Biological Half-life of Endogenous PMS Following Hysterectomy and Studies on Losses in Urine and Milk

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Abstract
Two Shetland ponies were hysterectomized on the 96th day of pregnancy. Nine blood samples were drawn from mare 209 over a 56-day period, and 8 samples were drawn from mare 294 over a 51-day period. On the basis of assay of these sera, the half-life of pregnant mare serum (PMS) in mare 209 was estimated to be 6.0 days: in mare 294 the estimate was 6.55 days. Comparing these data with earlier information on the half-life of PMS in the gelding, it is concluded that the ovaries of the mare do not play a significant role in metabolism of the hormone. The peak in concentration of PMS in the blood may be maintained for 40 days during normal pregnancy, and after the decline in activity begins it takes 9-18 days for the level of hormone to be reduced one half. Thus, it appears that PMS is slowly entering the blood from the endometrial cups during this period of decline. Concurrent samples of blood, urine and/or milk were obtained from 7 pony mares 63-128 days pregnant. It is estimated that excretion of hormone accounts for about one seventh of the loss of potency; metabolism presumably accounts for the remainder.

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