Concanavalin A blocks the Ca2+ -dependence of crayfish muscle fiber responses to glutamate

Inhibition of a voltage-dependent Ca current by concanavalin A

Incubation of the hypotrichous ciliate Stylonychia mytilus in fluorescein-labeled concanavalin A (Con A, 0.1-0.5 microgram/ml) produced a strong fluorescence of its membranelles, but comparatively weak fluorescence of the other compound cilia and of the somatic membrane. Compared to untreated cells, the frequency of spontaneous backward movements was reduced in the presence of 0.5 microgram/ml ConA. In electrophysiological experiments Con A altered the excitability of the cell membrane. The two-peak action potential lost its second component which is associated with voltage-dependent Ca channels in the membranelles. The corresponding Ca current (Ca current I) was inhibited by low concentrations of Con A (0.2-0.5 microgram/ml). A second voltage-dependent Ca current (Ca current II) was not affected. Reducing the K outward current by intracellular Cs and/or extracellular tetraethylammonium, or changing the holding potential, did not restore the Con A-sensitive Ca current I. Con A also inhibited this current when Ca was replaced by Ba. The inhibitory effect of Con A on the voltage-dependent Ca current I was prevented by 10-30 mM alpha-methyl-D-mannoside, and the lectin wheat germ agglutinin (20 micrograms/ml) did not affect the Ca currents, indicating that the Con A effect was mediated by binding to specific sugar residues on the excitable membrane. The succinylated dimeric derivative of Con A did not inhibit Ca current I up to concentrations of 5 micrograms/ml. It is concluded that the two voltage-dependent Ca currents in Stylonychia can be chemically isolated due to their different sensitivity to Con A, which appears to bind preferentially to sites near or at the Ca channel in the membranellar membrane.

Postsynaptic effects of nickel ions at the insect neuromuscular junction

The effect of extracellular nickel on the excitatory postsynaptic response at the insect neuromuscular junction was studied in the segmental muscle of the larval mealworm Tenebrio molitor. The response to L-glutamate applied iontophoretically (glutamate potential, GP) was potentiated in the presence of Ni2+ though the excitatory postsynaptic potential (EPSP) was reduced. It seems unlikely that Ni2+ acts at the same binding site as L-glutamate does since the value of the limiting slope of double logarithmic plots for the action of glutamate was increased in the presence of Ni2+. The potentiation of GP in the presence of Ni2+ cannot be ascribed to competition between Ni2+ and Ca2+ since GP amplitude did not show any dependence on the concentration of Ca2+. Nickel ions did not alter the reversal potential of excitatory postsynaptic current (EPSC) and glutamate current (GC) under the voltage clamp condition, whereas the amplitude of GC was potentiated in the presence of Ni2+. The time constant of the decay of EPSC showed a weak voltage dependency: the more depolarized the membrane, the more prolonged the time constant. In the presence of 1 mM Ni2+ the amplitude of miniature EPSCs (MEPSCs) increased and the half decay time was prolonged significantly. These results suggest that Ni2+ interacts with charged groups near the glutamate receptor-channel complex so that the kinetics of the channel are altered.

The effect of calcium ions on the glutamate response and its desensitization in crayfish muscle fibres

The responses of crayfish muscle fibres to bath application or long ionophoresis of L-glutamate were studied in normal and low Ca2+ solutions. The smaller responses recorded in low Ca2+ solutions have characteristics suggesting a faster desensitization. Desensitization and recovery have complex kinetics. Desensitization is faster and recovery slower when external Ca2+ concentration is reduced. Both components of the recovery phase, which can be fitted by the sum of two exponentials, are affected by the external Ca2+ concentration. Recovery can be accelerated by external Ca2+ ionophoresis onto desensitized glutamate receptors. Responses to brief glutamate pulses of low intensity are not affected by Ca2+ reduction. For higher intensities, signs of desensitization are detectable early in the rising phase of the response. Concanavalin A (Con A) blocks both desensitization and Ca2+ dependence with similar time courses. Whether or not the preparation has been treated with Con A, the slowly rising responses recorded in isotonic Ca2+ do not show signs of desensitization. Con A causes a partial blockade of the glutamate response. The Ca2+ dependence of the glutamate response can be explained by the Ca2+ dependence of the desensitization process, the cation acting at ectocellular sites of the muscle membrane.