Background and purpose::Several studies using radioligand binding assays, have shown that measurement of thermodynamic parameters can allow discrimination of agonists and antagonists (Weilandet al., 1979;Boreaet al., 1996a). Here we investigate whether agonists and antagonists can be thermodynamically discriminated at CCK2receptors in rat cerebral cortex.
Experimental approach::The pKLof [3H]‐JB93182 in rat cerebral cortex membranes was determined at 4, 12, 21 and 37°C in 50 mMTris‐HCl buffer (buffer B pH 6.96; containing 0.089 mMbacitracin). pKIvalues of ligands of diverse chemical structure and with differing intrinsic activity (α), as defined by the lumen‐perfused rat and mouse stomach bioassays, were determined in buffer B at 4, 12, 21 and 37°C.
Key results::[3H]‐JB93182 labelled a homogeneous population of receptors in rat cerebral cortex at 4, 12, 21 and 37°C and the pKLand Bmaxwere not altered by incubation temperature. [3H]‐JB93182 binding reached equilibrium after 10, 50, 90 and 220 min at 37, 21, 12 and 4°C, respectively. pKIvalues forR‐L‐365,260,R‐L‐740,093, YM220, PD134,308 and JB95008 were higher at 4°C than at 37°C. There was no effect of temperature on pKIvalues for pentagastrin, CCK‐8S,S‐L‐365,260, YM022, PD140,376 and JB93242.
Conclusions and implications::CCK2receptor agonists and antagonists at rat CCK2receptors cannot be discriminated by thermodynamic analysis using [3H]‐JB93182 as the radioligand.British Journal of Pharmacology(2007)151, 1352–1367; doi:10.1038/sj.bjp.0707355