Extracellular GTP causes membrane-potential oscillations through the parallel activation of Mg2+ and Na+ currents in Paramecium tetraurelia.

Paramecium tetraurelia responds to extracellular GTP (>/= 10 nm) with repeated episodes of prolonged backward swimming. These backward swimming events cause repulsion from the stimulus and are the behavioral consequence of an oscillating membrane depolarization. Ion substitution experiments showed that either Mg2+ or Na+ could support these responses in wild-type cells, with increasing concentrations of either cation increasing the extent of backward swimming. Applying GTP to cells under voltage clamp elicited oscillating inward currents with a periodicity similar to that of the membrane-potential and behavioral responses. These currents were also Mg2+- and Na+-dependent, suggesting that GTP acts through Mg2+-specific (IMg) and Na+-specific (INa) conductances that have been described previously in Paramecium. This suggestion is strengthened by the finding that Mg2+ failed to support normal behavioral or electrophysiological responses to GTP in a mutant that specifically lacks IMg ("eccentric"), while Na+ failed to support GTP responses in "fast-2," a mutant that specifically lacks INa. Both mutants responded normally to GTP if the alternative cation was provided. As IMg and INa are both Ca2+-dependent currents, the characteristic GTP behavior could result from oscillations in intracellular Ca2+ concentration. Indeed, applying GTP to cells in the absence of either Mg2+ or Na+ revealed a minor inward current with a periodicity similar to that of the depolarizations. This current persisted when known voltage-dependent Ca2+ currents were blocked pharmacologically or genetically, which implies that it may represent the activation of a novel purinergic-receptor-coupled Ca2+ conductance.