α- and β-adrenergic receptor mechanisms in spontaneous contractile activity of rat ileal longitudinal smooth muscle

Gastrointestinal motility is influenced by adrenergic modulation. Our aim was to identify specific subtypes of adrenergic receptors involved in inhibitory mechanisms that modulate gut smooth muscle contractile activity. Muscle strips of rat ileal longitudinal muscle were evaluated for spontaneous contractile activity and for equimolar dose-responses (10−7 to 3 × 10−5 M) to the adrenergic agents norepinephrine (nonselective agonist), phenylephrine (α1-agonist), clonidine (α2-agonist), prenalterol (β1-agonist), ritodrine (β2-agonist), and ZD7114 (β3-agonist) in the presence and absence of tetrodotoxin (nonselective nerve blocker). Norepinephrine (3 × 10−5 M) inhibited 65 ± 6% (mean ± SEM) of spontaneous contractile activity. The same molar dose of ritodrine, phenylephrine, or ZD7114 resulted in less inhibition (46 ± 7%, 31 ± 5%, and 39 ± 3%, respectively; P < 0.05). The calculated molar concentration of ZD7114 needed to induce 50% inhibition was similar to that of norepinephrine, whereas higher concentrations of phenylephrine or ritodrine were required. Clonidine and prenalterol had no effect on contractile activity. Blockade of intramural neural transmission by tetrodotoxin affected the responses to ritodrine and phenylephrine (but not to norepinephrine or ZD7114), suggesting that these agents exert part of their effects via neurally mediated enteric pathways. Our results suggest that adrenergic modulation of contractile activity in the rat ileum is mediated primarily by muscular β3-, β2-, and α1-receptor mechanisms; the latter two also involve neural pathways.