1. Albumin fractional synthetic rate was determined in five non-pregnant subjects and five normal pregnant subjects in late gestation after an overnight fast by simultaneous prime and intravenous infusion of two precursor amino acids, [N-15]glycine and L-[1-C-13]leucine, with additional priming of the large but, slowly turning over, urea pool with [N-15(2)]urea. 2. The two tracers yielded similar values of albumin fractional synthetic rate: 6.1 and 6.0%/day in non-pregnant subjects and 7.3 and 7.6%/day in pregnant subjects, for glycine and leucine, respectively. While plasma volume was greater and serum albumin concentration was significantly reduced during pregnancy, the calculated intravascular albumin mass was significantly increased in pregnant subjects. 3. The amount of albumin synthesized in the intravascular compartment was significantly greater at 8.8 and 9.5 g/day in pregnant subjects compared with 6.4 and 6.3 g/day in non-pregnant control subjects (glycine and leucine methods, respectively). Calculated whole-body protein turnover using glycine was not different between the two subject groups, but leucine flux was higher in pregnant subjects. Partitioning of nitrogenous products in urine revealed that pregnant subjects excreted less urea, less ammonia and less creatinine than the non-pregnant control subjects. 4. These findings suggest that whereas the serum albumin concentration decreases during pregnancy secondary to the large increase in plasma volume, there is an increase in albumin synthesis such that total intravascular albumin mass is increased in late pregnancy.
