Characterization of Adenosine Receptors in Mullus surmuletus

The intent of the present study was to investigate adenosine receptor sites in brain membranes of the saltwater teleost fish, Mullus surmuletus, using the A(1) receptor selective agonist, [H-3]CHA, and A(2a) receptor selective agonist [H-3]CGS 21680. The A(1) selective agonist, [H-3]CHA, bound saturably, reversibly and with high affinity to a single-class of binding sites (K-d 1.47 nM; B-max 100-190 fmol/mg protein, dependent on fish length). The A(2a) selective agonist, [H-3]CGS 21680, also bound saturably, reversibly and with relative high affinity to a single-class of binding sites (K-d 44.2 nM; B-max 150-300 fmol/mg protein dependent on fish length). In equilibrium competition experiments, adenosine analogous, NECA, CGS 21680, CHA, CPA, S-PIA, R-PIA, CPCA, DPMA, and xanthine antagonists, DPCPX, XAC, and THEO all displaced [H-3]CHA and [H-3]CGS 21680 specifically bound to brain membranes from Mullus surmuletus. Specific binding of both [H-3]CHA and [H-3]CGS 21680 was inhibited by GDP beta S. For [H-3]CHA the IC50 Value was 2.5 +/- 0.1 mu M, while for [H-3]CGS 21680 the IC50 value was 7.7 +/- 0.3 mu M. Our results indicate that the high affinity binding sites for [H-3]CHA have some pharmacological characteristics of mammalian A(1) adenosine receptors, while the binding sites for [H-3]CGS 21680 appear to be virtually identical to the binding sites for [H-3]CHA.