Extracellular ATP Induces Hyperpolarization and Motility Stimulation of Ciliary Cells

Cellular membrane potential and ciliary motility were examined in tissues cultures prepared from frog palate and esophagus epithelia. Addition of micromolar concentrations of extracellular ATP caused membrane hyperpolarization and enhanced the beat frequency. These two effects of ATP were 1) dose dependent, reaching a maximum at 10 microM ATP; 2) dependent on the presence of extracellular Ca2+ or Mg2+; 3) insensitive to inhibitors of voltage-gated calcium channels; 4) abolished after depleting the intracellular Ca2+ stores with thapsigargin; 5) attenuated by quinidine (1 mM), Cs+ (5-20 mM), and replacement of extracellular Na+ by K+; 6) insensitive to charybdotoxin (5-20 nM), TEA (1-20 microM), and apamin (0.1-1 microM); 7) independent of initial membrane potential; and 8) unaffected by amiloride. In addition, extracellular ATP induced an appreciable rise in intracellular Ca2+. Addition of thapsigargin caused an initial enhancement of the ciliary beat frequency and membrane hyperpolarization. These results strongly suggest the involvement of calcium-dependent potassium channels in the response to ATP. The results show that moderate hyperpolarization is closely associated with a sustained enhancement of ciliary beating by extracellular ATP.