Effect of the medium composition on the resting membrane potential in the earthworm longitudinal somatic muscle fibers

Norepinephrine or increased extracellular K+ hyperpolarize the membrane of the earthworm somatic muscle fibre, whereas removal of Cl- from external solution or a hypotonic solution depolarize the membrane. The dependence of the membrane resting potential on the extracellular K+ is quite characteristic against the background of ouabain action. A preliminary membrane depolarisation by ouabain eliminates the above effects on the membrane resting potential. The data obtained suggest that the ouabain-sensitive active ion pump directly contributes to the membrane resting potential value. This hypothesis is discussed with respect to existence of active Cl- transport combined with Na+, K(+)-pump which presumably takes part in the intracellular osmotic pressure regulation in the earthworm somatic muscle.     

A New TASK for Dipeptidyl Peptidase-like Protein 6

Dipeptidyl Peptidase-like Protein 6 (DPP6) is widely expressed in the brain where it co-assembles with Kv4 channels and KChIP auxiliary subunits to regulate the amplitude and functional properties of the somatodendritic A-current, I_SA. Here we show that in cerebellar granule (CG) cells DPP6 also regulates resting membrane potential and input resistance by increasing the amplitude of the I_K(SO) resting membrane current. Pharmacological analysis shows that DPP6 acts through the control of a channel with properties matching the K_2P channel TASK-3. Heterologous expression and co-immunoprecipitation shows that DPP6 co-expression with TASK-3 results in the formation of a protein complex that enhances resting membrane potassium conductance. The co-regulation of resting and voltage-gated channels by DPP6 produces coordinate shifts in resting membrane potential and A-current gating that optimize the sensitivity of I_SA inactivation gating to subthreshold fluctuations in resting membrane potential.     

爪蟾胚胎脊髓胆碱能神经元上两类非去极化激...

Single calcium channel current was studied on the identified spinal cholinergic neurons from embryonic Xenopus laevis with patch clamp method. The results indicate that some calcium channels show opening activity at resting membrane potential. According to the characteristics of conductance and kinetics of such channels, they could be divided into two types: a stretch sensitive Type-NS with a slope conductance of 7.5 pS (mean open time 0.58 ms at resting membrane potential) and a Type-NL with a slope conductance of 16.7 pS (and two opening times of 2 ms and 19.3 ms). Both types of channel are predominantly active at resting potential and negative membrane phases. It is suggested that they may be involved in the calcium-dependent neuronal events at resting state.     

The causal theory of the resting potential of cells

In this pedagogical article the causal theory of the resting potential of cells is presented, which for given extracellular ion concentrations predicts the intracellular ones simultaneously with the resting potential. In addition to the Na, K-pump, fixed charges on the membrane surfaces are taken into account. The equation determining the resting potential in the causal theory suggests a new explanation of the genesis of the resting potential. The usual criterion for an ion pump to be electrogenic is not relevant for the whole of the resting potential, and may therefore be misleading. The physical meaning of the Goldman-Hodgkin-Katz formula for the membrane potential as a diffusion potential is also explained and tested with numbers for the giant axon of the squid. A significant discrepancy between theory and experiment is found which calls for an experimental re-examination of the constitutive equations for passive potassium and sodium currents.     

Intracellular microelectrode measurements in small cells evaluated with the patch clamp technique.

Microelectrode penetration of small cells leads to a sustained depolarization of the resting membrane potential due to a transmembrane shunt resistance (Rs) introduced by the microelectrode. This has led to underestimation of the resting membrane potential of various cell types. However, measurement of the fast potential transient occurring within the first few milliseconds after microelectrode penetration can provide information about pre-impalement membrane electrophysiological properties. We have analyzed an equivalent circuit of a microelectrode measurement to establish the conditions under which the peak of the impalement transients (Ep) approaches the pre-impalement resting membrane potential (Em) of small cells most closely. The simulation studies showed that this is the case when the capacitance of the microelectrode is low and the membrane capacitance of the cell high. In experiments performed to assess the reliability of Ep as a measure of Em, whole-cell patch clamp measurements were performed in the current clamp mode to monitor, free from the effects of Rs, Em in cultured human monocytes. Microelectrode impalement of such patch clamped cells and measurement of Ep made it possible to detect correlation between Ep and Em and showed that for small cells such as human monocytes Ep is on average 6 mV less negative than the resting membrane potential.     

