Comparison of the binding properties of A1 adenosine receptors in brain membranes of two congeneric marine fishes living at different depths.

The binding properties of A1 adenosine receptors in brain membranes were compared in two congeneric marine teleost fishes which differ in their depths of distribution. Adenosine receptors were labeled using the A1 selective radioligand [3H]cyclohexyladenosine ([3H]CHA). The A1 receptor agonist [3H]CHA bound saturably, reversibly and with high affinity to brain membranes prepared from Sebastolobus altivelis and S. alascanus; however, the mean Kd values differed significantly (Figs. 1-3, Table 1). Saturation data fit to a one site model indicated that the A1 receptor in S. alascanus exhibited a higher affinity (Kd = 1.49 nM) for [3H]CHA whereas A1 receptors in S. altivelis exhibited a significantly lower affinity (Kd = 3.1 nM). Moreover, S. altivelis, but not S. alascanus, parameter estimates for [3H]CHA binding to two sites of receptor were obtained (Fig. 3, Table 1). The mean dissociation constant values for the high and low affinity sites for [3H]CHA in S. altivelis were 0.43 nM and 16.3 nM, respectively. In equilibrium competition experiments the adenosine analogs R-phenylisopropyladenosine (R-PIA), N-ethylcarboxamidoadenosine (NECA) and S-phenylisopropyladenosine (S-PIA) all displayed higher affinities for A1 receptors in S. alascanus as compared to S. altivelis brain membranes (Table 2, Fig. 6). The specific binding of [3H]CHA was significantly increased by 0.1 and 1.0 mM MgCl2 in both fishes; however, the sensitivity (95-131% increase) of S. altivelis to this effect was significantly greater than that of S. alascanus (48-91% increase) (Fig. 5). The results of kinetic, equilibrium saturation and equilibrium competition experiments all suggest that A1 adenosine receptors of S. altivelis and S. alascanus brain membranes differ with respect to their affinities for selected adenosine agonists.