Electrical excitability of artificial enzyme membranes. IV. Theoretical approach of the membrane potential of synthetic proteinic films

This paper deals with the theoretical approach of the membrane potential of artificial proteinic film. Programming techniques using finite difference simulations for the steady state and transient solutions of the Nernst-Planck and Poisson equations were used and solved by the collocation and corrector methods. This approach allows one to calculate the membrane potential without any discontinuity between the Donnan and the diffusion potentials, the thickness of the boundary layers being automatically determined by the intrinsic properties of the solution and of the membrane. The theoretical results are compared with experimental potentials measured on proteinic artificial films.

[cAMP-dependent phosphorylation inhibits potential-dependent passive transport of Ca2+ in myometrial sarcolemma vesicles]

It is established that Ca2+ transport from the predominantly inverted vesicles of pig myometrium sarcolemma depends on the value of the membrane potential which is created on vesicles by the K+-valinomycin system. It is shown that variations in the membrane potential from -60 to +30 mV cause acceleration of the calcium transport from the vesicles, the maximal transport being observed at delta psi from 0 up to +30 mV. The endogenic and exogenic cAMP-dependent phosphorylation of plasma membrane proteins inhibits the passive transport of calcium at all the membrane potential values studied. A degree of potential-dependent Ca2+ transport inhibition correlates with the value of cAMP-dependent phosphorylation of sarcolemma proteins.

Properties of single chloride selective channel from sarcoplasmic reticulum

The behavior of single chloride channels in sarcoplasmic reticulum of rabbit and trout skeletal muscle was examined by fusing isolated vesicle fractions into planar lipid bilayers. The channel exhibited a full open state with a unit conductance of 65 pS (in 100 mM Cl-) and several subconductance states with reversal potentials which were dependent on the chloride gradient across the bilayer. Open probability was 0.6-0.95 for membrane potentials ranging from -60 to +60 mV. The kinetic behaviour could be described by assuming one time constant for the fully conducting channel, and at least two time constants for the non-conducting channel. In the presence of methane sulfonate, sulfate and phosphate anions, a decrease in the unit current amplitude but not open time argued in favor of a competition between these anions and Cl- at the transport site of the channel. Chloride channel activity was not affected by variations of Ca2+ concentration in both chambers or by the presence of Mg2+. Similarly, neither millimolar ATP nor the presence of the drugs taurine (up to 10 mM), lidocaine (2-40 microM) or the calmodulin antagonist W7 (5-150 microM), modified channel behavior. Finally, pH variations between 6.8 to 8 were without effect.

[Trace postsynaptic reactions in neuromuscular junctions]

Factors which control the duration of postsynaptic responses in neuromuscular junctions are discussed together with a possibility of origin of trace postsynaptic reactions (potentiation and desensitization). In experiments on nerve-muscle preparations of various vertebrates (lamprey, frog, chicken, rat), the key role of functional activity of acetylcholinesterase was revealed in regulation of the duration of both short- and long-living postsynaptic responses. The significance of sufficient activity of acetylcholinesterase for normal realization of neuromuscular transmission presumably increases in the course of evolution of vertebrates. An analysis of possible molecular nature of trace reactions on the postsynaptic membrane was carried out.

Anion conductances of the giant axon of squid Sepioteuthis

Anion conductances of giant axons of squid, Sepioteuthis, were measured. The axons were internally perfused with a 100-mM tetraethylammonium-phosphate solution and immersed in a 100-mM Ca-salt solution (or Mg-salt solution) containing 0.3 microns tetrodotoxin. The external anion composition was changed. The membrane currents had a large amount of outward rectification due to anion influx across Cl- channels of the membrane (Inoue, 1985). The amount of outward rectification depended on the species of anion used and was strongly influenced by temperature and internal pH. In contrast to the anion conductances themselves, the conductance relative to Cl- (gA/gCl) was found to be quite stable against changes in the membrane potential, temperature, and pH. It is therefore suggested that each gA/gCl is an intrinsic quantity of the Cl- channel of the squid axon membrane. The sequence and values of gA/gCl obtained in this study were NO3- (1.80) greater than I- (1.40) greater than Br- (1.07) greater than Cl- (1.00) greater than MeSO3- (0.46) greater than H2PO2- (0.33) greater than CH3COO- (0.29) greater than SO4(2-) (0.06).

Membrane transport of electrolyte ions and time-dependent membrane potential

Equations are derived for the transport of a symmetrical electrolyte, consisting of cations and anions of equal valency, through a neutral membrane that separates two solutions of finite volume under quasi-steady-state conditions. The time-dependent membrane potential produced by the flow of ions is taken into account. Deviation of the time course of the solute concentrations from that of neutral solutes is found to be determined by the permeability ratio of cations and anions (when this ratio equals unity, the derived membrane transport equations reduce to those for neutral substances). Simple approximate expressions for the solute concentrations and of the membrane potential as functions of time are proposed, which are in excellent agreement with the exact numerical results.