Temperature Effects on Pacemaker Generation, Membrane Potential, and Critical Firing Threshold in Aplysia Neurons

Temperature increases cause a regular and reproducible increase in the frequency of generation of pacemaker potentials in most Aplysia neurons specialized for this type of activity which can only be explained as a direct stimulating effect of temperature upon the ionic mechanisms responsible for pacemaker potentials. At the same time all cells in the visceral ganglion undergo a membrane potential hyperpolarization of approximately 1–2 mv/°C warmed. In spite of the marked variation in resting membrane potential the critical firing threshold remains at a constant membrane potential level at all temperatures in the absence of accommodative changes. The temperature-frequency curves of all types of cells are interpreted as a result of the interaction between the effects of temperature on the pacemaker-generating mechanism and resting membrane potential. Previous observations on the effects of temperature on excitability of mammalian neurons suggest that other types of neurons may undergo similar marked shifts in resting membrane potential with temperature variation.     

Modulation of membrane potential by an acetylcholine-activated potassium current in trout atrial myocytes

Application of the current-clamp technique in rainbow trout atrial myocytes has yielded resting membrane potentials that are incompatible with normal atrial function. To investigate this paradox, we recorded the whole membrane current (I(m)) and compared membrane potentials recorded in isolated cardiac myocytes and multicellular preparations. Atrial tissue and ventricular myocytes had stable resting potentials of -87 +/- 2 mV and -83.9 +/- 0.4 mV, respectively. In contrast, 50 out of 59 atrial myocytes had unstable depolarized membrane potentials that were sensitive to the holding current. We hypothesized that this is at least partly due to a small slope conductance of I(m) around the resting membrane potential in atrial myocytes. In accordance with this hypothesis, the slope conductance of I(m) was about sevenfold smaller in atrial than in ventricular myocytes. Interestingly, ACh increased I(m) at -120 mV from 4.3 pA/pF to 27 pA/pF with an EC(50) of 45 nM in atrial myocytes. Moreover, 3 nM ACh increased the slope conductance of I(m) fourfold, shifted its reversal potential from -78 +/- 3 to -84 +/- 3 mV, and stabilized the resting membrane potential at -92 +/- 4 mV. ACh also shortened the action potential in both atrial myocytes and tissue, and this effect was antagonized by atropine. When applied alone, atropine prolonged the action potential in atrial tissue but had no effect on membrane potential, action potential, or I(m) in isolated atrial myocytes. This suggests that ACh-mediated activation of an inwardly rectifying K(+) current can modulate the membrane potential in the trout atrial myocytes and stabilize the resting membrane potential.     

The interactive effects of fatigue and pH on the ionic conductance of frog sartorius muscle fibers

The effects of fatigue on the membrane conductance of frog sartorius muscle at the resting potential and during an action potential were studied. When muscles were exposed to an extracellular pH of 8.0 the membrane conductance at the resting potential increased during fatigue by about 20% and returned to prefatigue level in about 20 min. The membrane conductance of muscle fibers exposed to pH 6.4 was about three times less than that of pH 8.0 and decreased further during fatigue. Furthermore, the recovery of a normal membrane conductance was slow at pH 6.4. Both the inward, depolarizing and the outward, repolarizing currents during the action potential are reduced in fatigue. In each case the effect is greater at pH 6.4 than at 8.0 and recovery towards normal values is slower at pH 6.4. It is concluded that the ionic conductance of the sarcolemmal membrane at the resting potential and during an action potential are modified by fatigue and that these changes are modulated by pHo.     

