Abstract
We measured contraction of muscle strips caused by endothelin (ET) isopeptides and binding of 125I-ET-1 to muscle cell membranes prepared from human and guinea-pig gallbladders. Visualization of 125I-ET-1 binding sites in tissue was performed by autoradiography. Results in human were similar to those in guinea-pig. ET-1 caused tetrodotoxin and atropine-insensitive contraction. The relative potencies for ET isopeptides to cause contraction were ET-1=ET-2>ET-3. ET-1 caused contraction was only slightly inhibited by BQ-123 (potent ET A receptor antagonist) and not by BQ-788 (potent ET B receptor antagonist). It was inhibited by the combination of both. Autoradiography localized 125I-ET-1 binding to the smooth muscle layer. Binding of 125I-ET-1 to muscle cell membranes was saturable and specific. Analysis of dose–inhibition curves demonstrated the presence of two classes of receptors. One class (ET A receptor) had a high affinity for ET-1 and ET-2 but a low affinity for ET-3, and the other (ET B receptor) a high affinity for ET-1, ET-2 and ET-3. These results demonstrate that similar to guinea-pig, human gallbladder possesses both ET A and ET B receptors cooperating to mediate muscle contraction.
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