title: Transmembrane outward hydrogen current in intracellularly perfused neurones of the snail Helix pomatia.
abstract: The ionic nature and pharmacological properties of the outward current activated by membrane depolarization were studied on isolated neurones of the snail Helix pomatia, placed in Na+- and Ca2+-free extracellular solutions and intracellularly perfused with K+-free solution ("nonspecific outward current"). It was shown that the amplitude and reversal potential of this current (estimated from instantaneous current-voltage characteristics) are determined mainly by the transmembrane gradient for H+ ions. Lowering of pHi induced an increase in the current amplitude and a shift of the reversal potential to more negative values; the shift magnitude was comparable with that predicted for the hydrogen electrode. Raising pHi, as well as lowering pHo, induced a decrease in the current amplitude and a displacement of the current activation curve to more positive potentials. Addition of EGTA (8 mmol/l) to the intracellular perfusate did not affect the current amplitude. Extracellular 4-aminopyridine (10 mmol/l), verapamil (0.25 mmol/l) or Cd2+ (0.5 mmol/l) blocked the current. It is concluded that the current studied is carried mainly by H+ ions. In the same neurones the nature of the fast decay of the calcium inward current was also studied (in the presence of extracellular Ca2+ ions). This decay considerably slowed when pHi was raised or pHo was lowered, and it became less pronounced upon extracellular application of 4-aminopyridine or upon intracellular introduction of phenobarbital (4 mmol/l) and tolbutamide (3 mmol/l). It is suggested that the fast decay of the calcium inward current is due to activation of a Ca-sensitive component of the hydrogen current which depends on accumulation of Ca2+ ions. The possible physiological role of the transmembrane hydrogen currents is discussed.
mesh_terms: D015761:4-Aminopyridine; D000631:Aminopyridines / Q000494:pharmacology; D000818:Animals; D002104:Cadmium / Q000494:pharmacology; D002118:Calcium / Q000494:pharmacology / Q000502:physiology; D002463:Cell Membrane Permeability / Q000187:drug effects; D006372:Helix, Snails; D006859:Hydrogen / Q000502:physiology*; D006863:Hydrogen-Ion Concentration; D008564:Membrane Potentials / Q000187:drug effects*; D009474:Neurons / Q000187:drug effects / Q000502:physiology*; D011188:Potassium / Q000494:pharmacology; D014700:Verapamil / Q000494:pharmacology
chemical_list: D000631:Aminopyridines; D002104:Cadmium; D006859:Hydrogen; D015761:4-Aminopyridine; D014700:Verapamil; D011188:Potassium; D002118:Calcium