Decrease of Clonidine Binding Affinity to α2‐Adrenoceptor by ADP‐Ribosylation of 41,000‐Dalton Proteins in Rat Cerebral Cortical Membranes by Islet‐Activating Protein

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Abstract
The IC50 value for inhibition of specific [3H]yohimbine binding to rat cerebral cortical membranes by clonidine was increased, and the Hill coefficient (nH) approached unity in the presence of 150 μM GTP. Pretreatment of membranes with islet‐activating protein (IAP) in the presence of NAD caused an increase in IC50 and nH values for clonidine compared with control membranes in the absence of GTP, the addition of which was without effect. Scatchard analysis showed that the Bmax value of the high‐affinity component in [3H]clonidine binding was decreased by pretreatment with IAP/NAD. GTP in a concentration range of 0.1 μM‐ I mM caused significant elevation of [3H]yohimbine binding. In IAP/NAD‐pretreated membranes, however, [3H]yohimbine binding was no longer affected by GTP, although IAP/NAD significantly (p < 0.01) increased [3H]yohimbine binding compared to control. IAP ADP‐ribosylated 41,000 dalton proteins of cerebral cortical membranes. From these results, it can be suggested that inhibitory guanine nucleotide regulatory protein with MI 41,000 couples to α2‐adrenoceptors to regulate binding affinity of agonists and antagonists in membranes of the rat cerebral cortex.