Anesthetic Effects on Resting Membrane Potential Are Voltage-dependent and Agent-specific

Like their chemical counterparts, electrical synapses show complex dynamics such as rectification and voltage dependence that interact with other electrical processes in neurons. The consequences arising from these interactions for the electrical behavior of the synapse, and the dynamics they create, remain largely unexplored. Using a voltage-dependent electrical synapse between a descending modulatory projection neuron (MCN1) and a motor neuron (LG) in the crustacean stomatogastric ganglion, we find that the influence of the hyperpolarization-activated inward current (Ih) is critical to the function of the electrical synapse. When we blocked Ih with CsCl, the voltage dependence of the electrical synapse shifted by 18.7 mV to more hyperpolarized voltages, placing the dynamic range of the electrical synapse outside of the range of voltages used by the LG motor neuron (-60.2 mV to -44.9 mV). With dual electrode current- and voltage-clamp recordings, we demonstrate that this voltage shift is due to a sustained effect of Ih on the presynaptic MCN1 axon terminal membrane potential. Ih-induced depolarization of the axon terminal membrane potential increased the electrical postsynaptic potentials and currents. With Ih present, the axon terminal resting membrane potential depolarized, shifting the dynamic range of the electrical synapse towards the functional range of the motor neuron. We thus demonstrate that the function of an electrical synapse is critically influenced by a voltage-dependent ionic current (Ih).

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K+ currents regulate the resting membrane potential, proliferation, and contractile responses in ventricular fibroblasts and myofibroblasts

AJP Heart and Circulatory Physiology ◽

10.1152/ajpheart.01220.2004 ◽

2005 ◽

Vol 288 (6) ◽

pp. H2931-H2939 ◽

Cited By ~ 143

Author(s):

L. Chilton ◽

S. Ohya ◽

D. Freed ◽

E. George ◽

V. Drobic ◽

...

Keyword(s):

Membrane Potential ◽

Reversal Potential ◽

Mechanical Activity ◽

Resting Membrane Potential ◽

Whole Cell Patch Clamp ◽

Voltage Dependent ◽

Primary Determinant ◽

Inwardly Rectifying ◽

Low K ◽

Ionic Basis

Despite the important roles played by ventricular fibroblasts and myofibroblasts in the formation and maintenance of the extracellular matrix, neither the ionic basis for membrane potential nor the effect of modulating membrane potential on function has been analyzed in detail. In this study, whole cell patch-clamp experiments were done using ventricular fibroblasts and myofibroblasts. Time- and voltage-dependent outward K+ currents were recorded at depolarized potentials, and an inwardly rectifying K+ (Kir) current was recorded near the resting membrane potential (RMP) and at more hyperpolarized potentials. The apparent reversal potential of Kir currents shifted to more positive potentials as the external K+ concentration ([K+]o) was raised, and this Kir current was blocked by 100–300 μM Ba2+. RT-PCR measurements showed that mRNA for Kir2.1 was expressed. Accordingly, we conclude that Kir current is a primary determinant of RMP in both fibroblasts and myofibroblasts. Changes in [K+]o influenced fibroblast membrane potential as well as proliferation and contractile functions. Recordings made with a voltage-sensitive dye, DiBAC3(4), showed that 1.5 mM [K+]o resulted in a hyperpolarization, whereas 20 mM [K+]o produced a depolarization. Low [K+]o (1.5 mM) enhanced myofibroblast number relative to control (5.4 mM [K+]o). In contrast, 20 mM [K+]o resulted in a significant reduction in myofibroblast number. In separate assays, 20 mM [K+]o significantly enhanced contraction of collagen I gels seeded with myofibroblasts compared with control mechanical activity in 5.4 mM [K+]o. In combination, these results show that ventricular fibroblasts and myofibroblasts express a variety of K+ channel α-subunits and demonstrate that Kir current can modulate RMP and alter essential physiological functions.

