An apparatus to assay opioid activity in the infused lumen of the intact isolated guinea pig ileum

Abstract

A modified apparatus is described that provides for the simultaneous bathing of the serosa of an intact piece of isolated guinea pig ileum while allowing infusion of the isolated lumen. The comparative compartmental potency of the opioid agonists morphine, casomorphins, and enkephalins to inhibit electrically driven contractions are described in this system. The rank-order potency for serosally applied opioid agonists was (IC50 values, nM): [d-Ala2,N-Me-Phe4,Gly-ol5]-enkephalin (DAMGO) (15)>[d-Ala2,d-Leu5]-enkephalin (DADLE) (35)≥morphine (46)≥[d-Ala2]-met-enkephalinamide (55)>[d-Ala2]-β-casomorphin[1–4] amide (122)>β-casomorphin[1–4] amide (940)>met- and leu-enkephalin (>6000). This contrasted to the rank-order potency for the luminally applied opioid agonists: DADLE (63)>DAMGO (135)>[d-Ala2]-met-enkephalinamide=morphine (4700)>[d-Ala2]-β-casomorphin[1–4] amide (29000). β-Casomorphin[1–4] amide, leu-enkephalin and met-enkephalin are mostly inactive when applied luminally. Furthermore, the opioid antagonists, casoxin 4 and [d-Ala2]-casoxin 4, when infused into the lumen, significantly overcame the inhibitory effect of morphine added to the serosal side. This model provides an assay and screening system to differentiate between the effects of chemical agents applied via the blood stream (serosa) or food side (lumen) on quiescent or electrically driven gut activity of the nervous plexi or receptor systems of the ileum.