Barium has been used as an electron-opaque substitute for calcium in a study of the distribution of divalent cations between organelles in homogenates or intact rat islets of Langerhans. These were incubated in the presence of barium acetate. Accumulation of electron-opaque deposits was stimulated during incubation of islets in the presence of high glucose concentrations and was diminished in conditions in which intracellular cyclic AMP levels were raised. Mitochondria were found to be the principal sites of accumulation of electron-opaque deposits. Addition of dinitrophenol to homogenates or intact islets abolished mitochondrial barium accumulation. X-ray microanalysis of the deposits in frozen sections showed them to consist predominantly of barium and phosphate. These experiments serve to emphasize further the critical role of mitochondria in the regulation of divalent cation accumulation in B cells, and to confirm that a direct effect on intracellular distribution of divalent cations may represent one important mechanism of action of cyclic AMP in regulating insulin secretion.

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