Receptors for cholecystokinin and insulin in isolated pancreatic acini: hormonal control of secretion and metabolism.

Both isolated pancreatic cells and isolated pancreatic acini have been prepared from mouse and rat pancreas. Although isolated cells are useful for the study of early events in stimulus-secretion coupling such as regulation of Ca2+ fluxes, they show a relatively poor ability to secrete digestive enzymes in response to secretagogues. Most likely this deficiency in secretion is due to the loss of specialization at the luminal plasma membrane surface. In contrast to isolated cells, isolated pancreatic acini can be used to study the secretion of digestive enzymes in response to hormones and neurotransmitters. Using isolated acini and a new preparation of radioiodinated cholecystokinin (CCK), we have been able to characterize pancreatic CCK receptors. Analysis of binding data reveals two orders of binding sites, a high affinity site (Kd = 64 pm) and a low affinity site (Kd = 21 nm). Stimulation of amylase release correlates with occupancy of the high affinity site. High affinity receptors for insulin are also present on isolated acini. After binding to these receptors, insulin stimulates protein and amylase synthesis. Insulin does not directly stimulate amylase secretion but rather potentiates the effects of hormonal secretagogues. These studies indicate, therefore, that isolated pancreatic acini can be employed to study in vitro various events in hormonal and neurotransmitter regulation of pancreatic enzyme secretion and metabolism.