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Ratiometry of transmembrane voltage-sensitive fluorescent dye emission in hearts

AJP Heart and Circulatory Physiology ◽

10.1152/ajpheart.2000.279.3.h1421 ◽

2000 ◽

Vol 279 (3) ◽

pp. H1421-H1433 ◽

Cited By ~ 73

Author(s):

Stephen B. Knisley ◽

Robert K. Justice ◽

Wei Kong ◽

Philip L. Johnson

Keyword(s):

Membrane Potential ◽

Action Potentials ◽

Signal To Noise Ratio ◽

Green Emission ◽

Motion Artifacts ◽

Resting Membrane Potential ◽

Baseline Drift ◽

Fluorescence Measurements ◽

Transmembrane Voltage ◽

Voltage Dependent

Transmembrane voltage-sensitive fluorescence measurements are limited by baseline drift that can obscure changes in resting membrane potential and by motion artifacts that can obscure repolarization. Voltage-dependent shift of emission wavelengths may allow reduction of drift and motion artifacts by emission ratiometry. We have tested this for action potentials and potassium-induced changes in resting membrane potential in rabbit hearts stained with di-4-ANEPPS [Pyridinium, 4-(2-(6-(dibutylamino)-2-naphthalenyl) ethenyl)-1-(3-sulfopropyl)-, hydroxide, inner salt] using laser excitation (488 nm) and a two-photomultiplier tube system or spectrofluorometer (resolution of 500–1,000 Hz and <1 mm). Green and red emissions produced upright and inverted action potentials, respectively. Ratios of green emission to red emission followed action potential contours and exhibited larger fractional changes than either emission alone ( P < 0.001). The largest changes and signal-to-noise ratio (signal/noise) were obtained with numerator wavelengths of 525–550 nm and denominator wavelengths of 650–700 nm. Ratiometry lessened drift 56–66% ( P < 0.015) and indicated decreases in resting membrane potential. Ratiometry lessened motion artifacts and increased magnitudes of deflections representing phase-zero depolarizations relative to total deflections by 123–188% in intact hearts ( P < 0.02). Durations of action potentials at different pacing rates, temperatures, and potassium concentrations were independent of whether they were measured ratiometrically or with microelectrodes ( P ≥ 0.65). The ratiometric calibration slope was 0.017/100 mV and decreased with time. Thus emission ratiometry lessens the effects of motion and drift and indicates resting membrane potential changes and repolarization.

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Physiological role of Ca(2+)-activated and voltage-dependent K+ currents in rabbit coronary myocytes

AJP Cell Physiology ◽

10.1152/ajpcell.1994.266.6.c1523 ◽

1994 ◽

Vol 266 (6) ◽

pp. C1523-C1537 ◽

Cited By ~ 67

Author(s):

N. Leblanc ◽

X. Wan ◽

P. M. Leung

Keyword(s):

Steady State ◽

Membrane Potential ◽

Physiological Role ◽

Cell Voltage ◽

Resting Membrane Potential ◽

Physiological Level ◽

Steady State Current ◽

Voltage Dependent ◽

A Minor ◽

And Function

The properties and function of Ca(2+)-activated K+ (KCa) and voltage-dependent K+ (IK) currents of rabbit coronary myocytes were studied under whole cell voltage-clamp conditions (22 degrees C). Inhibition of KCa by tetraethylammonium chloride (1-10 mM) or charybdotoxin (50-100 nM) suppressed noisy outward rectifying current elicited by 5-s voltage steps or ramp at potentials > 0 mV, reduced the hump of the biphasic ramp current-voltage relation, and shifted by less than +5 mV the potential at which no net steady-state current is recorded (Enet; index of resting membrane potential). Inhibition of steady-state inward Ca2+ currents [ICa(L)] by nifedipine (1 microM) displaced Enet by -11 mV. Analysis of steady-state voltage dependence of IK supported the existence of a "window" current between -50 and 0 mV. 4-Aminopyridine (2 mM) blocked a noninactivating component of IK evoked between -30 and -40 mV, abolished the hump current during ramps, and shifted Enet by more than +15 mV; hump current persisted during 2-min ramp depolarizations and peaked near the maximum overlap of the steady-state activation and inactivation curves of IK (about -22 mV). A threefold rise in extracellular Ca2+ concentration (1.8-5.4 mM) enhanced time-dependent outward K+ current (6.7-fold at +40 mV) and shifted Enet by -30 mV. It is concluded that, under steady-state conditions, IK and ICa(L) play a major role in regulating resting membrane potential at a physiological level of intracellular Ca2+ concentration, with a minor contribution from KCa. However, elevation of intracellular Ca2+ concentration enhances KCa and hyperpolarizes the myocyte to limit Ca2+ entry through ICa(L).

