Effects of reagents modifying carboxyl groups on the gating current of the myelinated nerve fiber

Abstract

The effect of the carboxyl group activating reagent N-ethoxy-carbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) on the gating current of the frog node of Ranvier was investigated. A 10-min treatment with 2 mM EEDQ (in the presence or absence of 10 mM ethylenediamine) irreversibly reduced the slope of the on charge-voltage relation Qon(E), shifted its midpoint potential Emid in the positive direction and reduced the maximum charge Qon max measured with strong depolarizing pulses. In six experiments, 2 mM EEDQ + 10 mM ethylenediamine increased the factor k (a reciprocal measure of the slope of the Qon(E) curve) from 16 to 22 mV. In five experiments, 2 mM EEDQ alone increased k from 16 to 23 mV. In a single experiment, 5 mM EEDQ + 10 mM ethylene diamine increased k from 17 to 31 mV. The reduction in slope suggests that EEDQ decreases the valence of the gating particles or reduces the fraction of the membrane field that they traverse. In addition, EEDQ (which inhibits inactivation of the sodium current, see M. Rack and K.H. Woll, J. Membrane Biol. 82:41-48, 1984) caused a small increase of the off charge Qoff, and a marked increase of the Qoff/Qon ratio, i.e. inhibited charge immobilization. Since the effects of EEDQ occurred regardless of the presence or absence of ethylenediamine, they are probably due to crosslinking reactions. The effects of EEDQ were compared with those of the water-soluble carbodiimide EDC. Treatment with 10 or 50 mM EDC (plus 10 or 50 mM ethylenediamine) caused a smaller increase of k than treatment with 2 mM EEDQ but reduced Qon max by the same amount.