Red cell volume and [erythropoietin] responses during exposure to simulated microgravity

In order to test the hypothesis that normal gravity is an important influence on human serum [Erythropoietin] ([Epo]), the hematologic response to 16 d of 6 degrees head-down tilt (HDT, n = 6 men) was compared with 16 d of normal gravity exposure (CON, n = 7 men). Prior to bed rest, CON and HDT subjects, respectively, were similar in the following characteristics (mean +/- SD): age = 40 +/- 3, 39 +/- 6 yr; height = 181 +/- 5, 182 +/- 6 cm; weight = 88.5 +/- 11.3, 81.7 +/- 12.0 kg; maximal oxygen consumption in supine 6 degrees head-down tilt position (VO2max) = 2.63 +/- 0.38, 2.67 +/- 0.52 L.min-1; hematocrit = (Hct) 41.6 +/- 2.4, 43.0 +/- 3.4%; hemoglobin ([Hb]) = 15.1 +/- 1.0, 14.5 +/- 1.0 g.100 ml-1; plasma volume (PV) = 3829 +/- 857, 3768 +/- 512 ml; and [Epo] = 11.6 +/- 2.9, 10.0 +/- 6.2 mU.ml-1. Calculated red cell volume (RCV) was greater in HDT than CON (2845 +/- 410 vs. 2139 +/- 253 ml, p < 0.05) at baseline. Decreases in PV (-15%, 580 ml, p < 0.05) and an insignificant decrease in RCV (-12%, 354 ml, p = 0.07) were observed in the HDT group, with a concurrent 6% increase in [Hb] (p < 0.05). PV, RCV and [Hb] remained unchanged in the CON group. [Epo] remained unchanged during HDT (12.2 +/- 3.2; 10.8 +/- 3.8; 11.2 +/- 3.1; 11.2 +/- 2.6 mU.ml-1 for HDT days 1, 2, 8 and 16, respectively). There was no difference between CON and HDT groups in [Epo] before or during HDT. It was concluded that the insignificant change (-12%) in RCV observed during HDT was insufficient to stimulate an increase in [Epo], probably because the content of oxygen in arterial blood remained unaffected. The observation that [Epo] remained unchanged despite this loss of RCV during HDT also suggests a possible decrease in the responsiveness of the erythropoietic system to [Epo].