title: Ionic mechanism of the outward current induced by intracellular injection of inositol trisphosphate into Aplysia neurons.
abstract: Inositol 1,4,5-trisphosphate (InsP3) has been proposed to be the intracellular second messenger in the mobilization of Ca2+ from intracellular stores in a variety of cell types. The ionic mechanism of the effect of intracellularly injected InsP3 on the membrane of identified neurons (R9-R12) of Aplysia kurodai was investigated with conventional voltage-clamp, pressure-injection, and ion-substitution techniques. Brief pressure injection of InsP3 into a neuron voltage-clamped at -40 mV reproducibly induced an outward current (10-60 sec in duration, 20-60 nA in amplitude) associated with a conductance increase. The current was increased by depolarization and decreased by hyperpolarization up to -80 mV, where it disappeared. Extracellular application of tetraethylammonium (TEA; 5 mM) blocked the InsP3-induced outward current, and the current was not affected by the presence of bath-applied 4-aminopyridine (4-AP; 5 mM). The InsP3-induced outward current recorded at a holding potential of -40 mV increased in amplitude in low-K+ solutions and decreased in amplitude in high-K+ solutions. Alteration of [Cl-]0, as well as perfusion with Ca2+ free plus 2 mM EGTA solution, did not affect the outward current. The InsP3-induced outward current was found to disappear when the neuron was injected with the Ca2+ chelator EGTA. The outward current evoked by repeated InsP3 injection at low doses exhibited summation and facilitation and, at high doses, was shown to desensitize. The calmodulin inhibitor N-(6-amino-hexyl)-5-chloro-1-naphthalene sulfonamide (W-7; 20-50 microM), inhibited both the InsP3-induced and the Ca2+-activated outward currents. An intracellular pressure injection of Ca2+ ions into the same identified neuron was shown to produce an outward current associated with a K+ conductance increase similar to the InsP3-induced current, and the current was blocked by bath-applied TEA (5mM). These results suggest that brief pressure injection of InsP3 into certain identified neurons of Aplysia induces a 4-AP-resistant, TEA-sensitive K+ current activated by increased intracellular free Ca2+ concentration, and this increase might be the result of the mobilization of Ca2+ from intracellular stores by InsP3.
mesh_terms: D015761:4-Aminopyridine; D000631:Aminopyridines / Q000494:pharmacology; D000818:Animals; D001048:Aplysia / Q000502:physiology*; D002118:Calcium / Q000502:physiology*; D002712:Chlorides / Q000502:physiology; D004533:Egtazic Acid / Q000494:pharmacology; D004553:Electric Conductivity; D015544:Inositol 1,4,5-Trisphosphate; D007295:Inositol Phosphates / Q000494:pharmacology*; D007473:Ion Channels / Q000187:drug effects / Q000502:physiology*; D008564:Membrane Potentials / Q000187:drug effects; D009474:Neurons / Q000187:drug effects*; D011188:Potassium / Q000378:metabolism*; D013403:Sugar Phosphates / Q000494:pharmacology*; D019789:Tetraethylammonium; D013757:Tetraethylammonium Compounds / Q000494:pharmacology
chemical_list: D000631:Aminopyridines; D002712:Chlorides; D007295:Inositol Phosphates; D007473:Ion Channels; D013403:Sugar Phosphates; D013757:Tetraethylammonium Compounds; D004533:Egtazic Acid; D019789:Tetraethylammonium; D015544:Inositol 1,4,5-Trisphosphate; D015761:4-Aminopyridine; D011188:Potassium; D002118:Calcium