Selective increase in the urinary excretion of protein 1 (Clara cell protein) and other low molecular weight proteins during normal pregnancy

The effect of normal pregnancy on the tubular transport of proteins has been studied by measuring four low molecular weight (Mr) proteins in the urine of pregnant women: protein 1 (a recently discovered urinary protein identical to Clara cell protein), beta 2-microglobulin, retinol-binding protein and alpha 1-microglobulin. The urinary excretion of albumin and beta-N-acetylglucosaminidase was also determined. One hundred and fourteen women with uncomplicated pregnancy were examined: 22 in the first trimester, 42 in the second and 50 in the third trimester. They were compared to 40 age-matched non-pregnant women. The urinary excretion of the four low Mr proteins was significantly increased during the second and third trimester of pregnancy. During the last trimester, the mean relative increases in the urinary excretion of these proteins ranged from 2.8 to 15.6 and prevalences of elevated values from 25 to 46%. This rise in low Mr urinary protein excretion was particularly important in some pregnant women, representing (e.g. for protein 1) more than a 100-fold increase above normal. The urinary excretion of beta-N-acetyl-D-glucosaminidase was also increased during pregnancy but the albuminuria remained stable. These changes in low Mr urinary proteins were reversible after delivery and most likely resulted from a transient decrease in the reabsorptive capacity of the proximal tubule associated with an increase of the filtered load. However, some women excreted high amounts of protein 1 which could not be accounted for by a decreased tubular reabsorption and which might originate from a secretion by the urogenital tract.