Search

Injection of NMDAR antagonist into the thalamus can produce delta frequency EEG oscillations in the thalamocortical system. It is surprising that an antagonist of an excitatory neurotransmitter should trigger such activity, and the mechanism is unknown. One hypothesis is that the antagonist blocks excitation of GABAergic cells, thus producing disinhibition. To test this hypothesis, we investigated the effect of NMDAR antagonist (APV) on cells of the nucleus reticularis (nRT) in rat brain slices, a thalamic nucleus that can serve as a pacemaker for thalamocortical delta oscillations and that is composed entirely of GABAergic neurons. We found, unexpectedly, that nRT cells are hyperpolarized by APV. This occurs because these cells have an unusual form of NMDAR (probably NR2C) that contributes inward current at resting potential in response to ambient glutamate. The hyperpolarization produced by APV is sufficient to deinactivate T-type calcium channels, and these trigger rhythmic bursting at delta frequency. The APV-induced delta frequency bursting is abolished by dopamine D2 receptor antagonist, indicating that dopamine and NMDAR antagonist work synergistically to stimulate delta frequency bursting. Our results have significant implications concerning the electrophysiological basis of schizophrenia and bring together the NMDAR hypofunction, dopamine, and GABA theories of the disease. Our results suggest that NMDAR hypofunction and dopamine work synergistically on the GABAergic cells of the nRT to generate the delta frequency EEG oscillations, a thalamocortical dysrhythmia (TCD) in the awake state that is an established abnormality in schizophrenia.

Abstract: The influence of angiotensin (Ang) (1-7) on potassium current (Kv) and resting potential of smooth muscle cells isolated from mesenteric artery of Sprague Dawley rats was investigated. Measurements of potassium current were performed using the whole cell configuration of pCLAMP. The results indicated that Ang (1-7) (10−9 M) increased the potassium current by 120% ± 2.6% (P < .05) and the resting potential of smooth muscle cells by 8 ± 2.8 mV (n = 23; P < .05). Ang II (10−9 M) administered to the bath reduced the potassium current by 35% ± 3.6% (n = 23; P < .05) and depolarized the arterial myocytes by 7.8 ± 2.1 mV (n = 25; P < .05). The effect of the heptapeptide on potassium current was inhibited by a Mas receptor inhibitor (A779; 10−8 M) as well as by a protein kinase A (PKA) inhibitor (10−9 M) dialyzed into the cell. Intracellular dialysis of the catalytic subunit of PKA (5 × 10−8 M) enhanced the potassium current by 38% ± 3.4% (n = 14; P < .05) but did not abolish the effect of Ang (1-7). On the other hand, Bis-1 (10−9 M), which is a specific inhibitor of PKC, suppressed the effect of Ang (1-7) on potassium current. In conclusion, Ang (1-7) counteracts the effect of Ang II on potassium current and membrane potential of smooth muscle cells from mesenteric arteries, which are resistance vessels involved in the regulation of peripheral resistance and blood pressure. The activation of the cAMP/PKA cascade is essential for the effect of the heptapeptide. Pathophysiological implications are discussed. [Copyright &y& Elsevier]

Abstract: Recently it has been shown that decreasing the extracellular pH of cells stimulates the formation of inward membrane invaginations and vesicles, accompanied by an enhanced uptake of macromolecules. This type of endocytosis was coined as proton-induced uptake (PIU). Though the initial induction of inward membrane curvature was rationalized in terms of proton-based increase of charge asymmetry across the membrane, the dependence of the phenomenon on plasma membrane characteristics is still unknown. The present study shows that depolarization of the membrane resting potential elevates PIU by 25%, while hyperpolarization attenuates it by 25%. Comparison of uptake in suspended and adherent cells implicates that the resting-potential affects PIU through remodeling the actin-cytoskeleton. The pH at the external interface of the cell membrane rather than the pH gradient across it determines the extent of PIU. PIU increases linearly upon temperature increase in the range of 4–36°C, in correlation with the membrane fluidity. The plasma membrane fluidity and the lipid phase order are modulated by enriching the cell's membrane with cholesterol, tergitol, dimethylsulfoxide, 6-ketocholestanol and phloretin and by cholesterol depletion. These treatments are shown to alter the extent of PIU and are better correlated with membrane fluidity than with the lipid phase order. We suggest that the lipid phase order and fluidity influence PIU by regulating the lipid order gradient across the perimeter of the lipid-condensed microdomains (rafts) and alter the characteristic tension line that separates the higher ordered lipid-domains from the lesser ordered ones. [Copyright &y& Elsevier]

