Studies of the Ca2+ requirements for glucose- and carbachol-induced augmentation of inositol trisphosphate and inositol tetrakisphosphate accumulation in digitonin-permeabilized islets. Evidence for a glucose recognition site in insulin secretion.
The metabolism of D-glucose is believed to initiate and regulate insulin secretion by islet beta-cells, although the identity of the metabolite which couples glucose metabolism to the cellular events involved in insulin secretion is unknown. An alternative hypothesis involves the presence of a glucoreceptor for which there has been no biochemical evidence. We have investigated whether glucose recognition by the beta-cell is coupled to phospholipase C. We have used digitonin-permeabilized, [3H]inositol-prelabeled islets to study glucose and carbachol activation of phospholipase C. In this model, carbachol recognition by its muscarinic receptor was coupled to phospholipase C activation. D-Glucose (but not L-glucose) also stimulated phospholipase C activity in these permeabilized islets. This effect was not due to glucose metabolism since glucose 6-phosphate did not affect phospholipase C activity and since phosphorylation of [3H]glucose was not detectable in digitonin-permeabilized islets. Glucose had no effect on the myo-inositol-1,4,5-trisphosphate-5-phosphatase or 3-kinase activities. In the absence of agonist, free Ca2+ concentrations between 0.1 and 1 microM (as determined with a Ca2+-specific electrode) did not influence phospholipase C activity. Stimulation of phospholipase C activity by either carbachol or glucose required Ca2+ in the submicromolar range and was optimal at 0.5 microM free Ca2+.myo-Inositol-1,3,4,5-tetrakisphosphate production from permeabilized islets was synergistically augmented by Ca2+ (0.5-10 microM) and glucose. Phospholipase C activity in islets is therefore not directly activated by free Ca2+ concentrations in the submicromolar range. Furthermore, glucose per se activates phospholipase C activity independently of glucose metabolism. A working hypothesis based on these findings is that glucose is recognized by a site which is coupled to phospholipase C in islets.
Wolf
B A
BA
Department of Pathology, Washington University School of Medicine, St. Louis, Missouri 63110.
Florholmen
J
J
Turk
J
J
McDaniel
M L
ML
D000818:Animals; D002118:Calcium / Q000494:pharmacology*; D002217:Carbachol / Q000494:pharmacology*; D002463:Cell Membrane Permeability; D002478:Cells, Cultured; D004072:Digitonin / Q000494:pharmacology*; D005947:Glucose / Q000494:pharmacology*; D015544:Inositol 1,4,5-Trisphosphate; D007295:Inositol Phosphates / Q000096:biosynthesis*; D007328:Insulin / Q000378:metabolism*; D000078790:Insulin Secretion; D007515:Islets of Langerhans / Q000187:drug effects / Q000378:metabolism*; D007700:Kinetics; D008297:Male; D051381:Rats; D011919:Rats, Inbred Strains; D011956:Receptors, Cell Surface / Q000378:metabolism*; D013403:Sugar Phosphates / Q000096:biosynthesis*
D007295:Inositol Phosphates; D007328:Insulin; D011956:Receptors, Cell Surface; D013403:Sugar Phosphates; C027786:glucose receptor; C081655:inositol-1,3,4,5-tetrakisphosphate; D015544:Inositol 1,4,5-Trisphosphate; D002217:Carbachol; D005947:Glucose; D004072:Digitonin; D002118:Calcium