Alpha-1 and alpha-2 adrenoceptor binding in cerebral cortex: Role of disulfide and sulfhydryl groups |
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Authors: | Tomás A Reader Richard Brière Louise Grondin |
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Institution: | (1) Centre de recherche en sciences neurologiques Département de physiologie Faculté de médecine, Université de Montréal, Succ. A, C. P. 6128, H3C 3J7 Montréal, Québec, CANADA |
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Abstract: | The triated adrenergic antagonists Prazosin (3H]PRZ) and Idazoxan (3H]IDA, or RX-781094) bind specifically and with high affinity to 1 and 2-adrenoceptors respectively, in membrane preparations from cerebral cortex. Saturation experiments performed to determine the density of receptors and the dissociation constant (K
d) were analyzed by the methods of Eadie Hofstee, iterative modelling, and the procedure of Hill, while the specificity of the labelling was verified by displacement experiments. Since receptors are proteins, we examined the role of disulfide (–SS–) bridges and sulfhydryl (–SH) groups in the specific combination of 3H]PRZ and 3H]IDA to the 1 and 2-adrenoceptors. Pretreatment of the membranes with the –SS– reactive DL-dithiothreitol (DTT) or the alkylating agent N-ethylmaleimide (NEM), alone or in combination, decreased specific binding of both ligands, with only minor changes in the non-specific counts. The 3H]IDA binding ( 2-sites) was more sensitive to both DTT and NEM than the 3H]PRZ sites ( 2-adrenoceptors), and the initial changes induced by alkylation of the 2-site were due to an important decrease in the affinity for 3H]IDA, as judged by the increase in theK
d. This modulation in the affinity caused by alkylation of a thiol group could explain the higher potency of the blocking agent tetramine disulfide benextramine at the 2-site. The results provide evidence for the participation of –SS– and –SH groups in the binding site of 1 and 2-adrenoceptors in the cerebral cortex. |
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