Evolutionary Temperature Adaptation of Agonist Binding to the A₁ Adenosine Receptor

¹ Department of Zoology and Physiology, Louisiana State University, Baton Rouge, Louisiana 70803
² College of Pharmacy, Oregon State University, Corvallis, Oregon 97331

The effects of temperature on the binding of the agonist [³H]cyclohexyladenosine to A₁ adenosine receptors were studied by equilibrium binding techniques in brain membranes from eight vertebrate species with average body temperatures from 1 to 40°C. K_d values for rat and chicken increased markedly as measurement temperature decreased. In contrast, the K_d values for six teleost species, including warm-adapted, cold-adapted, and deep-living species, were much less sensitive to temperature perturbation. At 5°C K_d values vary 30-fold among the species; however, at temperatures approximating the cell temperatures of the species there is only a four-fold range of values. Binding enthalpies varied in sign and magnitude among the species. Binding entropies were positive for all the species; values were largest for the warm-adapted species, and smallest for the deep-living fishes. B_max values were relatively insensitive to temperature changes. MgCl₂ significantly increased B_max values, and for two of three species, lowered K_d values. MgCl₂ did not alter the enthalpy changes. In equilibrium competition experiments at 5°C using brain membranes from the deep-living teleost Antimora rostrata, the adenosine analogs R-phenylisopropyladenosine, N-ethylcarboxamidoadenosine, and 2-chloroadenosine were approximately 23-fold more potent than S-phenylisopropyladenosine. Despite perturbation by low temperature of agonist binding to mammalian and avian A₁ adenosine receptors, agonist recognition and binding properties of the A₁ receptor have been retained in vertebrates adapted to different body temperatures. These adaptive trends mirror those noted in studies of soluble enzyme homologs and muscle actins from species adapted to different temperatures.