Effect of protein reagents on electrotonic coupling in crayfish septate axon

The lateral giant axons of the crayfish nerve cord are composed of segments contributed by each ganglion, which are electrotonically coupled by way of gap junctions. We have investigated the involvement of protein residues in regulating the resistance of crayfish junctional channels by determining effects of group-specific protein reagents. When applied to well-coupled axons, the sulfhydryl group reagents N-ethylmaleimide (NEM) and diamide uncoupled the segments; junctional resistance (R(j)) was increased without changing membrane resistance or axoplasmic pH (pH(i)). The uncoupling produced by NEM could be reversed by alkalinization of the cytoplasm (addition of ammonium chloride to the external medium). Another sulfhydryl reagent (p-chloromercuribenzoic acid) increased R(j) to a lesser extent. A disulfide reagent and three amino acid and three carboxyl group reagents had no effect on the R(j) of these axons. The effect of group-specific reagents on partially uncoupled axons was tested by applying the drugs to axons previously exposed to weak acids. N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline recoupled partially uncoupled axons by decreasing R(j) and prevented subsequent uncoupling of the junction by low pH(i). Another carboxyl group reagent, as well as sulfhydryl and amino group reagents, either had no effect or uncoupled the axons further by increasing R(j). These experimental results suggest that amino acid residues, possibly containing carboxyl and sulfhydryl groups, control the opening and closing of junctional channels and may thus be associated with the channels' active sites.