Effect of FK506 on ATP-induced intracellular calcium oscillations in cow tracheal epithelium.

To elucidate the effect of FK506 on Ca2+ oscillations in airway epithelium, we investigated cultured cow tracheal epithelial cells with a Ca2+ image-analysis system. ATP (1 microM) induced long-lasting Ca2+ oscillations, having nearly constant peak values (300-400 nM) and intervals (20-40 s) in subconfluent cells but not in confluent cells. These responses were gradually attenuated and abolished by the addition of FK506. Rapamycin, which binds the FK506-binding protein (FKBP), likewise inhibited Ca2+ oscillations, whereas cyclosporin A, a calcineurin inhibitor, did not. Treatment of cells with FK506 decreased Ca2+ content in thapsigargin-sensitive stores, suggesting that the partial depletion of the stores causes the inhibition of Ca2+ oscillations. Immunocytochemistry revealed the existence of cytoplasmic FKBP-like immunoreactivities. The expression of a 12-kDa FKBP was greater in subconfluent cells than in confluent cells as determined by Western blotting, suggesting that the 12-kDa FKBP may be one of the factors that regulates Ca2+ oscillations. Therefore, FK506 possesses an inhibitory action on the Ca2+ response via intracellular FKBP but not via calcineurin, which may result in modification of airway epithelial functions.