In animal cells, the resting potential is established by the concentration gradients of sodium and potassium ions and the different permeabilities of the cell membrane to them. The large concentration gradients of sodium and potassium ions are maintained by the Na/K pump. Under physiological conditions, the pump transports three sodium ions out of and two potassium ions into the cell per ATP hydrolyzed. However, unlike other primary or secondary active transporters, the Na/K pump does not work at the equilibrium state, so the pumping ratio is not a thermodynamic property of the pump. In this article, I propose a dipole-charging model of the Na/K pump to prove that the three Na to two K pumping ratio of the Na/K pump is determined by the ratio of the ionic mobilities of potassium to sodium ions, which is to ensure the time constant τ and the τ-dependent processes, such as the normal working state of the Na/K pump and the propagation of an action potential. Further, the concentration ratios of potassium ions outside and inside the cell to sodium ions inside and outside the cell are 0.3027 and 0.9788, respectively, and the sum of the potassium and sodium equilibrium potentials is −30.3 mV. A comparative study on these constants is made for some marine, freshwater and terrestrial animals. These findings suggest that the pumping ratio of the Na/K pump and the ion concentration ratios play a role in the evolution of animal cells. [ABSTRACT FROM AUTHOR]

In the present study, a practical activity is proposed to adopt an experimental approach to demonstrate the relationship between the equilibrium potential for K+ and transmembrane electrical potential without glass micropipettes. A conventional setup for recording contractile activity of isolated smooth muscle preparations was used based on the events elegantly described by Somlyo and Somlyo in the 1960s. They showed that, in response to a given stimulus, smooth muscle cells may contract, recruiting electromechanical or pharmacomechanical coupling by mechanisms that involve, or not, changes in transmembrane potential, respectively. By means of contractions and relaxations of a ring-like preparation from the rat mesenteric artery, it is possible to observe the functional consequences of handling K+ concentration in the extracellular compartment and the effects caused by opening K+ channels in that preparation, which are significant when the cell membrane establishes an electrical potential difference between intra- and extracellular compartments (driven mainly by K+ permeability under resting conditions). The effects observed by students fit well with values predicted by Nernst and Goldman-Hodgin-Katz equations, and we demonstrated that the activity is able to improve students' comprehension regarding basic principles of bioelectricity. [ABSTRACT FROM AUTHOR]

Although it is generally thought that sensory transduction occurs at or close to peripheral nerve endings, with action potentials subsequently propagating along the axons of dorsal root ganglia (DRG) neurons toward the central nervous system, the small diameter of nociceptive axons and their endings have made it difficult to estimate their membrane properties and electrogenic characteristics. Even the resting potentials of nociceptive axons are unknown. In this study, we developed the capability to record directly with patch-clamp electrodes from the small-diameter distal axons of DRG neurons in vitro. We showed using current-clamp recordings that 1) these sensory axons have a resting potential of -60.2 ± 1 mV; 2) both tetrodotoxin (TTX)-sensitive (TTX-S) and TTX-resistant (TTX-R) Na+ channels are present and available for activation at resting potential, at densities that can support action potential electrogenesis in these axons; 3) TTX-sensitive channels contribute to the amplification of small depolarizations that are subthreshold with respect to the action potential in these axons; 4) TTX-R channels can support the production of action potentials in these axons; and 5) these TTX-R channels can produce repetitive firing, even at depolarized membrane potentials where TTX-S channels are inactivated. Finally, using voltage-clamp recordings with an action potential as the command, we confirmed the presence of both TTX-S and TTX-R channels, which are activated sequentially during action potential in these axons. These results provide direct evidence for the presence of TTX-S and TTX-R Na+ channels that are functionally available at resting potential and contribute to electrogenesis in small-diameter afferent axons. [ABSTRACT FROM AUTHOR]

