Differential patterns of glucose-induced electrical activity and intracellular calcium responses in single mouse and rat pancreatic islets

Abstract

Although isolated rat islets are widely used to study in vitro insulin secretion and the underlying metabolic and ionic processes, knowledge on the properties of glucose-induced electrical activity (GIEA), a key step in glucose-response coupling, has been gathered almost exclusively from microdissected mouse islets. Using a modified intracellular recording technique, we have now compared the patterns of GIEA in collagenase-isolated rat and mouse islets. Resting membrane potentials of rat and mouse beta-cells were approximately -50 and -60 mV, respectively. Both rat and mouse beta-cells displayed prompt membrane depolarizations in response to glucose. However, whereas the latter exhibited a bursting pattern consisting of alternating hyperpolarized and depolarized active phases, rat beta-cells fired action potentials from a nonoscillating membrane potential at all glucose concentrations (8.4-22.0 mmol/l). This was mirrored by changes in the intracellular Ca2+ concentration ([Ca2+]i), which was oscillatory in mouse and nonoscillatory in rat islets. Stimulated rat beta-cells were strongly hyperpolarized by diazoxide, an activator of ATP-dependent K+ channels. Glucose evoked dose-dependent depolarizations and [Ca2+]i increases in both rat (EC50 5.9-6.9 mmol/l) and mouse islets (EC50 8.3-9.5 mmol/l), although it did not affect the burst plateau potential in the latter case. We conclude that there are important differences between beta-cells from both species with respect to early steps in the stimulus-secretion coupling cascade based on the following findings: 1) mouse beta-cells have a larger resting K+ conductance in 2 mmol/l glucose, 2) rat beta-cells lack the compensatory mechanism responsible for generating membrane potential oscillations and holding the depolarized plateau potential in mouse beta-cells, and 3) the electrical and [Ca2+]i dose-response curves in rat beta-cells are shifted toward lower glucose concentrations. Exploring the molecular basis of these differences may clarify several a priori assumptions on the electrophysiological properties of rat beta-cells, which could foster the development of new working models of pancreatic beta-cell function.