Ionic mechanisms underlying action potentials in myometrium

1. The ionic mechanisms underlying the simple spike action potential in longitudinal myometrium of pregnant rats and the complex action potential which occurs in the same layer of pregnant guinea-pigs are discussed. 2. The current during the upstroke of the simple spike is carried by calcium and repolarization results from inactivation of the calcium current and activation of a potassium current. 3. A slow inward current underlies the plateau component of the complex action potential and calcium is involved in carrying or regulating this current. 4. Single channel recordings from the longitudinal myometrium of pregnant guinea-pigs reveal large conductance (130-170 pS) potassium channels which are activated by depolarization of the membrane. The activation of these channels during the upstroke of the spike would contribute to the rapid termination of the spike. 5. The duration of the plateau component of the complex action potential closely correlates with the duration of contraction and it is suggested that sufficient calcium may enter the cell during the action potential to activate the contractile apparatus directly.

Long-term potentiation: persisting problems and recent results

In this paper we discuss recent experimental results pertinent to three unresolved issues regarding the long-term potentiation (LTP) effect: the nature of its enduring substrates, the biochemical mechanisms that produce it, and its potential role in memory. LTP appears to be triggered by a postsynaptic influx of calcium and is associated with alterations in the shape of dendritic spines and probably the formation of new synapses. We discuss the possibility that morphological reorganization also modifies membrane surface chemistry of synaptic elements. Evidence is presented that LTP is not associated with changes in presynaptic calcium currents. Activation of protein kinase C is shown to be insufficient for the induction of LTP, although it may play a modulatory role. The hypothesis that activation of a calcium-sensitive protease (calpain) is pivotal to the establishment of LTP is supported by experiments showing that a calpain inhibitor, leupeptin, blocks LTP. Furthermore, activation of NMDA receptors, an event implicated in LTP induction, is accompanied by calcium-sensitive proteolysis of spectrin, a major dendritic cytoskeletal protein. The finding that stimulation patterns designed to mimic naturally-occurring cell discharge patterns are highly effective for LTP induction greatly strengthens the hypothesis that LTP actually occurs during the encoding of information in cortical systems. Potential contributions of LTP to learning are explored using computer simulations of a simple cortical network.

Disruption of muscle reorganization by lesions of the peripheral nerve in transforming claws of snapping shrimps

We have performed surgical transections on nerves in the transforming claws of snapping shrimps. In normal transformation muscle restructuring occurs, involving degeneration of some fibers and biochemical changes in others. Surgical section of the entire second limb nerve root or of its distal, dorsal branch--both of which contain the motor axons to the closer muscle--prevents muscle restructuring, even though transformation of external claw morphology proceeds. Furthermore, nerve lesions must be performed within a specific time period after transformation has been triggered in order for the effects to be observed. We suggest that transformation involves an early sensitization of the targeted muscle and that this process depends upon an intact nervous pathway within the second nerve root.

Properties of human basophils isolated by fluorescence-activated cell sorting

The aim of the current study was twofold: (1) to consider the applicability of fluorescence-activated cell sorting (FACS) to basophil isolation from small blood volumes and (2) to compare basophils obtained from children with asthma to basophils from healthy children. With FACS, basophil suspensions were prepared with a purity of 84% (range, 75% to 95%) and a recovery of 20% (range, 15% to 30%). The purified basophils had a total histamine content of 1.6 +/- 0.12 pg per cell, not differing significantly from total histamine content observed in "total" leukocyte suspensions (1.4 +/- 0.07 pg per basophil). The same was true for IgE receptor-mediated histamine release (29 +/- 4% versus 27 +/- 4%) and for ionophore A23187-induced histamine release (41 +/- 6% versus 51 +/- 9%). Sorted basophils from subjects with asthma released more histamine after IgE receptor activation (0.67 +/- 0.09 pg per cell) than basophils from healthy children (0.40 +/- 0.04 pg per cell; p less than 0.02). Expressed as percent release, no significant difference was observed (37 +/- 3.2% versus 30 +/- 2.7%). Ionophore A23187-induced histamine release did not differ significantly between subjects with asthma and control subjects, neither expressed as picograms per cell (1.21 +/- 0.17 pg per cell versus 1.02 +/- 0.11 pg per cell) nor expressed as percent release (66 +/- 4.4% versus 74 +/- 3.2%).(ABSTRACT TRUNCATED AT 250 WORDS)