Resting state respiration of mitochondria: reappraisal of the role of passive ion fluxes

Rat liver mitochondria respiring under resting state conditions in the presence of oligomycin were rapidly blocked with cyanide and the dissipation of the membrane potential, measured with a tetraphenylphosphonium-sensitive electrode, was followed over time. The plot of the rate of membrane potential dissipation versus the actual value of the membrane potential was nonlinear and identical to the plot of resting state respiration (titrated with small amounts of a respiratory inhibitor) versus the membrane potential. The relationship between the respiratory chain activity and the proton-motive force in mitochondria oxidizing succinate with either oxygen or ferricyanide as electron acceptors was also found to be identical. These results are interpreted as an indication that the passive permeability of the inner mitochondrial membrane toward ions is far more significant in maintaining resting state respiration than is the molecular slippage of the pumps in the respiratory chain. These results also confirm the non-ohmic characteristics of the inner mitochondrial membrane.     

Plasma membrane potential of the alga dunaliella, and its relation to osmoregulation

A fluorescent dye sensitive to membrane potential was used to follow the plasma-membrane potential in the unicellular halo-tolerant alga Dunaliella salina. The signal observed during dissipation of the plasma membrane potential by the addition of excess K⁺ and valinomycin, or a protonophore, was taken as a measure of the preexisting potential. A resting potential of −85 to −100 millivolts (negative inside) was calculated. Following a hypertonic shock, the plasma membrane was rapidly hyperpolarized. This hyperpolarization was transient, and the algae resumed their resting potential about 30 minutes after the shock. The resting plasma membrane potential was decreased by vanadate and is concluded to be generated mostly by the plasma membrane ATPase of Dunaliella. The transient hyperpolarization following a hypertonic shock indicates, therefore, a transient activation of the ATPase. This is further corroborated by a rapid transient decrease in the intracellular ATP following a hypertonic shock and its inhibition by vanadate. It is suggested that activation of the plasma membrane ATPase may be the trigger for osmoregulation in Dunaliella.     

Effects of batrachotoxin on membrane potential and conductance of squid giant axons

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

Origin of Axon Membrane Hyperpolarization under Sucrose-Gap

One of the disadvantages of the sucrose-gap method for measuring membrane potentials with extracellular electrodes is a membrane hyperpolarization of the order of 30 to 60 mv, as compared with the resting potential obtained with intracellular microelectrodes in the absence of a sucrose-gap. In the present study the contribution of the sucrose-sea water junction potential to this hyperpolarization effect has been evaluated by comparing the effects on the resting potential of several anion and cation substitutions in the sea water bathing the lobster giant axon under sucrose-gap. Measurements with microelectrodes demonstrate a significant liquid junction potential between sucrose and standard artificial sea water. The value of this liquid junction potential as well as the measured resting membrane potential varies as a function of the anions and cations substituted in the sea water. Both the liquid junction potential and the sucrose-gap-induced hyperpolarization can be eliminated by using a low mobility anion to replace most of the chloride in sea water while the normal cation content remains unchanged. These data provide evidence that loop currents at the sucrose-sea water-axon junctions are at least partly responsible for membrane hyperpolarization under a sucrose gap.     

豚鼠耳蜗螺旋动脉平滑肌细胞和内皮细胞静息膜电位特性

目的:多种内耳疾病和内耳微循环障碍有关,但目前对提供内耳主要血供的耳蜗螺旋动脉平滑肌(SMC)和内皮细胞(EC)的生理学特性还不十分清楚,需要进一步研究。方法:本研究采用双细胞内微电极记录技术和细胞荧光染色技术,研究耳蜗螺旋动脉平滑肌和内皮细胞的膜电位特性和细胞间的通讯联系。结果:研究发现耳蜗螺旋动脉SMC和EC具有高、低两种静息膜电位(RP)状态,两种静息膜电位状态的细胞对乙酰胆碱和高K+的反应完全不同。双微电极可同时记录到EC-ECS、MC-SMC和SMC-EC不同类型的细胞,两个细胞的静息膜电位也可以是双高RP、双低RP和一高一低RP。实验所记录的一高一低RP均是SMC-EC类型,而且EC初始膜电位均为高电位,SMC初始膜电位均为低电位。而双高RP和双低RP可以是SMC-SMC或EC-EC或SMC-EC类型。结论:结果表明耳蜗螺旋动脉的SMC和EC在0.3~0.5 mm的范围内,同类细胞之间有很好的通讯联系,能很好的保持功能的协同和一致,血管壁异类细胞则不同。     

