Stimulus-secretion coupling remains a poorly understood phenomenon in pancreatic acinar cells as well as in other cell types. The issue is confused by the fact that binding of hormones and neurotransmitters to cell surface receptors stimulates a number of cellular responses only some of which are likely to be critical to secretion. Our knowledge is also limited by the fact that, until recently, cellular responses to stimulation could only be studied in preparations involving many secretory cells (e.g. dispersed acini, lobules, fragments) and, thus, the responses monitored in those studies represent the averaged response of many secretory units. In these proposed studies, the PI will exploit several recent developments in this field to further examine the phenomenon of stimulus-secretion coupling. These include: (a) the observation that a choline-deficient ethionine-supplemented diet blocks hormone-stimulated secretion by preventing phospholipase C generation of inositol trisphosphate and diacylglycerol; (b) the recent development of CCK analogs which interact as agonists at thigh affinity CCK receptors but which antagonize the effect of CCK at low affinity receptors; (c) the finding that replacement of extracellular Na+ with choline+ blocks sustained but not initial hormone stimulated digestive enzyme secretion; and (d) the recent development of methods with which some secretagogue- induced cellular responses can be studied in single acini. Thus, the specific goal of this project is to define hormone-stimulated acinar cell events which are critical to stimulus-secretion coupling. We will: (1) determine whether they are dependent upon events which precede or follow phospholipase C activation; (2) identify which events follow IP3 generation and which are triggered by diacylglycerol release: (3) define which events are mediated by high and which by low affinity CCK receptors; (4) determine which events are critical to the sustained as opposed to the initial phase of secretion. These studies will greatly extend our understanding of stimulus-secretion coupling in the exocrine pancreas as well as in other secretory systems.
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