title: Colchicine binding to bovine anterior pituitary slices and inhibition of growth-hormone release.
abstract: The uptake of [ring C-methoxyl-3H]colchicine into bovine anterior pituitary slices was studied. The data suggest that more than one site exists for the binding of colchicine. At low concentrations colchicine binds to saturable trypsin-sensitive site(s), with a dissociation constant of 3.1 +/- 0.69 mug. The binding capacity of these sites is 8.58 +/- 0.60 pmol of colchicine/mg of wet pituitary. At higher colchicine concentrations binding occurs predominantly to sites which exhibit non-saturation kinetics. Subcellular fractionation of colchicine-labelled slices shows that 90% of the saturable sites are present in the fraction containing cytosol, where the binding protein has a molecular weight of about 11.9 x 10(4) and constitutes 0.7% of the protein present. The nuclear fraction contains 10% of the saturable sites, and the mitochondria and granule fraction contain only non-saturable sites. The rate of colchicine uptake was studied at 0.84 mm- and 2mum-colchicine. At both concentrations the colchicine space exceeded the total tissue water within 10 min. Equilibration with the saturable binding sites was complete in 120 min at 2mum-colchicine. A concentration of colchicine (13.4 mum) which would give 81% maximum binding was found to decrease the length of observable microtubules in tissue fixed at 37 degrees C in glutaraldehyde by 83 +/- 4%. The colchicine-binding protein could be partially purified by using a standard procedure for isolation of brain tubulin. Colchicine inhibits the release of growth hormone in the presence of 3-isobutyl-1-methylxanthine (0.1 mm), but does not alter basal release. The concentration-dependence of colchicine inhibition is similar to that of colchicine binding, but maximum inhibition is only 35%.
mesh_terms: D000818:Animals; D001665:Binding Sites; D002417:Cattle; D002458:Cell Fractionation; D003078:Colchicine / Q000378:metabolism* / Q000494:pharmacology; D005260:Female; D013006:Growth Hormone / Q000378:metabolism*; D066298:In Vitro Techniques; D007700:Kinetics; D008970:Molecular Weight; D010902:Pituitary Gland / Q000378:metabolism*; D010903:Pituitary Gland, Anterior / Q000187:drug effects / Q000378:metabolism*; D011485:Protein Binding; D013997:Time Factors
chemical_list: D013006:Growth Hormone; D003078:Colchicine