Nitric oxide as a local mediator of prostaglandin F2alpha-induced regression in bovine corpus luteum: an in vivo study

To test whether nitric oxide (NO) is involved in prostaglandin (PG) F2alpha-induced regression of the bovine corpus luteum (CL) in vivo, heifers were treated as follows: Group 1, saline (3 ml/h); Group 2, dinoprost, an analogue of prostaglandin F2alpha (aPGF2alpha; 5 mg/0.5 h); Group III, Nomega-nitro-L-arginine methyl ester (L-NAME; 200 mg/4 h), an inhibitor of nitric oxide synthase; and Group IV, L-NAME (400 mg/4 h) and aPGF2alpha (5 mg/0.5 h). All treatments were administered by an intraluteal microdialysis system (MDS) on day 15 of the cycle. Perfusate and jugular plasma samples were collected at half-hour intervals; additionally, jugular plasma samples were collected once daily from day 16 to day 21 of the cycle. In the perfusate samples, aPGF2alpha increased P4 (P < 0.05), PGE2 (P < 0.001), and LTC4 (P < 0.05) concentrations; L-NAME increased P4 (P < 0.05) but did not change PGE2 and LTC4 (P > 0.05) concentrations as compared with the period before treatment. Simultaneous perfusion of CL with L-NAME and aPGF2alpha caused a further increase of P4 concentration (P < 0.05) induced by L-NAME or aPGF2alpha treatment and increased PGE2 and LTC4 (P < 0.001) concentrations to the level observed after aPGF2alpha treatment. Perfusion of CL with aPGF2alpha caused luteal regression within 24 h, while perfusion with L-NAME prolonged the life span of CL to day 21 (P < 0.05). Concomitant L-NAME and aPGF2alpha treatment partially counteracted (P < 0.05) the luteal regression caused by aPGF2alpha administration. These results show that NO is involved in the process of luteolysis in the bovine CL and suggest that the luteolytic effect of aPGF2alpha may be mediated by NO as an important component of an autocrine/paracrine luteolytic cascade.