title: Changes in intracellular pH affect calcium currents in Paramecium caudatum.
abstract: The relation between intracellular pH and membrane excitability was studied in the holotrich ciliate Paramecium caudatum. Intracellular pH (pHi) was measured with recessed-tip ion-sensitive microelectrodes (Thomas 1974) and electrical properties were examined by current stimulation and conventional two-electrode voltage clamp. Under normal conditions the resting pHi of Paramecium was 6.80 +/- 0.05. Intracellular alkalinization enhanced the early Ca current, while internal acidification depressed the Ca current. Both effects occurred in a voltage-independent manner. The late outward current was relatively unaffected by these alterations. Results obtained with replacement of extracellular Ca2+ by Ba2+ also support a direct effect of pHi on current through the Ca channel. Intracellular alkalinization to pH 7.15 converted graded, quasi-regenerative Ca responses elicited by injected current pulses into all-or-none action potentials. This change to all-or-none behaviour is presumed to be due to the increase in Ca current and a consequent change in the balance of inward and outward currents. Extracellular pH changes had little effect on pHi, resting membrane potential or the current-voltage relations. The intracellular pH was also independent of shifts in membrane potential. The results are consistent with a model in which Ca channel permeability is blocked by intracellular protonation of a single titratable site having an apparent dissociation constant of 6.2.
mesh_terms: D000085:Acetates / Q000494:pharmacology; D000818:Animals; D002118:Calcium / Q000502:physiology*; D002245:Carbon Dioxide; D006863:Hydrogen-Ion Concentration; D007473:Ion Channels / Q000502:physiology; D008564:Membrane Potentials; D010247:Paramecium / Q000502:physiology*
chemical_list: D000085:Acetates; D007473:Ion Channels; D002245:Carbon Dioxide; D002118:Calcium