Axonal surface charges: evidence for phosphate structure

Effects of chemical modification of carboxyl groups on the voltage-clamped nerve fiber of the frog

Voltage-clamped single nerve fibers of the frog Rana esculenta were treated with the carboxyl group activating reagent N-ethoxy-carbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) in the presence of different primary amines and without added amine. Carboxyl groups form stable amide bonds with primary amines in the presence of EEDQ. EEDQ treatment reduced the sodium current considerably and irreversibly, regardless of the presence of a primary amine in the Ringer's solution. The potassium current was also reduced. After modification the reduced sodium currents inactivated slowly and incompletely. The descending branch of the sodium current-voltage relation, INa(E), was shifted along the voltage axis in the depolarizing direction. The size of the shift was strongly dependent on the amine present during modification with EEDQ. The voltage-dependence of sodium inactivation, h infinity (E), was shifted to more positive values of membrane potential by EEDQ in the presence of ethylenediamine (11 mV) and glucosamine (3 mV). In contrast, a small shift to more negative potentials occurred in the presence of taurine (-3 mV) or without the addition of an amine (-2 mV). A tenfold increase of the calcium concentration still shifted the INa(E) and h infinity (E) curves of the chemically modified fibers. However, these shifts were smaller than those observed on untreated fibers. The currents remaining after the modification were completely blocked by tetrodotoxin; no change of the reversal potential occurred.

Interactions of uranyl ions with lipid bilayer membranes

Uranyl ions (UO2(2+)), once bound to the phosphate moieties of phospholipid head groups, stabilize bimolecular lipid membranes (BLMs) as well as decrease the nonactin-induced membrane conductance. UO2(2+) bind to a phosphatidyl choline-cholesterol (2:1, molar ratio) BLM surface with a dissociation constant of 2.3 microM and a maximum change in surface potential of 88 mV, which corresponds approximately to one uranyl ion per 31 nm2 surface area. Furthermore, uranyl ions can penetrate the lipid bilayers as neutral complexes such as uranyl acetate.

Screening of membrane surface charges by divalent cations: an atomic representation

To arrive at a clear and atomically realistic representation of the process of ionic screening, a model with the following necessary and justifiable constraints was devised. 1) The minimum internuclear distance (IND) between a negative site on the membrane and a cation screening the site is equal to the sum of the site's "equivalent" radius (rs) + the diameter of a water molecule (approximately 2.8 A) + crystal radius of the cation (rc). 2) The average value for the dielectric constant (D) over IND is given by D approximately 80 ((IND - rs - rc)/IND). When this simple atomic model for ionic screening is employed in conjunction with equilibrium ion-selectivity theory, it is possible to predict quantitatively, from coulombic energy calculations, the secondary stereospecific actions of certain alkaline-earth cations as well as the predominant screening effect of these divalent cations at the surfaces of different types of membranes. The model also successfully predicts the transition from a predominantly screening situation to a predominantly binding situation, which was observed experimentally when negative surface charge density was decreased in nerve.

Slow action of Ca on myelinated nerve fibres of Xenopus laevis

Single myelinated nerve fibres were isolated and the nodal currents were recorded with potential clamp technique. Rapid solution changes were performed by use of a recording chamber so shaped that it had a minimal dead space. A volume ten times larger than the dead space flowed past the node within about 20 ms. The half-time of the change in Na current (INa)amplitude associated with changes between solutions of different [Na]:s was 100-150 ms, whereas the half-time of the change in INa associated with changes between solutions of different [Ca]:s was 200-300 ms. The external [Ca] affects INa through its action on the Na permeability properties of the nodal membrane. The difference in rate of effect between changes in [Na] and [Ca] was also noticeable when simultaneous changes were performed. These findings are discussed with regard to the properties of the nodal gap.

Structural characteristics of the saxitoxin receptor on nerve

The effects of uranyl ion (UO22+; at low concentrations binds specifically to phosphate groups) and the cationic dye methylene blue (MB+; binds strongly to carboxyl groups) on saxitoxin (STX) potency in crayfish axon has been studied by means of intracellular microelectrodes. At pH 6.00 +/- 0.05 and 13.5 mM Ca2+, addition of 10.0 muM UO22+ + 5.0 nM STX had only slightly, if any, less effect on the spike's maximum rate of rise [0.79 +/- 0.04 (viz., mean +/- SEM) of control value] than did addition of 5.0 nM STX alone (0.72 +/- 0.05). Under the same conditions of pH and Ca2+ concentration, 1.0 mM MB+ had approximately the same effect: 1.0 mM MB+ + 5.0 nM STX, 0.76 +/- 0.03; 5.0 nM STX alone, 0.70 +/- 0.04. However, at pH 7.00 +/- 0.05 and lower Ca2+ concentrations, 1.0 mM MB+ significantly reduced STX potency. Using 6.0 mM Ca2+: 1.0 mM MB+ + 5.0 nM STX, 0.92 +/- 0.01; 5.0 nM STX alone, 0.68 +/- 0.08. Using 3.0 mM Ca2+, the corresponding values were 0.94 +/- 0.03 and 0.67 +/- 0.04. It is concluded that: (1) In accord with previous suggestions, the ionized acidic group known to exist in the Na channel (and to which a guanidinium group of STX appears to bind) is very likely a carboxyl group and not a phosphate group. (2) The accessible part of the Na channel mouth serving as the saxitoxin receptor probably does not include phospholipid in its structure proper.