Rat pancreatic islets contain an ionized or readily ionizable calcium fraction that can be determined by the metallochromic indicator glyoxal-bis-(2-hydroxyanil) (GBHA). This calcium fraction is mainly localized in the secretory granules. The relationship between the effects of glucose on 45Ca uptake and on this ionized calcium fraction was investigated. In addition, the effects of glucose on total islet calcium content were also studied. Stimulation of isolated islets for 30 min with 15 mM glucose in the presence of 2.5 mM CaCl2 increased the 45Ca uptake but decreased the GBHA-Ca content, while the total calcium content was not affected. Deletion of CaCl2 caused, at 2.5 mM glucose, an abrupt decrease of GBHA-Ca, which did not occur at 15 mM glucose. Total islet calcium content decreased slowly at 2.5 mM glucose, but this was not significantly affected by glucose stimulation. Islet GBHA-Ca can be reduced by 70% and total islet calcium by 30% by means of washing with calcium-free buffer. Reintroduction of calcium at 2.5 mM glucose partly restored, but glucose 15 mM completely restored, the GBHA-Ca level within 5 min. The total calcium content was restored within 15 min independent of the glucose concentration. The increase of the islet calcium content equalled the 45Ca uptake at 2.5 mM glucose. The 45Ca uptake at 15 mM glucose was higher than the increase of the islet calcium content.
The results indicate that the intragranular Ca2+ pool, as measured by GBHA, is rapidly and dramatically altered by glucose stimulation. The glucose-induced changes of this Ca2+ fraction are not directly related to changes of the 45Ca uptake or the total islet calcium content. It is suggested that intracellular factors, particularly cAMP (production of which is Ca2+ dependent) and Na+ concentration, alter the amount of granular Ca2+.