Kd. Clark et al., EXTRACELLULAR GTP CAUSES MEMBRANE-POTENTIAL OSCILLATIONS THROUGH THE PARALLEL ACTIVATION OF MG2-TETRAURELIA( AND NA+ CURRENTS IN PARAMECIUM), The Journal of membrane biology, 157(2), 1997, pp. 159-167
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.