Non-insulin-dependent diabetics have impaired insulin secretion. Our general hypothesis is that abnormal pancreatic islet electrical activity is an intrinsic lesion that contributes to this impairment. Islets are a syncytium of electrically-coupled cells containing different hormones. In elevated glucose, B-cell ion channels interact to produce bursting, and oscillatory electrical activity pattern which mediates cyclic Ca uptake and which is important for normal insulin secretion. Previous studies have attempted to understand bursting of whole islets based on the biophysical properties of the ion channels of single B-cells. However, it has not been clear whether single B-cells burst or how their different ion channels interact to mediate cell firing. These significant gaps in our knowledge prevent us from understanding how abnormal ion channel mechanisms underlie the abnormal bursting and secretion of diabetic animals. To address this issue, we will (1) apply voltage wave forms to single voltage-clamped mouse B-cells that simulate the changes in membrane potential and intracellular ions encountered in bursting (burstwave clamping) to determine how B-cell ion channel currents change during bursting; and (2) test how injecting different artificial voltage- and time-dependent ion channel currents into current-clamped B-cells affects cell firing (dynamic clamping). Preliminary data suggest that single B-cells have heterogeneous electrical activity, including bursting, and can be grouped into three subclasses. Modeling and artificial electrical coupling of independent cells with dynamic clamp will determine how coupling affects B-cell firing. A new model of islet bursting will be made by coupling cells having the heterogenous firing observed experimentally. Simultaneous patch clamping and amperometry will determine whether bursting vs. spiking preferentially augments insulin secretion. Accomplishment of these aims will identify the B-cell channels which underlie electrical activity in normal and diabetic islets and may lead to new therapies for diabetes based on manipulating these channels.
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