Gibbs-Donnan ratio and channel conductance of Tetrahymena cilia in mixed solution of K+ and Ca2+

A single cation-channel from Tetrahymena cilia was incorporated into planar lipid bilayers. This channel was voltage-independent and is permeable to K+ and Ca2+. In the experiments with mixed solutions where the concentrations of K+ and Ca2+ were varied, the single-channel conductance was found to be influenced by the Gibbs-Donnan ratio. The data are explained by assuming that the binding sites of this channel were always occupied by two potassium ions or one calcium ion under the present experimental conditions (5 mM-90 mM K+ and 0.5 mM-35 mM Ca2+) and these bound cations determined the channel conductivity.

Role of single-channel stochastic noise on bursting clusters of pancreatic beta-cells

To study why pancreatic beta-cells prefer to burst as a multi-cellular complex, we have formulated a stochastic model for bursting clusters of excitable cells. Our model incorporated a delayed rectifier K+ channel, a fast voltage-gated Ca2+ channel, and a slow Cai-blockable Ca2+ channel. The fraction of ATP-sensitive K+ channels that may still be active in the bursting regime was included in the model as a leak current. We then developed an efficient method for simulating an ionic current component of an excitable cell that contains several thousands of channels opening simultaneously under unclamped voltage. Single channel open-close stochastic events were incorporated into the model by use of binomially distributed random numbers. Our simulations revealed that in an isolated beta-cell [Ca2+]i oscillates with a small amplitude about a low [Ca2+]i. However, in a large cluster of tightly coupled cells, stable bursts develop, and [Ca2+]i oscillates with a larger amplitude about a higher [Ca2+]i. This may explain why single beta-cells do not burst and also do not release insulin.

The action of vasoactive intestinal peptide antagonists on peptidergic modulation of the squid Schwann cell

1. The effects of two specific antagonists of vasoactive intestinal peptide (VIP) receptors were investigated on the VIP-induced hyperpolarization of the membrane potential of the Schwann cell of the giant nerve fibre of the tropical squid. 2. Both (pCl-D-Phe6,Leu17)VIP and (N-Ac-Tyr1,D-Phe2)-GRF(1–29)amide competitively and reversibly blocked the effects of VIP in this preparation with the former compound being more potent than the latter. 3. The blocking actions of both antagonists were specific for the responses of this preparation to VIP. They did not block the actions of carbachol, DL-octopamine or substance P. 4. Both antagonists also reduced the effectiveness of an endogenous VIP-like component in the normal hyperpolarizing action of giant axon activity on the membrane potential of the Schwann cell, with the same potency ratio as for their actions on the effects induced by the exogenous application of VIP.

Active electrolyte transport in mammalian buccal mucosa

The transmural electrical potential difference (PD) was measured in vivo across the buccal mucosa of humans and experimental animals. Mean PD was -31 +/- 2 mV in humans, -34 +/- 2 mV in dogs, -39 +/- 2 mV in rabbits, and -18 +/- 1 mV in hamsters. The mechanisms responsible for this PD were explored in Ussing chambers using dog buccal mucosa. After equilibration, mean PD was -16 +/- 2 mV, short-circuit current (Isc) was 15 +/- 1 microA/cm2, and resistance was 1,090 +/- 100 omega.cm2, the latter indicating an electrically "tight" tissue. Fluxes of [14C]mannitol, a marker of paracellular permeability, varied directly with tissue conductance. The net fluxes of 22Na and 36Cl were +0.21 +/- 0.05 and -0.04 +/- 0.02 mueq/h.cm2, respectively, but only the Na+ flux differed significantly from zero. Isc was reduced by luminal amiloride, serosal ouabain, or by reducing luminal Na+ below 20 mM. This indicated that the Isc was determined primarily by active Na+ absorption and that Na+ traverses the apical membrane at least partly through amiloride-sensitive channels and exits across the basolateral membrane through Na+-K+-ATPase activity. We conclude that buccal mucosa is capable of active electrolyte transport and that this capacity contributes to generation of the buccal PD in vivo.

Theoretical study of the antiparallel double-stranded helical dimer of gramicidin as an ion channel

Recent experimental studies by Durkin, J. T., O. S. Andersen, F. Heitz, Y. Trudelle, and R. E. Koeppe II (1987. Biophys. J. 51:451a) have suggested that the antiparallel double-stranded helical (APDS) dimer of gramicidin can form a transmembrane cation channel. This article reports a theoretical study that successfully rationalizes the channel properties of the APDS dimer. As in the case of the head-to-head (HH) dimer, the APDS exhibits a high potential energy barrier as anions approach the channel mouth, according for the observation of valence selectivity. The calculated potential energies of cations show two binding sites near the channel mouths, a typical feature of the HH channel. The potential energies of hydrated cations in the APDS are generally higher than those in the HH channel and show a larger pseudoperiodicity and higher barriers, an observation which suggests that the APDS should exhibit lower single channel conductance.

