Erythropoietin: receptors and clinical use in rheumatoid arthritis.

Erythropoietin (Epo) receptors have been delineated using radioiodinated recombinant erythropoietin (rEpo) and highly purified murine and human erythroid colony-forming units (CFU-e). The murine CFU-e had 950 receptors/cell. One-third had a Kd of 0.09 nM while two-thirds had a Kd of 0.57 nM. Human CFU-e had 1,050 receptors/cell. Two hundred had a Kd of 0.10 nM and 850 had a Kd of 0.57 nM. 125I-rEpo was rapidly internalized and degraded by the CFU-e. Cross-linking of 125I-rEpo to human Epo receptors demonstrated two proteins of 100 and 90 kDa and proteolytic peptide mapping of each protein showed identical fragments indicating that they are very similar. rEpo was also used to treat patients with the anemia of rheumatoid arthritis, and one case is presented in which the patient's hematocrit rose from 32.5% to 44% in eight weeks. When the rEpo was discontinued, the hematocrit fell back to 33%.