Uterine vein prostaglandin levels in late pregnant dogs

We studied the uterine venous plasma concentrations of prostaglandins E₂, F_2α, 15 keto 13,14 dihydro E₂ and 15 keto 13,14 dihydro F_2α in late pregnant dogs in order to evaluate the rates of production and metabolism of prostaglandin E₂ and F_2α in pregnancy $\text{in vivo}$. We used a very specific and sensitive gas chromatography-mass spectrometry assay to measure these prostaglandins. The uterine venous concentrations of prostaglandin E₂ and 15 keto 13,14 dihydro E₂ were 1.35±.27 ng/ml and 1.89±.37 ng/ml, respectively; however, we could not find any prostaglandin F_2α and very little of its plasma metabolite in uterine venous plasma. Since uterine microsomes can generate prostaglandin F_2α and E₂ from endoperoxides, prostaglandin F_2α production $\text{in vivo}$ must be regulated through an enzymatic step after endoperoxide formation. Prostaglandin E₂ is produced by pregnant canine uterus in quantities high enough to have a biological effect in late pregnancy; however, prostaglandin F_2α does not appear to play a role at this stage of pregnancy.