Changes in intracellular pH affect calcium currents in Paramecium caudatum.

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.