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Sodium dependence of the thyrotrophin-induced depolarization in cultured porcine thyroid cells

Journal of Endocrinology ◽

10.1677/joe.0.1080225 ◽

1986 ◽

Vol 108 (2) ◽

pp. 225-230 ◽

Cited By ~ 1

Author(s):

T. A. Hambleton ◽

J. R. Bourke ◽

G. J. Huxham ◽

S. W. Manley

Keyword(s):

Membrane Potential ◽

Action Potentials ◽

Potassium Concentration ◽

Sodium Current ◽

Resting Membrane Potential ◽

Thyroid Cell ◽

Free Medium ◽

Thyroid Cells ◽

Voltage Dependent ◽

Sodium And Potassium

ABSTRACT Cultured porcine thyroid cells exhibit a resting membrane potential of about − 73 mV and depolarize to about − 54 mV on exposure to TSH. The depolarizing response to TSH was preserved in a medium consisting only of inorganic salts and buffers, but was abolished in sodium-free medium, demonstrating dependence on an inward sodium current. Increasing the potassium concentration of the medium resulted in a reduction in the resting membrane potential of 60 mV per tenfold change in potassium concentration, and a diminished TSH response. A hyperpolarizing TSH response was observed in a sodium- and bicarbonate-free medium, indicating that a hyperpolarizing ion current (probably carried by potassium) was also enhanced in the presence of TSH. Tetrodotoxin blocked the TSH response. We conclude that the response of the thyroid cell membrane to TSH involves increases in permeability to sodium and potassium, and that the thyroid membrane ion channels bear some similarity to the voltage-dependent sodium channels of excitable tissues, despite the absence of action potentials in the thyroid. J. Endocr. (1986) 108, 225–230

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Charybdotoxin block of Ca2+-activated K+ channels in colonic muscle depends on membrane potential dynamics

AJP Cell Physiology ◽

10.1152/ajpcell.1998.274.3.c673 ◽

1998 ◽

Vol 274 (3) ◽

pp. C673-C680 ◽

Cited By ~ 18

Author(s):

Bret W. Frey ◽

Andreas Carl ◽

Nelson G. Publicover

Keyword(s):

Membrane Potential ◽

Longitudinal Muscle ◽

Circular Muscle ◽

Resting Membrane Potential ◽

Dependent Manner ◽

Patch Clamp Technique ◽

Whole Cell Patch Clamp ◽

Rate Of Increase ◽

Voltage Dependent ◽

Simple Kinetic Model

Charybdotoxin (ChTX) is a specific blocker of Ca2+-activated K+ channels. The voltage- and time-dependent dynamics of ChTX block were investigated using canine colonic myocytes and the whole cell patch-clamp technique with step and ramp depolarization protocols. During prolonged step depolarizations, K+ current slowly increased in the continued presence of ChTX (100 nM). The rate of increase depended on membrane potential with an e-fold change for every 60 mV. During ramp depolarizations, the effectiveness of ChTX block depended significantly on the rate of the ramp (50% at 0.01 V/s to 80% at 0.5 V/s). Results are consistent with a mechanism in which ChTX slowly “unbinds” in a voltage-dependent manner. A simple kinetic model was developed in which ChTX binds to both open and closed states. Slow unbinding is consistent with ChTX having little effect on electrical slow waves recorded from circular muscle while causing depolarization and contraction of longitudinal muscle, which displays more rapid “spikes.” Resting membrane potential and membrane potential dynamics are important determinants of ChTX action.

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Characterization of K+ Currents Underlying Pacemaker Potentials of Fish Gonadotropin-Releasing Hormone Cells

Journal of Neurophysiology ◽

10.1152/jn.1999.81.2.643 ◽

1999 ◽

Vol 81 (2) ◽

pp. 643-653 ◽

Cited By ~ 19

Author(s):

Hideki Abe ◽

Yoshitaka Oka

Keyword(s):

