The sodium and potassium concentrations in the uterine fluid of the rat at the time of implantation

Rat uterine fluid Na+ and K+ concentrations were calculated from the ratio of the Na+ and K+ concentrations in uterine washings, on the assumption that the sum of Na+ and K+ (mequiv./l) is the same in blood serum and in uterine fluid. Uterine fluid volumes were likewise calculated from the total quantity of Na+ and K+ washed out of the uterine horn.

The mean uterine fluid volume was 7·6 μl per horn and the mean potassium concentration was 45·8 mequiv./l on day 6 of normal pregnancy. Both oophorectomy alone on day 2 and oophorectomy followed by daily injections of progesterone, cause a marked reduction of the uterine fluid volume on day 6 of pregnancy. Oophorectomy on day 2 caused a significant reduction in the uterine fluid K+ concentration. Administration of high and low doses of progesterone after oophorectomy on day 2 of pregnancy resulted in uterine fluid potassium concentrations higher and lower, respectively, than the normal day 6 fluid.

It is postulated that a high uterine fluid K+ concentration is essential for blastocyst-endometrial contact preceding implantation and that this may be significant with regard to certain types of delayed implantation.