Bovine Brain Coated Vesicles Contain Adenosine A1Receptors. Presence of Adenylate Cyclase Coupled to the Receptor

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Abstract
Clathrin-coated vesicles purified from bovine brain express adenosine A1 receptor binding activity. N6-Cyclohexyl[3H]adenosine ([3H]CHA), an agonist for the A1 receptor, binds specifically to coated vesicles. High and low agonist, affinity states of the receptor for the radioligand [3H]CHA with KD values of 0.18 and 4.4 nM, respectively, were detected. The high purity of coated vesicles was established by assays for biochemical markers and by electron microscopy. Binding competition experiments using agonists {N6CHA, N-cyclopentyladenosine, 5''-(N-ethylcarboxamido)adenosine, and N6-[(R)- and N6-[(S)-phenylisopropyl]adenosine} and antagonists (theophylline, 3-isobutyl-1-methylxanthine, and caffeine) confirmed the typical adenosine A1 nature of the binding site. This binding site presents stereospecificity for N6-phenylisopropyladenosine, showing 33 times more affinity for N6-[(R)- than for N6-[(S)-phenylisopropyl]adenosine. The specific binding of [3H]CHA in coated vesicles is regulated by guanine nucleotides. [3H]CHA specific binding was decreased by 70% in the presence of the hydrolysis-resistant GTP analogue guanyl-5-yl-imidodiphosphate. Bovine brain coated vesicles present adenylate cyclase activity. This activity was modulated by forskolin and CHA. The results of this study support the evidence that adenosine A1 receptors present in coated vesicles are coupled to adenylate cyclase activity through a Gi protein.