Mechanism whereby nitric oxide (NO) infused chronically intrauterine in ewes is antiluteolytic rather than being luteolytic

Abstract Nitric oxide (NO) has been reported to be luteolytic in vitro and in vivo in cows. However, an NO donor reversed PGF 2 α-induced inhibition of rat luteal progesterone secretion in vitro and an NO donor or endothelin-1 stimulated bovine luteal tissue secretion of prostaglandins E (PGE; PGE 1 , PGE 2 ) in vitro without affecting progesterone or PGF 2 α secretion. In addition, chronic infusion of an NO donor into the interstitial tissue of the ovarian vascular pedicle adjacent the luteal-containing ovary prevented the decline in circulating progesterone, while a nitric oxide synthase (NOS) inhibitor did not affect luteolysis. The objective of this experiment was to determine whether an NO donor or NOS inhibitor infused chronically intrauterine adjacent to the luteal-containing ovary during the ovine estrous cycle was luteolytic or antiluteolytic. Ewes were treated either with vehicle ( N  = 5), diethylenetriamine (DETA-control for DETANONOate; N  = 5), ( Z )-1-[2-(2-aminoethyl)- N -(2-ammonioethyl)amino]diazen-1-ium-1,2-diolate (DETANONOate-long acting NO donor; N  = 6), l -arginine ( N  = 5), l -nitro-arginine methyl ester ( l -NAME-NOS inhibitor; N  = 6), or NG-monomethyl- l -arginine acetate ( l -NMMA; NOS inhibitor; N  = 5) every 6 h from 2400 h (0 h) on day 8 through 1800 h on day 18 of the estrous cycle. Jugular venous blood and inferior vena cava plasma via a saphenous vein cathether 5 cm anterior to the juncture of the ovarian vein and inferior vena cava were collected every 6 h for analysis for progesterone and PGF 2 α and PGE, respectively, by RIA. Corpora lutea were collected at 1800 h on day 18 and weighed. Weights of corpora lutea were heavier ( P  ≤ 0.05) in DETANONOate-treated ewes when compared to vehicle, DETA, l -arginine, l -NAME, or l -NMMA-treated ewes, l -arginine luteal weights were heavier than vehicle, DETA, l -arginine, l -NAME, or l -NMMA-treated ewes, and luteal weights of vehicle, DETA, l -NAME, or l -NMMA-treated ewes did not differ amongst each other ( P  ≥ 0.05). Profiles of progesterone in jugular venous blood on days 8–18 differed ( P  ≤ 0.05) in DETANONOate-treated ewes when compared to vehicle, DETA, l -arginine, l -NMMA or l -NAME-treated ewes, which did not differ ( P  ≥ 0.05) amongst each other. The PGE:PGF 2 α ratio profile in inferior vena cava plasma of DETANONOate-treated ewes was increased ( P  ≤ 0.05) when compared to all other treatment groups. In a second experiment, conversion of [ 3 H PGE 2 ] to [ 3 H PGF 2 α] by day 15 ovine caruncular endometrium in vitro was determined in vehicle, DETA, or DETANONOate-treatment groups. Conversion of [ 3 H PGE 2 ] to [ 3 H PGF 2 α] was decreased ( P  ≤ 0.05) only by DETANONOate. It is concluded that NO is not luteolytic during the ovine estrous cycle, but may instead be antiluteolytic and prevent luteolysis by altering the PGE:PGF 2 α ratio secreted by the uterus.