Measurements of Cytoplasmic Ca2+ in Islet Cell Clusters Show That Glucose Rapidly Recruits β-Cells and Gradually Increases the Individual Cell Response

Abstract

The proportion of isolated single β-cells developing a metabolic, biosynthetic, or secretory response increases with glucose concentration (recruitment). It is unclear whether recruitment persists in situ when β-cells are coupled. We therefore measured the cytoplasmic free Ca2+ correction ([Ca2+]i) (the triggering signal of glucose-induced insulin secretion) in mouse islet single cells or clusters cultured for 1–2 days. In single cells, the threshold glucose concentration ranged between 6 and 10 mmol/l, at which concentration a maximum of ∼65% responsive cells was reached. Only 13% of the cells did not respond to glucose plus tolbutamide. The proportion of clusters showing a [Ca2+]i rise increased from ∼20 to 95% between 6 and 10 mmol/l glucose, indicating that the threshold sensitivity to glucose differs between clusters. Within responsive clusters, 75% of the cells were active at 6 mmol/l glucose and 95–100% at 8–10 mmol/l glucose, indicating that individual cell recruitment is not prominent within clusters; in clusters responding to glucose, all or almost all cells participated in the response. Independently of cell recruitment, glucose gradually augmented the magnitude of the average [Ca2+]i rise in individual cells, whether isolated or associated in clusters. When insulin secretion was measured simultaneously with [Ca2+]i, a good temporal and quantitative correlation was found between both events. However, β-cell recruitment was maximal at 10 mmol/l glucose, whereas insulin secretion increased up to 15–20 mmol/l glucose. In conclusion, β-cell recruitment by glucose can occur at the stage of the [Ca2+]i response. However, this type of recruitment is restricted to a narrow range of glucose concentrations, particularly when β-cell association decreases the heterogeneity of the responses. Glucose-induced insulin secretion by islets, therefore, cannot entirely be ascribed to recruitment of β-cells to generate a [Ca2+]i response. Modulation of the amplitude of the [Ca2+]i response and of the action of Ca2+ on exocytosis (amplifying actions of glucose) may be more important.