Multiple inhibitory mechanisms mediate non-adrenergic non-cholinergic relaxation in the circular muscle of the guinea-pig colon

Summary

The mechanisms responsible for nerve-mediated, non-adrenergic, non-cholinergic (NANC) relaxation in mucosa-free circular muscle strips from the proximal colon of the guinea-pig were investigated. Electrical field stimulation (EFS, 1–20 Hz, trains of 5 s duration, 100 V, 0.25 ms pulse width) in the presence of atropine (1 μmol/l) and guanethidine (3 μmol/l) evoked a triphasic motor response consisting of. (a) a primary relaxation, (b) a rebound contraction and (c) a secondary relaxation. These three responses were abolished by tetrodotoxin (1 μmol/l). Both apamin (0.01–0.3 μmol/l), a known blocker of low conductance, calcium-activated potassium channels in smooth muscles, and L-nitroarginine (L-NOARG) (1–100 μmol/l), a known blocker of nitric oxide (NO) synthase, increased the tone of the strips. Maximum effects on tone were observed with 0.1 μmol/l apamin (21 ± 3% of KCl-induced contraction) and 30 μmol/l L-NOARG (26 ± 4% of KCl response). The combined administration of 0.1 μmol/l apamin and 30 μmol/l L-NOARG produced an increase in tone (47 ± 5% of KCl response) that was larger than that produced by either compound alone. Neither apamin (0.1 μmol/l) nor L-NOARG (30 μmol/l) affected the isoprenaline-induced relaxation.

Apamin (0.1 μmol/l) depressed, but did not abolish, the primary relaxation to EFS at all frequencies without affecting the secondary relaxation. Apamin also enhanced the rebound contraction at a frequency of 1 Hz. L-NOARG (30 μmol/l) depressed, but did not abolish, the primary relaxation to EFS at all frequencies, had no effect on the rebound contraction and inhibited the secondary relaxation evoked at frequencies of 1–5 Hz, but not 10–20 Hz.

L-arginine (300 μmol/l) reversed the effect of L-NOARG on tone and the inhibitory effect on the EFS-evoked relaxation. In the presence of apamin and L-NOARG, the primary relaxation was suppressed at all frequencies; the secondary relaxation was inhibited at 1–5 Hz and unchanged at 10–20 Hz, as observed with L-NOARG alone. We conclude that three distinct mechanisms mediate the NANC relaxation of the circular muscle of the proximal colon of the guinea-pig in response to EFS. One mechanism can be operationally defined as apamin-sensitive and a second as L-NOARG-sensitive, the latter implying a possible role of NO as an inhibitory transmitter. A third NANC inhibitory mechanism, which is apamin- and L-NOARG-resistant, is also suggested.