Residual inhibition of epidermal growth factor binding by pancreatic secretagogues and phorbol ester in rat pancreas

Cholecystokinin‐octapeptide (CCK₈) inhibits ¹²⁵I‐labeled epidermal growth factor (EGF) cell‐associated radioactivity in pancreatic acini, ostensibly as a result of its ability to mobilize cellular Ca²⁺. The phorbol ester tetradecanoyl phorbol acetate (TPA), a compound that activates protein kinase C, mimics the inhibitory action of CCK₈. In the present study we examined the relationship between occupancy of the cholecystokinin (CCK) receptor, the subsequent inhibition of EGF binding, and the potential role of C‐kinase activation in mediating this inhibition. Proglumide and dibutyryl cyclic GMP (dbGMP), two distinct competitive antagonists of CCK₈, reversed the inhibitory actions of CCK₈. Analysis of steady‐state saturation kinetics of ¹²⁵I‐EGF binding indicated that CCK₈ decreased the apparent affinity of the EGF receptor, mainly as a result of a marked decrease in the amount of internalized ligand. TPA also inhibited ¹²⁵I‐EGF internalization. Removal of CCK₈ and TPA from incubation medium did not abolish their inhibitory actions. Carbachol, but not bombesin, exerted a similar residual inhibitory effect. It is suggested that in addition to acting via Ca²⁺, certain pancreatic secretagogues may also act through C‐kinase to regulate EGF binding.