Glucose-mediated Ca2+ signalling in single clonal insulin-secreting cells:evidence for a mixed model of cellular activation

Using clonal insulin-secreting BRIN-BD11 cells, we have assessed whether th e graded response of the whole cell population to glucose can be accounted for by a dose-dependent recruitment of individual cells, an amplification o f the response of the recruited cells or both. Cytosolic free Ca2+ concentr ation ([Ca2+](i)) is an established index of beta-cell function. We used fu ra-2 microfluorescence techniques to assess the [Ca2+](i) responsiveness of single BRIN-BD11 cells to glucose and other secretagogues. Glucose (1-16.7 mM) evoked oscillatory [Ca2+](i) rises in these cells resembling those fou nd in parental rat pancreatic beta-cells. The percentage of glucose-respons ive cells was 11% at 1 mM and increased to 40-70% at 3-16.7 mM glucose, as assessed by a single-stimulation protocol. This profile was unrelated to po ssible differences in the cell cycle, as inferred from experiments where th e cultured cells were synchronized by a double thymidine block protocol. In dividual cells exhibited variable sensitivities to glucose (threshold range : 1-5 mM) and a variable dose-dependent amplification of the [Ca2+](i) resp onses (EC50 range: 2-10 mM), as assessed by a multiple-stimulation protocol . Glyceraldehyde and alpha-ketoisocaproic acid had glucose-like effects on [Ca2+](i). The data support a mixed model for the activation of insulin-sec reting cells. Specifically, the graded secretory response of the whole cell population is likely to reflect both a recruitment of individual cells wit h different sensitivities to glucose and a dose-dependent amplification of the response of the recruited cells, (C) 2000 Elsevier Science Ltd. All rig hts reserved.