Abstract: Various transport models and mechanisms for ions from different compartments of the cell are considered. Compartments such as mitochondria, synaptic vesicles, sarco- and endoplasmic reticulum and vacuoles are considered. It is shown that an adequate description of the compartment-based substance transport can be developed using thermodynamically correct models. Such models are used to calculate both the concentrations of ions in such compartments and the resting potential on their membranes. The problem of the complexity of transport systems is also discussed. [Copyright &y& Elsevier]

Acute Myeloid Leukemia (AML) accounts for approximately one fifth of all childhood leukemia yet is responsible for a significant proportion of morbidity and mortality in this population. For this reason, research to identify novel targets for the development of effective AML therapeutics has intensified in the recent past. The THP-1 cell line, which was originally established from an infant diagnosed with AML, provides an experimental model for functional, pre-clinical therapeutics and target identification studies of AML. Here we show the expression of the voltage gated potassium channel Kv11.1 in THP-1 cells as opposed to normal hematopoietic stem cells. In addition, curcumin, a natural polyphenol derived from the plant Curcuma longa, effectively blocked Kv11.1 activity and also inhibited the proliferation of these cells. Curcumin was rapidly internalized by THP-1 cells and possibly exerts potential growth inhibitory activity by interacting with intracellular epitopes of the ion channel. Inhibition of ionic currents carried by Kv11.1 resulted in depolarization of cell membrane potential. We propose that the inhibition of Kv11.1 activity by curcumin may lead to interference with leukemic cell physiology and consequently the suppression of survival and proliferation of AML cells. Copyright © 2011 S. Karger AG, Basel [ABSTRACT FROM AUTHOR]

We have developed a chemical kinetics simulation that can be used as both an educational and research tool. The simulator is designed as an accessible, open-source project that can be run on a laptop with a student-friendly interface. The application can potentially be scaled to run in parallel for large simulations. The simulation has been successfully used in a classroom setting for teaching basic electrochemical properties. We have shown that this can be used for simulating fundamental molecular and chemical processes and even simplified models of predator–prey interactions. By giving the simulated entities spatial extent in the lattice, the particles do not interpenetrate, and clusters of particles can spatially exclude one another. Our simulation demonstrates that spatial inhomogeneity leads to different results than those that are obtained by using standard ordinary differential equation models, as previously reported. [ABSTRACT FROM PUBLISHER]

Abstract: Background and aims: Our aim was to investigate the effect of TNFα on muscle resting potential (RP) and then in muscle excitability and to demonstrate another mechanism implicated in intensive care units (ICU) acquired polyneuromyopathy. Methods: Experiments were carried out on adult female Wistar rats. After isolation of muscle fibres from peroneus longus, influence of TNFα was tested on RP by using intracellular microelectrodes. Digoxin and chelerythrin were used to determine the mechanism of TNFα action. Results: First, we found that TNFα induced a concentration dependent increase of muscle RP and that this mechanism, which was blocked by digoxin, was due to an effect on the Na/K ATPase. As it was also blocked by chelerythrin it was concluded that this effect was mediated by PKC activation of the Na/K ATPase. Conclusions: We demonstrated that TNFα leads to a PKC mediated increase in muscle RP. Depolarization needed to reach the threshold voltage for muscle action potential should then be higher and this could be involved in the decrease in muscle excitability observed in acquired polyneuromyopathy. [Copyright &y& Elsevier]

