Binding of A₁Adenosine Receptor Ligand [³H]8-Cyclopentyl-1,3-Dipropylxanthine in Coronary Smooth Muscle

Vascular smooth muscle has been reported to contain the A1 subtype of adenosine receptors, but the existence of such receptor(s) in coronary smooth muscle has not been established. In the present study, the 3H-labeled A1-selective antagonist [3H]8-cyclopentyl-1,3-dipropylxanthine ([3H]DPCPX) was used to demonstrate the specific binding in porcine coronary artery smooth muscle membranes. The binding was saturable with a Bmax of 6.43 +/- 1.02 fmol/mg protein. Scatchard analysis of the binding data provided a single binding site with a Kd of 0.21 +/- 0.025 nmol/L. In the competition experiments, adenosine receptor agonists and antagonists showed the following order of potency (nmol/L): S-N6-(2-endonorbornyl)adenosine (S-ENBA) 0.11 = R(-)-N6-phenylisopropyladenosine 0.32 > DPCPX 3.2 = xanthine amine congener 2.4 = N6-cyclopentyladenosine 2.67 > 5'-(N-ethylcarboxamido)-adenosine 7.35 >> 2-[p-(2-carboxyethyl)-phenethyl-amino]-5'-(N-ethylcarboxamido)- adenosine 1000 > theophylline 83,000. This order of potency fits the criteria for the A1 adenosine receptor. S-ENBA, a highly selective A1 receptor agonist, was used to investigate the effect on isoproterenol-mediated vasorelaxation and cAMP accumulation. S-ENBA (0.1 to 10 nmol/L) dose-dependently shifted 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)-stimulated cAMP by 25% in the coronary rings. These inhibitory effects of S-ENBA on isoproterenol-mediated cAMP-accumulation and vasorelaxation were abolished by pertussis toxin (100 ng/mL, overnight) treatment of the arteries.(ABSTRACT TRUNCATED AT 250 WORDS)