The Effect of a prostaglandin F₂alpha analogue, fenprostalene, and oxytocin on both in vivo and isolated uterine motility in the cow

With the discovery of the uterokinetic properties of prostaglandin F₂α much interest in the possible application of this substance in the expulsion of retained fetal membranes has arisen. Although the bovine uterus has been found to become refractory to PGF₂α, the development of a high-activity, long half-life PGF₂α analogue, fenprostalene, has renewed interest in the prostaglandin's usefulness in the expulsion of retained fetal membranes.

Uterine motility was measured on two consecutive days in eight early-postpartum and six open cycling cows; and in open isolated bovine uterine horns by the intrauterine balloon technique. Fenprostalene and/or oxytocin was administered and uterine contractility was measured by a physiograph.

Results indicate that the early-postpartum cow myometrium was not responsive to fenprostalene when administered intramuscularly or intravenously. However, the open cycling cow exhibited a slight (non significant, P>0.05) increase in average area under the tracing (1414.7±135.3 vs. 1555.5±95.9 mm^) and contraction amplitude (20.3±2.4 vs. 22.3±1.4 mm Kg) following intramuscular fenprostalene injection. Oxytocin injection by both routes caused a significantly (P<0.05) marked stimulation of uterine contractility in both postpartum, and open cow groups. In the isolated horn experiments oxytocin and fenprostalene were statistically equal (P>0.05) in the response observed following addition to the bath. Although the uterus was refractory to repeated dosages of the same drug, alternated treatments of fenprostalene and oxytocin on the same uterine horn increased (P<0.05) mean measures of motility for each treatment. This result is supportive of the proposed two receptor system. However, after halved dosages of each drug were instilled together into the bath solution, no additiveness or synergism was observed from the mixture affect on uterine contractility.

Results obtained in this study do not support the use of prostaglandins in cases where evacuation of uterine contents is desired.