Regulation of steady-state free Ca2+ levels by the ATP/ADP ratio and orthophosphate in permeabilized RINm5F insulinoma cells

• Published in: The Journal of biological chemistry 1988-03, Vol.263 (9), p.4247-4253
• Main Authors: Corkey, B E, Deeney, J T, Glennon, M C, Matschinsky, F M, Prentki, M
• Format: Article
• Language: English
• Subjects: Adenoma, Islet Cell - metabolism; Adenosine Diphosphate - metabolism; Adenosine Triphosphate - metabolism; Animals; Antimycin A - pharmacology; Biological and medical sciences; Calcium - metabolism; Cell Membrane Permeability; Cell physiology; Fundamental and applied biological sciences. Psychology; Insulinoma - metabolism; Magnesium - metabolism; Molecular and cellular biology; Oligomycins - pharmacology; Pancreatic Neoplasms - metabolism; Phosphates - metabolism; Phosphates - pharmacology; Secretion. Exocytosis; Tumor Cells, Cultured - metabolism
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1016/S0021-9258(18)68916-1

Stimulation of insulin secretion in the pancreatic beta-cell by a fuel such as glucose requires the metabolism of the fuel and is accompanied by increases in oxygen consumption and intracellular free Ca2+. A very early signal for these events could be a decrease in the cytosolic ATP/ADP ratio due to fuel phosphorylation. To test this hypothesis the regulation of free Ca2+ was evaluated in permeabilized RINm5F insulinoma cells that sequester Ca2+ and maintain a low medium free Ca2+ concentration (set point), between 100 and 200 nM, in the presence of Mg2+ and ATP. ATP, creatine, creatine phosphate, and creatine phosphokinase were added to the media to achieve various constant ratios of ATP/ADP. Free Ca2 was monitored using fura-2. The results demonstrated that the steady-state free Ca2+ concentration varied inversely with the ATP/ADP ratio and orthophosphate (Pi) levels. In contrast, no correlation between free Ca2+ and the phosphorylation potential (ATP/ADP.Pi) was found. Regulation of the Ca2+ set point by the ATP/ADP ratio was observed at ratios between 5 and 50 and at Pi concentrations between 1 and 7 mM, irrespective of whether mitochondria were participating in Ca2+ sequestration or were inhibited. Increasing the ATP/ADP ratio stimulated Ca2+ uptake by the nonmitochondrial pool but did not modify Ca2+ efflux. Glucose 6-phosphate (1 mM) had no effect on the Ca2+ set point. The data suggest that variations in the cytosolic ATP/ADP ratio induced by fuel stimuli may regulate Ca2+ cycling across nonmitochondrial compartments and the plasma membrane by modulating the activity of Ca2+ -ATPases. A mechanism linking fuel metabolism and cytosolic ATP/ADP ratio to activation of the Ca2+ messenger system in pancreatic beta-cells is proposed.