Structure and function of the endocrine cell types of the islets

Distinct intracellular Ca2+ response to extracellular adenosine triphosphate in pancreatic beta-cells in rats and mice

Extracellular adenosine triphosphate (ATP) has distinct effects on insulin secretion from pancreatic beta-cells between rats and mice. Using a confocal microscope, we compared changes between rats and mice in cytosolic free calcium concentration ([Ca2+]c) in pancreatic beta-cells stimulated by extracellular ATP. Extracellular ATP (50 microM) induced calcium release from intracellular calcium stores by activating P2Y receptors in both rat and mouse beta-cells. The intracellular calcium release stimulated by extracellular ATP is significantly smaller in amplitude and longer in duration in rat beta-cells than in mouse. In response to extracellular ATP, rat beta-cells activate store-operated calcium entry following intracellular calcium release. This response is lacking in mouse beta-cells. Rat and mouse beta-cells both responded to 9 mM glucose by increasing [Ca2+]c. This increase, however, was pronounced only in the rat beta-cells. In 9 mM glucose, extracellular ATP induced a pronounced calcium release above the increased level of [Ca2+]c in rat beta-cells. In mouse beta-cells, however, extracellular ATP did not exhibit calcium release on top of the increased level of [Ca2+]c in 9 mM glucose. These results demonstrate distinct responses between rat and mouse beta-cells to extracellular ATP under the condition of low and high glucose. Considering that extracellular ATP inhibits insulin secretion from mouse beta-cells but stimulates insulin secretion from rat beta-cells, we suggest that store-operated Ca2+ entry may be related to exocytosis in pancreatic rat beta-cells.

Do local immune-neuroendocrine disturbances initiate diabetes?

It has been suggested that there exists a local immune-neuroendocrine self-regulating system in the pancreas. The system consists of β-cells, nerve ganglia, intercellular fluid, connective tissue, and endothelial and immunocompetent cells. The local immune-neuroendocrine system governs the background level of insulin production by intrinsic mechanisms both in normal conditions and in a recovery period after different kinds of stress. The activity of this system by a complex of metabolic, environmental, nerve, and nonspecific immune factors has been determined. The local immune-neuroendocrine system is partially autonomous as a result of local integrative nerve circuits, morphological and functional substrates. Increased or decreased synthesis and release of some cytokines or biologically active substances (neurotransmitters, neuropeptides, γ-aminobutyric acid, metabolites, nitric oxide, ions, etc.) by various cell types in the local immune-neuroendocrine system above usual levels may result in disturbances of sensitivity and functions of β-cells. If the capability of the local immune-neuroendocrine system is insufficient for their compensation, the islet cell autoantigens may occur, the specific immune mechanisms are involved, and the pathological process becomes irreversible. Some ways for prevention of disturbances in the local immune-neuroendocrine system during the early and late phases of diabetes are presented.Key words: β-cells, diabetes, local immuneneuroendocrine system.