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

Abstract

Using clonal insulin-secreting BRIN-BD11 cells, we have assessed whether the graded response of the whole cell population to glucose can be accounted for by a dose-dependent recruitment of individual cells, an amplification of the response of the recruited cells or both. Cytosolic free Ca2+ concentration ([Ca2+]i) is an established index of [beta]-cell function. We used fura-2 microfluorescence techniques to assess the [Ca2+]i responsiveness of single BRIN-BD11 cells to glucose and other secretagogues. Glucose (1-16.7Â mM) evoked oscillatory [Ca2+]i rises in these cells resembling those found in parental rat pancreatic [beta]-cells. The percentage of glucose-responsive cells was 11% at 1Â mM and increased to 40-70% at 3-16.7Â mM glucose, as assessed by a single-stimulation protocol. This profile was unrelated to possible differences in the cell cycle, as inferred from experiments where the cultured cells were synchronized by a double thymidine block protocol. Individual cells exhibited variable sensitivities to glucose (threshold range: 1-5Â mM) and a variable dose-dependent amplification of the [Ca2+]i responses (EC50 range: 2-10Â 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-secreting cells. Specifically, the graded secretory response of the whole cell population is likely to reflect both a recruitment of individual cells with different sensitivities to glucose and a dose-dependent amplification of the response of the recruited cells.