Characterization of the in vitro responses of equine cecal longitudinal smooth muscle to endothelin-1

Objective—To characterize the in vitro response of equine cecal longitudinal smooth muscle (CLSM) to endothelin (ET)-1 and assess the role of ETA and ETB receptors in those ET-1–induced responses. Animals—36 horses without gastrointestinal tract disease. Procedure—To determine cumulative concentrationresponse relationships, CLSM strips were suspended in tissue baths containing graded concentrations of ET-1 (10 –9 to 10 –6 M) with or without BQ-123 (ETA receptor antagonist); with or without IRL-1038 (ETB receptor antagonist); or with both antagonists at concentrations of 10 –9 , 10 –7 , and 10 –5 M. To determine the percentage change in baseline tension of CLSM, the areas under the curve during the 3-minute periods before and after addition of each dose were compared. Also, the effects of ET-1 and a combination of selective ETA and ETB receptor antagonists on electrically evoked contractions were studied. Results—ET-1 caused sustained increases in CLSM tension in a concentration-dependent manner. Contractile responses to ET-1 were not significantly inhibited by either BQ-123 or IRL-1038 alone at any concentration; however, responses were significantly inhibited by exposure to the antagonists together at a concentration of 10 –5