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Studies were conducted to characterize receptors for prostaglandin (PG) F(2alpha) (PGF(2alpha)) and PGE(2), and the signalling pathways regulating total nitric oxide synthase activity and progesterone production in rabbit corpora lutea (CL) of different luteal stages. CL were obtained at days 4, 9 and 13 of pseudopregnancy and cultured in vitro for 2 h with PGF(2alpha) or PGE(2) and with activators and inhibitors of G protein (Gp), phospholipase C (PLC), protein kinase C (PKC), adenylate cyclase (AC) and protein kinase A (PKA). High affinity PGF(2alpha) receptor (K(d)=1.9+/-0.6 nM mean+/-s.e.m. ) concentrations increased (P< or =0.01) four- to five-fold from early to mid- and late-luteal phases (50.6+/-8.5, 188.3+/-36.1 and 231.4+/-38.8 fmol/mg protein respectively). By contrast, PGE(2) receptor (K(d)=1.6+/-0.5 nM) concentrations decreased (P< or =0.01) from day 4 to day 9 and 13 (27.5+/-7.7, 12.4+/-2.4 and 16.5+/-3.0 fmol/mg protein respectively). The Gp-dependent AC/PKA pathway was triggered only on day 4 CL, mimicking the PGE(2) treatment and increasing progesterone production. In both day 9 and day 13 CL, the Gp-activated PLC/PKC pathway evoked a luteolytic effect similar to that induced by PGF(2alpha). The time-dependent selective resistance to PGF(2alpha) and PGE(2) by rabbit CL is mediated by factors other than a lack of luteal receptor-ligand interactions.

Total activity of nitric oxide (NO) synthase (NOS) and expression of both endothelial (eNOS) and inducible (iNOS) isoforms were examined in corpora lutea (CL) of rabbits across pseudopregnancy by quantitative RT-PCR analysis, Western blot and immunohistochemistry. CL were collected at early- (day 4), mid- (day 9) and late- (day 13) luteal phases of pseudopregnancy. The PCR product of rabbit luteal eNOS was cloned and its direct sequence exhibited 90% homology with those of other species. The steady-state mRNA levels encoding eNOS remained fairly constant throughout both early- and mid-luteal stages of pseudopregnancy but dropped almost to half (P</=0.05) by day 13. By contrast, luteal eNOS proteins increased 2-fold (P</=0.05) from the early- to late-luteal phase. Independently of CL age, iNOS mRNA was very poorly expressed while protein levels gradually declined from the early- to late-luteal stage. Intense eNOS-like immunoreactivity was detected in large luteal cells, while iNOS staining was targeted to a few, isolated cells, probably macrophages. Basal NOS activity was greater in day 4 CL than in both day 9 and day 13 CL. These data are the first to characterize in rabbit CL the temporal expression patterns of NOS isoforms across different luteal stages of pseudopregnancy and, collectively, suggest the existence of an expressional control for this constitutive isoform, which might have a physiological role in regulating CL function during development.

In this study, we have examined the presence and the distribution of receptors for leptin (Ob-R) in the oviduct of rabbits, and the effects of leptin on the release of prostaglandin (PG) F2α and PGE2 and on the activity of nitric oxide (NO) synthase (NOS) by oviducts cultured in vitro. Rabbits were killed during the follicular phase and the oviducts were incubated in vitro with leptin, PGF2α, PGE2, NO donor and inhibitors of NOS and cyclo-oxigenase (COX). Using immunohistochemistry, Ob-R-like positive reaction was observed only in the cytoplasm of secretory cells, having stronger intensity in the infundibulum and ampulla tracts than in the isthmus. Both leptin and NO donor inhibited PGE2 release, whereas they enhanced PGF2α release; NOS inhibitor alone or with leptin increased PGE2 and decreased PGF2α production; NOS activity was enhanced by leptin, while PGs did not affect this enzyme. This study suggests that the oviduct could be a potential target for endocrine regulation by leptin, whose circulating levels may act as a metabolic signal modulating oviductal PG release through mediation of the NOS/NO system.