Somatostatin binding to its receptors on rat pancreatic acinar membranes was characterized with [125I-Tyr1]somatostatin. The COOH-terminal octapeptide of cholecystokinin (CCK8), when present at various concentrations in the reaction mixture for the binding study, reduced labeled somatostatin binding in a dose-dependent manner, whereas carbachol or Ca2+ ionophore did not affect the binding. By contrast, when pancreatic acini were first treated with carbachol and thereafter [125I-Tyr1]somatostatin binding to membranes prepared from these acini was examined, carbachol reduced subsequent somatostatin binding in a dose-dependent manner. Scatchard analysis of the labeled somatostatin binding revealed that carbachol pretreatment decreased the maximum binding capacity from 142 +/- 20 fmol/mg of membrane protein to 63.5 +/- 3.5 fmol/mg of membrane protein without significantly affecting the binding affinity. To test for the possibility that CCK8 also may affect labeled somatostatin binding through an intracellular process, pancreatic acini were first treated with CCK8 and then the membrane bound CCK8 was washed out. Subsequent labeled somatostatin binding to membranes from these acini was also decreased. When 1 mM EDTA was present in the pretreatment medium, the inhibitory effect of carbachol or CCK8 was partially abolished, suggesting that an intracellular process to modulate somatostatin binding is dependent on Ca2+. On the other hand, pretreatment of acini with Ca2+ ionophore almost failed to affect subsequent labeled somatostatin binding. Results therefore suggest that CCK8 can modulate labeled somatostatin binding to pancreatic acinar membranes not only acting through an intracellular process but also at membrane sites and carbachol- or CCK8-activated intracellular process to modulate somatostatin binding is dependent on Ca2+, but Ca2+ mobilization itself is not sufficient to affect subsequent somatostatin binding.