Steady State ◽

Membrane Potential ◽

Gonadotropin Releasing Hormone ◽

Resting Membrane Potential ◽

Potential Range ◽

Releasing Hormone ◽

Voltage Dependent ◽

Pacemaker Potentials ◽

Low Threshold

Characterization of K+ currents underlying pacemaker potentials of fish gonadotropin-releasing hormone cells. Endogenous pacemaker activities are important for the putative neuromodulator functions of the gonadotropin-releasing hormone (GnRH)-immunoreactive terminal nerve (TN) cells. We analyzed several types of voltage-dependent K+ currents to investigate the ionic mechanisms underlying the repolarizing phase of pacemaker potentials of TN-GnRH cells by using the whole brain in vitro preparation of fish (dwarf gourami, Colisa lalia). TN-GnRH cells have at least four types of voltage-dependent K+currents: 1) 4-aminopyridine (4AP)-sensitive K+current, 2) tetraethylammonium (TEA)-sensitive K+ current, and 3) and 4) two types of TEA- and 4AP-resistant K+ currents. A transient, low-threshold K+ current, which was 4AP sensitive and showed significant steady-state inactivation in the physiological membrane potential range (−40 to −60 mV), was evoked from a holding potential of −100 mV. This current thus cannot contribute to the repolarizing phase of pacemaker potentials. TEA-sensitive K+ current evoked from a holding potential of −100 mV was slowly activating, long lasting, and showed comparatively low threshold of activation. This current was only partially inactivated at steady state of −60 to −40 mV, which is equivalent to the resting membrane potential. TEA- and 4AP-resistant sustained K+ currents were evoked from a holding potential of −100 mV and were suggested to consist of two types, based on the analysis of activation curves. From the inactivation and activation curves, it was suggested that one of them with low threshold of activation may be partly involved in the repolarizing phase of pacemaker potentials. Bath application of TEA together with tetrodotoxin reversibly blocked the pacemaker potentials in current-clamp recordings. We conclude that the TEA-sensitive K+ current is the most likely candidate that contributes to the repolarizing phase of the pacemaker potentials of TN-GnRH cells.

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Voltage-dependent effects of isoproterenol on cytosolic Ca concentration in rat heart

AJP Heart and Circulatory Physiology ◽

10.1152/ajpheart.1987.252.4.h697 ◽

1987 ◽

Vol 252 (4) ◽

pp. H697-H703 ◽

Cited By ~ 2

Author(s):

S. S. Sheu ◽

V. K. Sharma ◽

M. Korth

Keyword(s):

Membrane Potential ◽

Calcium Concentration ◽

Ventricular Myocytes ◽

Cytosolic Calcium ◽

Resting Membrane Potential ◽

Beta Adrenoceptor ◽

Adrenoceptor Agonist ◽

Voltage Dependent ◽

K Concentration ◽

Ca2 Channels

The effect of the beta-adrenoceptor agonist, isoproterenol, on cytosolic calcium concentration ([Ca2+]i) was studied with the Ca2+-sensitive fluorescent indicator quin 2 in enzymatically dissociated rat ventricular myocytes. Under conditions in which cells have normal polarized resting membrane potential, isoproterenol (1 microM) produced a decrease in [Ca2+]i. In contrast, in the depolarized cells (by raising extracellular K+ concentration to 50 mM), isoproterenol (1 microM) caused an increase in [Ca2+]i. This isoproterenol-induced increase in [Ca2+]i in depolarized cells could be reversed by prior exposure of the cells to the Ca2+ channel blocker, verapamil (5 microM). The results indicate that isoproterenol can either decrease or increase [Ca2+]i depending on membrane potential. The actual effect of isoproterenol on [Ca2+]i at any given membrane potential probably reflects the relative contributions of isoproterenol-induced stimulation of Ca2+ buffering or effluxing activities (which favor a decrease in [Ca2+]i) and enhancement of Ca2+ influx through voltage-sensitive Ca2+ channels (which favors an increase in [Ca2+]i).

