The gall bladder cholecystokinin receptor exists in two guanine nucleotide-binding protein-regulated affinity states

To study proximal events in cholecystokinin (CCK) action on bovine gall bladder smooth muscle, we used the hormone analogue D-Tyr-Gly-[(N1e^28,31)CCK-26-32]-phenethyl ester (OPE), which has unique biological properties. This fully efficacious agonist differs from native CCK by not expressing supramaximal inhibition of cell shortening, yet it clearly interacts with the same receptor molecule. This was demonstrated in binding and affinity labeling studies, where both peptides label the same M_r 70,000-85,000 protein and both fully compete for binding of the other ligand. Further, its relatively high affinity for the low affinity CCK receptor permits the clear demonstration of two affinity states of a CCK receptor on a membrane preparation and makes possible evaluation of the molecular basis of these affinity states and their regulation. Analysis of homologous and heterologous binding curves performed with both CCK and OPE peptides and radioligands demonstrated the presence of two affinity states, with CCK being able to distinguish them (K_d1 = 0.48 ± 0.04 nM and K_d2 = 56.5 ± 7.4 nM) and OPE recognizing them equally (K_d1 = 0.94 ± 0.31 nM and K_d2 = 0.96 ± 0.23 nM). In the presence of nonhydrolyzable GTP analogues, there was a shift in distribution of receptors toward the low affinity state, with the total number of receptors and their absolute affinities for each peptide remaining constant. Thus, the gall bladder CCK receptor is a single molecule capable of assuming two interconvertible affinity states, regulated by a guanine nucleotide-binding protein. Two full agonists are capable of interacting with this molecule to yield different biological responses via different molecular events.