Alpha 2-adrenergic, but not imidazole, agonists activate NaCl cotransport in rabbit tracheal epithelial cells

The adrenergic agonist clonidine activates NaCl cotransport in rabbit tracheocytes. With the use of the high-affinity analogue p-[125I]iodoclonidine, binding of clonidine to cells was determined to fit a two-site model, with one site of high specificity for alpha 2-adrenergic (alpha 2-AR) and the other with a high affinity for I1-imidazol(in)e (I1) receptors. Total density of binding sites for both receptors was similar at 18 fmol/mg protein. Moxonidine displayed a 166-fold greater specificity for I1 receptors compared with cimetidine. Bumetanide-sensitive Na or Cl transport was stimulated by the alpha 2-AR agonists clonidine or guanabenz but not by the I1 agents cimetidine or moxonidine. I1 agonists-stimulated Na transport was detected only in the presence of bumetanide. Prazosin did not block clonidine-stimulated NaCl uptake or efflux, indicating the presence of an alpha 2A-AR subtype. Addition of clonidine either before or after incubation with l-isoproterenol or forskolin did not attenuate the time- and dose-dependent increase in adenosine 3',5'-cyclic monophosphate (cAMP) levels. Thus clonidine stimulates NaCl cotransport in rabbit tracheocytes through an alpha 2A-AR mechanism that does not require cAMP for signal transduction. In addition, I1-imidazol(in)e receptors stimulate Na transport in rabbit tracheocytes through an unidentified pathway.