ATP INDUCES 2 CHOLECYSTOKININ BINDING-AFFINITY STATES IN PERMEABILIZED RAT PANCREATIC ACINI

The influence of adenine and guanine nucleotides on cholecystokinin (CCK) receptor binding was examined in streptolysin O-permeabilized rat pancreatic acini. Specific binding of tracer to intact acini was 12.1 +/- 0.4% per milligram protein, while permeabilized acini bound 34.6 +/- 2.9% (n = 7). The increase in binding was also seen when normalized to DNA. Binding to permeabilized acini was reduced by the presence of 1 mM ATP to 23.0 +/- 1.3%. Analysis of competitive inhibition of tracer binding by unlabeled CCK-8 was consistent with binding to two affinity states on intact acini, with the equilibrium dissociation constants for the high (K(dH))- and low (K(dL))-affinity states equal to 41 +/- 5 pM and 5.2 +/- 0.4 nM, respectively; permeabilized acini displayed a single binding site with K(d) = 598 +/- 40 pM. In the presence of 1 mM ATP, two states were seen on permeabilized acini with K(dH) = 85 +/- 11 pM and K(dL) = 2.7 +/- 0.6 nM. ATP, ATP-gamma-S, GTP, and GTP-gamma-S all inhibited binding, with half-maximal inhibition occurring at >1 mM, 21-mu-M, 5-mu-M, and 0.4-mu-M, respectively. GTP-gamma-S (1-mu-M) also induced two affinity states with K(dH) = 112 +/- 7 pM and K(dL) = 1.5 +/- 0.2 nM (n = 3). Binding of CCK to pancreatic membranes was also decreased by ATP, and a similar regeneration of two binding affinity states was observed. ATP also decreased binding of [I-125-Tyr4]bombesin to permeabilized acini, but in contrast did not generate two measurable binding affinity states. The results suggest that the CCK receptor on rat pancreatic acini exists as a single entity capable of existing in two interconvertible affinity states, modulated by physiological concentrations of adenine and guanine nucleotides.