Phorbol ester desensitization of clonal insulin-releasing cell response to carbachol involves depletion of an intracellular calcium pool

The neurotransmitter ATP triggers Ca2+ responses promoting coordination of pancreatic islet oscillations

OBJECTIVES

Pulsatile insulin release into the portal vein is critically dependent on entrainment of the islets in the pancreas into a common oscillatory phase. Because the pulses reflect periodic variations of the cytoplasmic Ca concentration ([Ca]i), we studied whether the neurotransmitters adenosine triphosphate (ATP) and acetylcholine promote synchronization of [Ca]i oscillations between islets lacking contact.

METHODS

Medium-sized and small mouse islets and cell aggregates were used for measuring [Ca]i with the indicator fura-2.

RESULTS

Exposure to acetylcholine resulted in an initial [Ca]i peak followed by disappearance of the [Ca]i oscillations induced by 11-mmol/L glucose. The effect of ATP was often restricted to an elusive [Ca]i peak. The incidence of distinct [Ca]i responses to ATP increased under conditions (accelerated superfusion, small islets, or cell aggregates) intended to counteract purinoceptor desensitization owing to intercellular accumulation of ATP. Attempts to imitate neural activity by brief (15 seconds) exposure to ATP or acetylcholine resulted in temporary synchronization of the glucose-induced [Ca]i oscillations between islets lacking contact.

CONCLUSIONS

The data support the idea that purinergic signaling has a key role for coordinating the oscillatory activity of the islets in the pancreas, reinforcing previous arguments for the involvement of nonadrenergic, noncholinergic neurons.

Protein Kinase C Modulates Agonist-sensitive Release of Ca2+ from Internal Stores in HEK293 Cells Overexpressing the Calcium Sensing Receptor

This study examined the mechanism of Ca2+ entry and the role of protein kinase C (PKC) in Ca2+ signaling induced by activation of the calcium sensing receptor (CaR) in HEK293 cells stably expressing the CaR. We demonstrate that influx of Ca2+ following CaR activation exhibits store-operated characteristics in being associated with Ca2+ store depletion and inhibited by 2-aminoethoxydiphenyl borate. Inhibition of PKC with GF109203X, Go6983, or Go6976 and down-regulation of PKC activity enhanced the release of Ca2+ from internal stores in response to the polyvalent cationic CaR agonist neomycin, whereas activation of PKC with acute 12-O-tetradecanoylphorbol-13-acetate treatment decreased the release. In contrast, overexpression of wild type PKC-alpha or -epsilon augmented the neomycin-induced release of Ca2+ from internal stores, whereas dominant negative PKC-epsilon strongly decreased the release, but dominant negative PKC-alpha had little effect. Prolonged treatment of cells with 12-O-tetradecanoylphorbol-13-acetate effectively down-regulated immunoreactive PKC-alpha but had little effect on the expression of PKC-epsilon. Together these results indicate that diacylglycerol-responsive PKC isoforms differentially influence CaR agonist-induced release of Ca2+ from internal stores. The fundamentally different results obtained when overexpressing or functionally down-regulating specific PKC isoforms as compared with pharmacological manipulation of PKC activity indicate the need for caution when interpreting data obtained with the latter approach.

Gastrin-releasing peptide mobilizes calcium from intracellular stores in HIT-T15 cells

The mechanism by which gastrin-releasing peptide (GRP) increases cytoplasmic calcium [Ca2+]ic was studied in insulin-producing HIT-T15-cells. At zero glucose, GRP (100 nM) rapidly increased [Ca2+]ic in the presence and absence of extracellular Ca2+. The effect was potentiated by glucose, impaired by the inhibitor of microsomal Ca(2+)-ATPase, thapsigargin, and abolished by the inhibitor of phospholipase C U73122. In contrast, the inhibitor of Ca2+ induced Ca2+ release, ryanodine, was without effect. Furthermore, the GRP-induced increase in [Ca2+]ic was potentiated by forskolin and impaired by activation of protein kinase C (PKC) by 12-O-tetradecanoyl-phorbol-13-acetate (TPA). Based on these results, we conclude: 1) that GRP mobilizes Ca2+ from a thapsigargin-sensitive intracellular Ca2+ pool through activation of phospholipase C, and 2) that the GRP-induced mobilization of Ca2+ is potentiated by cyclic AMP.