三叉神经中脑核神经元膜的电学特性

用大鼠脑干脑片,给三叉神经中脑核79个神经元作了细胞内记录,测算了20个神经元膜的电学特性:静息电位-60.3±5.6mV;输入阻抗为10.5±5.4MΩ;时间常数1.3±0.5ms。电刺激可诱发动作电位,测算32个神经元的有关参数:阈电位-50—-55mV;波幅69.5±6.1mV;超射11.9±3.6mV;波宽0.8±0.2ms。TTX(0.3μmol/L)或无钠使之消失。通以长时程矩形波电流可引起200—250Hz的2—15个重复放电,但在通电停止前终止,TEA或4-AP可延长放电。膜电位-60—-55mV时在动作电位之后可看到阈下电位波动,它不受TTX的影响,无钙时消失,TEA或4-AP使波幅增大。静息电位去极化可使45个神经元中的40个发生外向整流作用,并被TEA,4-AP或无钙抑制,超极化则发生内向整流作用,Cs或无钠抑制之。灌流液中加入各种钾通道阻断药时神经元的稳态I-V曲线发生相应变化,提示I_(DR),l_A,I_(K(Ca))及I_Q可能都与静息时的膜电导有关。     

The control of membrane ionic currents by the membrane potential of muscle

Comparisons between electrotronic potentials and certain predicted curves allow the identification of the membrane potential at which the sodium and potassium currents are switched on in frog sartorius. The activation potentials (the membrane potentials at which the ionic currents are great enough to be resolved by the method) are functions of the resting potential and time but not of ionic concentration. In the normal fiber, the activation potential for sodium lies nearer the resting potential and depolarizations set off sodium currents and action potentials. Below a resting potential of 55 to 60 mv. sodium activation is lost and conduction is impossible. A tenfold increase of calcium concentration lowers (moves further from the resting potential) the sodium activation potential by 20 to 25 mv. whereas the potassium activation potential is lowered by only 15 mv. Certain consequences of this are seen in the behavior of the muscle cell when it is stimulated with long duration shock.     

Resting membrane potential of rat ventroposteriomedial thalamic neurons during postnatal development

Resting membrane potential is a critical parameter determining tonic or bursting mode of the thalamic neurons. Previous studies using whole-cell recordings showed that immature ventroposteriomedial (VPM) and lateral geniculate thalamic neurons are strongly depolarized and have resting membrane potential near ?50 mV. Yet, whole-cell recordings are associated with an introduction of the shunting conductance via the gigaseal that may lead to membrane depolarization in small neurons with high, in the gigaohm range, membrane resistance. Therefore, we have performed measurements of resting potential of VPM neurons in slices obtained from neonatal rats of postnatal days P2-P7 using cell-attached recordings of NMDA channels as voltage sensors. Because currents through the NMDA channels reverse near 0 mV, we assumed that the resting potential should equal the reversal potential of currents through NMDA channels in cell-attached recordings. Analysis of the current-voltage relationships of NMDA currents revealed that the resting potential in the immature VPM neurons is around ?74 mV and that it does not change during the first postnatal week. This suggests that VPM neurons may operate in the bursting mode during the early postnatal period and support the oscillatory activity (spindle-like bursts) in the developing thalamocortical networks.     

Membrane Potential Measurements in Cultured Intestinal Villi

Fragmented epithelia of newborn rat small intestine were successfully cultured for periods of up to 4 weeks. Stable intracellular recordings of membrane potential were obtained from these cultured cells. Membrane resting potential varied according to cell location along a villus. The potentials ranged from -70 to -15 mV, being highest at the tip of the villus. The mean resting potential and membrane resistance were -72.4 mV and 8.6 M Ω, respectively. The membrane potential was markedly dependent on the extracellular K⁺ concentration ([K]₀], but not significantly on [Na]₀ and [Cl]₀-Deprivation of Ca²⁺ from the surrounding medium depolarized the membrane by 20 mV. When the cells were cooled down to 6°C, membrane potential was reduced by 40 mV. Based on these data, basic mechanisms underlying the resting potential are discussed in connection with cell differentiation or maturation.     

