CCK receptor dysfunction in muscle membranes from human gallbladders with cholesterol stones

Human gallbladders with cholesterol stones exhibit impaired muscle contraction induced by agonists that act on transmembrane receptors, increased membrane cholesterol content, and abnormal cholesterol-to-phospholipid ratio compared with those with pigment stones. The present study was designed to investigate the functions of the CCK receptor of gallbladder muscle membranes by radioreceptor assay and cross-linking. 125I-labeled CCK-8 binding was time-dependent, competitive, and specific. Scatchard analysis showed that the maximum specific binding (Bmax) was significantly decreased in cholesterol compared with pigment stone gallbladders (0.18 +/- 0. 07 vs. 0.38 +/- 0.05 pmol/mg protein, P < 0.05). In contrast, the affinity for CCK was higher in cholesterol than pigment stone gallbladders (0.18 +/- 0.06 vs. 1.2 +/- 0.23 nM). Similar results were observed in binding studies with the CCK-A receptor antagonist [3H]L-364,718. Cross-linking and saturation binding studies also showed significantly less CCK binding in gallbladders with cholesterol stones. These abnormalities were reversible after incubation with cholesterol-free liposomes. The Bmax increased (P < 0.01) and the dissociation constant decreased (P < 0.001) after incubation with cholesterol-free liposomes. In conclusion, human gallbladders with cholesterol stones have impaired CCK receptor binding compared with those with pigment stones. These changes are reversed by removal of the excess membrane cholesterol. These receptor alterations may contribute to the defective contractility of the gallbladder muscle in patients with cholesterol stones.