Histochemical study of the lumbar colonic nerve supply to the internal anal sphincter and its physiological role in dogs.

The sympathetic innervation of the internal and sphincter (IAS) and its physiological role in maintaining sphincter tone were histochemically and mechanically studied in dogs anesthetized with pentobarbital sodium. Numerous catecholamine-fluorescent nerve fibers with varicosities were identified in the IAS of normal dogs. Such fibers were markedly reduced at one week and one month after resection of the hypogastric nerves (HGNs) or the lumbar colonic nerve (LCN), and disappeared after combined HGN and LCN resection. IAS tone decreased to 37.2% of baseline at 1 h after LCN resection and to 69.9% after HGN resection. It returned to the preoperative level at one week and one month after resection. The restored IAS tone was decreased again by acute transection of the previously intact HGNs or LCN. Combined LCN and HGN resection also caused a marked reduction of IAS tone (36.9%) at 1 h after the procedure, and was then restored to the preoperative level with time. The restored tone was not decreased by phentolamine administration. These findings confirmed that both the LCN and the HGN innervate the IAS and play a physiological role in the development of resting tone. The restoration of IAS tone after denervation may be due to intrinsic myogenic properties of the sphincter.