Characterization of beta-adrenoceptors of tilapia erythrocytes with hydrophobic and hydrophilic radioligands.

Tilapia are widely used for to investigate stress physiology, and beta-adrenoceptors in fish erythrocytes play important roles in response to hypoxia and other environmental stresses. As the beta-adrenoceptor in tilapia erythrocytes has never been investigated, we characterized beta-adrenoceptors in intact red blood cells of a tilapia (Oreochromis mossambicus) in this study by radioligand binding assay with a hydrophobic beta-adrenoceptor antagonist, 1-[4,6-propyl-3H]dihydroalprenolol ([3H]DHA), and a hydrophilic ligand (-)-4-(3-t-butylamino-2-hydroxypropoxy)-[5,7-3H] benzimidazol-2-one ([3H]CGP-12177). The equilibrium dissociation constant (KD) for [3H]CGP-12177 calculated from kinetic experiments (KD = 5.8 +/- 4.8 nM) was comparable to that obtained from Scatchard analysis (KD = 1.22 +/- 0.18 nM). However, the KD for [3H]DHA obtained from kinetic studies (KD = 0.41 +/- 0.12 nM) was much smaller than that from a Scatchard plot (KD = 7.8 +/- 1.6 nM). The hydrophobic [3H]DHA bound to two sites (KD = 7.8 nM, Bmax = 12,500 sites/cell and KD = 77.4 nM, Bmax = 70,550 sites/cell), whereas the hydrophilic [3H]CGP-12177 bound to one site (KD = 1.2 nM, Bmax = 1,900 sites/cell). The results indicate that high-affinity beta-adrenoceptors are located both on the surface of and inside tilapia erythrocytes, and low-affinity receptors exist only inside erythrocytes.