Changes in uterine sialic acid and glycogen during early pregnancy in the rat.

Changes in uterine sialic acid and glycogen were studied in rats during days 1-6 of pregnancy. There was a sharp decline in uterine glycogen following mating, and thereafter increased gradually to reach a peak level on day 4 (4 PM). Coinciding with the entry of blastocysts into the uterus [between day 4 (10 PM) and day 5 (10 AM)] uterine glycogen decreased to a low level which was maintained up to 4 PM on day 6 of pregnancy. Uterine sialic acid was maximal on day 0 (proestrus, 10 PM) and was maintained at this level up to day 1 of pregnancy. Thereafter, uterine sialic acid concentration declined gradually to the lowest level by day 4 (9 AM). There was a sharp increase in uterine sialic acid between 4 and 10 PM Ofl day 4, and was followed by a marked decline between day 4 (10 PM) and day 5 (9 AM); it increased again on the evening of day 5 and attained significantly higher levels by day 6 (4 PM). The fluctuations in uterine sialic acid and glycogen during early pregnancy appear to be a sequel to the fluctuating levels of estrogens during proestrus, estrus, and early pregnancy. A possible role for sialic acid in attachment of blastocysts to the uterus is postulated.