Oscillatory membrane potential response to glucose in islet beta-cells: a comparison of islet-cell electrical activity in mouse and rat

Glucose induces synchronous mitochondrial calcium oscillations in intact pancreatic islets

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

Hyperpolarization-activated cyclic nucleotide-gated channels in pancreatic beta-cells

Hyperpolarization-activated cyclic nucleotide-modulated (HCN) channels mediate the pacemaker current (Ih or If) observed in electrically rhythmic cardiac and neuronal cells. Here we describe a hyperpolarization-activated time-dependent cationic current, beta-Ih, in pancreatic beta-cells. Transcripts for HCN1-4 were detected by RT-PCR and quantitative PCR in rat islets and MIN6 mouse insulinoma cells. beta-Ih in rat beta-cells and MIN6 cells displayed biophysical and pharmacological properties similar to those of HCN currents in cardiac and neuronal cells. Stimulation of cAMP production with forskolin/3-isobutyl-1-methylxanthine (50 microM) or dibutyryl-cAMP (1 mM) caused a significant rightward shift in the midpoint activation potential of beta-Ih, whereas expression of either specific small interfering (si)RNA against HCN2 (siHCN2b) or a dominant-negative HCN channel (HCN1-AAA) caused a near-complete inhibition of time-dependent beta-Ih. However, expression of siHCN2b in MIN6 cells had no affect on glucose-stimulated insulin secretion under normal or cAMP-stimulated conditions. Blocking beta-Ih in intact rat islets also did not affect membrane potential behavior at basal glucose concentrations. Taken together, our experiments provide the first evidence for functional expression of HCN channels in the pancreatic beta-cell.