Heterogeneity of external surface charges near sodium channels in the nodal membrane of frog nerve

[BIOELECTRIC ACTIVITY OF INTERNEURONES OF SPINAL CORD IN THE EXPERIMENTAL MENOPAUSE IN FEMAL RATS]

We analyzed the amplitude-time characteristics of the components of the potential dorsal surface of the spinal cord (PDS SC) in experimental menopause. The excitation threshold increased at 102.14% and the latent period at 94.12% in animals with experimental menopause. In the context of supramaximal stimulation of dorsal root L5 identified N1 amplitude growth at 10.14%, N2 at 11.82%, N3 at 48.28%, and P-wave 31.58% and to increase component N3 26.54% in the experimental group. At condition of paired stimulation pulses on the time interval from 2 to 3 ms there was a significant increase, and from 6 to 30 ms--a depression of N1-component of the second PDS SC in the group of animals with experimental menopause. Thus, our results suggest the presence of changes in nerve structures of the posterior horn of the spinal cord at conditions of estrogen deficiency.

Modulation of voltage-gated ion channels by sialylation

Control and modulation of electrical signaling is vital to normal physiology, particularly in neurons, cardiac myocytes, and skeletal muscle. The orchestrated activities of variable sets of ion channels and transporters, including voltage-gated ion channels (VGICs), are responsible for initiation, conduction, and termination of the action potential (AP) in excitable cells. Slight changes in VGIC activity can lead to severe pathologies including arrhythmias, epilepsies, and paralyses, while normal excitability depends on the precise tuning of the AP waveform. VGICs are heavily posttranslationally modified, with upward of 30% of the mature channel mass consisting of N- and O-glycans. These glycans are terminated typically by negatively charged sialic acid residues that modulate voltage-dependent channel gating directly. The data indicate that sialic acids alter VGIC activity in isoform-specific manners, dependent in part, on the number/location of channel sialic acids attached to the pore-forming alpha and/or auxiliary subunits that often act through saturating electrostatic mechanisms. Additionally, cell-specific regulation of sialylation can affect VGIC gating distinctly. Thus, channel sialylation is likely regulated through two mechanisms that together contribute to a dynamic spectrum of possible gating motifs: a subunit-specific mechanism and regulated (aberrant) changes in the ability of the cell to glycosylate. Recent studies showed that neuronal and cardiac excitability is modulated through regulated changes in voltage-gated Na(+) channel sialylation, suggesting that both mechanisms of differential VGIC sialylation contribute to electrical signaling in the brain and heart. Together, the data provide insight into an important and novel paradigm involved in the control and modulation of electrical signaling.

Deglycosylation of the beta1-subunit of the BK channel changes its biophysical properties

Large-conductance Ca(2+)-activated potassium (BK) channels are composed of pore-forming alpha-subunits and auxiliary beta-subunits. The alpha-subunits are widely expressed in many cell types, whereas the beta-subunits are more tissue specific and influence diverse aspects of channel function. In the current study, we identified the presence of the smooth muscle-specific beta1-subunit in murine colonic tissue using Western blotting. The native beta1-subunits migrated in SDS-PAGE as two molecular mass bands. Enzymatic removal of N-linked glycosylations from the beta1-subunit resulted in a single band that migrated at a lower molecular mass than the native beta1-subunit bands, suggesting that the native beta1-subunit exists in either a core glycosylated or highly glycosylated form. We investigated the functional consequence of deglycosylating the beta1-subunit during inside-out single-channel recordings. During inside-out single-channel recordings, with N-glycosidase F in the pipette solution, the open probability (P(o)) and mean open time of BK channels increased in a time-dependent manner. Deglycosylation of BK channels did not affect the conductance but shifted the steady-state voltage of activation toward more positive potentials without affecting slope when Ca(2+) concentration was <1 microM. Treatment of myocytes lacking the beta1-subunits of the BK channel with N-glycosidase F had no effect. These data suggest that glycosylations on the beta1-subunit in smooth muscle cells can modify the biophysical properties of BK channels.

Relationship between age of menopause and cell-mediated immune hypersensitivity in right- and left-handed women

The aim of this work was to study the relationship between age of menopause and cell-mediated immune hypersensitivity in right- (N = 32) and left-handed (N = 15) women who had experienced menopause after age of 34 at least one year before interview. Age of menopause was higher in right-handed than left-handed women. Cell-mediated immunity was higher in left-handers than right-handers. Hand-preference correlated with age of menopause, but inversely correlated with tuberculin reaction and percentages of CD4+ and CD8+ lymphocytes; age of menopause correlated with cell-mediated immunity. The results suggested that early menopause in left-handed women may be due to a more active immune system, especially cell-mediated immunity.

