Effect of nitric oxide synthase inhabitors on ovulation in hCG-stimulated mares

Recent studies suggest that nitric oxide (NO) may have a role in regulating ovarian physiology. To investigate the role of NO during ovulation in mares, inhibitors of the nitric oxide synthase (NOS) were administered to estrous mares. Forty cycling mares (20 horses and 20 pony mares) were allotted to one of the three treatment groups. Once a follicle was at least 27 mm in diameter, but smaller than 35 mm, mares were given one of the following treatments: saline solution 0.9% (n = 20, w/v, i.v., every 12 h), Nomega-nitro-L-arginine methyl ester hydrochloride (L-NAME; n = 10, 148 micromol/kg, i.v., every 12 h), or aminoguanidine hemisulfate (AG; n = 10, 406 micromol/kg, i.v., every 12 h). When a follicle >30 mm was present on one of the ovaries, ovulation was induced with hCG (2,500 IU, i.v.). The median time of ovulation (+/-6 h) after hCG administration for the treatment groups was 42, 84 and 54 h for mares treated with saline solution, L-NAME and AG, respectively. There was no significant difference between the groups treated with AG or L-NAME (P = 0.06); however, these groups were different from the control group (P < 0.05). The delayed ovulation caused by the administration of NOS inhibitors suggests a role for NO in follicular growth and ovulation in horses.