Glucose regulation of free Ca(2+) in the endoplasmic reticulum of mouse pancreatic beta cells.
Free Ca(2+) was measured in organelles of individual mouse pancreatic beta cells loaded with the low affinity indicator furaptra. After removal of cytoplasmic indicator by controlled digitonin permeabilization the organelle Ca(2+) was located essentially in the endoplasmic reticulum (ER), >90% being sensitive to inhibition of sarco(endo)plasmic reticulum Ca(2+)-ATPases. The Ca(2+) accumulation in the ER of intact beta cells depended in a hyperbolic fashion on the glucose concentration with half-maximal and maximal filling at 5.5 and >20 mM, respectively. Also elevation of cytoplasmic Ca(2+) by K(+) depolarization significantly enhanced the Ca(2+) accumulation. In permeabilized beta cells 1-3 mM ATP caused rapid Ca(2+) filling of the ER reaching almost 500 microM. At 50 nM, Ca(2+) ER became half-maximally filled at 45 microM ATP, whereas only 3.5 microM ATP was required at 200 nM Ca(2+). Inositol 1,4,5-trisphosphate induced a rapid release of about 65% of the ER Ca(2+), and its precursor phosphatidylinositol 4,5-bisphosphate was found to slowly mobilize 75% by another mechanism. It is concluded that glucose is an efficient stimulator of Ca(2+) uptake in the ER of pancreatic beta cells both by increasing ATP and cytoplasmic Ca(2+). Because physiological concentrations of cytoplasmic ATP are in the mM range, Ca(2+) sequestration can be anticipated to be modulated by factors reducing its ATP sensitivity.
Tengholm
A
A
Department of Medical Cell Biology, Uppsala University, SE-751 23 Uppsala, Sweden.
Hellman
B
B
Gylfe
E
E
D000255:Adenosine Triphosphate / Q000494:pharmacology; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D000252:Calcium-Transporting ATPases / Q000502:physiology; D004721:Endoplasmic Reticulum / Q000378:metabolism*; D005947:Glucose / Q000494:pharmacology*; D015544:Inositol 1,4,5-Trisphosphate / Q000494:pharmacology; D007515:Islets of Langerhans / Q000378:metabolism*; D051379:Mice; D019269:Phosphatidylinositol 4,5-Diphosphate / Q000494:pharmacology
D019269:Phosphatidylinositol 4,5-Diphosphate; D015544:Inositol 1,4,5-Trisphosphate; D000255:Adenosine Triphosphate; D000252:Calcium-Transporting ATPases; D005947:Glucose; D002118:Calcium