Blocking effect of lanthanum on delayed-rectifier K+ current in Drosophila neurons

The delayed-rectifier potassium current (IKDR) is important in regulating neuronal excitability. The authors characterized the neurotoxic effect of lanthanum on IKDR. The conventional whole-cell patch-clamp technique was applied to cultured Drosophila neurons derived from embryonic neuroblasts. IKDR was measured from neurons before and after application of 0.1 mM lanthanum to the external saline. IKDR was smaller in the lanthanum-containing saline (441 +/- 57 pA) than in the control saline (680 +/- 35 pA) (p <.001). Activation and inactivation of IKDR were unchanged by lanthanum. Because these results suggest that lanthanum acts as a potent blocker of IKDR, neuronal excitability may be altered during lanthanum neurotoxicity.

Expression of Kv1.1 delayed rectifier potassium channels in Lec mutant Chinese hamster ovary cell lines reveals a role for sialidation in channel function

Kv1.1 potassium (K+) channels contain significant amounts of negatively charged sialic acids. To examine the role of sialidation in K+ channel function, Chinese hamster ovary cell lines deficient in glycosylation (Lec mutants) were transfected with rat brain Kv1.1 cDNA. The K+ channel was functionally expressed in all cell lines, but the voltage dependence of activation (V1/2) was shifted to more positive voltages and the activation kinetics were slower in the mutant cell lines compared with control. A similar positive shift in V1/2 was recorded in control cells expressing Kv1.1 following treatment with sialidase or by raising extracellular Ca2+. In contrast, these treatments had little or no effect on the Lec mutants, which indicates that channel sialic acids appear to be the negative surface charges sensitive to Ca2+. The data suggest that sialic acid addition modifies Kv1.1 channel function, possibly by influencing the local electric field detected by its voltage sensor, but that these carbohydrates are not required for cell surface expression.

Effects of gadolinium on ion channels in the myelinated axon of Xenopus laevis: four sites of action

The action of gadolinium (Gd3+) on ion currents in myelinated axons of Xenopus laevis was investigated with the voltage clamp technique. The analysis revealed the following effects. (i) The potential-dependent parameters of both Na and K channels were shifted. The shift was equally large for activation, inactivation, and activation time constant curves (+9 mV for 100 microM Gd3+). The effects could be explained by screening of fixed surface charges at a density of -1.2 e nm-2. (ii) The rate of gating for both Na and K channels was reduced more than predicted from the shift. This effect could be quantified as a scaling (by a factor 3 and 5 respectively at 100 microM Gd3+) of the activation time constant curves. (iii) An activation- and inactivation-independent block of both Na and K channels, obeying 1:1 stoichiometry with a Kd value of about 70 microM potential-independent block of leakage current, obeying 1:2 stoichiometry with a Kd value of 600 microM. (iv) The analysis suggests separate binding sites for the effects, comprising high affinity modulatory and blocking sites on the channel proteins and low affinity receptors on phospholipids, associated with the effect on the leakage current.

Intra and extracellular surface charges near Ca2+ channels in neurons and neuroblastoma cells

The properties of low (LVA) and high (HVA) voltage-activated calcium currents were investigated in rat sensory neurons and a murine neuroblastoma cell line exposed to various concentrations of intra- or extracellular monovalent ([c+]i/o) and trivalent ([c3+]i/o) cations. In neurons, when [c+]i was changed from 150 to 20 mM, positive shifts of 18-28 mV were observed in activation curves of both LVA and HVA currents, as well as in LVA inactivation curves. Extracellularly, in divalent-free solutions, [c+]o of 20-50 mM produced medium (12-22 mV) negative shifts of the LVA channel properties. These data were used to estimate, by a "screening" model, a negative surface charge density around neuron's calcium channels of 1/1,000 and 1/1,325 eA-2 at the outside or inside face, respectively. In the presence of physiological concentrations of divalent cations, [c+]o of 20-60 mM caused smaller (4-11 mV) negative shifts of the activation and inactivation curves, which can be explained by assuming a partial neutralization of negative charges by divalent cations. By applying the above procedure to LVA channels of neuroblastoma cells, the ratio of extra- to intracellular surface charge density turned out to be more than tenfold higher than in neurons. Effects produced by [c3+]i/o were not in agreement with expectations based on screening or binding models.

