Antigenic variation in trypanosomes

Abstract

The relation between intracellular pH and membrane excitability was studied in the holotrich ciliate Paramecium caudatum. Intracellular pH (pH$_{\text{i}}$) 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$_{\text{i}}$ of Paramecium was 6.80 $\pm $ 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$_{\text{i}}$ 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$_{\text{i}}$, 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.