Effects of noradrenaline (NA) on the isolated rectal circular muscle of the cats were studied in comparison with the effects on the internal anal sphincter (IAS). NA (10(-8)-10(-7) g/ml) caused tonic contraction in four of 15 strips of the rectum taken from 15 animals, and in all 15 strips of the IAS. Phenylephrine also induced rectal and IAS contraction. Rectal contraction induced by NA was resistant to phentolamine, yohimbine, propranolol, hexamethonium and tetrodotoxin, but blocked by atropine. IAS contraction induced by NA was resistant to propranolol, atropine, hexamethonium and tetrodotoxin, but blocked by phentolamine and yohimbine. It is suggested that an atropine-sensitive excitatory adrenergic mechanism other than the excitatory alpha-adrenergic mechanism exists in the rectal circular muscle.</p
