Dependence of mu-conotoxin block of sodium channels on ionic strength but not on the permeating [Na+]: implications for the distinctive mechanistic interactions between Na+ and K+ channel pore-blocking toxins and their molecular targets
Abstract
Mu-conotoxins (mu-CTXs) are Na+ channel-blocking, 22-amino acid peptides produced by the sea snail Conus geographus. Although K+ channel pore-blocking toxins show specific interactions with permeant ions and strong dependence on the ionic strength (mu), no such dependence has been reported for mu-CTX and Na+ channels. Such properties would offer insight into the binding and blocking mechanism of mu-CTX as well as functional and structural properties of the Na+ channel pore. Here we studied the effects of mu and permeant ion concentration ([Na+]) on mu-CTX block of rat skeletal muscle (mu1, Nav1.4) Na+ channels. Mu-CTX sensitivity of wild-type and E758Q channels increased significantly (by approximately 20-fold) when mu was lowered by substituting external Na+ with equimolar sucrose (from 140 to 35 mm Na+); however, toxin block was unaltered (p > 0.05) when mu was maintained by replacement of [Na+] with N-methyl-d-glucamine (NMG+), suggesting that the enhanced sensitivity at low mu was not due to reduction in [Na+]. Single-channel recordings identified the association rate constant, k(on), as the primary determinant of the changes in affinity (k(on) increased 40- and 333-fold for mu-CTX D2N/R13Q and D12N/R13Q, respectively, when...Continue Reading
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