Glucose induces synchronous mitochondrial calcium oscillations in intact pancreatic islets

Mitochondria shape Ca2+ signaling and exocytosis by taking up calcium during cell activation. In addition, mitochondrial Ca2+ ([Ca2+]M) stimulates respiration and ATP synthesis. Insulin secretion by pancreatic β-cells is coded mainly by oscillations of cytosolic Ca2+ ([Ca2+]C), but mitochondria are also important in excitation–secretion coupling. Here, we have monitored [Ca2+]M in single β-cells within intact mouse islets by imaging bioluminescence of targeted aequorins. We find an increase of [Ca2+]M in islet-cells in response to stimuli that induce either Ca2+ entry, such as extracellular glucose, tolbutamide or high K+, or Ca2+ mobilization from the intracellular stores, such as ATP or carbamylcholine. Many cells responded to glucose with synchronous [Ca2+]M oscillations, indicating that mitochondrial function is coordinated at the whole islet level. Mitochondrial Ca2+ uptake in permeabilized β-cells increased exponentially with increasing [Ca2+], and, particularly, it became much faster at [Ca2+]C > 2 μM. Since the bulk [Ca2+]C signals during stimulation with glucose are smaller than 2 μM, mitochondrial Ca2+ uptake could be not uniform, but to take place preferentially from high [Ca2+]C microdomains formed near the mouth of the plasma membrane Ca2+ channels. Measurements of mitochondrial NAD(P)H fluorescence in stimulated islets indicated that the [Ca2+]M changes evidenced here activated mitochondrial dehydrogenases and therefore they may modulate the function of β-cell mitochondria. Diazoxide, an activator of KATP, did not modify mitochondrial Ca2+ uptake.