Temporal patterns of changes in ATP/ADP ratio, glucose 6-phosphate and cytoplasmic free Ca²⁺ in glucose-stimulated pancreatic β-cells

Closure of ATP-sensitive K⁺ (K_ATP) channels is part of the stimulus-secretion coupling mechanism in the pancreatic β-cell, leading to membrane depolarization and influx of Ca²⁺ through voltage-sensitive L-type Ca²⁺ channels. The elevated ATP/ADP ratio seen in the presence of high levels of glucose has been postulated to mediate the glucose-induced closure of the K_ATP channels and rise in cytoplasmic free Ca²⁺ concentration ([Ca²⁺]_i), or alternatively to be a consequence of activation of mitochondrial dehydrogenases by the increase in [Ca²⁺]_i. To distinguish between these two possibilities, the time course of the change in the ATP/ADP ratio was determined in comparison with that of [Ca²⁺]_i. We here show that a severalfold rise in the ATP/ADP ratio occurs rapidly on stimulation of suspensions of mouse pancreatic β-cells with glucose. The change in the ATP/ ADP ratio is an early event that begins within 20-40 s and precedes the rise in [Ca²⁺]_i. The temporal relationship indicates that the adenine nucleotide changes cannot be a consequence of the [Ca²⁺]_i changes and may indeed be the connecting link between glucose metabolism and [Ca²⁺]_i changes. When the cells were sequentially treated with high glucose concentration, clonidine and finally high extracellular Ca²⁺ concentration to induce synchronized oscillations in [Ca²⁺]_i in the cell suspension, corresponding oscillations in the ATP/ADP ratio were observed. Glucose 6-phosphate levels oscillated out of phase with the ATP/ADP ratio. These results support the hypothesis that the Ca²⁺ oscillations previously observed in glucose-stimulated single islets or β-cells may reflect oscillations in the ATP/ADP ratio that accompany oscillatory glycolysis.