Immature Purkinje neurons are particularly vulnerable cells. They survive in cerebellar slice cultures under treatment by the synthetic steroid mifepristone (RU486) that depolarizes diem at this age. The present study aims at understanding die mechanism underlying this neuroprotective effect. In the developing cerebellum, die role of γ-aminobutyric acid (GABA) in neuron survival is unknown. In 3-d-old mouse cerebellar slice cultures, we show that GABAA receptor activation is depolarizing and excitatory. Antagonists of GABAA receptors rescue Purkinje neurons, demonstrating that GABA is endogenously released hi tins preparation and is toxic. Mifepristone likely protects these neurons by reversing GABAA receptor-mediated chloride fluxes and reducing their driving force. Neuroprotection by mifepristone is dose-dependently decreased by the agonist of GABAA receptors muscimol and by caffeine, an agonist of internal calcium store release. Moreover, the survival induced by neomycin, an inhibitor of calcium release, is partially reversed by muscimol. The p38 mitogen-activated protein kinase (MAPK) inhibitor SB239063 also rescues Purkinje neurons. In summary, we propose that when GABA is depolarizing, mifepristone protects Purkinje neurons by shunting GABA responses and probably chloride fluxes, by inhibiting p38 MAPK activity and likely internal calcium store release. A new and nonhormonal effect of mifepristone is thus revealed. [ABSTRACT FROM AUTHOR]

Copyright of Exacta is the property of Exacta - Engenharia de Producao and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

model of the active transport of ions in the Cascinodiscus wailesii diatom cell is constructed taking into account the transport of H, Na, K, Ca, $\mathrm{NO}_{3}^{-}$, Cl, and $\mathrm{NH}_{4}^{+}$ ions. This model allows calculating intracellular concentrations of basic ions and the biomembrane resting potential. A hierarchical algorithm 'one ion-one transport system' is used in the model. The dependence of the resting potential on the extracellular concentration of potassium is plotted in terms of the model. The calculated values are in good agreement with the corresponding experimental data. [ABSTRACT FROM AUTHOR]

Abstract: An analytical model, which describes the stationary transformation of light energy to the energy of pigment electronic excitation, has been constructed. A proton pump of the thylakoid membrane has been considered as a two-level conformon. The difference between the energies of the excited and ground states of both the pigment and the protein complex is assumed to be the energy of an absorbed photon. It has been found how the concentration of ions in a lumen and the potential across the thylakoid membrane depend on the concentration of ions in the stroma and the brightness temperature of absorbed radiation. Conditions for the maximum efficiency of the photosynthesis process have been analyzed. This model has been used to determine the electric potential (φ≈6.7mV) at the chloroplast thylakoid membrane. The calculated value of the electric potential is in good agreement with the experimental data. A limitation on the stoichiometric coefficient of the proton transport through ATP-synthase, m>3, has been found theoretically. [Copyright &y& Elsevier]

For a new class of neuron models we demonstrate here that typical membrane action potentials and spike-bursts are only transient states but appear to be asymptotically stable; and yet such metastable states are plastic — being able to dynamically change from one action potential to another with different pulse frequencies and from one spike-burst to another with different spike-per-burst numbers. The pulse and spike-burst frequencies change with individual ions' pump currents while their corresponding metastable-plastic states maintain the same transmembrane voltage and current profiles in range. It is also demonstrated that the plasticity requires two one-way ion pumps operating in opposite transmembrane directions to materialize, and if only one ion pump is left to operate, the plastic states will be lost to a rigid asymptotically stable state either as a resting potential, or a limit cycle with a fixed pulse frequency, or a spike-burst with a fixed spike-per-burst number. [ABSTRACT FROM AUTHOR]

