CHARACTERIZATION OF THE 3 DIFFERENT STATES OF THE CHOLECYSTOKININ (CCK) RECEPTOR IN PANCREATIC ACINI

By measuring binding of [I-125]CCK-8 and [H-3]L-364,718 to rat pancreatic acini we demonstrated directly that the pancreatic CCK receptor can exist in three different affinity states with respect to CCK - high affinity, low affinity and very low affinity. Binding of [I-125]CCK-8 reflects interaction of the tracer with the high and low affinity states, whereas binding of [H-3]L-364,718 reflects interaction of the tracer with the low and very low affinity states. Treating acini with carbachol abolished the high affinity state of the CCK receptor and converted approximately 25% of the low affinity receptors to the very low affinity state. Carbachol treatment was particularly useful in establishing the values of K-d for the high and low affinity states for different CCK receptor agonists and antagonists. Of the various CCK receptor agonists tested, CCK-8 had the highest affinity for the high affinity state (K-d similar to 1 nM), whereas CCK-JMV-180 had the highest affinity for the low (K-d 7 nM) and very low affinity (K-d 200 nM) states. Gastrin and de(SO4)CCK-8 had affinities for the high and low affinity states of the receptor that were 100- to 400-fold less than those of CCK-8 but had affinities for the very low affinity state that were only 3- to 10-fold less than that of CCK-8. CCK receptor antagonists showed several patterns in interacting with the different states of the CCK receptor. L-364,718 had the same affinity for each state of the CCK receptor. CR1409 and Bt(2)cGMP each had similar affinities for the high and low affinity states and lower affinity for the very low affinity state. L-365,260 and CCK-JMV-179 had the highest affinity for the low affinity state and lower affinities for the high and very low affinity states. Different CCK receptor agonists caused the same maximal stimulation of amylase secretion but showed different degrees of amplification in terms of the relationship between their abilities to stimulate amylase secretion and their abilities to occupy the low affinity state of the CCK receptor. When amplification was expressed quantitatively as the value of K-d for the low affinity state divided by the corresponding EC(50) for stimulating amylase secretion the values were CCK-8 (1000), de(SO4)CCK-8 (1500), gastrin (100) and CCK-JMV-180 (Menozzi, D., Vinayek, R., Jensen, R.T. and Gardner, J.D. (1991) J. Biol. Chem. 266, 10385-1091). Studies using carbachol-treated acini, CCK-8 and CCK-JMV-180 indicated that occupation of the low affinity state of the CCK receptor by CCK-8 or CCK-JMV-180 produces oscillations of [Ca2+](i) whereas occupation of the very low affinity state by CCK-8 causes a large, transient increase in [Ca2+](i) and this increase can be prevented by CCK-JMV-180. Thus, our results indicate that the pancreatic CCK receptor exists in three different, interconvertible affinity states for CCK-8, that each state recognizes unique structural features of various CCK receptor agonists and antagonists and that each state activates characteristic signal transduction mechanisms that, in turn, produce state-specific changes in pancreatic acinar cell function.