Octopamine relaxes rabbit jejunal smooth muscle by selective activation of dopamine D1 receptors.

The effect of octopamine on intestinal smooth muscle of rabbit isolated jejunum has been studied. Octopamine induced a dose-dependent decrease of muscle tone and this reproducible relaxation was not modified by tetrodotoxin or by agents that acted on adrenergic nerve terminals. Adrenoceptor antagonists, at concentrations sufficient to block each adrenoceptor type, did not reduce the actions of octopamine. On the other hand, octopamine-induced relaxations were affected by agents that have the ability to change cyclic AMP (cAMP) content; such as alloxan (an adenylate cyclase inhibitor), imidazole (a stimulator of phosphodiesterase), and isobutyl methylxanthine (an inhibitor of phosphodiesterase). Direct stimulation of adenylate cyclase by octopamine was demonstrated using radioimmunoassay of cAMP. Furthermore, haloperidol and perphenazine at concentration required to block dopamine receptor sites attenuated both smooth muscle relaxation and the formation of cAMP induced by octopamine. The effect of octopamine was totally blocked by SCH 23390, an antagonist of dopamine D-1 receptors. The lack of effect of domperidone and sulpiride, antagonists of dopamine D-2 receptors, on the actions of octopamine excludes the involvement of dopamine D-2 receptors. These results suggest that octopamine acts on intestinal dopamine D-1 receptor sites to produce relaxation of rabbit jejunum through an increase of cAMP.