Excitatory 5‐hydroxytryptamine Receptors in the Cat Urinary Bladder Are of the M‐and 5‐ht₂‐type

In anaesthetized cats, a biphasic contraction of the urinary bladder, consisting of an initial spike followed by a longer-lasting contracture, was elicited by 5-hydroxytryptamine (5-HT) in doses of 3-30 micrograms/kg, i.a. Administration of MDL 72222 (30, 100 and 300 micrograms/kg, i.v.), which specifically antagonizes the M-type 5-HT receptors, selectively blocked the spike phase of the 5-HT response. Ketanserin (30, 100 and 300 micrograms/kg, i.v.), on the other hand, eliminated only the second phase of the 5-HT-induced bladder contraction. Injection of MDL 72222 (300 micrograms/kg) after ketanserin (300 micrograms/kg), or of 100 micrograms/kg of methysergide or cyproheptadine, or 300 micrograms/kg of ketanserin after MDL 72222 (300 micrograms/kg), led to blockade of both phases of the response to 5-HT. The contractile response to dimethylphenylpiperazinium (DMPP) remained unaffected. These results clearly show that the early and late phases of the cat urinary bladder contraction elicited by 5-HT are mediated by M and 5-HT2 receptors, respectively.