Evolutionary Temperature Adaptation of Agonist Binding to the A1Adenosine Receptor

Abstract

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. Kd values for rat and chicken increased markedly as measurement temperature decreased. In contrast, the Kd values for six teleost species, including warm-adapted, cold-adapted, and deep-living species, were much less sensitive to temperature perturbation. At 5°C Kd 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. Bmax values were relatively insensitive to temperature changes. MgCl2 significantly increased Bmax values, and for two of three species, lowered Kd 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.