[Calcium hypothesis of Alzheimer disease].

Alzheimer's disease is the most common neurodegenerative disorder characterized by progressive memory and cognitive abilities loss. The etiology of Alzheimer's disease is poorly understood. In this regard, there is no effective treatment for the disease. Various hypotheses to explain the nature of the pathology of Alzheimer's disease led to the development of appropriate therapeutics. Despite of decades of research and clinical trials available therapeutics, at best, can only slow down the progression of the disease, but cannot cure it. This review dedicated to the one of modern hypotheses of Alzheimer's disease pathogenesis implied the impairment of calcium homeostasis as a key event for the development of neurodegenerative processes.

[Two types of store-operated channels in A431 cells]. / Dva tipa depoupravliaemykh kanalov v kletkakh A431.

Activation of phospholipase C-coupled receptors leads to the release of Ca2+ from Ca2+ stores, and subsequent activation of store-operated cation (SOC) channels, promoting sustained Ca2+ influx. The most studied SOC channels are CRAC ("calcium-release activated calcium") channels exhibiting a very high selectivity for Ca2+. However, there are many SOC channels permeable for Ca2+ but having a lower selectivity. And while Ca2+ influx is important for many biological processes, little is known about the types of SOC channels and mechanisms of SOC channel activation. Previously, we described store-operated Imin channels in A431 cells. Here, by whole-cell recordings, we demonstrated that the store depletion activates two types of current in A431 cells--highly selective for divalent cations (presumably, ICRAC), and moderately selective (ISOC supported by Imin channels). These currents can be registered separately and have different developing time and amplitude. Coexisting of two different types of SOC channels in A431 cells seems to facilitate the control of intracellular Ca(2+)-dependent processes.

Low-conductivity calcium channels in the macrophage plasma membrane: activation by inositol-1,4,5-triphosphate.

Local voltage clamping was applied to mouse macrophage plasma membrane to study calcium channels activated by inositol-1,4,5-triphosphate (IP3) and blocked by heparin. These channels were clearly distinguished from IP3-activated channels of the endoplasmic reticulum by their low conductivity (about 1 pSm for 100 mM Ca2+), high selectivity for Ca2+ relative to K+ (P(Ca):P(K) > 1000), calcium inactivation, and activation on hyperpolarization; these properties allowed them to be assigned to the I(CRAC) family. On the other hand, the properties of the IP3 receptors of these channels (IP3R), i.e., the dose-dependent effect of IP3, the IP3 desensitization of the receptor, and the sensitivity to micromolar concentrations of heparin and arachidonic acid were close to those of the endoplasmic reticulum IP3 receptor. The most likely interpretation of these data is that IP3R are not located in the endoplasmic reticulum, but, acting via some kind of conformational change occurring on binding of IP3, transmit a signal from the endoplasmic reticulum to the highly selective Ca2+ channels. This point of view is in agreement with the published "coupling model" [1].

[Low conductivity calcium channels in the plasmatic membrane of macrophages: activation with inositol 1,4,5-triphosphate]. / Kal'tsievye kanaly nizkoi provodimosti v plazmaticheskoi membrane makrofagov: aktivatsiia inozitol (1,4,5)-trifosfatom.

Using patch-clamp technique we have shown that the plasma membrane of mouse macrophages contains calcium channels that are activated by inositol (1, 4, 5)-trisphosphate (IP3) and blocked by heparine. Their conductivity properties strongly differentiate them from IP3-activated channels of endoplasmic reticulum, but make it possible to include them to the ICRAC family. By the other hand, properties of the IP3 receptor (IP3R) of our channels are similar to those of endoplasmic IP3R. Basing on these data we suggest that IP3R could be located out of the plasma membrane, and by some conformational changes transduces the signal to the high selective Ca2+ channel in the plasma membrane. This model well conforms with the known in the literature "coupling model" of calcium signalling [1].

[A new type of IP3-sensitive highly selective calcium channels of low conductance in the plasma membrane of carcinoma A 431 cells]. / Novyi tip IP3-chuvstvitel'nykh vysokoselektivnykh kal'tsievykh kanalov nizkoi provodimosti v plazmaticheskoi membrane kletok kartsinomy A 431.

