TANNIN INHIBITS CAMP PATHWAYS IN BOVINE AIRWAY EPITHELIUM

Tannin, isolated from aqueous extracts of cotton bracts, inhibits chloride secretion in airway epithelial cells. The effect of tannin on the epinephrine- and bradykinin-stimulated rise in intracellular free calcium and cyclic adenosine monophosphate (cAMP) was examined using bovine tracheal epithelial cells in suspension and culture. Basal intracellular calcium levels were 33 +/- 11 nM (mean +/- SEM, n = 54) and increased 13- to 15-fold after addition of epinephrine (10(-6) M) or bradykinin (2 x 10(-6) M). Tannin pretreatment blunted the subsequent response to epinephrine beginning at a tannin concentration of 10 Ag/ml. Pretreatment with 100 mug/ml tannin completely inhibited the rise in intracellular free calcium in response to epinephrine but had no effect on the calcium response to bradykinin. In the absence of tannin, both bradykinin and epinephrine increased intracellular levels of cAMP. At a tannin concentration of 10 mug/ml, tannin inhibited the rise in intracellular cAMP in cells stimulated with either epinephrine or bradykinin 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 cells in a dose-dependent manner. R(o) decreased from 948 +/- 69 fmol/mg protein under control conditions (n = 4) to 587 +/- 131 fmol/mg protein in the presence of 25 mug/ml tannin (n = 3). Tannin had no effect upon the K(d) for epinephrine binding (132 +/- 12 pM). Tannin had no effect on bradykinin binding to airway epithelial cells. We conclude that tannin inhibits chloride secretion in airway epithelial cells in part by inhibiting cAMP pathways and that this effect is due to inhibition of epinephrine binding to airway epithelial cells.