Role of calcium and calmodulin in ciliary stimulation induced by acetylcholine

The goal of this work was to elucidate the molecular events underlying stimulation of ciliary beat frequency (CBF) induced by acetylcholine (ACh) in frog esophagus epithelium. ACh induces a profound increase in CBF and in intracellular Ca²⁺ concentration ([Ca²⁺]_i) through M₁ and M₃ muscarinic receptors. The [Ca²⁺]_i slowly decays to the basal level, while CBF stabilizes at an elevated level. These results suggest that ACh triggers Ca²⁺-correlated and -uncorrelated modes of ciliary stimulation. ACh response is abolished by the phospholipase C (PLC) inhibitor U-73122 and by depletion of intracellular Ca²⁺ stores but is unaffected by reduction of extracellular Ca²⁺ concentration and by blockers of Ca²⁺ influx. Therefore, ACh activates PLC and mobilizes Ca²⁺ solely from intracellular stores. The calmodulin inhibitors W-7 and calmidazolium attenuate the ACh-induced increase in [Ca²⁺]_i but completely abolish the elevation in CBF. Therefore, elevation of [Ca²⁺]_i is necessary for CBF enhancement but does not lead directly to it. The combined effect of Ca²⁺ elevation and of additional factors, presumably mobilized by Ca²⁺-calmodulin, results in a robust CBF enhancement.