Tolbutamide stimulation of pancreatic β-cells involves both cell recruitment and increase in the individual Ca2+ response

1 Individual pancreatic beta -cells are functionally heterogeneous. Their sensitivity to glucose is variable, so that the proportion of active cells increases with the glucose concentration (recruitment). 2 We have investigated whether sulphonylureas also recruit beta -cells, by measuring cytoplasmic Ca2+ ([Ca2+](i))- the triggering signal of insulin secretion-in single cells and clusters of cells prepared from mouse islets. 3 In 4 mM glucose, the threshold concentration of tolbutamide inducing a [Ca2+](i) rise was variable (5-50 muM). 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 mM (EC50, of similar to 14 and similar to4 muM tolbutamide respectively). 4 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 [Ca2+](i) oscillations were synchronous in all cells of each cluster, whereas there was no synchrony between clusters or individual cells. 5 Independently of cell recruitment, tolbutamide gradually augmented the magnitude of the [Ca2+](i) rise in single cells and clusters. This increase occurred over a broader range of concentrations than did recruitment (EC50 of similar to 50 and 25 muM tolbutamide at 4 and 5 mM glucose respectively). 6 Tolbutamide (10 muM) accelerated the recruitment of single cells and clusters brought about by increasing glucose concentrations (range of 3-7 mM instead of 4-10 mM glucose), and potentiated the amplification of the individual responses that glucose also produced. 7 In conclusion, both metabolic (glucose) and pharmacologic (sulphonylurea) inhibition of K+-ATP channels recruits beta -cells to generate a [Ca2+](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.