Nitric oxide causes contraction in the rat isolated small intestine

The effect of authentic nitric oxide (NO) was investigated on rat ileal myenteric plexus-longitudinal muscle strips in the presence of atropine. NO (10(-10) to 3 x 10(-6) M) caused a quick, transient contraction of the smooth muscle, with an optimum concentration of 10(-7) M, as well as a slower contraction with a higher optimum concentration. In preparations sub-maximally pre-contracted with 5-hydroxytryptamine, NO (10(-7) to 3 x 10(-6) M) also caused a quick relaxant response. These responses to NO (10(-6) M) were not influenced by tetrodotoxin, indicating a direct effect on the smooth muscle.