Py. Chen et al., GLUCOSE-6-PHOSPHATE AND CA2+ SEQUESTRATION ARE MUTUALLY ENHANCED IN MICROSOMES FROM LIVER, BRAIN, AND HEART, Diabetes, 47(6), 1998, pp. 874-881
Microsomes prepared from three rat tissues were examined for their abi
lity to import glucose-6-phosphate (G-6-P). Microsomes from liver, whi
ch possess a high level of glucose-6-phosphatase activity, mere compar
ed with those from cerebral cortex and cardiac muscle, which are not i
nvolved in the export of glucose and in which glucose-6-phosphatase ac
tivity is relatively low In all three, a selective permeability to G-6
-P was detected by light scattering. However, the sugar-phosphate spec
ificity of the transport process differed. G-6-P was able to enhance A
TP-dependent Ca2+ sequestration in all three types of microsomes, In a
ddition, enzymatic detection of G-6-P after the rapid filtration of mi
crosomes determined that, in the absence of Ca2+ and ATP, a level of i
ntramicrosomal G-6-P approaching a passive equilibrium with the extram
icrosomal G-6-P concentration was rapidly achieved in all three tissue
s. However under conditions in which Ca2+ was being actively accumulat
ed, the intramicrosomal levels of G-6-P exceeded the equilibrium value
by three-to fourfold. This enhanced sequestration was not observed in
the presence of Ca2+ or ATP alone or in the presence of a Ca2+ ionoph
ore or an inhibitor of the microsomal Ca2+ ATPase. These data are cons
istent with a selective import pathway into the endoplasmic/sarcoplasm
ic reticulum for G-6-P independent of glucose-g-phosphatase activity.
In addition, they suggest an alternate explanation for the enhanced se
questration of Ca2+ by the endoplasmic/sarcoplasmic reticulum of intac
t cells seen when extracellular glucose is increased.