ADENOSINE PROMOTES REGULATION OF CORNEAL HYDRATION THROUGH CYCLIC ADENOSINE-MONOPHOSPHATE

Purpose. To investigate the cellular mechanisms whereby adenosine incr eases net transendothelial fluid transport by the endothelial cells of the cornea. Methods. Rabbit corneas were isolated and the endothelial surface was superfused while thickness was measured with the specular microscope. Cyclic adenosine monophosphate (cAMP) was measured in end othelia from fresh and incubated corneas, and adenylyl cyclase and pho sphodiesterase activities were measured in homogenates or the particul ate fraction of endothelia from bovine or rabbit. Adenosine, adenosine -receptor agonists, dibutyryl cAMP, forskolin, and phosphodiesterase i nhibitors were used to modulate physiological and biochemical paramete rs. Result. Adenosine, N-ethyl(carboxamido)adenosine, dibutyryl cAMP, forskolin, and phosphodiesterase inhibitors all promoted deturgescence of swollen corneas and maintained fresh corneas at lower steady state thicknesses than in controls. These effects were abolished in the pre sence of ouabain or 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid o r after complete removal of HCO3- from the media. Intracellular cAMP w as significantly increased by forskolin and phosphodiesterase inhibito rs and, to a lesser extent, by agonists. Increases in cAMP concentrati on declined rapidly with time. Cyclase activity in the bovine tissue w as enhanced by agonists and by G-protein activators. Dose-response cur ves of corneal swelling indicated a greater sensitivity to N-ethyl(car boxamido) adenosine than to the A(2 alpha) specific agonist CGS 21680. Conclusions. Adenosine increases net endothelial fluid transport thro ugh an increase in cAMP. The effects are mediated by stimulation of ad enylyl cyclase through a G-protein coupled to an adenosine receptor, w hich is most probably of the A(2 beta) subtype, Results suggest that t he regulation of corneal hydration by adenosine is more probably throu gh stimulation of active transport than through a change in permeabili ty, involving either transmembrane fluxes of Na+ or HCO3- or another s tep tightly coupled to these primary events in fluid movement.