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 preparationg of the guinea-pig in a concentration-dependent manner (threshold 2.5 muM). The relaxant effect did not show tachyphylaxis.2 The relaxation was not affected by tetrodotoxin (0.3 muM), guanethidine (17 muM) nor by a combination of phentolamine (36 muM) and propranolol (4 muM)3 In taenia caeci preparations suspended in K(+)-depolarizing, Ca(2+)-free Ringer, addition of Ca(2+) (0.1 to 30 mM) resulted in a slow contraction. PIT (50 muM) and papaverine (15 muM) antagonized these contractions, whereas indomethacin (28 muM) was ineffective.4 Although PIT (50 muM 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 muM 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.