Application of 0.2-15.0 mM inositol 1,4,5-trisphosphate (IP3) to excised plasma membrane patches from A 431 cells induced activity of low conductance channels that were highly selective for Ca2+ and Ba2+. With 100 mM Ca2+ or 105 mM Ba2+, the channel conductance was 1.1 pS for the minor conductance substrate. The dose response curve gave an apparent binding constant of 0.18 microM IP3. The channel open probability displayed a strong potential dependence: it decreased markedly upon depolarization, and the half-maximum value was achieved at 73 mV. The dependence of channel activity on the intracellular free Ca2+ concentration was bell shaped, but markedly different from that reported for endoplasmic IP3 receptor. The same channels were detected in intact cells upon application of calcium mobilizing reagents. The activity induced in cell attached mode disappeared after patch excision but could be resumed by application of IP3. The data obtained argue for IP3 acting directly on plasma membrane Ca2+ channels.

[Activation of the calcium channels of A-431 cells by epidermal growth factor]. / Aktivatsiia kal'tsevykh kanalov kletok A-431 épidermal'nym faktorom rosta.

The cell-attached version of patch-clamp technique was used to search for calcium permeable channels in human carcinoma A-431 cells. With 100 mM CaCl2 in pipette, the inward currents were recorded having the mean unitary conductance of 2.8 pS and the reversal potential (obtained by linear extrapolation) equal to +25.5 mV. Application of the epidermal growth factor (EGF) into the bath extracellular solution produced a transient increase in probability for these channels to be open. The effect developed with a delay of about 20 seconds to last thereafter for 36 seconds (mean values). We propose that these channels mediate EGF-induced increase in concentration of cytosolic free calcium.

[Effect of glutaraldehyde on the activation and inactivation of sodium channels in frog nerve fibers]. / Vliianie glutaral'degida na aktivatsiiu i inaktivatsiiu natrievykh kanalov nervnogo volokna liagushki.

Effect of glutaraldehyde on gating of sodium channels was studied in the nodal membrane of the frog nerve fibre. After treatment the inactivation became slower, incomplete and the steepness of its voltage-dependence considerably decreased. The activation became slower and the steepness of its voltage-dependence decreased though both effects for the activation were less pronounced than for the inactivation. Effective charge of the activation as determined from the limiting logarithmic slope of the activation curve did not change significantly. Possible explanations of the phenomena observed are discussed.

[Activation and inactivation of batrachotoxin-modified sodium channels of nerve fiber membranes in the frog]. / Aktivatsiia i inaktivatsiia modifitsirovannykh batrakhotoksinom natrievykh kanalov membrany nervnogo volokna liagushki.

Currents through batrachotoxin (BTX)-modified sodium channels in frog myelinated nerve were measured under voltage-clamp conditions. Nonlinearity of "instantaneous" current-voltage relations was taken into account when determining steady-state parameters of channel activation. BTX induces the shift of voltage dependence of channel activation towards more negative potentials by 67 mV, without changes in its steepness. Current kinetics and effect of preceding depolarization on current size suggest that BTX-modified channels are capable for partial inactivation. High level of steady-state conductance of BTX-modified channels can be explained by suggestion that open state of the channel is energetically more profitable than inactivated one. It is concluded that effect of BTX on inactivation is different in principle from that of pronase and protein reagents.

[Potential-dependent interaction of chemical reagents with the sodium channel gating mechanism in a nerve fiber]. / Potentsialozavisimoe vzaimodeistvie khimicheskikh reagentov s vorotnym mekhanizmom natrievogo kanala v nervnom volokne.

Effect of two water-soluble carbodiimides (WRC) and Woodward's reagent K (RK) on gating in sodium channels of nodal membrane was examined. The WRC treatment (pH 4.8-5.2) at holding potential from -80 to -100 mV resulted in a considerable slowing down of activation and inactivation processes, and rendered the gating machinery less sensitive to membrane potential changes. Effective charge of activation (Zef) determined from limiting logarithmic slope of activation curve at potentials where channels just begin to open was reduced by the factor of 1.7. The same treatment but at zero holding potential did not reduce Zef, and changed other parameters of gating function to a less degree. RK at high negative holding potentials induced changes in properties of gating machinery similar to those induced by WRC at zero potential. RK effects were less potential-dependent than those of WRC. The results suggest that gating machinery of sodium channel incorporates at least two types of carboxyl groups. The first type: "mobile", play the role of gating particles; during depolarization they move from the surface into the membrane causing opening or inactivation of the channel. The second type: "immobile", is always exposed to the external solution regardless of the channel state.

