Expression of endothelial and inducible nitric oxide synthases is modulated in the endometrium of cyclic and early pregnant mares.

The researchers examined the changes in expression of two types of nitric oxide synthases (eNOS and iNOS) in the endometrium of horses, both during a regular cycle and early pregnancy. The findings indicate these proteins, which help control blood flow and glandular activity, are influenced by hormonal changes and could play a crucial role in preparing the endometrium for successful pregnancy.

Key Findings and Methods

• The study used real-time polymerase chain reaction and immunohistology to examine the expression of eNOS and iNOS in the endometrium of cyclic and pregnant mares.
• Concentration of eNOS mRNA varied throughout the oestrous cycle, with significantly higher transcripts on Day 5, whereas iNOS transcription did not significantly change over time. However, in early pregnant mares, both eNOS and iNOS mRNA increased between Days 12 and 15.

Evaluating Protein Presence with Immunocytochemistry

• They also examined the presence of eNOS and iNOS proteins using immunocytochemical methods. In cyclic mares, eNOS protein was detected in endometrial epithelia, the basement membrane, the endothelial layer, and smooth muscle cells of the vasculature.
• However, iNOS protein was undetectable in the endometrium of cyclic mares but could be found in pregnant mares.
• The endometrial epithelia of pregnant mares were positive for both proteins, with a stronger presence for iNOS.

Implications of the Research

• The researchers suggest that the changes in eNOS and iNOS expression during the oestrous cycle and early pregnancy indicate these proteins are regulated by ovarian steroids. The presence of eNOS protein in certain parts of the endometrium indicates it may be involved in the regulation of endometrial cyclicity.
• The increased presence of both eNOS and iNOS during early gestation implies they could play a role in controlling the endometrial vascular bed and glandular activity, helping to create a suitable microenvironment for a successful pregnancy.

This study is novel in that it describes for the first time the changes in eNOS and iNOS expression in the mare’s endometrium during the oestrous cycle and early pregnancy. It provides significant insights into the potential role of nitric oxide synthases in endometrial physiology and successful equine pregnancy.