Erythropoietin binding to the red cell membranes

Some characteristics of Epo binding to the red cell membranes were investigated in a system in which the hormone was added to a suspension of cells in a mixture of normal serum phosphate buffered saline (1:7 (V/V)), testing the unbound Epo. Epo binding shows a gradual decrease during the maturation of red cells (0.15 U/ml packed erythrocytes, 0.285 U/ml reticulocytes and 0.375 U/ml erythroblasts from a total of 1.4 U/ml). The binding is also time- and pH-dependent. The amounts of Epo bound within 60 min are independent of temperature, but after 120 min a dissociation of the hormone-receptor complex occurred at 23 and 37 degrees; at 4 degrees C the binding continue, although at a very low rate. RBC membranes bound the hormone at pH 7.8-8 and release it at pH 6.5. The "in vivo" stimulation of beta receptors with isoproterenol increases the erythropoietic response caused by Epo, suggesting a possible relation with the adenylcyclase system.