[Selectivity and sensitivity to blocking by hydrogen ions of batrachotoxin-modified sodium channels in nerve fiber membranes]. / Selektivnost' i chuvstvitel'nost' k blokirovaniiu ionami vodoroda modifitsirovannykh batrakhotoksinom natrievykh kanalov v membrane nervnogo volokna.

Currents through normal and batrachotoxin-modified sodium channels in frog nerve were measured under voltage clamp conditions. Measured reversal potentials and the Goldman equation were used to calculate relative permeabilities. The permeability ratios were: PNa: PNH4: PK = 1: 0.47: 0.19. Hydrogen-to-sodium permeability ratio was estimated from reversal potential measurements in Na-free acid (pH 3.7-3.8) solutions. It was 528 +/- 46 for batrachotoxin-modified sodium channels. Modified channels were less sensitive to hydrogen block as compared with normal ones. The difference in apparent pKa for acid group between normal and modified channels was about 0.40.

[Potential-dependent changes in the ionic selectivity of batrachotoxin-modified sodium channels of a frog nerve fiber]. / Potentsialozavisimye izmeneniia ionnoi selektivnosti modifitsirovannykh batrakhotoksinom natrievykh kanalov nervnogo volokna liagushki.

Currents through batrachotoxin-modified sodium channels in frog myelinated fibres were measured under voltage-clamp conditions. Reversal potential (Erev) of steady-state currents is shown to be about 5 mV less positive than Erev of initial (peak) currents. Control experiments with procaine and tetrodotoxin in external solutions showed that this shift of Erev during depolarizing pulse cannot be accounted for by the presence of unmodified sodium channels, unblocked potassium channels, nonlinearity of the leakage or any changes in transmembrane gradients of current-carrying cations. "Instantaneous" current measurements showed that Erev becomes less positive as amplitude and duration of preliminary depolarization increase. The results obtained are consistent with assumption that sodium-potassium selectivity of the batrachotoxin-modified channels depends on potential.

[Ion currents through batrachotoxin-modified sodium channels of node of Ranvier membranes at high positive and negative potentials]. / Ionnye toki cherez modifitsirovannye batrakhotoksinom natrievye kanaly membrany perekhvata Ranv'e pri vysokikh polozhitel'nykh i otritsatel'nykh potentsialakh.

Ionic currents through batrachotoxin-modified sodium channels in frog nerve fibres were measured over a wide range of membrane potentials. At potentials above +80 mV currents decay in time and their steady-state level decreased as potentials increased. "Instantaneous" current measurements have shown that this phenomenon was due to the decrease in net channel conductance. Scorpion toxin affected current kinetics only slightly at these potentials, which suggested that these decays were not caused by usual inactivation process. Externally applied procaine induced slow (tens of ms) potential-dependent block of batrachotoxin-modified channels at large positive potentials. At large negative potentials (above -100 mV) "instantaneus" currents decreased due to fast voltage-dependent block of the channels by calcium ions.

[Hydrogen ion currents through sodium channels in myelinated nerve fiber membranes]. / Toki ionov vodoroda cherez natrievye kanaly v membrane mielinovogo nervnogo volokna.

Ionic currents in the nodal membrane of myelinated frog nerve fibre were measured under voltage clamp conditions when the Ranvier node was bathed in solutions containing impermeant cations instead Na. At pH lower than 4.0 small (less than 0.1 nA) currents were detected which rose to peak and then decayed more slowly. Kinetics and voltage range of activation of these currents were similar to those of usual sodium currents at low pH. These currents were reversibly blocked by benzocaine (1 mM). All this permitted identifying them as currents through sodium channels. Experiments in which concentrations of substituting cations (tris+, choline+), Ca2+ and H+ ions were varied showed that the inward currents observed are carried by hydrogen (or hydronium) ions. According to reversal potential measurements the relative permeability of the channels (PH/PNa) is equal to 203 +/- 14 on the average. It is concluded that the energy barriers for H+ in sodium channel are much lower than for Na+, but their passage through the channel is slow because of binding to an acidic group in the channel.

[Hydrogen currents through aconitine-modified sodium channels in nerve fiber membranes]. / Vodorodnye toki cherez modifitsirovannye akonitinom natrievye kanaly v membrane nervnogo volokna.

