Abstract
Regulation of insulin release, membrane potential, transmembrane 45Ca fluxes and cytoplasmic free Ca2+ concentration, [Ca2+]i, was examined using suspensions of transplantable NEDH rat insulinoma cells previously cultured for 2-3 days to eliminate necrotic tumour cells and counter prior hypoglycaemia. Insulinoma cells displayed a resting [Ca2+]i of 94 +/- 8 nM (n = 17) and released 104 +/- 15 ng insulin 10(-6) cells (n = 7) during 60 min incubations with uptake of 2.7 +/- 0.2 nmol 45Ca 10(-6) cells (n = 7). High concentrations of glucose did not affect membrane potential, transmembrane 45Ca fluxes, [Ca2+]i or insulin release by insulinoma cells. K+ at 25 mM depolarised the plasma membrane, induced a small increase in 45Ca efflux and increased [Ca2+]i by 65%. This modest action was not associated with demonstrable effects on 45Ca uptake and insulin release. The effect of 25 mMK+ on [Ca2+]i was counteracted by D-600, but this blocker of voltage-activated Ca2+ channels and verapamil lacked effects on transmembrane 45Ca fluxes and insulin release. The Ca2+-calmodulin antagonist, trifluoroperazine, was also without effect on 45Ca fluxes and insulin release. Ca2+ ionophore ionomycin increased [Ca2+]i, whereas A23187 and X537A did not affect transmembrane 45Ca fluxes. Moreover, insulin release was independent of extracellular Ca2+ over the range 0-20.4 mM despite marked affects on transmembrane 45Ca fluxes and a greater than 4-fold change of [Ca2+]i. Dibutyryl cyclic AMP increased insulin release by 55% without affecting transmembrane 45Ca fluxes or [Ca2+]i. The phosphodiesterase inhibitor, theophylline, also enhanced insulin release by 10-36% with no change of 45Ca uptake. The effectiveness of theophylline was independent of extracellular Ca2+ over the range 0-10.2 mM. These results indicate that inappropriate Ca2+ regulation is a key pathogenic feature underlying the inappropriate insulin secretion of rat insulinoma cells.
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Flatt, P., Abrahamsson, H., Arkhammar, P. et al. Measurements of membrane potential, transmembrane 45Ca fluxes, cytoplasmic free Ca2+ concentration and insulin release by transplantable rat insulinoma cells maintained in tissue culture. Br J Cancer 58, 22–29 (1988). https://doi.org/10.1038/bjc.1988.154
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DOI: https://doi.org/10.1038/bjc.1988.154