Regulation of nitric oxide synthase isoforms and role of nitric oxide during prostaglandin F-2 alpha-induced luteolysis in rabbits

Total activity of nitric oxide synthase (NOS) and the gene expression of both endothelial NOS (eNOS) and inducible NOS (iNOS) isoforms in corpora lutea of pseudopregnant rabbits were examined during prostaglandin F(2alpha) (PGF(2alpha))-induced luteolysis. Corpora lutea were collected at 0, 6, 12, 24 and 48 h after an injection of PGF(2alpha) at day 9 of pseudopregnancy. At 12 h after PGF(2alpha) administration, luteal mRNA encoding eNOS decreased (P<or=0.05) by 40% and remained low throughout the subsequent 36 h, whereas eNOS protein increased (P<or=0.05) two- to threefold. By contrast, expression of mRNA encoding iNOS was poor and remained fairly constant, but transcription increased eightfold (P<or=0.01) within 6 h after PGF(2alpha) treatment and then decreased to values similar to those of controls. Total NOS activity increased twofold (P<or=0.01) at 6 h after treatment and remained high thereafter, whereas progesterone concentrations in explanted corpora lutea decreased (P<or=0.01) from 302.4+/-42.3 pg x mg(-1) at day 9 to 58.6+/-8.3 at 48 h later, and peripheral plasma concentrations of progesterone declined too. Long-term administration of Nomega-nitro-L-arginine methyl ester (0.6 g l(-1) per os) from day 2 of pseudopregnancy onward partially blocked the luteolytic action of PGF(2alpha) administered at day 9 of pseudopregnancy. In nitric oxide (NO)-deficient rabbits, progesterone concentrations remained higher (P<or=0.01) than in controls at 24-48 h after PGF(2alpha) administration (4.5 to 3.2 ng x ml(-1), respectively). These data are the first to characterize NOS activity. The time course of expression of eNOS and iNOS in rabbit corpora lutea during PGF(2alpha)-induced luteolysis gives additional support to a physiological role of NO in the regulation of regression of corpora lutea in rabbits.