EFFECTS OF CHOLECYSTOKININ ON CHICKEN CECAL MOTILITY - MECHANISMS INVOLVED

The aims of this work are to characterize the effects of cholecystokin in (CCK) on chicken ceca and to study in vitro the mechanisms through which such actions are mediated. Longitudinal and circular cecal strip s kept in vitro in organ baths were responsive to CCK sulphated octape ptide (CCK-8s). On longitudinal strips the response consisted of a fas t phasic contraction followed by a sustained increase in tone which wa s dose dependent and decreased markedly in the presence of tetrodotoxi n (TTX). Ketanserin (10(-5) M) also caused a decrease in the CCK-8s re sponse. CCK tetrapeptide (CCK-4) and CCK unsulphated octapeptide (CCK- 8ns) induced slightly less contractile effects at concentrations of 2 x 10(-6) M only. L365,260 and L364,718 decreased the response of longi tudinal strips to CCK-8s with similar efficacy. On circular strips CCK -8s caused rhythmic phasic contractions of dose dependent amplitude an d frequency, and both effects were resistant to TTX. The EC50 for the amplitude was about 4 times higher than that for the frequency. CCK-8n s (2x 10(-6) M) also caused phasic contractions, whereas the same conc entrations of CCK-4 did not elicit any motor effects. L365,260 and L36 4,718 showed different efficacy in decreasing amplitude or frequency o f contraction. These results suggest that 1) Both muscularly and neura lly located CCK receptors are present on the longitudinal layer of chi cken ceca whereas only muscular receptors are present on the circular muscle. 2) 5HT2 receptors seem to be involved in the neurally mediated CCK-8s response observed in the longitudinal layer. 3) The different potency of CCK-8s, CCK-8ns and CCK4 to induce contractile effects and of the CCK-A and CCK-B antagonists to block such effects suggests the existence of two different CCK receptors on the circular layer.