EXTERNAL GTP ALTERS THE MOTILITY AND ELICITS AN OSCILLATING MEMBRANE DEPOLARIZATION IN PARAMECIUM-TETRAURELIA

Paramecium, a unicellular ciliated protist, alters its motility in res ponse to various stimuli. Externally added GTP transiently induced alt ernating forward and backward swimming interspersed with whirling at a concentration as low as 0.1 muM. ATP was 1000-fold less active, where as CTP and UTP produced essentially no response. The response to the n onhydrolyzable GTP analogs guanosine 5'-[gamma-thio]triphosphate and g uanosine 5'-[beta, gamma-imido]triphosphate was indistinguishable from that to GTP. This behavioral response was correlated with an unusual transient and oscillating membrane depolarization in both wild-type ce lls and the mutant pawn B, which is defective in the voltage-dependent Ca2+ current required for action potentials. This is a specific effec t of external GTP on the excitability of a eukaryotic cell and, to our knowledge, is the first purinergic effect to be discovered in a micro organism.