MYOMETRIAL ARGINASE ACTIVITY INCREASES WITH ADVANCING PREGNANCY IN THE GUINEA-PIG

OBJECTIVES: Arginase has been suggested to play an important role in c ellular growth and development, particularly important to the fetus, b y supplying L-ornithine for the synthesis of polyamines. The purpose o f this investigation was to determine whether pregnancy alters myometr ial arginase activity and whether estradiol was responsible for the ch ange. STUDY DESIGN: Myometrium and kidney were obtained from nonpregna nt and pregnant guinea pigs of known gestational age. Arginase activit y was measured under physiologic conditions by the conversion carbon 1 4-labeled guanidino-L-arginine to carbon 14-labeled urea. The concentr ations of the enzyme's substrate, L-arginine, and its principal metabo lite, L-omithine, were measured in myometrium from near-term pregnant animals by use of an amino acid analyzer. Finally, a group of random c ycle guinea pigs received 500 mu g/kg estradiol for 5 days before the myometrium was removed. RESULTS: Myometrial arginase activity in pregn ant animals was more than double that of myometrium from nonpregnant a nimals by the time the first measurement was made at 0.14 gestation. I t continued to rise, peaking at values >25-fold higher than the nonpre gnant activity by 0.90 gestation. Arginase activity in the myometrium underlying the placental implantation site was >25 fold higher (p < 0. 05) than myometrium from nonpregnant animals when first studied at 0.6 3 gestation and 10-fold higher than the contralateral fundal myometriu m at the same time of gestation. Myometrial arginase activity in the s terile horn of six pregnant animals was half that of the horn containi ng one or more pups, but still five times higher than that of nonpregn ant animals. Renal arginase activity also rose with advancing pregnanc y, but the magnitude of the increase (up to 2-fold) was much smaller t han that observed in either the fundal or placental implantation site myometrium. Estradiol had no significant effect on myometrial arginase activity. CONCLUSIONS: These studies demonstrate that pregnancy incre ases myometrial arginase activity and that the presence of placenta or fetus is necessary for the maximal effect.