Direct characterization ofΒ-adrenoceptors in membranes of immature red blood cells from rats

Summary By means of the radioactive antagonist ligand (³H) (–) dihydroalprenolol (DHAP) specific binding sites were identified in membrane preparations from red blood cells from rats. These specific sites were characterized as-adrenoceptors because of the following reasons: Specific binding of DHAP (in contrast to unspecific binding) was dependent on temparature and time of incubation. Furthermore, specific binding of DHAP showed saturability, temperature-dependent reversibility and high affinity (K_D-value of DHAP = 6.51 nM). Specific binding of DHAP was competitively inhibited by-adrenergic antagonists (pindolol > alprenolol propranolol > practolol) and agonists (isoprenaline > adrenaline). The (–) enantiomers of pindolol and isoprenaline showed pronounced higher affinities for the receptor sites than the respective (+) enantiomers. The receptor density in the membrane preparations (pmoles/mg protein) was strongly dependent on the degree of reticulocytosis: The B_max-values increased more than 4 to 5 fold without alteration of the respective K_D-values when reticulocyte counts were enhanced from 3 to 80 % treatment of the animals with increasing doses of acetyl phenylhydrazine.Preliminary accounts were presented at the meeting of the Deutsche Gesellschaft für HÄmatologie [19]