Administration of a nitric oxide synthase inhibitor counteracts prostaglandin F2-induced luteolysis in cattle

The objective of this study was to determine whether nitric oxide (NO) is produced locally in the bovine corpus luteum (CL) and whether NO mediates prostaglandin F-2alpha (PGF(2alpha))-induced regression of the bovine CL in vivo. The local production of NO was determined in early 1, early 11, mid, late, and regressed stages of CL by determining NADPH-d activity and the presence of inducible and endothelial NO synthase immunolabeling. To determine whether inhibition of NO production counteracts the PGF(2alpha)-induced regression of the CL, saline (10 ml/h; n = 10) or a nonselective NOS inhibitor (Nomega-nitrO-L-arginine methyl ester dihydrochloride [L-NAME]; 400 mg/h; n = 9) was infused for 2 h on Day 15 of the estrous cycle into the aorta abdominalis of Holstein/Polish Black and White heifers. After 30 min of infusion, saline or cloprostenol, an analogue of PGF(2alpha) (aPGF(2alpha); 100 mug) was injected into the aorta abdominalis of animals infused with saline or L-NAME. NADPH-diaphorase activity was present in bovine CL, with the highest activity at mid and late luteal stages (P < 0.05). Inducible and endothelial NO synthases were observed with the strongest immunolabeling in the late CL (P < 0.05). Injection of aPGF(2alpha) increased nitrite/nitrate concentrations (P < 0.01) and inhibited P-4 secretion (P < 0.05) in heifers that were infused with saline. Infusion of L-NAME stimulated P-4 secretion (P < 0.05) and concomitantly inhibited plasma concentrations of nitrite/nitrate (P < 0.05). Concentrations of P-4 in heifers infused with L-NAME and injected with aPGF(2alpha) were higher (P < 0.05) than in animals injected only with aPGF(2alpha). The PGF(2alpha) analogue shortened the cycle length compared with that of saline (17.5 +/- 0.22 days vs. 21.5 +/- 0.65 days P < 0.05). L-NAME blocked the luteolytic action of the aPGF(2alpha) (22.6 +/- 1.07 days vs. 17.5 +/- 0.22 days, P < 0.05). These results suggest that NO is produced in the bovine CL. NO inhibits luteal steroidogenesis and it may be one of the components of an autocrine/paracrine luteolytic cascade induced by PGF(2alpha).