Erythropoietin does not demonstrate circadian rhythm in healthy men

The hormone erythropoietin (Epo) produced in the kidneys in response to hypoxia stimulates the production of red blood cells. We measured serum Epo levels in 26 healthy men over a 24-h period to determine whether Epo is secreted according to a circadian rhythm. Samples were collected every 2-4 h, and Epo was measured by using a radioimmunoassay (INCSTAR Epo-trac). To determine whether blood collection had any effect on Epo production, one-half of the subjects began the collection period in the morning and the other half in the evening. Exercise and the use of prostaglandin inhibitors were prohibited to eliminate fluctuations in Epo production in response to discrete stimuli. The daily Epo concentration for all subjects combined was 15.5 +/- 5.3 (SD) U/l. No significant circadian variation in serum Epo concentrations was observed in either group or when both groups were combined; however, a measure of individual variance was observed (mean deviation = 0.8 +/- 0.4 U/l).