+ [( Ca2+]o) concentrations. The Na+‐ or Ca2+‐induced current was recorded in single ventricular cells enzymatically dispersed from guinea‐pig hearts, using the technique of whole‐cell voltage clamp combined with internal perfusion. 2. In the presence of 10‐40 mM‐Na+ and 55‐803 nM‐Ca2+ in the internal solution, an increase of [Ca2+]o from 0.1 to 0.5‐20 mM or an increase of [Na+]o from 30 to 50‐140 mM induced an extra current associated with an increase in membrane conductance. The reversal potential of these extra currents was determined from an intersection of the current‐voltage (I‐V) relations obtained in the absence and presence of a Na+‐Ca2+ exchange blocker, Ni2+ (2 mM). 3. Ba2+ in the external solution failed to induce the extra current, but inhibited the background conductance having a reversal potential at around 0 mV. Thus, 1 mM‐Ba2+ was added to all external solutions, so that a change in the background current was minimized during application of Ca2+ or Ni2+. 4. The relation between [Ca2+]o and amplitude of the Ca2+‐induced current was examined in the presence and absence of Ni2+. Lineweaver‐Burk analysis revealed that the action of Ni2+ on the extra current might be a mixed type of competitive and non‐competitive inhibition. 5. During the application of Ca2+, the Ca2+‐induced outward current decayed in a time‐dependent manner, resulting in a shift of the I‐V relations towards positive potentials. This current decay was inhibited by increasing the capacity of the internal Ca2+‐buffer, using BAPTA (1,2‐bis(o‐aminophenoxy)ethane‐N,N,N',N'‐tetraacetic acid) or higher concentrations of EGTA. The result indicates that [Ca2+]i, at least under the cell membrane, changes due to ion fluxes through the Na+‐Ca2+ exchange and that control of the ion concentrations within the cell is prerequisite for measuring the reversal potential of the Na+‐Ca2+ exchange. 6. The shift of both the holding current and the I‐V relations during stimulation of the exchange was suppressed, when the membrane potential was clamped at the equilibrium potential of 3Na+:1Ca2+ exchange.(ABSTRACT TRUNCATED AT 400 WORDS)
Transient calcium current of retinal bipolar cells of the mouse.
Authors: Kaneko A , Pinto L H , Tachibana M
Published: December 2014

1. Isolated bipolar cells were obtained by enzymic (papain) dissociation of the adult mouse retina. The membrane voltage was clamped and the membrane currents were measured by the whole‐cell version of the patch‐clamp technique. Isolated bipolar cells and horizontal cells of the goldfish retina were also studied for comparison. 2. Hyperpolarization from the holding voltage, Vh, of ‐46 mV evoked a slowly activating, Cs+‐sensitive, inward current (probably an h‐current), and depolarization evoked a TEA‐ and Cs+‐sensitive outward current (probably a combination of K+ currents). 3. Depolarization from a more negative Vh (e.g. ‐96 mV) evoked a transient inward current that had maximal amplitude between ‐40 and ‐20 mV. This current was identified as a Ca2+ current (ICa): its amplitude was increased with elevated [Ca2+]o and was decreased with reduced [Ca2+]o, and it was blocked by 4 mM‐Co2+, but not by 5 microM‐TTX. 4. Both the perikaryon and the axon terminal generated ICa with similar properties. 5. The plot of Ca2+ conductance (gCa) against membrane voltage (activation curve) was sigmoidal: in 10 mM [Ca2+]o, gCa increased for membrane voltages more positive than ‐65 mV, was half‐maximal at about ‐25 mV, and reached saturation at about +30 mV. The plot of inactivation of gCa against membrane voltage was also sigmoidal: with 1 s conditioning depolarization in 10 mM [Ca2+]o, gCa decreased for membrane voltages more positive than ‐80 mV, was half‐maximal at about ‐50 mV, and was fully suppressed for voltages greater than ‐30 mV. 6. ICa in the mouse bipolar cells was insensitive to 50 microM‐Cd2+, 10 microM‐nifedipine and 10 microM‐Bay K 8644. In contrast, the calcium currents of bipolar and horizontal cells of the goldfish retina were markedly suppressed by 50 microM‐Cd2+ and 10 microM‐nifedipine, and were augmented several fold by 10 microM‐Bay K 8644. The calcium currents of goldfish bipolar and horizontal cells were sustained, and were activated in a more positive range of potentials than the ICa of mouse bipolar cells. 7. The voltage range at which the ICa of mouse bipolar cells is activated includes the presumed range of membrane potentials spanned during light‐evoked responses; thus, this current may participate in synaptic transmission. The transient character of ICa may also help to shape transient responses of ganglion cells.
Transient, steady‐state and rebreathing responses to carbon dioxide in man, at rest and during light exercise.
Authors: Jacobi M S , Patil C P , Saunders K B
Published: December 2014

1. The transient ventilatory response to CO2, measured using short pulses at constant inflow, was compared with the steady‐state response at rest and during exercise at 50 W, and with the rebreathing response at rest, in nine healthy subjects. At rest CO2 was given at flow rates of 0.2 and 0.4 l min‐1 and during exercise, to compensate for the smaller inhaled CO2 fraction as ventilation increased, at flow rates of 0.4 and 0.8 l min‐1. 2. We calculated two indexes of gain for the transient response: the ratio of the peaks of the ventilation and PCO2 pulses, and the ratio of their integrals. 3. The steady‐state response was greater than the transient response at rest and during exercise, but there was no correlation between the two. The rebreathing response was greater than both. Both the transient and the steady‐state responses were greater during exercise than at rest. 4. To assess alinearity, the steady‐state responses to the two CO2 flow rates were compared. At rest, there was no significant difference. During exercise, the response was greater to 0.4 than 0.8 l min‐1, indicating alinearity concave downwards. 5. We conclude that the transient response as we calculate it is not representative of steady‐state gain, and that the CO2 response in light exercise is steeper, and concave downwards in shape. The rebreathing technique overestimates CO2 sensitivity near the control point.
The electrophysiological effects of neurotensin on neurones of guinea‐pig prevertebral sympathetic ganglia.
Authors: Stapelfeldt W H , Szurszewski J H
Published: December 2014

