Background/aims: To clarify the functional differences of vasoactive intestinal peptide (VIP) in the human internal anal sphincter (IAS) between the proximal and distal parts from the dentate line, we investigated the VIPergic nerve responses of normal proximal and distal IAS in vitro.
Methodology: Normal IAS specimens derived from 20 patients with lower rectal cancer (14 men and 6 women aged from 48 to 77 years, average 66.5 years) were used. These IAS muscles were divided into 2 parts [oral site IAS from dentate line; proximal part (PIAS; n=20), anal site IAS from dentate line; distal part (DIAS; n=20)]. A mechanogram was used to evaluate in vitro muscle strip responses to VIP after treatment with various autonomic nerve blockers.
Results: 1) Response to VIP after blockade of the adrenergic and cholinergic nerves: VIP showed relaxation reaction in both PIAS and DIAS. The percentage of relaxation responses via VIP nerves in the PIAS was significantly greater than that in the DIAS (p=0.0350). 2) VIP responses in the PIAS and DIAS were not completely blocked by tetrodotoxin. These results indicated that VIP acts through both nerves and directly on the IAS muscle.
Conclusions: There are functional differences in the regulation of VIPergic nerves between the PIAS and DIAS. Relaxation reaction via VIPergic nerves was mainly involved in the regulation of enteric nerve responses in the PIAS.