Beta-adrenergic-mediated Cl secretion: evidence for additional non-cAMP-dependent pathway of effect

It has been suggested that beta-adrenergic receptor antagonists with intrinsic sympathomimetic activity, like pindolol, are weak partial agonists for beta-adrenergic-stimulated adenylyl cyclase activation. To evaluate this possibility, beta-adrenergic-mediated chloride secretion was studied in tracheal epithelial cells maintained in primary culture. Pindolol caused a dose-dependent increase in chloride secretion with a half-maximal effective concentration of 91 pM to a maximum that was 30 +/- 3% that of isoproterenol. Pindolol-induced chloride secretion was antagonized by the beta-adrenergic antagonist nadolol. However, in contrast to isoproterenol, pindolol did not stimulate adenosine 3',5'-cyclic monophosphate (cAMP) accumulation, adenylyl cyclase activity, or protein kinase A activation. Further studies examined the coupling of beta-adrenergic stimulation of cAMP accumulation to beta-adrenergic stimulation of chloride secretion. Coincubation of cells with the phosphodiesterase inhibitor RA233 increased maximal isoproterenol-stimulated cAMP accumulation eightfold but did not significantly increase the potency or maximal effect of isoproterenol for chloride secretion. It is clear that beta-adrenergic-stimulated elevations in cAMP mediate chloride secretion. These studies also demonstrate that pindolol, a drug with intrinsic sympathomimetic activity, mediates a beta-adrenergic receptor-specific increase in chloride secretion without increasing adenylyl cyclase nor protein kinase A activities. Thus intrinsic sympathomimetic activity may represent a non-cAMP-dependent mechanism of beta-adrenergic effect.