Mechanism of noncholinergic excitation of canine ileal circular muscle by motilin

In the isolated perfused canine ileal segment, exogenous motilin infused for 9 min, at concentrations from 10(-10) M and 10(-8) M, increased circular muscle motility concomitant with inhibiting tonic VIP release, maximum at 10(-8) M. Both effects increased with increasing motilin concentrations. Atropine 10(-7) M pretreatment did not alter these responses. Naloxone 10(-7) M pretreatment eliminated both the increase in motor activity and the inhibition of VIP levels. Thus the nonmuscarinic neural pathway responsible for motor activation by motilin probably involves the stimulation of release of opiates, which in turn inhibit the release of VIP. Reduction of tonic inhibition of the muscle by continuous VIP release may in part account for increases in motor activity induced by motilin.