Figure 3. Enrichments of isotopic tracers and products before, during and after bed rest in red blood cells.

Red blood cells enrichments of isotopic tracers ([²H₂] and [¹⁵N]glycine) and products ([²H₂] and l-[¹⁵N]glutathione). A, steady state for [²H₂]glycine (•) precursor pool is shown while steady state condition for [¹⁵N]glycine (□) pool is assumed. Diversity between tracer isotope steady state values is due to intrinsic metabolic differences between ²H₂ and ¹⁵N isotopes. B, l-[²H₂]glutathione (•) enrichment slope increase reflects linear tracer incorporation into glutathione products. Diversity between l-[²H₂]glutathione and l-[¹⁵N]glutathione (□) product enrichments measured at the end of infusions (7th hour) is the result of different tracer incorporation times. [²H₂]glycine was, in fact, infused for 7 h while [¹⁵N]glycine was infused for 3 h.