A mathematical model of the transport of basic ions (K+, Na+, Cl−) across the hepatocyte membrane has been created using the previously constructed models of active ion transport in biomembranes. The dependence of the resting potential on extracellular ion concentration has been established. Using the model, it is possible to independently calculate the resting potential at the biomembrane and the concentrations of sodium, potassium, and chlorine ions in the cell. The calculated internal concentrations of the ions are in good agreement with the corresponding experimental values. [ABSTRACT FROM AUTHOR]

Electronic circuit analogues of biological membranes are presented, by which the passive channels are modeled as parallel and serial combinations of resistors (Ohmic conductors) and diffusors (negative non-Ohmic conductors), and the active channels of the one-way ion pumps are modeled as one-way inductors. We show that our simple analog models can reproduce known phenomena of membrane excitability such as the generation of action potentials and spike bursts. [ABSTRACT FROM AUTHOR]

In the early postnatal period, energy metabolism in the suckling rodent brain relies to a large extent on metabolic pathways alternate to glucose such as the utilization of ketone bodies (KBs). However, how KBs affect neuronal excitability is not known. Using recordings of single NMDA and GABA-activated channels in neocortical pyramidal cells we studied the effects of KBs on the resting membrane potential ( Em) and reversal potential of GABA-induced anionic currents ( EGABA), respectively. We show that during postnatal development (P3–P19) if neocortical brain slices are adequately supplied with KBs, Em and EGABA are both maintained at negative levels of about −83 and −80 mV, respectively. Conversely, a KB deficiency causes a significant depolarization of both Em (>5 mV) and EGABA (>15 mV). The KB-mediated shift in EGABA is largely determined by the interaction of the NKCC1 cotransporter and Cl−/HCO3 transporter(s). Therefore, by inducing a hyperpolarizing shift in Em and modulating GABA signaling mode, KBs can efficiently control the excitability of neonatal cortical neurons. [ABSTRACT FROM AUTHOR]

Non-adapting superior cervical ganglion (SCG) neurones with a clustering activity and sub-threshold membrane potential oscillations were occasionally recorded, suggesting the presence of a persistent sodium current ( INaP). The perforated-patch technique was used to establish its properties and physiological role. Voltage-clamp experiments demonstrated that all SCG cells have a TTX-sensitive INaP activating at about −60 mV and with half-maximal activation at about −40 mV. The mean maximum INaP amplitude was around −40 pA at −20 mV. Similar results were achieved when voltage steps or voltage ramps were used to construct the current–voltage relationships, and the general INaP properties were comparable in mouse and rat SCG neurons. INaP was inhibited by riluzole and valproate with an IC50 of 2.7 and 3.8 μM, respectively, while both drugs inhibited the transient sodium current ( INaT) with a corresponding IC50 of 34 and 150 μM. It is worth noting that 30 μM valproate inhibited the INaP by 70% without affecting the INaT. In current clamp, valproate (30 μM) hyperpolarised resting SCG membranes by about 2 mV and increased the injected current necessary to evoke an action potential by about 20 pA. Together, these results demonstrate for the first time that a persistent sodium current exists in the membrane of SCG sympathetic neurones which could allow them to oscillate in the sub-threshold range. This current also contributes to the resting membrane potential and increases cellular excitability, so that it is likely to play an important role in neuronal behaviour. [ABSTRACT FROM AUTHOR]

Glucose sensing by hypothalamic neurons triggers adaptive metabolic and behavioral responses. In orexin neurons, extracellular glucose activates a leak K+ current promoting electrical activity inhibition. Sensitivity to external acidification and halothane, and resistance to ruthenium red designated the tandem-poreK+ (K2P ) channel subunit TASK3 as part of the glucose-induced channel. Here, we show that glucose inhibition and its pH sensitivity persist in mice lacking TASK3 or TASK1, or both subunits. We also tested the implication of another class of K2P channels activated by halothane. In the corresponding TREK1/2/TRAAK triple knock-out mice, glucose inhibition persisted in hypothalamic neurons ruling out a major contribution of these subunits to the glucose-activated K+ conductance. Finally, block of this glucose-induced hyperpolarizing current by low Ba2+ concentrations was consistent with the conclusion that K2P channels are not required for glucosensing in hypothalamic neurons. [ABSTRACT FROM AUTHOR]

