Evidence for the induction of repetitive action potentials in synaptosomes by K+-channel inhibitors: an analysis of plasma membrane ion fluxes.
The effects of four K+-channel inhibitors on synaptosomal free Ca2+ concentrations and 86Rb+ fluxes are analysed. 4-Aminopyridine, alpha-dendrotoxin, charybdotoxin, and tetraethylammonium all increase the free Ca2+ concentration, although their potencies differ widely. In each case, the elevation in free Ca2+ concentration is reversed by the subsequent addition of tetrodotoxin. The transient 86Rb+ efflux from preequilibrated synaptosomes induced with high concentrations of veratridine is partially inhibited by 4-aminopyridine and alpha-dendrotoxin. In contrast, when 4-aminopyridine or alpha-dendrotoxin is added to polarized synaptosomes, and enhanced 86Rb+ flux is seen, both for uptake and for efflux with no change in the total 86Rb+/K+ content of the synaptosomes and with only a slight time-averaged plasma membrane depolarization (6.4 and 3.3 mV, respectively). The enhancements of flux by 4-aminopyridine or alpha-dendrotoxin are sensitive to ouabain and/or to tetrodotoxin. Furthermore, these flux changes show the same concentration dependencies as the blocked component of veratridine-stimulated 86Rb+ efflux, the elevation of free Ca2+ concentration, and the facilitation of glutamate exocytosis that are elicited by 4-aminopyridine or alpha-dendrotoxin. It is concluded that these findings support the proposal of spontaneous, repetitive firing of synaptosomes evoked by K+-channel inhibitors and that the enhanced 86Rb+ flux is a consequence of the activity of 4-aminopyridine- and alpha-dendrotoxin-insensitive K+ channels during these action potentials.
Tibbs
G R
GR
Department of Biochemistry, Imperial College, South Kensington, London, England.
Dolly
J O
JO
Nicholls
D G
DG
D015761:4-Aminopyridine / Q000494:pharmacology; D000200:Action Potentials / Q000187:drug effects* / Q000502:physiology; D000818:Animals; D002462:Cell Membrane / Q000737:chemistry / Q000502:physiology; D018999:Charybdotoxin / Q000494:pharmacology; D004546:Elapid Venoms / Q000494:pharmacology; D016257:Fura-2; D006168:Guinea Pigs; D007473:Ion Channels / Q000502:physiology; D010455:Peptides / Q000494:pharmacology; D026902:Potassium Channel Blockers; D012414:Rubidium Radioisotopes / Q000378:metabolism; D013574:Synaptosomes / Q000187:drug effects* / Q000502:physiology; D019789:Tetraethylammonium; D013757:Tetraethylammonium Compounds / Q000494:pharmacology; D013779:Tetrodotoxin / Q000494:pharmacology; D014701:Veratridine / Q000494:pharmacology
D004546:Elapid Venoms; D007473:Ion Channels; D010455:Peptides; D026902:Potassium Channel Blockers; D012414:Rubidium Radioisotopes; D013757:Tetraethylammonium Compounds; D018999:Charybdotoxin; C081506:dendrotoxin K; D013779:Tetrodotoxin; D019789:Tetraethylammonium; D014701:Veratridine; D015761:4-Aminopyridine; D016257:Fura-2