Maternal and fetal plasma levels of 3-methylhistidine in pregnant nonhuman primates.

Available data indicate little reutilization of 3-methylhistidine (3-MH) in the rat and man. These data led to the use of urinary 3-MH excretion as a measure of muscle protein catabolism in those animal species. However, 3-MH excretion does not accurately measure protein catabolism in the sheep, pig, and rabbit. This is due, at least in part, to the fact that renal amino acid (AA) transport systems reabsorb 3-MH from the glomerular filtrate. The monkey differs from man in that its plasms contains significant quantities of 3-MH, suggesting an active renal transport system for this AA. The present study measured maternal and fetal plasma 3-MH levels in 33 pregnant rhesus monkeys to determine whether the non-human primate placenta contained transport sites concentrating this AA to the fetal plasma. Mean fetal plasma 3-MH concentrations were 16.4 +/- 6.71 micrometers/100 ml, while maternal levels were 9.45 +/- 3.69 micrometers/100 ml. The fetal to maternal gradient was maintained between 1.6 to 1.7 during the course of maternal infusions of various AA. Since placental AA transport systems are similar to those in the kidney and intestine, the data also suggest the presence of AA transport systems for 3-MH in the monkey, indicating that urinary 3-MH excretion would be a poor method for measuring muscle protein catabolism in this species.