BINDING OF A(1) ADENOSINE RECEPTOR-LIGAND [H-3] 8-CYCLOPENTYL-1,3-DIPROPYLXANTHINE IN CORONARY SMOOTH-MUSCLE

Vascular smooth muscle has been reported to contain the A(1) subtype o f adenosine receptors, but the existence of such receptor(s) in corona ry smooth muscle has not been established. In the present study, the H -3-labeled A(1)-selective antagonist [H-3]8-cyclopentyl 1,3-dipropylxa nthine ([H-3]DPCPX) was used to demonstrate the specific binding in po rcine coronary artery smooth muscle membranes. The binding was saturab le with a B-max of 6.43+/-1.02 fmol/mg protein. Scatchard analysis of the binding data provided a single binding site with a K-d Of 0.21+/-0 .025 nmol/L. In the competition experiments, adenosine receptor agonis ts and antagonists showed the following order of potency (nmol/L): S-N -6-(2-endonorbornyl)adenosine (S-ENBA) 0.11=R(-)-N-6-phenylisopropylad enosine 0.32>DPCPX 3.2=xanthine amine congener 2.4=N-6-cyclopentyladen osine 2.67>5'-(N-ethylcarboxamido)adenosine 7.35>>2-[p-(2-carboxyethyl )-phenethyl-amino]-5'- (N-ethylcarboxamido)-adenosine 1000>theophyllin e 83 000. This order of potency fits the criteria for the A(1) adenosi ne receptor. S-ENBA, a highly selective A(1) receptor agonist, was use d to investigate the effect on isoproterenol-mediated vasorelaxation a nd cAMP accumulation. S-ENBA (0.1 to 10 nmol/L) dose-dependently shift ed the isoproterenol-mediated (10(-8) to 10(-5) mol/L) vasorelaxation to the right in vascular rings, S-ENBA (10 nmol/L) inhibited the basal cAMP levels by 36% and attenuated the isoproterenol (10(-5) mol/L)-st imulated cAMP by 25% in the coronary rings. These inhibitory effects o f S-ENBA on isoproterenol-mediated cAMP-accumulation and vasorelaxatio n were abolished by pertussis toxin (100 ng/mL, overnight) treatment o f the arteries. Similarly, the effects of S-ENBA on isoproterenol-medi ated responses were antagonized by DPCPX. This study provides the evid ence suggesting the existence of A(1) adenosine receptors linked to ad enylyl cyclase in an inhibitory manner through pertussis toxin-sensiti ve G proteins in the coronary artery.