Effects of transmural field stimulation in isolated smooth muscle of human rectum and internal anal sphincter

Smooth muscle preparations from the circular muscle layer of the most distal rectum and the proximal and distal human internal anal sphincter (IAS) mounted in organ baths to record isometric tension developed spontaneous tension. Transmural electrical field stimulation (TMS) induced frequency- and impulse duration-dependent relaxations sensitive to tetrodotoxin in the stimulation range of 0.5-40 Hz and 0.04-0.6 ms. Poststimulus contractions were most frequent and prominent in rectal preparations. Maximal relaxations were comparable in the three locations and were achieved at 10 Hz and 0.4 ms. The frequency inducing half-maximal response was lower in rectal strips compared with LAS. Phentolamine (10(-6) M) enhanced relaxations and diminished off-contractions at 40 Hz in distal LAS. N-omega-nitro-L-arginine (L-NNA) concentration dependently inhibited both relaxations and off-contractions (10 Hz, 0.4 ms). The pD(2) values (-log EC50) Of L-NNA were lower in rectal muscle compared with those in IAS. L-Arginine (10(-4) M) inhibited the blocking effect of L-NNA. In one-half of the preparations, L-NNA reversed the relaxations to duration contractions (15-40 Hz), which were inhibited by atropine in rectal preparations and by phentolamine in LAS. In conclusion, excitatory innervation of the LAS is alpha-adrenergic and cholinergic in the rectum. A product of the L-arginine-nitric oxide pathway mediates the TMS-induced inhibition of the muscle and is also involved in poststimulus contractions.