A study has been made of changes in the amount and concentration of some chemical constituents of the rat uterus during the latter part of pregnancy and post-partum involution in both lactating and non-lactating animals. Dry, defatted, uterine tissue was analysed for hexose, hexosamine, uronic acid, nucleic acid and collagen. Samples of serum were analysed for free hydroxyproline and serum mucoprotein.
Up to the 3rd day post partum the rate of uterine involution (by weight) was similar in both lactating and non-lactating rats. During this period the overall rate of tissue loss was found to be a first order process kinetically, the fraction of tissue lost per day being about one-third. Thereafter there was a marked divergence in the two groups; hyperinvolution occurred in the lactating group but not in the non-lactating animals.
The disappearance of collagen showed a distinct lag during the first 24 hr post partum in both groups. At 5 to 8 days post partum, uteri from nonlactating animals contained 12% more collagen than non-pregnant controls and about 100% more than uteri from lactating rats.
The total hexose content showed the largest relative decrease post partum of all the chemical components studied; the hexose concentration fell sharply during involution to about half the value found on the 20th day of pregnancy. Hexosamine and uronic acid concentrations, however, showed only slight decreases while the collagen concentration varied considerably during the involution period. The serum hydroxyproline showed a slight increase post partum in contrast to the serum mucoprotein level which was markedly elevated during the latter part of pregnancy and the whole of the involution period studied.
Administration of oestrone prevented hyperinvolution in lactating rats.