Cation gating and selectivity in a purified, reconstituted, voltage-dependent sodium channel

[14C]guanidinium ion influx into Na+ channel preparations from mouse cerebral cortex

[14C]Guanidinium ion influx into Na+ channel preparations from mouse and rat cerebral cortex (purified synaptosomes, and synaptoneurosomes) was characterized and its properties were compared with those for 22Na+ influx. Tetrodotoxin-sensitive influx of [14C]guanidinium ion was stimulated by aconitine, veratridine, and batrachotoxin with a K0.5 of 7, 5 and 0.3 microM, respectively, the maximal influx being the same with all toxins. Scorpion venom shifted the activation curve of veratridine to the left, but did not increase the maximal influx. The potency of the local anesthetic drugs cocaine and tetracaine in inhibiting [14C]guanidinium ion influx depended upon the concentration of veratridine used to activate the Na+ channels. The mechanism of inhibition was of a competitive nature. Other local anesthetic drugs and cocaine congeners inhibited [14C]guanidinium ion influx with potencies very similar to those for inhibition of 22Na+ influx. The results show that [14C]guanidinium ion influx is a valid model for 22Na+ influx through voltage-dependent Na+ channels although there are some differences between the two influx assays. The guanidinium ion assay offers the convenience of the 14C isotope as compared with the strongly radiating 22Na+ isotope.

Characterization and purification of a soluble protein controlling Ca-channel activity in paramecium

The analysis of the voltage-sensitive Ca++ channel of the unicellular eucaryote, Paramecium has been extended to a biochemical level based on recent observations that the transfer of cytoplasm from wild-type cells into mutants lacking Ca++-channel function ("pawn" in P. tetraurelia and "CNR" in P. caudatum) causes mutant cells to regain Ca++-channel function. We have microinjected various cytoplasmic fractions into mutant cells and measured the restored Ca++-channel function using a convenient behavioral assay. Following the "curing" activity, we characterized and purified the component from wild-type cytoplasm that can restore the function missing in cells carrying mutations in the cnrC gene. The curing factor is not an RNA, but a heat-labile, -SH-containing protein that appears to affect existing mutant channels on the ciliary membrane. We have purified this factor over 500-fold from the soluble cytoplasm using conventional techniques. The protein is of low apparent molecular weight (less than 30,000 daltons), acidic, soluble, and does not have the properties of calmodulin.