Interactions of neosaxitoxin with the sodium channel of the frog skeletal muscle fiber.

Abstract

Neosaxitoxin (neoSTX) differs structurally from saxitoxin (STX) in that the hydrogen on N-1 is replaced by a hydroxyl group. On single frog skeletal muscle fibers in the vaseline-gap voltage clamp, the concentrations for reducing the maximum sodium current by 50% (EDs0) at pH's 6.50, 7.25, and 8.25 are, respectively, 4.9, 5.1, and 8.9 nM for STX and 1.6, 2.7, and 17.2 nM for neoSTX. The relative potencies of STX at the different pH's closely parallel the relative abundance of the protonated form of the 7,8,9 guanidinium function, but the relative potencies of neoSTX at the same pH's vary with the relative abundance of the deprotonated N-1 group. In constant-ratio mixtures of the two toxins, the observed EDs0's are consistent with the notion that the two toxins compete for the same site. At pH's 6.50 and 7.25, the best agreement between observed and computed values is obtained when the efficacy term (c) for either toxin is 1. At pH 8.25 the best agreement is obtained if the efficacy is 1 for STX but 0.75 for neo-STX. The marked pH dependence of the actions of neoSTX probably reflects the presence of a site in the receptor that interacts with the N-1 -OH, in addition to those interacting with the 7,8,9 guanidinium and the C-12 hydroxyl groups. Considering the three-dimensional structure of the STX and neoSTX molecules, the various site points are probably located in a fold or a crevice of the channel protein, where the extracellular orifice of the sodium channel is located.

The Sodium Channel of Human Excitable Cells is a Target for Gambierol

“…Keeping in mind that the effect produced by gambierol was sodium dependent, we decided to verify that this toxin is acting on the voltage-gated sodium channel by using a specific inhibitor of Na + channels, such as neosaxitoxin [32][33][34], a paralytic toxin that blocks sodium channel by binding to site 1. Data confirmed that voltagegated Na + channels are involved in the depolarizing effect produced by 30 µM gambierol since neosaxitoxin was successful in inhibiting the toxin induced depolarization.…”

Specificity for block by saxitoxin and divalent cations at a residue which determines sensitivity of sodium channel subtypes to guanidinium toxins.

“…Neosaxitoxin (NEO) is a naturally occurring derivative of STX that differs from STX by substitution of a single hydroxyl group for a hydrogen atom at the N-1 posi-tion of the C-2 guanidinium group (Shimizu, Hsu, Fallon, Oshima, Miura, and Nakanishi, 1978). With respect to equilibrium blocking affinity at pH ~7, NEO has previously been found to be an equipotent or slightly more potent Na-channel blocker than STX (Hu and Kao, 1991). However, the blocking kinetics of STX and NEO differ markedly (Strichartz, 1984).…”

Actions of chiriquitoxin on frog skeletal muscle fibers and implications for the tetrodotoxin/saxitoxin receptor.

“…The biological studies were performed on short segments of single isolated frog skeletal muscle fibers in a Vaseline gap voltage-clamp method (Hille and Campbell, 1976). Details of the procedures and methods of data acquisition and analysis can be found in other papers (Hu and Kao, 1991;Yang et al, 1992a). The actions of CqTX on both 1N~ and IK were studied.…”

“…As a result, the ENa was at +20 to +30 mV. The lower external Na ÷ concentration does not alter the EDs0 of the toxin (Hu and Kao, 1991). Current traces showing maximum INa and at ENa were selected for detailed comparisons.…”