A mathematical model of the active transport of main ions in cells of archaebacteria has been constructed. A set of equations has been developed and solved for ion fluxes through the bacterium membrane. The model is based on the principle "one ion—one transport system." Considering experimental data, the major transport mechanism was determined for each ion and the balance equation was written on the basis of this mechanism in the stationary state. This allowed calculating values of the membrane potential and intracellular concentrations of the ions independently. The calculated values of the intracellular concentrations and resting potential are in qualitative agreement with the corresponding experimental values for cells of extremely halophilic archaea. [ABSTRACT FROM AUTHOR]

Summary: Hyperkalemia may cause cardiac pacemaker (PMK) malfunctioning due to a reduction of the electronegativity of the resting myocardial potential. Both sensing and capture mechanisms could be temporarily affected, with possible life-threatening effects. Calcium chloride may counteract almost immediately such a dangerous effect of hyperkalemia, restoring the normal pacemaker function. We describe the case of a 75-year-old woman, observed in the Emergency Department for extreme weakness, very large QRS complexes, and a pacemaker type II exit block, associated with sensing failure and “spike-on-T” phenomenon. She was managed with immediate i.v. calcium chloride, followed by insulin/glucose and sodium bicarbonate infusions; the ECG recordings show an almost immediate correction of the PMK malfunctioning and serial improvement of the intraventricular conduction. This case supports the feasibility and effectiveness of i.v. calcium administration, as expected on the basis of electrophysiological ionized calcium effect on the threshold potential. [Copyright &y& Elsevier]

Abstract: A model of the active transport of ions in a cardiac muscle cell, which takes into account the active transport of Na+, K+, Ca2+, Mg2+, HCO3 − and Cl− ions, has been constructed. The model allows independent calculations of the resting potential at the biomembrane and concentrations of basic ions (sodium, potassium, chlorine, magnesium and calcium) in a cell. For the analysis of transport processes in cardiac cell hierarchical algorithm “one ion–one transport system” was offered. The dependence of the resting potential on concentrations of the ions outside a cell has been established. It was shown, that ions of calcium and magnesium, despite their rather small concentration, play an essential role in maintenance of resting potential in cardiac cell. The calculated internal concentrations of ions are in good agreement with the corresponding experimental values. [Copyright &y& Elsevier]

Glucose sensing by hypothalamic neurons triggers adaptive metabolic and behavioral responses. In orexin neurons, extracellular glucose activates a leak K+ current promoting electrical activity inhibition. Sensitivity to external acidification and halothane, and resistance to ruthenium red designated the tandem-pore K+ (K2P ) channel subunit TASK3 as part of the glucose-induced channel. Here, we show that glucose inhibition and its pH sensitivity persist in mice lacking TASK3 or TASK1, or both subunits. We also tested the implication of another class of K2P channels activated by halothane. In the corresponding TREK1/2/TRAAK triple knock-out mice, glucose inhibition persisted in hypothalamic neurons ruling out a major contribution of these subunits to the glucose-activated K+ conductance. Finally, block of this glucose-induced hyperpolarizing current by low Ba2+ concentrations was consistent with the conclusion that K2P channels are not required for glucosensing in hypothalamic neurons. [ABSTRACT FROM AUTHOR]

