A method for dissecting two site receptor interactions in ligand binding studies

In vitro selectivity of agonists and antagonists for beta 1- and beta 2-adrenoceptor subtypes in rat brain

A radioreceptor binding assay was developed to determine the selectivity of beta-adrenoceptor agents in rat brain. This was achieved by using the highly selective unlabelled antagonists CGP 20712A and ICI 118-551 to block beta 1- or beta 2-sub-populations respectively in rat cerebral cortex membranes. This permitted the selective labelling of beta-adrenoceptors with the antagonist (-)-[125I]pindolol. Using this method, compounds could be routinely screened and selectivity profiles for binding in the CNS determined with a high degree of sensitivity and resolution.

Effects of temperature and ethanol on agonist and antagonist binding to rat heart muscarinic receptors in the absence and presence of GTP

The effect of temperature on the binding of four agonists and three antagonists to rat heart muscarinic receptors was studied in the absence and presence of GTP. The binding of agonists to two states (or classes) of receptors, in the absence of GTP, led to enthalpy and entropy changes that decreased sharply above 25 degrees C, suggesting that agonists induced 'isomerization' reactions (large conformational changes and/or receptor-effector association). Both temperature increase and ethanol decreased hydrophobic interactions, thereby hindering binding and/or agonist-induced 'isomerization' reactions. Addition of GTP to the incubation medium also appeared to reverse (or prevent) 'isomerization' reactions. For agonist binding to the low-affinity state, in the presence of GTP, and for antagonist binding, the thermodynamic parameters observed could be readily explained by simple receptor-ligand associations; large entropy increases and small enthalpy increases, provoked by hydrophobic and ionic interactions, were partly neutralized by entropy and enthalpy decreases, due to hydrogen bonds and van der Waals interactions. The muscarinic antagonists used (atropine, n-methylscopolamine and dexetimide), being more hydrophobic molecules than the agonists tested (carbamylcholine, oxotremorine and pilocarpine), induced larger entropy changes or more negative enthalpy changes.