THE MECHANISM OF THE RELAXANT EFFECT OF 2‐2′‐PYRIDYLISATOGEN ON THE ISOLATED TAENIA OF THE GUINEA‐PIG CAECUM

1 2‐2′‐Pyridylisatogen tosylate (PIT) slowly relaxed taenia caeci preparations of the guinea‐pig in a concentration‐dependent manner (threshold 2.5 μm). The relaxant effect did not show tachyphylaxis. 2 The relaxation was not affected by tetrodotoxin (0.3 μm), guanethidine (17 μm) nor by a combination of phentolamine (36 μm) and propranolol (4 μm) 3 In taenia caeci preparations suspended in K+‐depolarizing, Ca2+‐free Ringer, addition of Ca2+ (0.1 to 30 mm) resulted in a slow contraction. PIT (50 μm) and papaverine (15 μm) antagonized these contractions, whereas indomethacin (28 μm) was ineffective. 4 Although PIT (50 μm for 30 min) caused a relaxation of the taenia, and, when the tone of the preparations was restored with carbachol, antagonized adenosine 5′‐triphosphate (ATP)‐induced relaxations, relaxation of the taenia with papaverine (30 μm for 5 min) did not antagonize ATP‐induced relaxations. It is concluded that the relaxant and ATP‐receptor blocking actions of PIT are independent properties of the compound.