Inhibition of ciliary activity by phorbol esters in rabbit tracheal epithelial cells

To study the effect of protein kinase C activation on respiratory ciliary activity, we measured ciliary beat frequency (CBF) by a photoelectric technique in response to phorbol esters and cell-permeable diglyceride in cultured tracheal epithelial cells from rabbits. Phorbol 12-myristate 13-acetate (PMA) resulted in a concentration- and time-dependent inhibition of CBF (half maximum inhibitory concentration (IC50) = 3 x 10(-10) M) with the maximal decrease being 21.0 +/- 1.4% (mean +/- SE, p less than 0.001) observed at 10(-6) M. L-alpha-dioctanoylglycerol (DiC8), another known activator of protein kinase C, likewise reduced CBF in a dose-dependent fashion. In contrast, phorbol 12,13-didecanoate, a non-tumor-promoting phorbol ester that does not stimulate protein kinase C, produced no significant changes in CBF. The decrease in CBF induced by PMA was not affected by blockade of arachidonic acid metabolism with indomethacin and nordihydroguaiaretic acid, but was antagonized by pretreatment with H-7, a specific inhibitor of protein kinase C (p less than 0.01). Maximal ciliary inhibition with either PMA or DiC8 was not accompanied by a decrease in intracellular concentration of cyclic AMP. These results indicate that activation of protein kinase C has a significant depressive effect on ciliary activity, and hence the airway mucociliary transport function, presumably through a regulatory pathway that is not dependent on cyclic AMP or arachidonic acid metabolites.