RESUMEN
<p><b>OBJECTIVE</b>To investigate the change pattern of substance P (SP) in the portal vein during the rectoanal inhibitory reflex (RAIR), and its physiologic significance; the influence of external splanchnic nerve of rectum and anal to the RAIR.</p><p><b>METHODS</b>The rats were divided into seven groups, among them there were six groups, which were first divided into two big groups according to whether the external splanchnic nerve to the rectum and anal were cut off, one is no cut-off external splanchnic nerve group, the other is cut-off external splanchnic nerve group. Each group were further divided, according to the distance of the balloon-sac on Foley's tube in the rectum away from anal verge, into 2, 4, 6 centimeter groups; A control group with Foley's tube put into the rectum, but the balloon-sac on Foley's tube did not pumped up with water. Measure and compare the value and change of SP in the portal vein during the RAIR.</p><p><b>RESULTS</b>The comparison of SP in portal vein, among the 2, 4 centimeter groups with cut-off external splanchnic nerve, all groups with intact external splanchnic nerve supply and control group, had no statistic difference (P>0.05). The comparison between the 6 centimeter group with intact external splanchnic nerve group and the 2, 4 centimeter groups with cut-off external splanchnic nerve, P<0.01, the statistic difference was significant. The comparison between 6 centimeter group of intact and cut-off external splanchnic nerve, P<0.01, the difference was significant.</p><p><b>CONCLUSION</b>The reason for the stimulation on upper rectum dose not induce the RAIR is related with this stimulation result in the release of SP, the exciting mediator to internal sphincter. The external splanchnic nerve supply of rectum and anal canal have influence on the change of SP of the portal vein during RAIR.</p>