UNCHANGED INVIVO P-50 AT HIGH-ALTITUDE DESPITE DECREASED ERYTHROCYTE AGE AND ELEVATED 2,3-DIPHOSPHOGLYCERATE

We measured hematological and erythrocyte O2 transport parameters in whole blood and density-separated erythrocytes in 11 mountaineers before and during 5 days of exposure to high altitude (4,559 m). We determined the in vivo (arterial pH(blood) and PCO2) and standard (pH(blood) = 7.4, PCO2 = 40 Torr) O2 tension at 50% O2 saturation of hemoglobin and (P(50,vv) and P(50,st)) and Bohr coefficients (BC) for fixed acid (H+) and CO2 and examined the contribution of the altered average age of circulating erythrocytes due to the stimulation of erythropoiesis on whole blood 2,3-diphosphoglycerate (2,3-DPG) and P(50,st). At altitude, whole blood P(50,vv) remained almost unchanged, whereas P(50,st) and 2,3-DPG increased significantly (+4 Torr; 3.5 μmol/g hemoglobin). BC(CO2) was elevated significantly at altitude. Serum erythropoietin increased transiently fourfold, iron utilization increased, and serum iron decreased by 66%. Reticulocyte counts increased, but other hematological parameters were unchanged. In density-separated erythrocytes, P(50,st) and 2,3-DPG increased with decreasing cell density but were higher in fractions with comparable reticulocyte counts in cells prepared at altitude than in those from control studies. Our data show that, despite the increase in 2,3-DPG and the decrease in average erythrocyte age, the in vivo hemoglobin-O2 affinity remains unchanged. P(50,st) values reflect the elevation of 2,3-DPG, and ~50% of the increase in both parameters can be ascribed to the increase in the number of reticulocytes and young erythrocytes.