Identification of Adenosine Receptor in Rat Pineal Gland: Evidence for A-2 Selectivity

We have examined the binding of the adenosine agonist radioligands [³H]cyclohexyIadenosine (³H]CHA), R-N⁶-[³H]phenylisopropyladenosine ([³H]R-PIA), and 5′-N-ethylcarboxamido[³H]adenosine ([³H]NECA) to membranes prepared from rat pineal gland. The results showed that the A-1-selective ligands (CHA and R-PIA) had ±10% specific binding. By contrast, [³H]NECA, a nonselective A-l/A-2 li-gand, gave 72% specific binding of the total binding. This specific binding was insensitive to cyclopentyladenosine (50 M) or R-PIA (50 μM). To characterize this binding, we sed the N-ethylmaleimide pretreatment method. Under these conditions, this binding was of high affinity with a K_D of 51 ± 10 nM and an apparent B_max of 1,060 ± 239 fmol/ mg of protein. Specific binding was unaffected by the presence of MgCl₂ (10 mM) but was sensitive to guanylyliinidodi- phosphate (100 μM) (-25%), a result suggesting the involvement of an N-protein mechanism in the coupling of the adenosine receptor labeled by [³H]NECA to other components of the receptor complex. The rank of activity of adenosine analogues in displacing specific [³H]NECA binding was NECA ± 2-chloroadenosine ±S-adenosyl-L-homocysteine ± CHA. Binding was also displaced by 3-isobutyl-l-meth-ylxanthine (IC₅₀= 23.6 μM). These findings are consistent with the selective labeling by [³H]NECA of an A-2-type adenosine receptor in rat pineal membranes.