  Glucose homeostasis is controlled by the islets of Langerhans which are
equipped with alpha-cells increasing the blood glucose level, beta-cells
decreasing it, and delta-cells the precise role of which still needs
identifying. Although intercellular communications between these endocrine
cells have recently been observed, their roles in glucose homeostasis have not
been clearly understood. In this study, we construct a mathematical model for
an islet consisting of two-state alpha-, beta-, and delta-cells, and analyze
effects of known chemical interactions between them with emphasis on the
combined effects of those interactions. In particular, such features as
paracrine signals of neighboring cells and cell-to-cell variations in response
to external glucose concentrations as well as glucose dynamics, depending on
insulin and glucagon hormone, are considered explicitly. Our model predicts
three possible benefits of the cell-to-cell interactions: First, the asymmetric
interaction between alpha- and beta-cells contributes to the dynamic stability
while the perturbed glucose level recovers to the normal level. Second, the
inhibitory interactions of delta-cells for glucagon and insulin secretion
prevent the wasteful co-secretion of them at the normal glucose level. Finally,
the glucose dose-responses of insulin secretion is modified to become more
pronounced at high glucose levels due to the inhibition by delta-cells. It is
thus concluded that the intercellular communications in islets of Langerhans
should contribute to the effective control of glucose homeostasis.

Tissues and Organs