Calcium exchangeability in subcellular fractions of pancreatic islet cells.

The distribution and exchangeability of calcium within subcellular compartments of rat pancreatic islet cells was determined in pulse-chase experiments following isotopic labelling with 45Ca2+. In unstimulated islets the mitochondrial fraction showed the greatest, and the insulin secretory granule fraction the least, uptake of 45Ca2+; subsequently the rate of loss from each compartment during 120 min was in the order mitochondrial greater than microsomal greater than secretory granule fraction. On exposure to D-glucose, 16.8 mM, the 45Ca labelling of all 3 subcellular compartments was increased after 5 and 30 min stimulation. In contrast, theophylline 10 mM caused a significant decrease in mitochondrial 45Ca content with little effect on other compartments. These results are discussed in relation to the importance of calcium disposition, exchangeability, and mobility for the initiation and control of insulin secretion.