Surface expression of the K2P3.1 two-pore domain potassium channel is regulated by phosphorylation-dependent binding of 14-3-3, leading to suppression of coatomer coat protein I (COPI)-mediated retention in endoplasmic reticulum (ER). Here, we investigate the nature of the macromolecular regulatory complexes that mediate forward and retrograde transport. We demonstrate that (i) the channel employs two separate but interacting COPI binding sites on the N- and C-termini; (ii) disrupting COPI binding to either site interferes with the ER retention; (iii) p11 and 14-3-3 do not interact on their own; (iv) p11 binding to the C-terminal retention motif is dependent on 14-3-3; and (v) p11 is coexpressed in only a subset of tissues with K2P3.1, while 14-3-3 expression is ubiquitous. We conclude that K2P3.1 forward transport requires 14-3-3 suppression of COPI binding, whereas p11 serves a modulatory role. [ABSTRACT FROM AUTHOR]

Abstract: Models of the active transport of neurotransmitters in synaptic vesicles were constructed. The models were used to determine the resting potential at membranes of synaptic vesicles: 40mV (monoamines and acetylcholine) and −40mV (glutamate). The potential at the membrane of a synaptic vesicle was almost absent for the transport of GABA and glycine. The neurotransmitter concentration of a cell was 0.1–18mM at the concentration of neurotransmitters in a vesicle equal to 0.5M. This result is in qualitative agreement with the relevant experimental data. [Copyright &y& Elsevier]

A model of the active transport of ions through internal membranes of mitochondria is proposed. If concentrations of ions in a cell are known, this model allows calculating concentrations of all main ions (H+, Ca+2, K+, Mg2+, Na+, Cl−) in the mitochondrion matrix and the resting potential across the membrane. The theoretical values satisfactorily agree with available experimental data on the concentrations and the potentials, including different operating regimes of the adenosine triphosphate (ATP) synthetase (the main regime, short circuiting or ATP synthetase blocking). The active transport of Mg2+ ions in exchange for protons was assumed. In accordance with the model, the ATP synthetase operation is possible only if the stoichiometric coefficient of protons is 3. [ABSTRACT FROM AUTHOR]

1. 5-Hydroxytryptamine (5-HT) has an important role in the pathogenesis of irritable bowel syndrome. To investigate the effects of 5-HT on the contractile activity of myocytes of the guinea-pig proximal colon, cell imaging before and after contraction was undertaken and images were analysed using image-analysis software. Ion currents and membrane potentials were measured. Cytoplasmic free Ca2+ was recorded using a confocal microscope followoing loading of the cells with the fluorescent probe Fura-2AM. 2. 5-Hydroxytryptamine reduced cell length in a dose-dependent manner (EC50 = 0.189 µmol/L). Under current clamp, 10 µmol/L 5-HT reduced action potential amplitude (measured as peak height) and decreased action potential duration, as well as depolarizing the resting potential from −68.4 ± 3.6 to −22.96 ± 4.65 mV. Iberiotoxin (1 µmol/L) blocked the effects of 5-HT in reducing the time to repolarization (T90) and nicardipine (5 µmol/L) blocked the effects of 5-HT in reducing action potential amplitude. 3. In the whole-cell mode, 5-HT enhanced L-type Ca2+ currents, large conductance K+ channel (BKCa) currents and spontaneous transient outward currents (STOC). In addition, 5-HT increased intracellular Ca2+ levels. Ondansetron (10 µmol/L) blocked the effects of 5-HT in enhancing L-type Ca2+ currents, BKCa currents and STOC. 4. In conclusion, 5-HT induces contraction of colonic myocytes, mostly as a result of Ca2+ release from the sarcoplasmic reticulum (SR) following activation of 5-HT3 receptors and the inositol 1,4,5-trisphosphate pathway. In addition, the effect of 5-HT in decreasing action potential amplitude is mediated by the release of Ca2+ from the SR, as well as by enhanced L-type Ca2+ current. 5-Hydroxytryptamine decreased action potential duration by enhancing BKCa current. [ABSTRACT FROM AUTHOR]

