Dose Response Relationships for the “Early” and “Late” Response to Erythropoietin ∗

Summary

The dose response curves measured 24 hours (“early”) after erythropoietin injection differ significantly from those measured 48 hours after (“late”). The slope of the curve relating increase in iron uptake by total erythroid tissue to EP dose is greater (p < 0.01) in the 48-hour response. Total uptake saturates above 8 units in the “early” response but not in the “late.” Marrow uptake drops at higher doses (16 units) in the “early” response but does not do so in the “late.” The curve for uptake by circulating erythrocytes shows only one slope in the “early” but two distinct slopes in the “late” response: one from 1-4 units the other much steeper from 4-16 units. The results are discussed in relation to the possibility of different mechanisms being involved in the two responses.