THE A(2B) ADENOSINE RECEPTOR MEDIATES CAMP RESPONSES TO ADENOSINE RECEPTOR AGONISTS IN HUMAN INTESTINAL EPITHELIA

Adenosine is thought to be a major effector in immunological stimulati on of Cl- secretion in intestinal epithelia. Previous studies indicate that both apical and basolateral domains of intestinal epithelial cel ls possess functionally defined adenosine receptors. However, it is un clear whether the same receptor subclass is expressed, what the recept or subclass(es) is, or how the receptors signal the Cl- secretory resp onse. We now characterize the intestinal epithelial adenosine receptor subtype using the model epithelium, T84. Both apical and basolateral adenosine receptor agonist response profiles revealed a hierarchy (ED( 50)) of 5'-(N-ethylcarboxamido)adenosine > adenosine > CGS-21680. Simi larly, inhibition studies revealed identical ID50 hierarchies for apic al and basolateral antagonism by xanthine amine congener > 1,3-diethyl -8-phenylxanthine > aminophylline. Analyses of both agonist and antago nist pharmacological hierarchies in Chinese hamster ovary cells stably expressing the A(2b) receptor revealed these same hierarchies, Northe rn blots performed on RNA extracted from polarized T84 monolayers demo nstrated no detectable message for A(1) or A(2a) adenosine receptor, b ut strong hybridization was detected for the A(2b) adenosine receptor. Subsequent Northern blots of RNA prepared from human alimentary tract revealed that A(2b) adenosine receptor message was heavily expressed throughout the colon, in the appendix, and more modestly expressed in the small intestine (ileum), Analyses of cAMP generation in T84 cells in response to adenosine indicated that the basolateral A(2b) receptor elicits Cl- secretion through this signaling pathway, Stimulation of Cl- secretion through the apical A(2b) receptor exhibited relatively s mall but significant increases in cAMP compared with basolateral stimu lation, The protein kinase A inhibitor H-89, used at concentrations th at did not affect short circuit current responses to the Ca2+-mediated agonist carbachol, effectively inhibited short circuit current elicit ed by either apical or basolateral adenosine, These data suggest that the major intestinal epithelial adenosine receptor is the A(2b) subcla ss, which is positively coupled to adenylate cyclase, Such observation s have potentially important implications for the treatment of diarrhe al diseases.