The purpose of the proposed research is to investigate the mechanism by which pancreatic endocrine cells recognize glucose. Understanding how B-cells sense the ambient concentration of glucose and translate this information into the modulation of hormone release is an important question in diabetes research. In mammalian islets, glucokinase was proposed to function as a glucose-sensor that determines the rate of glycolysis within the physiological range of glucose concentrations. A metabolite of glycolysis is then thought to regulate insulin release. Glucokinase most likely changes conformation dependent on the concentration of glucose and it could therefore itself act as an intracellular signal for hormone release. Both glucokinase and a metabolite may also act in concert to stimulate hormone release. Newly discovered features of the in vitro perfusion Brockmann body (i.e., the principal pancreatic islet) of channel catfish (Ictalurus panctatus), which include its secretory response to 2-deoxy- glucose and the selective pH-dependence of its glucose recognition mechanism make it an attractive model for studying the carbohydrate sensing mechanism. One objective of this investigation is to test the hypothesis that the principal islet of catfish also contains glucokinase; preliminary experiments suggest that this is true. Other experiments with the perfused Brockmann body show that 2-deoxy-glucose, a substrate of glucokinase that cannot be further metabolized, potently and reproducibly induced insulin and somatostatin release. This finding may indicate that glucokinase itself acts as an intracellular signal molecule. Two mutally exclusive hypothesis may explain the observation and will be tested: A) that 2-deoxy-glucose acts directly on endocrine cells via glucokinase; B) that 2-deoxyglucose acts via neurons on endocrine cells. In order to test hypothesis B part of this research will include investigating the innervation of the principal islet and testing the effect of neurotransmitting on hornome release. In the absence of extracellular sodium, or at a low extracellular pH glucose recognition by A, B, and D cells of the catfish islet is selectivity abolished. Therefore, the carbohydrate sensing mechanism in the catfish islet will be further investigated by measuring the regulation of the intracellular pH of endocrine cells and by determining its effect on the concentration of glycolytic metabolites. Thus, the following hypotheses will be tested: A) the absence of extracellular sodium causes the intracellular pH of pancreatic endocrine cells to fall; B) glycolysis is inhibited at a low intracellular pH.
