Characterization of the binding of [3H]-(+/-)-L-364,718: a new potent, nonpeptide cholecystokinin antagonist radioligand selective for peripheral receptors.

1986 
(3H)-(+/-)-L-364,718 a new, potent and selective nonpeptide peripheral cholecystokinin (CCK) antagonist bound saturably and reversibly to rat pancreatic membranes. The radioligand recognized a single class of binding sites with a high affinity (Kd = 0.23 nM). The binding of (/sup 3/H)-(+/-)-L-364,718 was stereospecific in that the more biologically active (-)-enantiomer demonstrated greater potency than the (+)-enantiomer. The rank order of potency of various CCK agonists and antagonists in displacing (/sup 3/H)-(+/-)-L-364,718 correlated with their ability to displace (/sup 125/I)CCK-8 and their known pharmacological activities in peripheral tissues. However, the absolute potencies of agonists were greater in displacing (/sup 125/I)CCK-8 than (/sup 3/H)-(+/-)-L-364,718. As described for other physiologically relevant receptor systems, the potency for displacement of (/sup 3/H)-(+/-)-L-364,718 binding by CCK agonists, but not antagonists, was reduced by guanosine 5'-(beta, gamma-imido)triphosphate and NaCl and enhanced by MgCl/sub 2/. (/sup 3/H)-(+/-)-L-364,718 also demonstrated specific binding to bovine gall bladder tissue but not guinea pig brain or gastric glands, consistent with its selectivity as a peripheral CCK antagonist. (/sup 3/H)-(+/-)-L-364,718 binding to pancreatic membranes was not affected by various pharmacological agents known to interact with other common peptide and nonpeptide receptor systems. These data indicate that (/sup 3/H)-(+/-)-L-364,718 represents a new potent nonpeptidemore » antagonist radioligand for the study of peripheral CCK receptors which may allow differentiation of agonist and antagonist interactions.« less
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