Ionic currents in nodal membrane treated with aconitine were measured under voltage clamp conditions when nodes were bathed in Na-free solutions. At pH lower than 4.6 inward ionic currents were detected which had kinetics and voltage range of activation analogous to those of aconitine-modified sodium channels at low pH. These currents were blocked by benzocaine (2 mM). Experiments with various concentrations of Ca2+, tris+, TEA+, choline+ ions showed that these ions are essentially impermeable both at normal and acidic pH. It is concluded that the inward currents observed are carried by H+ (or H3O+) ions through aconitine-modified sodium channels. From reversal potential measurements relative permeability (PH/PNa) of sodium channels is estimated to be 1059 +/- 88. The results suggest that the aconitine-modified channel is a rather wide water-filled pore and the rate of H+ passing through the channel is limited by its binding to an acidic group.

[Kinetics of sodium current decay during normal axon membrane repolarization and in the presence of scorpion toxin]. / Kinetika spada natrievogo toka pri repoliarizatsii aksonnoi membrany v norme i v prisuststvii toksina skorpiona.

Decay of sodium currents in repolarization ("tail current") was studied in from axonal membrane. The decay in the membrane repolarization to -40 divided by -60 mV has two exponential components: fast and slow. The fraction of the slow component in the total "tail current" (theta M) decreases as the repolarization potential (Vp) becomes more negative; at Vp more negative than -80 mV "tail" follows practically one-exponential time course. When lengthening the test pulse (at the given Vp) the fraction of the fast component in the "tail" decreases quicker than that of the slow component, following approximately the kinetics of inactivation during the tests pulse. Scorpion toxin treatment results in slowing down "tail" kinetics mainly at the expense of increasing the fraction of the slow component. A kinetic diagram assuming two open state for the channel is suggested. A hypothesis is advanced that scorpion toxin, DDT and trinitrophenol have a common "site" to interact with the gating mechanism of the sodium channel.

[Features of the kinetic and stationary characteristics of aconitine-modified sodium channels]. / Nekotorye osobennosti kineticheskikh i statsionarnykh kharakteristik natrievykh kanalov, modifitsirovannykh akonitinom.

Kinetic and steady-state characteristics of aconitine-modified sodium channels were studied in the Ranvier node membrane. Aconitine-modified sodium channels are shown to be inactivated only partially. The voltage dependence of the fraction of noninactivated channels (h infinity) may be described by a three-state model of the channel with closed, open and inactivated states. A reasonable agreement with the data was obtained when parameters of the inactivated state were supposed to be not changed after aconitine modification of the channels. The membrane repolarization to -70 divided by -110 mV, after long (10 ms) depolarizing shift induces firstly fast current decay ("tail") and then its rather slow increase to a steady-state level. Kinetics of this current requires two or more open states to be postulated.

[Kinetics of the reaction between scorpion toxin and the sodium channels of nodes of Ranvier]. / Kinetika vzaimodeistviia toksina skorpiona s natrievym kanalom membrany perekhvata Ranv'e.

The kinetics of the scorpion Buthus eupeus venom toxin binding to sodium channels at holding potential -120 mV and subsequent dissociation of the toxin-channel complex with the membrane potential shifts to -60 divided by +120 mV were studied under voltage clamp conditions in the Ranvier node membrane. The dissociation rate is shown to increase exponentially when shifting the membrane potential towards more positive values, with an e-fold increase for each 32.3 mV. The results are consistent with the suggestion that the dissociation of the toxin-channel complex is due to a difference in electric energy between normal and toxin-modified inactivated sodium channels.

[Effect of toxins from the scorpion Buthus eupeus on the sodium channels of the membranes of nodes of Ranvier]. / Deistvie toksinov skorpiona Buthus eupeus na natrievye kanaly membrany perekhvata Ranv'e.

Effect of three toxins M3, M7 and M8 from Buthus eupeus poison on sodium channels of Ranvier node membrane of the frog was studied under potential fixation. Toxin M3 was found to produce no effect on sodium currents of Ranvier node membrane of the frog. Inactivation kinetics difference between normal and modified sodium channels was used in estimating a fraction of toxin-modified channels. The toxins were found to interact with the channels in the ratio 1 : 1, the dissociation constants for M7 and M8 equalling 1.7 . 10(-7) M and 0.9 . 10(-9) M respectively. Dependence of the effect of scorpion toxins on the membrane potential is stated.