It is well established that purines influence gastrointestinal motility, secretion and absorption by acting on smooth muscle and the epithelium; moreover they are involved in synaptic transmission in the myenteric and submucosal ganglia (Burnstock, 2001). However, the role of pyrimidines in gastro-intestinal functioning is less well established. Three metabotropic receptors of the P2Y class are activated by pyrimidines, P2Y₂ and P2Y₄ by uridine triphosphate and P2Y₆ by uridine diphosphate. Here we have examined the immunolocalisation of the UTP-specific receptor, P2Y₄, in human full thickness bowel.

Archival, paraffin-embedded bowel tissue (2 colon, 3 ileum) was obtained from the Department of Pathology, Glasgow Royal Infirmary and sectioned at 5 mm. Immunohistochemistry protocols were applied and, following microwave antigen retrieval, the P2Y₄ receptor was localized using the avidin-biotin complex (ABC) method. Rabbit monoclonal primary antibody (Alomone) was applied at 1:125 for 16 h at 4°C and immunoreactive sites visualized by sequential application of horse biotinylated secondary antibody, ABC complex and diaminobenzidine (Vector Laboratories). Sections were counterstained with haemotoxylin, dehydrated, cleared and mounted. Horse pre-immune serum was used as negative control.

In all specimens, strong immunoreactivity (IR) was observed in neurones in the myenteric, inner and outer submucosal ganglia. The longitudinal and circular muscle layers were negative for P2Y₄-like IR. Strong IR was also observed in the endothelium of the blood vessels traversing the submucosal layer. In ileum the epithelial cells were negative or showed only very faint IR. However, throughout the epithelium, specific strong IR was observed in a subpopulation of epithelial cells. These positively stained cells were found toward the base of crypts, although not exclusively, and the majority occurred singly or in pairs. The positively staining flame-like cells demonstrated a granular appearance, either apically, basolaterally or both. No evidence of this positively staining epithelial subpopulation was observed in the colon.

Here, we report that P2Y₄ receptor IR was found in all patients in subpopulations of ganglion cells of the myenteric and submucosal plexuses, suggesting that synaptic transmission or modulation may be mediated by pyrimidines. The subpopulation of epithelial cells observed remains to be identified but may represent either Paneth or enteroendocrine cells. Since the majority of cells occur singularly or in pairs and are not exclusively restricted to the base of crypts they are likely to be enteroendocrine in origin.

All procedures accord with current local guidelines.