A(2) ADENOSINE RECEPTORS IN MONGOLIAN GERBIL MIDDLE-EAR EPITHELIUM AND THEIR REGULATION OF CL- SECRETION

The present study investigates the effects of adenosine and its analog ues on CI- secretion in primary cultures of gerbil middle ear epitheli um. Short-circuit current (I-sc), an index of transepithelial active t ransport, was measured on the same cells cultured on porous filters. B aseline I-sc and transepithelial resistance were 27.0 +/- 0.7 mu A cm( -2) and 275 +/- 7 Omega cm(2), respectively (n = 178). Extracellular a denosine and its analogues elicited a sustained increase in I-sc when added to apical or basolateral surfaces. Both the A(2A) selective agon ist -carboxyethyl)phenethylamino-5'-N-ethylcarboxamido adenosine and t he A(2A)/A(2B) nonselective agonist 5'-(N-ethyl-carboxamido)adenosine (NECA) increased I-sc, but NECA was more effective than CGS21680. A(1) selective antagonist 8-cyclopentyl-1,3-dipropylxanthine did not reduc e NECA-induced I-sc. These results suggest the presence of both APA an ti A(2B) receptors. NECA did not stimulate a rise in the intracellular Ca2+ concentration ([Ca2+](i)) in single middle ear epithelial cells cultured on glass coverslips. Dibutyryl cAMP (dbcAMP) induced an initi al transient increase in I-sc followed by the sustained plateau. Addit ion of dbcAMP also caused a transient increase in [Ca2+](i). The prote in kinase A inhibitor, omocinnamylamino)ethyl]-5-isoquinolinesulfonami de, greatly reduced the increase in the I-sc responses to NECA. 1,2-Bi s-(2-aminophenoxy)ethane N,N,N',N' tetraacetic acid-acetoxymethyl este r influenced neither the NECA-induced increase in I-sc nor the dbcAMP- induced sustained phase of I-sc, but greatly inhibited the dbcAMP-indu ced transient increase in I-sc(.) Glibenclamide, a cystic fibrosis tra nsmembrane conductance regulator (CFTR) channel inhibitor, reduced the NECA-induced I-sc. These results indicate that extracellular adenosin e and its analogues activate the cAMP-protein kinase A system, but not intracellular Ca2+-dependent mechanisms, leading to Cl- secretion, po ssibly through the CFTR Cl- channels in the cultured gerbil middle ear epithelium.