Role of hypogastric nerve activity in opossum internal anal sphincter function: influence of surgical and chemical denervation.

The exact role of the hypogastric nerve (HGN) in the regulation of basal internal anal sphincter pressures (IASP) and rectoanal reflex (RAR)-induced internal anal sphincter (IAS) relaxation is not known. The studies were performed to investigate the effect of electrical stimulation of HGN (HGNS) on IASP and RAR-induced fall in IASP, simultaneously record the HGN activity (HGNA) and IASP in response to different volumes of rectal balloon distension (RBD) to mimic RAR and determine the neural pathway involved in RBD-induced changes in HGNA. The recording of multifiber unit efferent HGNA was carried out after ipsilateral deafferentation in animals. HGNS produced a frequency-dependent rise in IASP and suppression of RBD-induced fall in IASP. Hexamethonium markedly attenuated the basal HGNA by 86% without causing a significant change in the basal IASP. Five cc of RBD caused a fall in IASP of 70.8 +/- 4.8% without any significant change in HGNA. Further increases in the RBD volume caused a volume-dependent increase in the basal HGNA and a biphasic systemic arterial pressure response (an initial fall followed by an elevation). These responses were suppressed by sympathectomy or sacral denervation. Sympathectomy plus sacral denervation caused complete obliteration of these responses. The data suggest that in the basal state, HGN may not play a significant role in the resting IASP and RAR-induced IAS relaxation. However, there was a significant sympathoexcitation in response to higher volumes of RBD (supramaximal stimulus for RAR). Hypogastric and sacral nerves may participate in the afferent pathways for the RBD-induced sympathoexcitation.