Relationship of cholecystokinin receptor binding to regulation of biological functions in pancreatic acini.

Cholecystokinin (CCK) was conjugated to 125I-Bolton-Hunter reagent (125I-BH-CCK), and the binding of this ligand to CCK receptors in isolated mouse pancreatic acini was correlated with the regulation by CCK of both amylase release and the transport of 2-deoxyglucose and alpha-aminoisobutyric acid. Stimulation of amylase release by CCK was biphasic. At low CCK concentrations (less than 200 pM), amylase release was progressively stimulated, whereas at higher CCK concentrations (greater than 200 pM), amylase release was progressively reduced. In contrast, stimulation of 2-[3H]deoxyglucose transport and inhibition of alpha-[3H]aminoisobutyric acid transport were monophasic, being one-half maximal at 0.85 and 0.44 nM, respectively. Under incubation conditions identical to those employed for measuring biological functions, the binding of 125I-BH-CCK to receptors in acini was rapid and reversible. Competition-inhibition curves and Scatchard plots of equilibrium binding were compatible with two orders of binding sites. Employing a computer program for analysis of multiple binding sites, a high-affinity, low-capacity binding component having a Kd of 26 pM and a lower-affinity, higher-capacity binding component having a component Kd of 2.2 nM were resolved. Regulation of 2-[3H]deoxyglucose and alpha-[3H]aminoisobutyric acid uptake appeared, therefore, to be the result of fractional occupancy of the lower-affinity CCK receptors. Regulation of amylase released was more complex and appeared to be due to the concomitant occupancy of both the high- and low-affinity CCK receptors.