Changes in desmosine and pyridinoline crosslinks during rapid synthesis and degradation of elastin and collagen in the rat uterus.

The wet weight of the rat uterus increased 8-fold during pregnancy and fell by 70% within 5 days postpartum. Uterine collagen increased about 5-fold during pregnancy and also fell by 70% within 5 days. The crosslink pyridinoline remained constant at 0.28 mole/mole collagen at every time point, with the possible exception of 11-12 days of pregnancy. The pyridinoline link can therefore form within the short time span of a few days, a feature presumed to be necessary to maintain the full mechanical strength of the uterus during labor. Uterine elastin increased about 8-fold during pregnancy, but the desmosines did not keep pace and fell from a normal value of 1.43 mole/mole elastin to a low of 0.89 at term. Moreover, elastin content reached a maximum several days prior to parturition and then declined continuously to 5 days postpartum. During this decline there was a selective loss of the poorly crosslinked elastin. The desmosines cannot be used as a direct measure of uterine elastin content, because of their continuously changing levels. Desmosines and pyridinoline were measured both by ELISA and by the amino acid analyzer. The two methods gave almost identical results when elastin and collagen were first separated from each other.