1. The membrane effects of neurotensin on neurons of guinea‐pig prevertebral ganglia were investigated by means of intracellular recording techniques in vitro. 2. Neurotensin (2‐5 microM) applied by superfusion caused depolarizing responses in fifty‐seven of seventy‐four neurones tested in the inferior mesenteric ganglion and thirty‐seven of forty‐seven neurones tested in the coeliac plexus. The remaining neurones tested showed no membrane response. 3. Responses to neurotensin could be discriminated into two different types of membrane depolarizations on the basis of their different time courses and pharmacological characteristics: a steady‐state type of depolarization and a transient type of depolarization. Seven of fifty‐seven responsive neurones tested in the inferior mesenteric ganglion and ten of thirty‐seven responsive neurones tested in the coeliac plexus responded to neurotensin with a depolarization which was maintained constant as long as neurotensin was superfused over the preparation (steady‐state type). Forty‐eight of fifty‐seven responsive neurones tested in the inferior mesenteric ganglion and twenty of thirty‐seven responsive neurones tested in the coeliac plexus responded with a transient depolarization which was followed by a repolarization in the maintained presence of neurotensin (transient type). A combination of both types of responses was observed in two neurones tested in the inferior mesenteric ganglion and in seven neurones tested in the coeliac plexus. 4. Steady‐state type responses were characterized by a slowly developing membrane depolarization which reached a plateau and lasted throughout the presence of neurotensin. Amplitude and time course of this response were not altered in a solution containing hexamethonium (10 microM) and atropine (10 microM) or by a solution low in calcium (1 mM) and high in magnesium (15 mM). 5. Transient type depolarizations evoked by neurotensin were faster in reaching their maximum and were followed by a repolarization during the maintained presence of neurotensin. Responses similar in time course and amplitude were obtained in solutions containing hexamethonium (10‐100 microM) and atropine (10 microM). However, transient responses were abolished in a solution low in calcium (1 mM) and high in magnesium (15 mM) and were markedly attenuated in ganglia treated with capsaicin (3 microM). 6. Both types of depolarizations were associated with increases in membrane input resistance. Both responses converted subthreshold depolarizing electrotonic potentials and subthreshold fast EPSPs to action potentials. 7. Both types of depolarizations were observed when the C‐terminal hexapeptide fragment neurotensin 8‐13 was used.(ABSTRACT TRUNCATED AT 400 WORDS)
Tetrodotoxin block of single germitrine‐activated sodium channels in cultured rat cardiac cells.
Authors: Dugas M , Honerjäger P , Masslich U
Published: December 2014

1. The open time of single Na+ channels in excised (outside‐out) patches from cultured late‐fetal rat ventricular myocytes was prolonged to several minutes by germitrine (0.5 mM) in order to analyse tetrodotoxin (TTX) blocking kinetics. 2. The germitrine modification appeared during depolarizing pulses that activated normal Na+ channels. Following repolarization to ‐100 mV, the modified Na+ channel remained activated for 136 +/‐ 186 s (mean +/‐ S.D., n = 54) with an open‐channel current amplitude of ‐0.5 pA. The predominant open state with a mean open time of 0.13 s was interrupted by brief closing events lasting for milliseconds. Replacing extracellular Na+ by Cs+ decreased the current amplitude to ‐0.1 pA. 3. Extracellular superfusion with TTX (3 x 10(‐7) M) of a single germitrine‐activated Na+ channel induced full channel closures lasting seconds (blocked events) separated by channel reopenings (unblocked events) that were indistinguishable in terms of amplitude and gating kinetics from the germitrine‐activated state in the absence of TTX. 4. Cumulative probability histograms of blocked and unblocked events (n greater than 140) collected during long‐lasting germitrine modifications at 10(‐7) and 3 x 10(‐7) M‐TTX are well described by single exponentials. The 3‐fold increase in [TTX] decreased the time constant of the unblocked state, tau o, from 11.9 to 4.7 s, while the time constant of the blocked state, tau c, was not significantly altered from 8.6 to 9.7 s. A microscopic association rate constant of 7.7 x 10(5) M‐1 s‐1, dissociation rate constant of 0.11 s‐1, and equilibrium dissociation constant of 1.4 x 10(‐7) M (at ‐100 mV) were calculated (20 degrees C). 5. Increasing [TTX] to 10(‐5) M decreased tau o to 86 ms. This argues against the existence of a slower conformational step interposed between the binding of TTX to an open channel and the resultant channel closure. 6. Setting the membrane potential to ‐50 or 0 mV subsequent to a germitrine modification at ‐100 mV did not significantly alter TTX (3 x 10(‐7) M) blocking kinetics: tau o was 6.7 s at ‐50 mV and 5.2 s at 0 mV; tau c was 8.9 and 8.1 s, respectively. 7. These results suggest that blocked events correspond to the random times that a TTX molecule resides on the Na+ channel before it dissociates, and unblocked events correspond to the random waiting times of an unoccupied channel before it binds another toxin molecule.(ABSTRACT TRUNCATED AT 400 WORDS)