ATP reception and chemosensory adaptation in Tetrahymena thermophila

Micromolar concentrations of adenosine triphosphate (ATP) and its non-hydro lyzable analog beta-gamma-methylene ATP are both effective depolarizing che morepellents in Tetrahymena thermophila. Chemorepellent behavior consists o f repeated bouts of backward swimming (avoidance reactions) that can easily be quantified to provide a convenient bioassay for purinergic reception st udies, Chemosensory adaptation occurs following prolonged exposure (10 min) to the repellents, and cells regain normal swimming behavior. Adaptation i s specific since cells that are behaviorally adapted to either ATP or beta- gamma-methylene ATP still retain full responsiveness to the chemorepellents GTP and lysozyme. However, cross adaptation occurs between ATP and beta-ga mma-methylene ATP, suggesting that they involve the same receptor. Behavior al sensitivity to both ATP and beta-gamma-methylene ATP is increased by the addition of Na+, but addition of either Ca2+ or Mg2+ dramatically decrease s the response to ATP, These ionic effects are correlated with in vivo ATP hydrolysis, suggesting that divalent ions decrease purinergic sensitivity b y activating a Ca2+- or Mg2+-dependent ecto-ATPase to hydrolyze the ATP sig nal. In vivo [P-32]ATP binding studies and Scatchard analysis suggest that the behavioral adaptation is due to a decrease in the number of surface bin ding sites, as represented by decreased B-max values, All these changes are reversible (de-adaptation) after 12 min in a repellent-free buffer. Electr ophysiological analysis showed that both beta-gamma-methylene ATP (10 mu mo l l(-1)) and ATP (500 mu mol l(-1)) elicited sustained, reversible depolari zations while GTP (10 mu mol l(-1)) produced a transient depolarization, su ggesting that the chemosensory response pathways for ATP and GTP reception may differ. There may be separate ATP and GTP receptors since ATP and GTP r esponses do not cross-adapt and 'cold' (unlabeled) GTP is not a good inhibi tor of [P-32]ATP binding. These results suggests that T. thermophila posses s high-affinity surface receptors for ATP that are downregulated during che mosensory adaptation. These ATP receptors may act as chemorepellent recepto rs to enable T. thermophila to recognize recently lysed cells and avoid a p ossibly deleterious situation. This is the simplest eukaryotic organism to show an electrophysiological response to external ATP.