Expression patterns of endothelial and inducible nitric oxide synthase isoforms in corpora lutea of pseudopregnant rabbits at different luteal stages.

Total activity of nitric oxide (NO) synthase (NOS) and expression of both endothelial (eNOS) and inucible (iNOS) isoform were examined in corpora lutea (CL) of rabbits across pseudopregnancy by quantitative reverse transcription-polymerase chain reaction (RT-PCR) analysis, Western blot and immunohistochemistry. CL were collected at early- (day 4), mid- (day 9) and late- (day 13) luteal phase 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 two-fold (P£ 0.05) from early- to late-luteal phase. Independently of CL age, iNOS mRNA was very poorly expressed while protein levels gradually declined from 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 (18930 ± 535 d.p.m./mg wet tissue) than in both day 9 and day 13 CL (10460 ± 394 and 15430 ± 485 d.p.m., respectively). 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.