COVALENT BINDING OF A SELECTIVE AGONIST IRREVERSIBLY ACTIVATES GUINEA-PIG CORONARY-ARTERY A2 ADENOSINE RECEPTORS

Experiments employing guinea pig heart Langendorff preparations compar ed the coronary vasoactivity of a functionalized congener of adenosine , )phenylethylamino]-5'-N-ethylcarboxamidoadenosine, APEC, with the va soactivity of the product of the reaction of APEC with 1,4-phenylenedi isothiocyanate, 4-isothiocyanatophenylaminothiocarbonyl-APEC (DITC-APE C). Previous experiments showed that whereas APEC binds reversibly to the A2A adenosine receptor of brain striatum, DITC-APEC binds irrevers ibly. APEC caused concentration-dependent coronary vasodilation that p ersisted unchanged when agonist administration continued for up to 165 min, but promptly faded when the agent was withdrawn. The unselective adenosine receptor antagonist 8-(4-sulfophenyl) theophyline (8-SPT) a ntagonized the vasoactivity of APEC. By contrast, DITC-APEC (0.125 - 1 .0 nM) caused progressive, concentration-independent vasodilation that persisted unchanged for as long as 120 min after the agent was stoppe d and that was insensitive to antagonism by subsequently applied 8-SPT . However, perfusion of the heart with buffer containing 0.1 mM 8-SPT strongly antagonized the coronary vasodilatory action of DITC-APEC giv en subsequently. Such observations indicate that the covalent binding of DITC-APEC causes irreversible activation of the guinea pig coronary artery A2A adenosine receptor. Neither APEC nor DITC-APEC appeared to desensitize the coronary adenosine receptor during two or more hours of exposure to either agonist.