Excretion of MPA in the milk of lactating ewes treated for synchronization of estrus

Ten mature lactating ewes of the Chios island breed 3.5 +/- 0.5 (Mean +/- SEM) yr of age and weighing 51.9 +/- 1.6 kg (Mean +/- SEM) were synchronized for estrus with intravaginal sponges impregnated with 60 mg 6a-methyl-17-acetoxyprogesterone (MPA). The sponges remained in place for 14 d and 500 IU im PMSG were injected at their withdrawal. Daily milk samples (3 d pretreatment, 14 d on treatment, and 5 d posttreatment) were collected and analyzed by a double antibody RIA procedure for MPA. The concentration of MPA (Mean +/- SEM) in the milk increased to 5.05 +/- 0.11 ng/ml within the first day of sponge insertion, then declined and remained at a constant level (3.08 +/- 0.26 ng/ml) while the sponge was in place, eventually dropping to the background level (0.65 +/- 0.05 ng/ml) 24 h following sponge withdrawal. The curve for the quantity of MPA excreted in the milk was identical to that of MPA concentrations, showing significant differences among experimental days and among ewes. Finally, there was a significant relationship between milk production and MPA excretion into the milk (r = +0.581( * *)). It is concluded that only a very small percentage (0.08 +/- 0.01) of MPA contained in each sponge is excreted into the milk from the moment of sponge insertion until 5 d after its removal.