Microfluidic glucose stimulation reveals limited coordination of intracellular Ca²⁺ activity oscillations in pancreatic islets

The pancreatic islet is a functional microorgan involved in maintaining normoglycemia through regulated secretion of insulin and other hormones. Extracellular glucose stimulates insulin secretion from islet β cells through an increase in redox state, which can be measured by NAD(P)H autofluorescence. Glucose concentrations over ∼7 mM generate synchronous oscillations in β cell intracellular Ca²⁺ concentration ([Ca²⁺]_i), which lead to pulsatile insulin secretion. Prevailing models assume that the pancreatic islet acts as a functional syncytium, and the whole islet [Ca²⁺]_i response has been modeled in terms of islet bursting and pacemaker models. To test these models, we developed a microfluidic device capable of partially stimulating an islet, while allowing observation of the NAD(P)H and [Ca²⁺]_i responses. We show that β cell [Ca²⁺]_i oscillations occur only within regions stimulated with more than ∼6.6 mM glucose. Furthermore, we show that tolbutamide, an antagonist of the ATP-sensitive K ⁺ channel, allows these oscillations to travel farther into the nonstimulated regions of the islet. Our approach shows that the extent of Ca²⁺ propagation across the islet depends on a delicate interaction between the degree of coupling and the extent of ATP-sensitive K ⁺-channel activation and illustrates an experimental paradigm that will have utility for many other biological systems.