Dendrotoxin and charybdotoxin increase the cytosolic concentration of free Ca 2+in cerebrocortical synaptosomes: An effect not shared by apamin

Nanomolar concentrations of charybdotoxin or dendrotoxin increase the cytoplasmic free Ca 2+concentration in isolated central nerve terminals. The effects of the two toxins, normally considered to be blockers of K +channels controlled by voltage in a Ca 2+-sensitive or -insensitive manner, respectively, show only marginal additivity. Apamin, an inhibitor of low conductance Ca 2+-activated K +channels, was without effect in either the absence or presence of dendrotoxin. The effect of charybdotoxin on polarized, isolated central nerve terminals seems to be mediated largely by a block of K +channels sensitive to dendrotoxin. Apparently, these voltage-operated K +channels make a more significant contribution to maintaining the polarized potential of synaptosomes than do those activated by Ca 2+.