Decrease in participation of nitric oxide in nonadrenergic, noncholinergicrelaxation of rat intestine with age

Participation of nitric oxide in the electrical field stimulation-induced n onadrenergic, noncholinergic (NANC) relaxation in various intestinal region s was studied in 2- to 50-week-old Wistar rats. In the jejunum of 2-week-ol d rats, the extent of the nitric oxide-mediated component of the relaxation of longitudinal muscle was approximately 60-70%, whereas the component was 40-50% in 4-week-old rats and was absent in 8- and 50-week-old rats. Thus, nitric oxide seems to be the most important mediator at young ages but its significance is lost with age. The same tendency as that in the jejunum wa s also shown in longitudinal muscle of the ileum, proximal and distal colon , and rectum. The tendency was also shown in the circular muscle of the rec tum. Sensitivity of the longitudinal muscle of the jejunum and proximal col on to exogenously added nitric oxide was high in younger rats. Immunoreacti ve structures for nitric oxide synthase were observed in the circular muscl e layer of the rectum. The population of the structures was denser in 4-wee k-old than that in 50-week-old. The results suggest that NANC relaxation in every region of the intestine at 2-week-old is almost solely mediated by n itric oxide, and its significance as an inhibitory mediator gradually or ra pidly decreases with age.