Mechanisms of extracellular divalent and trivalent cation block of the sodium current in canine cardiac Purkinje cells

1. Single canine cardiac Purkinje cells were internally perfused and voltage clamped with a large-bore perfusion pipette for measurement of sodium ionic current (INa) in the absence and presence of extracellular group IIA divalent cations (Mg2+, Ba2+ and Ca2+), transition divalent cations (CO2+, Mn2+ and Ni2+), group IIB divalent cations (Cd2+ and Zn2+), and the trivalent cation La3+. 2. Open channel block of cardiac INa by external Ca2+, assessed from instantaneous INa-voltage (I-V) relationships, has been well described by a two-barrier, one-well model with a dissociation constant at 0 mV, KB(0), of 37 mM and an electrical distance, z' = delta, of 0.34. At the most negative test potentials there was less block of INa than predicted by the model, but correction of INa for the contribution of Na+ channel gating current (Ig) to the peak current improved the fit by the model. 3. The divalent cations Ba2+, Mg2+, CO2+ and Mn2+ produced voltage-dependent, open channel block of INa, which by the two-barrier, one-well model predicted a similar z' about one-third into the membrane field. The relative efficacy for voltage-dependent block was CO2+ > Mn2+ > Ca2+ > Mg2+ > Ba2+ with respective KB(0)s of 11, 13, 37, 43 and 61 mM. 4. Cd2+, Zn2+ and La3+ produced block of INa at low concentrations that was nearly voltage independent with z' < or = 0.13. Fits of single-site binding curves to peak INa in response to step depolarizations at positive test potentials gave the following apparent KD values: Zn2+ 0.14 mM, Cd2+ 0.27 mM and La3+ 0.50 mM. 5. In the presence of Cd2+, INa tail current relaxations were much faster than could be accounted for by Cd2+ binding to and/or screening of extracellular surface charges. Fits of the data to a model that assumed voltage-dependent open channel block during the tail current relaxations estimated the KB(0) for Cd2+ to be 0.80 mM. 6. Both z' and KB(0) for Ni2+ from fits of the two-barrier, one-well model to instantaneous I-V relationships varied as a function of [Ni2+], consistent with the hypothesis that Ni2+ blocked with similar affinity at a voltage-dependent and a voltage-independent site. At [Ni2+] > or = 5 mM, KB(0) was 7.6 mM and z' was 0.21.(ABSTRACT TRUNCATED AT 400 WORDS)

Lipid surface charge does not influence conductance or calcium block of single sodium channels in planar bilayers

We have studied the effects of membrane surface charge on Na+ ion permeation and Ca2+ block in single, batrachotoxin-activated Na channels from rat brain, incorporated into planar lipid bilayers. In phospholipid membranes with no net charge (phosphatidylethanolamine, PE), at low divalent cation concentrations (approximately 100 microM Mg2+), the single channel current-voltage relation was linear and the single channel conductance saturated with increasing [Na+] and ionic strength, reaching a maximum (gamma max) of 31.8 pS, with an apparent dissociation constant (K0.5) of 40.5 mM. The data could be approximated by a rectangular hyperbola. In negatively charged bilayers (70% phosphatidylserine, PS; 30% PE) slightly larger conductances were observed at each concentration, but the hyperbolic form of the conductance-concentration relation was retained (gamma max = 32.9 pS and K0.5 = 31.5 mM) without any preferential increase in conductance at lower ionic strengths. Symmetrical application of Ca2+ caused a voltage-dependent block of the single channel current, with the block being greater at negative potentials. For any given voltage and [Na+] this block was identical in neutral and negatively charged membranes. These observations suggest that both the conduction pathway and the site(s) of Ca2+ block of the rat brain Na channel protein are electrostatically isolated from the negatively charged headgroups on the membrane lipids.

Ion (H+, Ca2+, Co2+) and temperature effects on a hyperpolarization-activated membrane current in the lobster stretch receptor neurone

The effects of some ions (H+, Ca2+, Co2+) and temperature on the Q-channel (a channel which conducts a hyperpolarization-activated membrane current, IQ) was investigated in the slowly adapting lobster stretch receptor neurone. It was found: (1) that increases in pH cause an increase in the maximum channel conductance; (2) that Co2+ (1 mM) and increases in extracellular Ca2+ produce a positive shift of the relationship between the membrane voltage and the steady-state channel activation along the voltage scale; (3) that Co2+ and decreases in extracellular pH cause a decrease in rate of the voltage-dependent step of the channel's gating reaction; (4) that none of the ions has an effect on the rate of the voltage-independent step of the gating reaction; and (5) that Co2+ and K+ do not interfere with each other's actions on the channel's gating reaction and maximum ion conductivity. Increases in temperature from 13 to 23 degrees C were found: (1) to cause an increase in the maximum Q-channel conductance by a factor of 1.7; (2) to have no effect on the voltage dependence of the channel's gating reaction; but (3) to lead to an increase in rate of the gating reaction by a factor of about five for the voltage-dependent, and by a factor of about three for the voltage-independent gating step. The temperature sensitivity of the IQ mechanism proved to be mathematically reproducible by a previously developed, and slightly modified Q-channel model (Edman et al. 1987, Edman & Grampp 1989), and to be of no significance in the control of membrane excitability in a thermally unstable environment.

