EXTRACELLULAR GTP CAUSES MEMBRANE-POTENTIAL OSCILLATIONS THROUGH THE PARALLEL ACTIVATION OF MG2-TETRAURELIA( AND NA+ CURRENTS IN PARAMECIUM)

Paramecium tetraurelia responds to extracellular GTP (greater than or equal to 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 depolarizati on. Ion substitution experiments showed that either Mg2+ or Na+ could support these responses in wild-type cells, with increasing concentrat ions of either cation increasing the extent of backward swimming. Appl ying GTP to cells under voltage clamp elicited oscillating inward curr ents 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 (I-Mg) and Na+-specif ic (I-Na) conductances that have been described previously in Parameci um. 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 I-Mg (''eccentric''), while Na+ fail ed to support GTP responses in ''fast-2,'' a mutant that specifically lacks I-Na. Both mutants responded normally to GTP if the alternative cation was provided. As I-Mg and I-Na are both Ca2+-dependent currents , the characteristic GTP behavior could result from oscillations in in tracellular Ca2+ concentration. Indeed, applying GTP to cells in the a bsence of either Mg2+ or Na+ revealed a minor inward current with a pe riodicity similar to that of the depolarizations. This current persist ed when known voltage-dependent Ca2+ currents were blocked pharmacolog ically or genetically, which implies that it may represent the activat ion of a novel purinergic-receptor-coupled Ca2+ conductance.