Changes in intracellular pH affect calcium currents in Paramecium caudatum

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Abstract
The relation between intracellular pH and membrane excitability was studied in the holotrich ciliate Paramecium caudatum . Intracellular pH (pH j ) 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 pH i 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 Ca 2+ by Ba 2+ also support a direct effect of pH i 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 pH i , 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.