Glucose sensing of individual pancreatic beta-cells involves transitions between steady-state and oscillatory cytoplasmic Ca2+

Glucose stimulation of individual pancreatic beta-cells is associated with a rise of the cytoplasmic Ca2+ concentration ([Ca2+]i) manifested either as large amplitude oscillations (0.2-0.5/min) or as a sustained increase. Determinants for the transitions between the basal and the two stimulated states have now been studied using dual-wavelength fluorometric measurements on individual ob/ob mouse beta-cells loaded with the Ca2+ indicator Fura-2. The transition from the basal state to large amplitude oscillations was induced by raising the glucose concentration to 7 mM or above. The frequencies and shapes of the [Ca2+]i cycles remained largely unaffected when raising glucose as high as 40 mM. However, in some cells the oscillatory pattern was transformed into a sustained increase of [Ca2+]i at high glucose concentrations. Although the peak values for the oscillations exceeded the steady-state increase, the time average [Ca2+]i was higher during the latter phase. Both types of glucose-induced transitions were facilitated by the presence of 1-100 nM glucagon. Protein kinase C activation by 10 nM of the phorbol ester TPA resulted in a transformation of the glucose-induced oscillations into a sustained increase of [Ca2+]i but the levels reached were considerably lower than obtained with glucose alone. It is concluded that the glucose sensing of the individual beta-cell is based on sudden transitions between steady-state and oscillating cytoplasmic Ca2+. It is these transitions rather than alterations of the oscillatory characteristics which determine the average [Ca2+]i regulating insulin release.

Calcium oscillations in guinea-pig pancreatic acinar cells exposed to carbachol, cholecystokinin and substance P

1. Cytoplasmic Ca2+ ([Ca2+]i) responses were studied in guinea-pig pancreatic acinar cells during stimulation with cholecystokinin octapeptide (CCK-8), substance P (SP) and carbachol. 2. Individual cells exhibited [Ca2+]i responses to all three agonists. 3. In the absence of external Ca2+, all the agonists initiated [Ca2+]i peaks which, particularly at high agonist concentrations, rapidly declined. 4. SP induced repetitive monophasic [Ca2+]i transients which started from basal [Ca2+]i even after elevation of the external Ca2+ concentration. 5. CCK-8 triggered similar oscillations, which particularly at high agonist concentration or after elevating external Ca2+ became superimposed upon a sustained elevation of [Ca2+]i. 6. Carbachol-induced oscillations were more complex with [Ca2+]i transients superimposed on slower waves. 7. At high carbachol concentrations or elevation of external Ca2+ the slow waves fused into a sustained increase of [Ca2+]i. 8. The protein kinase C (PKC) activator 12-O-tetradecanoylphorbol-13-acetate attenuated the agonist-induced [Ca2+]i responses, and this effect was reversed by the PKC activator staurosporine. 9. The results indicate that oscillations of [Ca2+]i induced by SP, CCK-8 and carbachol involve intracellular mobilization of Ca2+. 10. CCK-8 and carbachol also cause a rise of [Ca2+]i by a mechanism more directly dependent on the presence of extracellular Ca2+. 11. In the case of carbachol the latter component is subject to oscillatory control. 12. The transition from oscillatory [Ca2+]i to sustained increase may be associated with inhibition of amylase release.

Insulin secretagogues induce Ca(2+)-like changes in cytoplasmic Mg2+ in pancreatic beta-cells

The effects of insulin secretagogues on the cytoplasmic Mg2+ concentration ([Mg2+]i) of pancreatic beta-cells were studied in suspensions and in individual beta-cells using dual-wavelength fluorometry and the indicator mag-fura-2. Average [Mg2+]i was in the 800-900 microM range in a medium containing 3 mM glucose. When the sugar concentration was raised to 20 mM, the cells reacted with an initial lowering of [Mg2+]i followed by an increase. The sugar apparently also stimulated leakage of the Mg2+ indicator. Addition of 100 microM tolbutamide or raising the K+ concentration by 25 mM caused relatively rapid increases of [Mg2+]i. Methoxyverapamil prevented the [Mg2+]i-increasing actions of glucose, K+ and tolbutamide. The greatest change in [Mg2+]i was obtained when beta-cells were exposed to 100 microM carbachol. In this case there was a more than 10% lowering, which was reversed upon removal of the agonist. Measurements of [Mg2+]i are important not only for understanding fluctuations of this ion, but may also aid to elucidate the mechanisms involved in the regulation of cytoplasmic Ca2+.

Activation of protein kinase C assists insulin producing cells in recovery from raised cytoplasmic Ca2+ by stimulating Ca2+ efflux

The effects of protein kinase C (PKC) activation on the cytoplasmic free Ca2+ concentration ([Ca2+]i) were studied in clonal insulin-producing RINm5F cells, using the fluorescent Ca2+ indicators quin-2 and fura-2. Both under basal and stimulatory conditions PKC activation lowered [Ca2+]i in these cells by promoting an active extrusion of Ca2+ to the extracellular space. PKC activation therefore assists insulin-producing cells in recovery from raised [Ca2+]i. Such an effect might be part of the signal regulating the insulin secretory process.