Glucose-6-phosphate and Ca2+ sequestration are mutually enhanced in microsomes from liver, brain, and heart.
Microsomes prepared from three rat tissues were examined for their ability to import glucose-6-phosphate (G-6-P). Microsomes from liver, which possess a high level of glucose-6-phosphatase activity, were compared with those from cerebral cortex and cardiac muscle, which are not involved in the export of glucose and in which glucose-6-phosphatase activity is relatively low. In all three, a selective permeability to G-6-P was detected by light scattering. However, the sugar-phosphate specificity of the transport process differed. G-6-P was able to enhance ATP-dependent Ca2+ sequestration in all three types of microsomes. In addition, enzymatic detection of G-6-P after the rapid filtration of microsomes determined that, in the absence of Ca2+ and ATP, a level of intramicrosomal G-6-P approaching a passive equilibrium with the extramicrosomal G-6-P concentration was rapidly achieved in all three tissues. However, under conditions in which Ca2+ was being actively accumulated, 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+ ionophore or an inhibitor of the microsomal Ca2+ ATPase. These data are consistent with a selective import pathway into the endoplasmic/sarcoplasmic reticulum for G-6-P independent of glucose-6-phosphatase activity. In addition, they suggest an alternate explanation for the enhanced sequestration of Ca2+ by the endoplasmic/sarcoplasmic reticulum of intact cells seen when extracellular glucose is increased.
Chen
P Y
PY
Department of Cell Biology, the University of Alabama at Birmingham, 35294-0005, USA.
Csutora
P
P
Veyna-Burke
N A
NA
Marchase
R B
RB
D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002540:Cerebral Cortex / Q000378:metabolism*; D005260:Female; D005952:Glucose-6-Phosphatase / Q000378:metabolism; D019298:Glucose-6-Phosphate / Q000378:metabolism*; D006706:Homeostasis; D007700:Kinetics; D008027:Light; D008861:Microsomes / Q000378:metabolism*; D008862:Microsomes, Liver / Q000378:metabolism*; D009206:Myocardium / Q000378:metabolism*; D009928:Organ Specificity; D051381:Rats; D017207:Rats, Sprague-Dawley; D012542:Scattering, Radiation
D019298:Glucose-6-Phosphate; D000255:Adenosine Triphosphate; D005952:Glucose-6-Phosphatase; D002118:Calcium