Selective block of calcium current by lanthanum in single bullfrog atrial cells

A single suction microelectrode voltage-clamp technique was used to study the actions of lanthanum ions (La3+) on ionic currents in single cells isolated from bullfrog right atrium. La3+, added as LaCl3, blocked the "slow" inward Ca2+ current (ICa) in a dose-dependent fashion; 10(-5) M produced complete inhibition. This effect was best fitted by a dose-response curve that was calculated assuming 1:1 binding of La3+ to a site having a dissociation constant of 7.5 x 10(-7) M. La3+ block was reversed (to 90% of control ICa) following washout and, in the presence of 10(-5) M La3+, was antagonized by raising the Ca2+ concentration from 2.5 to 7.5 mM (ICa recovered to 56% of the control). However, the latter effect took approximately 1 h to develop. Concentrations of La3+ that reduced ICa by 12-67%, 0.1-1.5 x 10(-6) M, had no measurable effect upon the voltage dependence of steady state ICa inactivation, which suggest that at these concentrations there are no significant surface-charge effects of La3+ on this gating mechanism. Three additional findings indicate that doses of La3+ that blocked ICa failed to produce nonspecific effects: (a) 10(-5) M La3+ had no measurable effect on the time-independent inwardly rectifying current, IK1; (b) the same concentration had no effect on the kinetics, amplitude, or voltage dependence of a time- and voltage-dependent K+ current, IK; and (c) 10(-4) M La3+ did not alter the size of the tetrodotoxin-sensitive inward Na+ current, INa, or the voltage dependence of its steady state inactivation. Higher concentrations (0.5-1.0 mM) reduced both IK1 and IK, and shifted the steady state activation curve for IK toward more positive potentials, presumably by reducing the external surface potential. Our results suggest that at a concentration of less than or equal to 10(-5) M, La3+ inhibits ICa selectively by direct blockade of Ca channels rather than by altering the external surface potential. At higher concentrations, La3+ exhibits nonspecific effects, including neutralization of negative external surface charge and inhibition of other time- and voltage-dependent ionic currents.

Kinetics of sodium current and gating current in the frog node of Ranvier

The experiments were done on voltage-clamped nodes of Ranvier of the frog. The aim was to study the kinetics of sodium current INa and gating current Igat over a large potential range (-92 to -12 mV) and to compare the time constants for the turning-on of INa or Igat with those for the turning-off measured at the same potential. Sodium tail currents were recorded at different postpulse potentials. Inactivation was inhibited by a few min treatment with 0.5 mM chloramine-T (Wang 1984). The sodium permeability was activated by a 0.4 ms pulse from holding potential (-92 mV) to about 0 mV. At the peak of INa the membrane was repolarized to postpulse potentials between -92 and -12 mV. At E greater than -60 mV the tail currents decayed with two time constants, tau 1 and tau 2, reflecting presumably the turning-off and the inactivation of the sodium permeability. The relation between tau 1 and postpulse potential was bellshaped with a maximum at -32 mV. The tail currents could also be fitted by the Hodgkin-Huxley equation with the sodium activation variable m raised to the second or third power. At E less than -50 mV tau m off was equal to 2 tau 1 or 3 tau 1, respectively, whereas at E greater than -25 mV tau m off was equal to tau 1. In addition, the time constant of the turning-on of sodium activation m (tau m on) was determined, assuming INa approximately m2 (with a small initial delay) or INa approximately m3 (without an initial delay). At -22 mV and -12 mV the ratio tau m off/tau m on was close to 1.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of various cations and anions on the action of tetrodotoxin and saxitoxin on frog myelinated nerve fibers

The influence of Mg2+, La3+, NO-3, and SCN- on the equilibrium effect of tetrodotoxin (TTX) and saxitoxin (STX) on single myelinated nerve fibres of the frog Rana esculenta was studied under voltage clamp conditions. Mg2+ and La3+ reduce the sodium permeability, shift the voltage dependence of the Na permeability PNa towards more positive potentials and reduce the effectiveness of TTX and STX. NO-3 and SCN- reduce the sodium permeability too, but shift the voltage dependence of PNa towards more negative potentials and increase the action of TTX and STX. In all experiments the change in effectiveness is larger for the divalent STX than for the monovalent TTX. It is concluded that changes of the monovalent TTX. It is concluded that changes of the external surface potential induced by Mg2+, La3+, NO-3 and SCN- affect the TTX and STX binding to toxin receptors. The apparent potential change at the toxin receptor is only a fraction of the change "seen' by the Na channel gates.