BINDING AND BIOLOGIC ACTIVITY OF NEUROTENSIN IN GUINEA-PIG ILEUM

Experiments were performed to relate receptor binding to biologic acti vity for the contractile effect of neurotensin (NT) in guinea pig ileu m. The contractile response was examined on pieces of ileum under 1 g tension in a 5 mi bath of oxygenated Tyrode's at 38 degrees C. NT cont racted the longitudinal muscle (ED(50), similar to 0.3 nM), the 2-3 g response peaking at I min and fading rapidly. In the presence of atrop ine (1 mu M), greater than or equal to 50% of the response was blocked and the residual effect gave an ED(50) of similar to 1.4 nM. In the p resence of atropine and CP-96,345, a substance P receptor antagonist ( 0.2 mu M), no contraction was observed at 20 nM NT. Thus, there were t wo components to the response, one involving acetylcholine (ED(50), 0 3 nM) and one substance P (ED(50) 1.4 nM). Using membrane preparations and I-125-labeled NT, specific, high affinity receptors for NT were d emonstrated in the muscle and myenteric plexus. Scatchard analyses ind icated the presence of two binding sites (K(d)s: similar to 0.1 nM sim ilar to 2 nM). Sodium ion and GTP analogs inhibited binding. Binding a nd biologic activity were similar in regard to dependence on specific groups within NT and sensitivity to metal ions. The high potency of Hg ++ was consistent with an involvement of free sulfhydryl group(s) in t he binding reaction; this was supported by work with SH-directed agent s. The results suggest that two receptor types or configurations may m ediate the two components of the contractile effect of NT on guinea pi g ileum.