Excitatory and inhibitory responses of Oddi's sphincter in guinea pigs

We examined the intrinsic motor innervation of the guinea pig choledochoduodenal junction and actions of cholecystokinin octapeptide (CCK-OP) on contractile and membrane activity of circular and longitudinal smooth muscles from three different areas: close to the choledochal sphincter (I); central area in the ampulla (II); and close to the duodenal papilla (III). In response to electrical field stimulation, circular muscle strips showed an initial twitchlike contraction followed by relaxation in areas I and II and only a transient relaxation in the muscle strips prepared from area III. In the longitudinal strips, the regional differences in response to the field stimulation were not prominent, and biphasic twitchlike contractions were observed in areas I, II, and III. Electric field stimulation evoked excitatory junction potentials (EJPs), inhibitory junction potentials (IJPs), or biphasic membrane response (initial EJP followed by an IJP) in the circular and longitudinal smooth muscle cells. Prominent regional differences were observed in areas I, II, and III. Namely, in area III both the circular and longitudinal muscle layers IJPs predominated, whereas in area I the response was predominantly excitatory. CCK-OP (greater than 10-8M) evoked repetitive action potentials in the circular muscle cells, and CCK-OP increased the frequency of slow waves or the spontaneous action potentials in longitudinal muscle cells. CCK-OP enhanced the amplitude of the IJPs and EJPs in both muscle layers. It would thus appear that bile flow is controlled by complex combinations of contraction and relaxation of the smooth muscle that may be due to regional differences in excitatory and inhibitory innervations.