Tolbutamide stimulation of pancreatic β‐cells involves both cell recruitment and increase in the individual Ca 2+ response

Individual pancreatic β‐cells are functionally heterogeneous. Their sensitivity to glucose is variable, so that the proportion of active cells increases with the glucose concentration (recruitment). We have investigated whether sulphonylureas also recruit β‐cells, by measuring cytoplasmic Ca 2+ ([Ca 2+ ] i ) – the triggering signal of insulin secretion – in single cells and clusters of cells prepared from mouse islets. In 4 m M glucose, the threshold concentration of tolbutamide inducing a [Ca 2+ ] i rise was variable (5 – 50 μ M ). The proportion of responsive cells and clusters therefore increased with the tolbutamide concentration, to reach a maximum of 90% of the cells and 100% of the clusters. This recruitment occurred faster when the glucose concentration was increased from 4 to 5 m M (EC 50 of ∼14 and ∼4 μ M tolbutamide respectively). Within responsive clusters little recruitment was observed; when a cluster was active, all or nearly all cells were active probably because of cell coupling. Thus, tolbutamide‐induced [Ca 2+ ] i oscillations were synchronous in all cells of each cluster, whereas there was no synchrony between clusters or individual cells. Independently of cell recruitment, tolbutamide gradually augmented the magnitude of the [Ca 2+ ] i rise in single cells and clusters. This increase occurred over a broader range of concentrations than did recruitment (EC 50 of ∼50 and 25 μ M tolbutamide at 4 and 5 m M glucose respectively). Tolbutamide (10 μ M ) accelerated the recruitment of single cells and clusters brought about by increasing glucose concentrations (range of 3 – 7 m M instead of 4 – 10 m M glucose), and potentiated the amplification of the individual responses that glucose also produced. In conclusion, both metabolic (glucose) and pharmacologic (sulphonylurea) inhibition of K + ‐ATP channels recruits β‐cells to generate a [Ca 2+ ] i response. However, the response is not of an all‐or‐none type; it increases in amplitude with the concentration of either glucose or tolbutamide.British Journal of Pharmacology (2001) 133 , 575–585; doi: 10.1038/sj.bjp.0704108