Barium spikes are generated in the spines of the sea urchin Diadema antillarum.
In the presence of calcium, Ba2+ ions, at concentrations as low as 1-2 mM, block the action potentials (a.p.'s) elicited by the electrical stimulation of the primary spines of Diadema antillarum. Lower concentrations of barium (0.1 mM) potentiate the a.p.'s recorded from spines equilibrated with Ca-Free artificial sea water (ASW). Exposure of the spines to a saturated solution of EDTA in Ca-free, unbuffered ASW reversibly blocks their electrical activity. Spines blocked by EDTA continue to generate a.p.'s following their equilibration with Ca-free ASW containing 1 mM of Ba2+ ions. The time course of the a.p.'s recorded from spines equilibrated with normal ASW is only slightly affected by the combined action of 15 mM of tetraethylammonium (TEA) and 5 mM of 4-aminopyridine (4-AP). In contrast, the duration of the a.p.'s recorded from spines blocked by EDTA and placed in 1 mM barium is markedly increased by the combined actions of TEA and 4-AP at the above concentrations. We conclude that the Ca-channels of the neurites present in Diadema spines are not, at least qualitatively, an exception with regard to their permeation by Ba2+. The blocking action of low concentrations of these ions, particularly in the presence of calcium, may be explained by the extremely high surface to volume ratio prevailing in neurites with a diameter of only less than 0.1-0.3 micron.
D015761:4-Aminopyridine; D000200:Action Potentials / Q000187:drug effects; D000631:Aminopyridines / Q000494:pharmacology; D000818:Animals; D001369:Axons / Q000187:drug effects / Q000502:physiology*; D001464:Barium / Q000502:physiology*; D002118:Calcium / Q000502:physiology*; D004492:Edetic Acid / Q000494:pharmacology; D004533:Egtazic Acid / Q000494:pharmacology; D012617:Sea Urchins / Q000502:physiology*; D019789:Tetraethylammonium; D013757:Tetraethylammonium Compounds / Q000494:pharmacology
D000631:Aminopyridines; D013757:Tetraethylammonium Compounds; D001464:Barium; D004533:Egtazic Acid; D019789:Tetraethylammonium; D004492:Edetic Acid; D015761:4-Aminopyridine; D002118:Calcium