Regulation of Na+/Ca2+ exchange in the rat pancreatic B cell

Computational modeling of the effect of temperature variations on human pancreatic β-cell activity

The effect of temperature variations on the pancreatic β-cell activity and the role of different model compartments in temperature sensing have been investigated using a computational modeling approach. The results of our study show that temperature variations by several degrees can change the dynamical states of the β-cell system. In addition, temperature variations can alter the characteristic features of the membrane voltage, which correlates with insulin secretion. Simulation results show that the ion channels such as the L-type calcium, the hERG potassium, sodium channels and the glycolysis pathway are the possible sites for sensing temperature variation. Results indicate that for a small temperature change, even though the frequency and amplitude of electrical activity are altered, the area under the membrane potential curve remains almost unchanged, which implies the existence of a thermoregulatory mechanism for preserving the amount of insulin secretion. Furthermore, the computational analysis shows that the β-cell electrical activity exhibits a bursting pattern in physiological temperature (37 °C) while in vitro studies reported almost the spiking activity at lower temperatures. Since hormone-secreting systems work more efficient in bursting mode, we propose that the pancreatic β-cell works better in the physiological temperature compared with the reference temperature (33 °C).

Significance of Na/Ca exchange for Ca2+ buffering and electrical activity in mouse pancreatic beta-cells

We have combined the patch-clamp technique with microfluorimetry of the cytoplasmic Ca2+ concentration ([Ca2+]i) to characterize Na/Ca exchange in mouse beta-cells and to determine its importance for [Ca2+]i buffering and shaping of glucose-induced electrical activity. The exchanger contributes to Ca2+ removal at [Ca2+]i above 1 microM, where it accounts for >35% of the total removal rate. At lower [Ca2+]i, thapsigargin-sensitive Ca2+-ATPases constitute a major (70% at 0.8 microM [Ca2+]i) mechanism for Ca2+ removal. The beta-cell Na/Ca exchanger is electrogenic and has a stoichiometry of three Na+ for one Ca2+. The current arising from its operation reverses at approximately -20 mV (current inward at more negative voltages), has a conductance of 53 pS/pF (14 microM [Ca2+]i), and is abolished by removal of external Na+ or by intracellularly applied XIP (exchange inhibitory peptide). Inhibition of the exchanger results in shortening (50%) of the bursts of action potentials of glucose-stimulated beta-cells in intact islets and a slight (5 mV) hyperpolarization. Mathematical simulations suggest that the stimulatory action of glucose on beta-cell electrical activity may be accounted for in part by glucose-induced reduction of the cytoplasmic Na+ concentration with resultant activation of the exchanger.

Identification, expression pattern and potential activity of Na/Ca exchanger isoforms in rat pancreatic B-cells

In the pancreatic B-cell, Na/Ca exchange displays a quite high capacity and participates in the control of cytosolic free Ca2+ concentration. The Na/Ca exchanger was recently cloned in various tissues. Two genes coding for two different exchangers (NCX1 and NCX2) have been identified and evidence for several isoforms for NCX1 shown. To characterize the isoform(s) expressed in pancreatic B-cells, a RT-PCR analysis was performed on mRNA from rat pancreatic islets, purified B-cells and insulinoma B-cells (RINm5F cells). PCR amplification did not yield the expected NCX2 DNA fragment but yielded 2 NCX1 bands, corresponding to NaCa3 and NaCa7, in the three preparations. NaCa3 and NaCa7 were equally expressed in pancreatic islets and purified B-cells. In RINm5F cells, NaCa3 expression did not differ from that in islet and purified B-cells but NaCa7 was 3 times less expressed. This lower expression was accompanied by a 3 times lower Na/Ca exchange activity in RINm5F cells compared to islet cells. Our data indicate the existence of 2 NCX1 isoforms but not of NCX2 in pancreatic B-cells. The difference in both the expression patterns of NCX1 isoforms and the activity of Na/Ca exchange in islet cells and RINm5F cells is compatible with a difference in activity between NaCa3 and NaCa7.

Evidence for a Na+-Ca2+ exchanger in rat pancreatic ducts

Only recently has it been recognized that intracellular Ca2+ is an important cellular mediator in pancreatic ducts. The aim of the present study was to characterize the Ca2+ efflux pathway in ducts freshly prepared from rat pancreas. Lowering of extracellular Na+ concentration resulted in a significant increase in intracellular Ca2+. This effect was fast, reversible, dependent on the extracellular Na+ concentration and did not correlate with intracellular pH changes. It was abolished in Ca2+-free solutions, indicating that the outwardly directed Na+ gradient was directly coupled to a flufenamate insensitive Ca2+ influx. Removal and reintroduction of extracellular Na+ induced transient hyperpolarization and depolarization of Vm, respectively. Taken together, our data indicate that pancreatic ducts possess an electrogenic Na+-Ca2+ exchanger, which under control conditions is responsible for transporting Ca2+ out of resting duct cells.

The pH dependence of interleukin-1 beta effects on insulin secretion in HIT cells

We have examined a hypothesis that the effects of recombinant human interleukin-1 beta (IL-1) on insulin secretion may depend upon the condition of intracellular pH levels in hamster clonal beta-cell line, HIT-T 15 cells. In the first experiment, the addition of 5 and 20 microM monensin, a Na+/H+ electroneutral exchange ionophore, did not attenuate a short-stimulatory effect of IL-1 on insulin secretion at 0-4 h period. At the concentration of 20 microM, monensin deleted the IL-1-induced reduction of insulin secretion at 4-24 h period, while 5 microM did not. Furthermore, 100 microM monensin blocked the bimodal effects of IL-1 on insulin secretion. In the second experiment, IL-1 significantly stimulated insulin secretion, although it did not affect cyclic AMP production at 0-4 h period. At 4-24 h period, IL-1 dose-dependently inhibited cyclic AMP production, accompanied with a significant reduction of insulin secretion. Alkalinization of circumstantial pH levels to 8.0 deleted the IL-1 effects on insulin secretion at 4-24 h period. However, alkalinization to pH 8.0 failed to attenuate the reduction of cyclic AMP production by IL-1. These data indicated that maintaining adequate intracellular pH levels may be important in the IL-1 effects on insulin secretion by the mechanism in which cyclic AMP may not be involved.

Inhibition of Na/Ca exchange stimulates insulin release from isolated rat pancreatic islets

Na/Ca exchange was recently shown to regulate cytosolic free Ca2+ concentration ([Ca2+]i) in the pancreatic B-cell. The aim of the present study was to provide direct evidence that inhibition of the activity of the exchange may also increase insulin release. In the presence of extracellular Na+, caffeine stimulated 45Ca outflow but did not increase insulin release from islets perifused in the presence of 2.8 mM glucose. By contrast, in the absence of extracellular Na+, caffeine almost failed to increase 45Ca outflow and reversibly stimulated insulin release despite the fact that the absence of extracellular Na+ per se reduced basal insulin release. Similar findings were observed in islets perifused at a higher glucose concentration (8.3 mM) except that, in the presence of extracellular Na+, caffeine more markedly increased 45Ca outflow and stimulated insulin release. Our data provide direct evidence that inhibition of Na/Ca exchange with resulting blockade of Ca2+ outflow may increase insulin release from the pancreatic B-cell under suitable experimental conditions.