Effects of glucose on 45Ca2+ uptake by pancreatic islets as studied with the lanthanum method

Abstract

1. Fluxes of 45Ca2+ were studied in pancreatic islets from non-inbred ob/ob-mice. Because La3+ blocked the transmembrane fluxes of 45Ca2+ in islet cells, incubations aimed at measuring glucose-induced changes of the intracellular Ca2+ were ended by washing the islets with 2 mM-La3+ for 60 min. 2. Uptake of 45Ca2+ progressed for 2 hr; the intracellular concentration of exchangable Ca2+ was about 7 m-mole/kg dry wt., as estimated from the isotope distribution at apparent equilibrium in islets exposed to 3 mM D-glucose. Raising the D-glucose concentration to 20 mM enhanced the 45 Ca2+ uptake whether or not the islets had first been equilibrated with the isotope. The stimulatory effect of D-glucose was observed in Tris buffer containing no anions but Cl- as well as in polyanionic bicarbonate buffer. The effect could not be reproduced with equimolar L-glucose. 3. The rate of 45Ca2+ release was the same whether the islets had been pre-loaded in the presence of 3 or 20 mM D-glucose. Thus the 45Ca2+ that had been taken up in response to 20 mM D-glucose appeared to be released much more slowly than the bulk of intracellular 45Ca2+. The release of 45Ca2+ was not significantly influenced by D-glucose during the release period. Incubation for 30 min was require for half of the radioactivity to be released. 4. The rates of insulin secretion were about the same in uni-anionic Tris buffer as in polyanionic bicarbonate buffer. A marked insulin secretory response to 20 mM D-glucose was observed in either buffer. 5. It is concluded that 20 mM D-glucose causes a net uptake of Ca2+ from the extracellular fluid into the interior of the beta-cells. This uptake is probably not regulated at the level of the plasma membrane but more likely reflects an increased affinity of some intracellular phase or compartment for the ion. Because the observed uptake and release of intracellular 45Ca2+ are slow processes in comparison with the rapid effects of extracellular Ca2+ on insulin secretion, insulin secretion may also depend on a more superficial and La3+-displacable Ca2+ pool.