Vn. Civelek et al., TEMPORAL SEQUENCE OF METABOLIC AND IONIC EVENTS IN GLUCOSE-STIMULATEDCLONAL PANCREATIC BETA-CELLS (HIT), Biochemical journal, 315, 1996, pp. 1015-1019
Stimulation of insulin release by glucose requires increased metabolis
m of glucose and a rise in cytosolic free Ca2+ in the pancreatic beta-
cell. It is accompanied by increases in respiratory rate, pyridine and
flavin nucleotide reduction state, intracellular pH and the ATP/ADP r
atio. To test alternative proposals of the regulatory relationships am
ong free Ca2+, mitochondrial metabolism and cellular energy state, we
determined the temporal sequence of these metabolic and ionic changes
following addition of glucose to clonal pancreatic beta-cells (HIT). C
ombined measurements of the native fluorescence of reduced pyridine nu
cleotides and oxidized flavin, intracellular pH, and free Ca2+ were pe
rformed together with simultaneous measurement of O-2 tension or remov
al of samples for assay of the ATP/ADP ratio. The initial changes were
detected in three phases. First, decreases occurred in the ATP/ADP ra
tio (< 3 s) and increases in pyridine (2 +/- 1 s) and flavin (2 +/- 1
s) nucleotide reduction. Next, increases in the O-2 consumption rate (
20 +/- 5 s), the ATP/ADP ratio (29 +/- 12 s) and internal pH (48 +/- 5
s) were observed. Finally, cytosolic free Ca2+ rose (114 +/- 10 s). M
aximal changes in the ATP/ADP ratio, O-2 consumption and pyridine and
flavin nucleotide fluorescence preceded the beginning of the Ca2+ chan
ge. These relationships are consistent with a model in which phosphory
lation of glucose is the initial event which generates the signals tha
t lead to an increase in respiration, a rise in the ATP/ADP ratio and
finally influx of Ca2+. Our results indicate that Ca2+ does not functi
on as the initiator of increased mitochondrial respiration.