TANNIN INHIBITS CAMP PATHWAYS IN BOVINE AIRWAY EPITHELIUM

Tannin, isolated from aqueous extracts of cotton bracts, inhibits chlo ride secretion in airway epithelial cells. The effect of tannin on the epinephrine- and bradykinin-stimulated rise in intracellular free cal cium and cyclic adenosine monophosphate (cAMP) was examined using bovi ne tracheal epithelial cells in suspension and culture. Basal intracel lular calcium levels were 33 +/- 11 nM (mean +/- SEM, n = 54) and incr eased 13- to 15-fold after addition of epinephrine (10(-6) M) or brady kinin (2 x 10(-6) M). Tannin pretreatment blunted the subsequent respo nse to epinephrine beginning at a tannin concentration of 10 Ag/ml. Pr etreatment with 100 mug/ml tannin completely inhibited the rise in int racellular free calcium in response to epinephrine but had no effect o n the calcium response to bradykinin. In the absence of tannin, both b radykinin and epinephrine increased intracellular levels of cAMP. At a tannin concentration of 10 mug/ml, tannin inhibited the rise in intra cellular cAMP in cells stimulated with either epinephrine or bradykini n but had no effect on bradykinin-stimulated prostaglandin E2 release. Tannin alone (10 mug/ml) increased prostaglandin E2 release. In other studies, tannin inhibited epinephrine binding to airway epithelial ce lls in a dose-dependent manner. R(o) decreased from 948 +/- 69 fmol/mg protein under control conditions (n = 4) to 587 +/- 131 fmol/mg prote in in the presence of 25 mug/ml tannin (n = 3). Tannin had no effect u pon the K(d) for epinephrine binding (132 +/- 12 pM). Tannin had no ef fect on bradykinin binding to airway epithelial cells. We conclude tha t tannin inhibits chloride secretion in airway epithelial cells in par t by inhibiting cAMP pathways and that this effect is due to inhibitio n of epinephrine binding to airway epithelial cells.