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Properties of a tonically active, sodium-permeable current in mouse urinary bladder smooth muscle

AJP Cell Physiology ◽

10.1152/ajpcell.00501.2003 ◽

2004 ◽

Vol 286 (6) ◽

pp. C1246-C1257 ◽

Cited By ~ 27

Author(s):

Kevin S. Thorneloe ◽

Mark T. Nelson

Keyword(s):

Membrane Potential ◽

Urinary Bladder ◽

Action Potential ◽

Action Potentials ◽

Resting Membrane Potential ◽

Action Potential Generation ◽

Potential Generation ◽

Bladder Smooth Muscle ◽

Voltage Dependent ◽

Urinary Bladder Smooth Muscle

Urinary bladder smooth muscle (UBSM) elicits depolarizing action potentials, which underlie contractile events of the urinary bladder. The resting membrane potential of UBSM is approximately −40 mV and is critical for action potential generation, with hyperpolarization reducing action potential frequency. We hypothesized that a tonic, depolarizing conductance was present in UBSM, functioning to maintain the membrane potential significantly positive to the equilibrium potential for K+ ( EK; −85 mV) and thereby facilitate action potentials. Under conditions eliminating the contribution of K+ and voltage-dependent Ca2+ channels, and with a clear separation of cation- and Cl−-selective conductances, we identified a novel background conductance ( Icat) in mouse UBSM cells. Icat was mediated predominantly by the influx of Na+, although a small inward Ca2+ current was detectable with Ca2+ as the sole cation in the bathing solution. Extracellular Ca2+, Mg2+, and Gd3+ blocked Icat in a voltage-dependent manner, with Ki values at −40 mV of 115, 133, and 1.3 μM, respectively. Although UBSM Icat is extensively blocked by physiological extracellular Ca2+ and Mg2+, a tonic, depolarizing Icat was detected at −40 mV. In addition, inhibition of Icat demonstrated a hyperpolarization of the UBSM membrane potential and decreased the amplitude of phasic contractions of isolated UBSM strips. We suggest that Icat contributes tonically to the depolarization of the UBSM resting membrane potential, facilitating action potential generation and thereby a maintenance of urinary bladder tone.

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Inhibition of voltage-dependent Ca2+influx by extracellular ATP in salivary cells of the leech Haementeria ghilianii

Journal of Experimental Biology ◽

10.1242/jeb.199.6.1335 ◽

1996 ◽

Vol 199 (6) ◽

pp. 1335-1341

Author(s):

W Wuttke ◽

T Munsch ◽

J Deitmer

Keyword(s):

Membrane Potential ◽

Voltage Clamp ◽

Extracellular Atp ◽

Threshold Concentration ◽

Resting Membrane Potential ◽

Voltage Gated ◽

Voltage Dependent ◽

K Concentration ◽

Gamma S ◽

Ca2 Channels

The effects of extracellular ATP on intracellular free Ca2+ concentration ([Ca2+]i) and depolarization-induced elevations of [Ca2+]i were investigated in salivary cells of the leech Haementeria ghilianii using the fluorescent Ca2+ indicator Fura-2. Simultaneously, the membrane potential was monitored or controlled by voltage-clamp. The cell membrane was depolarized either by transient elevations of the extracellular K+ concentration ([K+]o) to 90 mmol l-1 or by depolarizing steps under voltage-clamp. The resulting transient elevations of [Ca2+]i (Ca2+ transients) could be repeatedly elicited with little variability in amplitude. Ca2+ transients were completely inhibited by 2 mmol l-1 Ni2+ or in Ca2+-free saline. The transients are, therefore, dependent on Ca2+ influx from the external medium through voltage-gated Ca2+ channels. The Ca2+ influx was rapidly and reversibly inhibited by extracellular application of ATP. The effect was dose-dependent with a threshold concentration below 10(-7) mol l-1. A 50 % reduction in the amplitude of Ca2+ transients was obtained by application of 12 µmol l-1 ATP or ATP-gamma-S (apparent IC50, 1.6 µmol l-1 ATP) and Ca2+ transients were almost completely inhibited by 30100 µmol l-1 ATP. Resting [Ca2+]i, the resting membrane potential and membrane potential changes induced by 90 mmol l-1 [K+]o were not affected by ATP. Adenosine (10 µmol l-1) did not affect resting [Ca2+]i, the resting membrane potential or membrane potential changes induced by 90 mmol l-1 [K+]o and had little effect on Ca2+ transients. Suramin, an antagonist of vertebrate P2 receptors, was without effect on the inhibitory actions of ATP. We conclude that activation of a suramin-insensitive purinoceptor by ATP inhibits Ca2+ influx through voltage-gated Ca2+ channels in the salivary cells of Haementeria ghilianii.

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