Dysregulation of calcium channels decreases parasecretion in pancreatic ß-cells in rats born small for gestational age.

OBJECTIVE: To investigate the role of intrauterine malnourishment in the development and function of pancreatic islet ß-cells. METHODS: Whole-cell patch clamping was used to record voltage-gated calcium channel (VGCC)-mediated currents. Insulin secretion was detected by measuring capacitance using a sequence of sine wave stimuli. VGCC currents and insulin secretion were measured in the small for gestational age (SGA) group treated with human recombinant growth hormone (hGH). RESULTS: The membrane capacitance in the SGA group (6.4 ± 0.9 fF/Pf) was significantly reduced. Calcium current density and peak current density in the SGA group were also markedly decreased, whereas other measurements of calcium channels were unaltered. Treatment with hGH significantly rescued the membrane capacitance, whereas calcium channels were not affected. CONCLUSION: Our data suggest that decreased ß-cell secretion is caused by a decreased expression of calcium channels and reduced calcium currents. hGH restores ß-cell secretion in SGA animals, possibly independently of VGCC.

AMPA receptors regulate exocytosis and insulin release in pancreatic ß cells.

Ionotropic glutamate receptors (iGluRs) are expressed in islets and insulinoma cells and involved in insulin secretion. However, the exact roles that iGluRs play in ß cells remain unclear. Here, we demonstrated that GluR2-containing α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) were expressed in mouse ß cells. Glutamate application increased both cytosolic calcium and the number of docked insulin-containing granules, which resulted in augmentation of depolarization-induced exocytosis and high-glucose-stimulated insulin release. While glutamate application directly depolarized ß cells, it also induced an enormous depolarization when K(ATP) channels were available. Glutamate application reduced the conductance of K(ATP) channels and increased voltage oscillations. Moreover, actions of AMPARs were absent in Kir6.2 knock-out mice. The effects of AMPARs on K(ATP) channels were mediated by cytosolic cGMP. Taken together, our experiments uncovered a novel mechanism by which AMPARs participate in insulin release.

ATP regulates sodium channel kinetics in pancreatic islet beta cells.

Pancreatic beta cells act as glucose sensors, in which intracellular ATP ([ATP](i)) are altered with glucose concentration change. The characterization of voltage-gated sodium channels under different [ATP](i) remains unclear. Here, we demonstrated that increasing [ATP](i) within a certain range of concentrations (2-8 mM) significantly enhanced the voltage-gated sodium channel currents, compared with 2 mM cytosolic ATP. This enhancement was attenuated by even high intracellular ATP (12 mM). Furthermore, elevated ATP modulated the sodium channel kinetics in a dose-dependent manner. Increased [ATP](i) shifted both the current-voltage curve and the voltage-dependent inactivation curve of sodium channel to the right. Finally, the sodium channel recovery from inactivation was significantly faster when the intracellular ATP level was increased, especially in 8 mM [ATP](i), which is an attainable concentration by the high glucose stimulation. In summary, our data suggested that elevated cytosolic ATP enhanced the activity of Na(+) channels, which may play essential roles in modulating ß cell excitability and insulin release when blood glucose concentration increases.

[Glutamate receptors and gamma-aminobutyric acid receptors in pancreatic cells].

Glutamate and gamma-aminobutyric acid (GABA) receptors are mainly expressed in central nervous system and play critical roles in neural signal transduction. It has been demonstrated that glutamate and GABA receptors are also found in pancreatic islets. Interestingly, almost all of glutamate and GABA receptor subunits are present in islets. Here, we summarize current progresses of these receptors in islets, focusing on there expressions, physiological implications, interactions, as well as a novel approach to investigate roles of the receptors in islets slice. All these investigations will potentially supply new understanding of working mechanism of these receptors in islet and also shed a new insight for neuroscientific research.

[Progesterone Promotes Human Bone Marrow Mesenchymal Stem Cells to Synthesize Fibronectin via ERK Pathway].

OBJECTIVE: To investigate whether the progesterone can promote fibronection (FN) synthesis by human bone marrow mesenchymal stem cells (MSCs) and to explore the potential underlying mechanism. METHODS: The human bone marrow MSCs were cultured in a serum-free medium with progesterone for 72 hours, the MTT test was performed to observe the proliferation status and adhension ability of the treated cells. Western blot was used to detect the content of FN in MSDs with GAPDH as the internal reference, the phosphorylation of ERK1/2, as well as the FN content in MSC treated by PD98059, a specific inhibitor of ERK1/2. RESULTS: The progesterone at a range of certain doses not effect on the proliferation of human bone marrow MSCs. Progesterone (25 µg/L) treatment enhanced the FN expression and adherent ability of marrow MSCs. Progesterone could induce prompt phosphorylation of ERK 1/2 and its promoting effects on FN synthesis was reversed by PD98059. CONCLUSION: The progesterone can promote FN synthesis by human bone marrow MSCs via ERK 1/2 pathway, and it might be used to culture MSCs in serum-free medium.

Chronic zinc exposure decreases the surface expression of NR2A-containing NMDA receptors in cultured hippocampal neurons.

BACKGROUND: Zinc distributes widely in the central nervous system, especially in the hippocampus, amygdala and cortex. The dynamic balance of zinc is critical for neuronal functions. Zinc modulates the activity of N-methyl-D-aspartate receptors (NMDARs) through the direct inhibition and various intracellular signaling pathways. Abnormal NMDAR activities have been implicated in the aetiology of many brain diseases. Sustained zinc accumulation in the extracellular fluid is known to link to pathological conditions. However, the mechanism linking this chronic zinc exposure and NMDAR dysfunction is poorly understood. METHODOLOGY/PRINCIPAL FINDINGS: We reported that chronic zinc exposure reduced the numbers of NR1 and NR2A clusters in cultured hippocampal pyramidal neurons. Whole-cell and synaptic NR2A-mediated currents also decreased. By contrast, zinc did not affect NR2B, suggesting that chronic zinc exposure specifically influences NR2A-containg NMDARs. Surface biotinylation indicated that zinc exposure attenuated the membrane expression of NR1 and NR2A, which might arise from to the dissociation of the NR2A-PSD-95-Src complex. CONCLUSIONS: Chronic zinc exposure perturbs the interaction of NR2A to PSD-95 and causes the disorder of NMDARs in hippocampal neurons, suggesting a novel action of zinc distinct from its acute effects on NMDAR activity.