The effect of Atrial Natriuretic Peptide II (ANPII) on the magnitude of colonic spontaneous phasic contraction with and without epithelium in the guinea pig
The effect of atrial natriuretic peptide II on spontaneous phasic contraction (SPC) was investigated on guinea-pig colon in vitro. ANPII reduced the amplitude of SPC of isolated colon, which lasted for more than three minutes, and this relaxant was dose-dependent. In denuded preparations ANPII (10^-7 M) decreased phasic contractile force by 34% (P<0.01), which was significantly smaller than intact segments (63.9%, P<0.01). ANPII, at a concentration of (10^-7 M), reduced the amplitude of the SPC of colonic longitudinal muscle by 78% (P<0.01). Relaxation caused by ANPII (10^-7 M) on unrubbed tissue was increased in the presence of tetrodotoxin (P<0.05). Opiate antagonist, naloxone and dopaminergic (D1/D2) antagonist cis-flupentixal did not change the relaxant response to ANPII, but the latter reduced the frequency of SPC. These results suggest that the relaxant response of ANPII on guinea-pig isolated colon can be through the direct effect on smooth muscle cells and via epithelium, and the possibility that the nervous system opposes the relaxant effect of ANPII may exist.