Mechanisms of conduction slowing during myocardial stretch by ventricular volume loading in the rabbit

Acute ventricular loading by volume inflation reversibly slows epicardial electrical conduction, but the underlying mechanism remains unclear. This study investigated the potential contributions of stretch-activated currents, alterations in resting membrane potential, or changes in intercellular resistance and membrane capacitance. Conduction velocity was assessed using optical mapping of isolated rabbit hearts at end-diastolic pressures of 0 and 30 mmHg. The addition of 50 microM Gd3+ (a stretch-activated channel blocker) to the perfusate had no effect on slowing. The effect of volume loading on conduction velocity was independent of changes in resting membrane potential created by altering the perfusate potassium concentration between 1.5 and 8 mM. Bidomain model analysis of optically recorded membrane potential responses to a unipolar stimulus suggested that the cross-fiber space constant and membrane capacitance both increased with loading (21%, P = 0.006, and 56%, P = 0.004, respectively), and these changes, when implemented in a resistively coupled one-dimensional network model, were consistent with the observed slowing (14%, P = 0.005). In conclusion, conduction slowing during ventricular volume loading is not attributable to stretch-activated currents or altered resting membrane potential, but a reduction of intercellular resistance with a concurrent increase of effective membrane capacitance results in a net slowing of conduction.     

Membrane Potential Hyperpolarization in Mammalian Cardiac Cells by Synchronization Modulation of Na/K Pumps

In previously reported work, we developed a new technique, synchronization modulation, to electrically activate Na/K pump molecules. The fundamental mechanism involved in this technique is a dynamic entrainment procedure of the pump molecules, carried out in a stepwise pattern. The entrainment procedure consists of two steps: synchronization and modulation. We theoretically predicted that the pump functions can be activated exponentially as a function of the membrane potential. We have experimentally demonstrated synchronization of the Na/K pump molecules and acceleration of their pumping rates by many fold through use of voltage-clamp techniques, directly monitoring the pump currents. We further applied this technique to intact skeletal muscle fibers from amphibians and found significant effects on the membrane resting potential. Here, we extend our study to intact mammalian cardiomyocytes. We employed a noninvasive confocal microscopic fluorescent imaging technique to monitor electric field–induced changes in ionic concentration gradient and membrane resting potential. Our results further confirm that the well-designed synchronization modulation electric field can effectively accelerate the Na/K pumping rate, increasing the ionic concentration gradient across the cell membrane and hyperpolarizing the membrane resting potential.     

Progesterone induces hyperpolarization after a transient depolarization phase in human spermatozoa

Progesterone (P4) induces a membrane depolarization and various ion fluxes (chloride efflux, sodium and calcium influxes), which are required for the human sperm acrosome reaction (AR). By use of the potentiometric fluorescent dye DiSC3(5) and two different technical approaches, the present study aimed to quantify and further analyze P4-induced modifications in membrane potential in capacitated human spermatozoa. Spectrofluorimetric analysis revealed that the mean resting membrane potential of sperm was -58 +/- 2 mV (n = 12). When 10 microM P4 was added, the sperm membrane depolarized by approximately +15 mV, partly driven by a Cl- efflux. It subsequently repolarized to reach a significant lower potential than the initial resting potential in two thirds of the tested samples. The flow cytometry analysis showed a heterogeneous resting membrane potential and revealed that the depolarization-hyperpolarization events concerned only subpopulations, between 3% and 40% of the sperm cells according to the samples (n = 7). We hypothesize that P4 has a beneficial effect on the ability of zona pellucida to promote the AR in a sperm subpopulation by increasing the number of hyperpolarized cells presenting a membrane potential that is compatible with the opening of T-type calcium channels by subsequent zona pellucida-induced depolarization.