The auditory hair cell resting potential is critical for proper translation of acoustic signals to the CNS, because it determines their filtering properties, their ability to respond to stimuli of both polarities, and, because the hair cell drives afferent firing rates, the resting potential dictates spontaneous transmitter release. In turtle auditory hair cells, the filtering properties are established by the interactions between BK calcium-activated potassium channels and an L-type calcium channel (electrical resonance). However, both theoretical and in vitro recordings indicate that a third conductance is required to set the resting potential to a point on the ICa and IBK activation curves in which filtering is optimized like that found in vivo. Present data elucidate a novel mechanism, likely universal among hair cells, in which mechanoelectric transduction (MET) and its calcium-dependent adaptation provide the depolarizing current to establish the hair cell resting potential. First, mechanical block of the MET current hyperpolarized the membrane potential, resulting in broadband asymmetrical resonance. Second, altering steady-state adaptation by altering the [Ca2+] bathing the hair bundle changed the MET current at rest, the magnitude of which resulted in membrane potential changes that encompassed the best resonant voltage. The Ca2+ sensitivity of adaptation allowed for the first physiological estimate of endolymphatic Ca2+ near the MET channel (56 ± 11 µM), a value similar to bulk endolymph levels. These effects of MET current on resting potential were independently confirmed using a theoretical model of electrical resonance that included the steady-state MET conductance. [ABSTRACT FROM AUTHOR]

The auditory hair cell resting potential is critical for proper translation of acoustic signals to the CNS, because it determines their filtering properties, their ability to respond to stimuli of both polarities, and, because the hair cell drives afferent firing rates, the resting potential dictates spontaneous transmitter release. In turtle auditory hair cells, the filtering properties are established by the interactions between BK calcium-activated potassium channels and an L-type calcium channel (electrical resonance). However, both theoretical and in vitro recordings indicate that a third conductance is required to set the resting potential to a point on the ICa and IBK activation curves in which filtering is optimized like that found in vivo. Present data elucidate a novel mechanism, likely universal among hair cells, in which mechanoelectric transduction (MET) and its calcium-dependent adaptation provide the depolarizing current to establish the hair cell resting potential. First, mechanical block of the MET current hyperpolarized the membrane potential, resulting in broadband asymmetrical resonance. Second, altering steady-state adaptation by altering the [Ca2+] bathing the hair bundle changed the MET current at rest, the magnitude of which resulted in membrane potential changes that encompassed the best resonant voltage. The Ca2+ sensitivity of adaptation allowed for the first physiological estimate of endolymphatic Ca2+ near the MET channel (56 ± 11 µM), a value similar to bulk endolymph levels. These effects of MET current on resting potential were independently confirmed using a theoretical model of electrical resonance that included the steady-state MET conductance. [ABSTRACT FROM AUTHOR]

Abstract: A closed model of the active transport was constructed taking into account ATP-dependent opening and closing of barriers to ions and the relationship between the membrane potential and the work of ionic pumps under the condition of electroneutrality inside the cell. The internal consistency of the model was verified by the fulfillment of Onsager''s reciprocity relation. It was demonstrated that at the limit of large energy barriers the operation of the system of the active transport is equivalent to the “turning segment” model, which was proposed by the authors earlier. Values of the resting potential and the intracellular concentration of ions were obtained for different types of cells. These results were in qualitative agreement with relevant experimental data. [Copyright &y& Elsevier]

Requirements on models of the active transport of ions in biomembranes have been formulated. The basic requirements include an explicit dependence of the resting potential and intracellular concentrations of ions on the difference of ATP-ADP chemical potentials, a consideration of the reversibility of the ionic pump operation, a correlation between theoretical and experimental data on the resting potential and intracellular concentrations of ions for different types of cells, the pump efficiency approaching 100%, and a tendency of the resting potential to the Donnan potential if the active transport is blocked. A model satisfying the aforementioned requirements has been proposed by the authors as an example. [ABSTRACT FROM AUTHOR]