Ca2+ binding to calmodulin in the pCa range 5.5-7.0 exposes hydrophobic sites that bind hydrophobic inhibitory ligands, including calmodulin antagonists, some Ca2+-antagonists and calmodulin-binding proteins. The binding of these hydrophobic ligands to calmodulin can be followed by the approx. 80% fluorescence increase they produce in dansylated (5-dimethylaminonaphthalene-1-sulphonylated) calmodulin (CDRDANS). In the presence of Ca2+, calmodulin binds the calmodulin inhibitor, R24571, with an affinity of approx. 2-3 nM and hydrophobic ligands, including trifluoperazine (TFP), W-7 [N-(6-aminohexyl)-5-chloronaphthalene-1-sulphonamide], fendiline, felodipine and prenylamine, with affinities in the micromolar range. This binding is strongly Ca2+-dependent and Mg2+-independent. Calmodulin shows a reasonably high degree of specificity in its binding of these ligands over other ligands tested. CDRDANS, therefore, provides a convenient and simple means of monitoring the interaction of a variety of hydrophobic ligands with the Ca2+-dependent regulatory protein, calmodulin. CDRDANS binds to phospholipid vesicles made of (dimyristoyl)phosphatidylcholine (DMPC) or (dipalmitoyl)phosphatidylcholine (DPPC) and produces fluorescence increases only in the presence of Ca2+ and at temperatures above their gel-to-liquid crystalline phase transition. Although the fluorescence changes in CDRDANS accurately report phase transitions in these liposomes, its binding to these vesicles is weak. Calmodulin probably requires a high-affinity lipid-bound receptor protein for its high-affinity binding to natural membranes.
2,3-Dialkyl(dimethylamino)indoles, synthesized via the Fisher indole synthesis, were found to weakly bind to 5HT1 and 5HT2 sites in brain cortical membranes (IC50 greater than 1 microM at both sites for all compounds). These (dimethylamino)indoles were relatively potent antagonists of the serotonin receptor in the rat stomach fundus. At higher concentrations, several of the compounds were weak agonists at this receptor. For direct comparison with data obtained in the isolated rat fundus, antagonism of serotonin-induced contractions at 5HT2 receptors in the rat jugular vein was also examined. Several of the compounds showed good selectivity for the fundus receptor relative to the 5HT2 receptor; together with minimal affinity for 5HT1 and 5HT2 binding sites in brain cortical membranes, these results support the idea that the serotonin receptor in the stomach fundus is distinct from 5HT1 and 5HT2 binding sites.
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