Electronic characteristics and membrane properties of neurons in the inferior mesenteric ganglion in guinea-pig

Intracellular recordings were made from neurons (n = 75) in the inferior mesenteric ganglion (IMG) in guinea-pig to study the electronic characteristics and membrane properties of IMG cells which receive an excitatory, cholinergic input from mechanoreceptors in the gastrointestinal tract. An excitatory, cholinergic innervation from the periphery served as an index to identify the sympathetic neurons involved in the reflex inhibition of muscle tone when the gut is distended. Functionally identified neurons in the IMG were categorized into 4 subclasses (I, II, III and IV). Subclasses I and II comprised neurons which fired phasically (rapidly adapting), with the neurons in subclass II showing a voltage relaxation in the electronic potentials elicited by depolarizing current-clamps. Subclasses III and IV comprised neurons which fired tonically (slow adapting), with the neurons in subclass III also showing relaxation of electronic potentials. Active and passive membrane properties were determined for neurons in each of the 4 subclasses of IMG cells. Measured values for the charging time-constant, the threshold current and the voltage threshold for firing (as well as calculated values for the input capacitance, specific membrane resistance, total surface area, cell diameter and cell space-constant) distinguished the neurons classed as phasic-firing from the neurons classed as tonic-firing. There were no statistical differences between the membrane properties of subclass I and II phasic neurons, or the membrane properties of subclass III and IV tonic neurons, to explain why the neurons in subclasses II and III showed a relaxation in electrotonic potentials during current-clamp. In the light of recent voltage-clamp data on the IMG cells the actions of time conductances for potassium ions are discussed to account for the variations in the electrotonic behavior of these IMG cells.

[Analysis of an equivalent electric circuit of the hepatocyte during stopping and restoration of the blood flow]

An analysis of an electric scheme of hepatocyte under conditions of stopping and restoring the blood flow taking into account the morphological and biochemical characteristics of the normal and injured liver was carried out. It is shown that the fast phase of restoring the membrane potential (MP) when restoring the blood flow following ischemia is stipulated by restoring extracellular concentrations of potential-generating ions; and slow restoring of the normal liver MP is connected with the phase of potassium efflux hyperpolarization. The injured liver MP stability to ischemia is higher because of the transmembrane ion gradient decrease before ischemia and next effect of depolarisation of passive currents weakening under conditions of closed extracellular space, and total ATPase activity decrease, i.e. a more economical ATP expenditure by the cell at ischemia.

Fresh and cultured human lens epithelial cells: an electrophysiological study of cell coupling and membrane properties

Microelectrode studies of fresh human and rabbit lens epithelia revealed stable membrane potentials [VR (human) = -36 mV; VR (rabbit) = -45 mV] and low input resistances [Ri (human) = 10 M omega; Ri (rabbit) = 20 M omega]. Coupling studies, using two voltage microelectrodes, demonstrated that the low input resistance of the fresh epithelial tissue was due to electrotonic coupling, which was found to be extremely labile and sensitive to perfusion of the apical (fibrefacing) surface of the epithelium. The intercellular coupling could be stabilized by raising the calcium concentration of the perfusate. Studies performed on confluent monolayers of cultured human lens epithelial (HLE) cells demonstrated a membrane potential (VR = -33 mV) and input resistance (Ri = 29 M omega) similar to their fresh counterparts. The intercellular coupling of these cells was found to be much more robust. Ultrastructural studies revealed that the apical junction of cultured HLE cells was less complex than that found in fresh tissue, the latter exhibiting multiple interdigitations and folds. The cultured monolayer was dissociated into single cells by a variety of methods and the membrane properties of individual cells were studied. Single cells were found to have a lower membrane potential (-20 to -25 mV) and an input resistance in the range 110-170 M omega, depending on the method of dissociation. Channel blocking and ion replacement studies revealed significant conductance pathways for potassium, sodium and chloride and a cell-attached patch clamp investigation revealed three distinct channel types. Of the two channels with inward currents at the resting potential, one, with a conductance of 25 pS, is identified as a non-selective cation channel, and the other, with a conductance of 14 pS and reversal potential of - 14 mV, is a possible candidate for a chloride channel but has yet to be characterized. A third channel with an outward current at the resting potential is identified as a potassium channel with a conductance of 49 pS. A link between epithelial uncoupling and certain types of cataract is proposed.